Chief Mate Orals Revision Notes

Orals Revision

Notes

Task – Revision Notes:

Manoeuvring

Interaction

A vessel moving along experiences unwanted forces caused by the flow of water and unequal pressure around the hull.

 A build up of positive pressure builds up ahead of ship caused by water piling up ahead of the vessel trying to resist its fwd movement.

 The water down the sides of the ship creates a balancing area of negative pressure.

 The negative pressure over compensates the positive pressure so another smaller area of positive pressure astern is created.

 These pressure zones not only go outwards they also act vertically downwards.

Bank Effect

This occurs when a vessel is passing a gently shelving bank, the positive

pressure forward pushes the bow away from the bank. The Negative pressures draw the stern towards the bank. If the forces are very strong then it may cause the vessel to roll towards the bank increasing the vessels draught.

 To control the effect you have to constantly correct the amount of helm that is being applied.

Bow Cushion effect

When the river banks sides are steep sided the vessel will experience

constructive forces. The forward pressure area is partially constrained on the obstructed side and therefore creates a cushion at the bow.

As long as the stern is kept far enough away that the negative forces do not create a suction then you can balance the outward turning force of the bow cushion with the inward turning force at the stern, this is done by applying helm towards the bank.

If you are navigating in a narrow channel which is constrained on both sides then the bow cushion forces the vessel to take the center line of the channel. If you pass an opening in the channel this loss of pressure will cause the bow to turn towards the opening.

Squat

 The restriction at the bow causes the speed of water to flow under the fore foot of the vessel to increase

 This increase of water causes a low pressure area under the fore foot to form which in turn leads to a loss of buoyancy at the fore foot.

 Due to the loss of buoyancy the bow will dip

 Due to the bow dipping an increasing amount of water will build up in resistance

 This forces the bow deeper dipping the bow further until the buoyancy is equal to the downward weight of the vessel

 In addition to what is happening forward the speed of the flow of the water around the vessel also suffers an increase as it is drawn both along the ships side and down under the hull.

 This fall in water level causes the loss of the under keel clearance called SQUAT

 Another effect of squat is the reduced steering lever, during a turn this may lead to a sudden or rapid sheer of the vessel. If a vessel does experience sheer then a good burst of power is required to correct the sheer before reducing speed to reduce squat.

Squat = Cb x V 2

K

Cb = the block coefficient

V = speed in knots

K = a constant depending upon the depth : draught ratio

K = When the available depth of water is greater than twice the deepest draught then K may be assumed to be 100

K = Where the ratio is between 1 : 1.15 and 1 : 2 then K may be assumed to be 50

Ship to Ship Interaction

If two vessels meet head to head the combined positive bow pressures will cause the bows to be repelled

When the vessels are abeam the negative pressure zones cause a suction towards each other

As the vessels are stern to stern the negative pressure causes the vessels to be drawn together

Shallow water

 In shallow water the size of the turning circle and a loss of speed.  This is because in shallow water there is a much greater build up of

lateral resistance caused by the restriction of under keel clearance.  This causes the pivot point to move aft shortening the turning lever  The longitudinal water flow under the keel is increased and so the vessel

has to use more power to maintain speed

 There is also a restricted lateral flow caused by the increased draught on the outside of the turn.

 The result is that the rudder is less effective, the turn is dramatically reduced therefore the angle of drift is much shallower.

Ship Speed

The following factors effect ships speed: Boundary Layer

 Vessels carry a body of water surrounding the whole body of the ship, this water is on mm thick but it causes a loss of power

Shallow water effect on bow and stern waves

 In addition to the reduction of under keel clearance there is an increase of water around the hull which produces a larger bow and stern wave.

 In addition to squat the vessel will also have to increase speed to over come the bow and stern waves generated

Stopping

There are six main ways of stopping a vessel using just engines and rudders:  Crash stop

 Low frequency rudder cycling  High frequency rudder cycling  Turning under full helm

 Controlled speed reduction  Inertia stop

Crash stop – Putting engines from Full ahead to Full astern There is a immediate loss of control

Much better to reduce to slow ahead then slow astern, the chance of cavitation is reduced and control is maintained for longer

Low frequency rudder cycling –

This is designed to take greatest advantage of drift angle which develops as a vessel enters a turn.

High frequency rudder cycling –

This relies on the drag on the rudder to reduce headway Turning under full helm –

Good to use when the vessel is not restricted by sea room or depth.

This is probably the quickest and most efficient method of taking way off due to the drag on the hull created by the lateral

resistance to the turn Controlled Speed reduction –

The most practical way of taking way off the vessel in confined waters

Speed is reduced in stages until both vessel and engines are dead slow ahead

When at Slow ahead then Slow astern is sufficient to stop the vessel

Inertia Stop

This is simply stopping the engines and allowing the ship to stop The use of anchors for stopping

Dredging is particularly effective in eliminating speed and directional control. Having both anchors out shifts the pivot point to a position between the two anchors – this gives improved steering and makes it easier to control the bow. The additional drag created by the anchors is often sufficient to take all way off the vessel

Clearing a Foul Anchor

If the anchor becomes fouled on the sea bed then there are couple of methods that you have to try and break it out –

 Heave short and steam slowly over the anchor

 If this fails pay out some cable and steam around the anchor position, this should rotate the shank allowing it to break out

 If this fails you will need to buoy and break the cable.

Clearing a foul hawse

When a vessel has both anchors out it is inevitable that the vessel will swing round it moorings due to tide and wind. This will result in the anchor cables becoming crossed.

Clearing these turns can be tricky, you can do it by gentle engine moments and rudder movements steam round un-twisting the cable.

If this fails the foul must be cleared manually.

 Heave in so that the foul turns above the water

 Lash the cables together using natural fiber rope below the turns  Pass a preventer wire through the sleeping cable after the turns and

lashing

 Heave up the preventer wire to act as a slip wire and turn it up on the bits

 Walk back on the sleeping cable to expose a joining shackle

 Make fast the cable below the joining shackle and break the cable  Pass a wire messenger from the port side

 Make a half turn around the riding cable in the opposite direction to the turns

 Pass a wire up the stbd hawse pipe and attach it to the sleeping cable  Heave up on the messenger wire and slack on the easing wire

 This will remove the turns, one half turn at a time

 When all the turns have been removed, retrieve the sleeping cable by heaving on the easing wire

 Reconnect the joining shackle and remove the preventer wire  Move the lashing between the two cables

 Heave up and secure the anchors for sea

Hanging off an anchor

When a vessel is to moor to a buoy with its own cable it is therefore necessary to hang the anchor off

 Walk the anchor out till is clear of the hawse pipe  Secure anchor using wires and bottle screws

 Guillotines should be left in place

 Pass a wire through the D shackle on the anchor secure one end to the bits and the other end should be put on the windlass

 Rig a preventer wire in a slack position

 Walk back on the anchor till the weight is taken by the wire

 The wire will now be in the up down position and the preventer will be tight

 Pay out anchor until the first join shackle appears on deck

 Rig an easing wire below the joining shackle and then break the shackle  Slack back on the easing wire until the cable is clear of the hawse pipe  The vessel can now use the broken cable to moor to a buoy.

Open moor

The vessel should approach with the wind and weather approximately six points on the bow with sufficient headway, but not too fast it will cause damage to the anchors

 Walk both anchors to the waterline  Let go the windward anchor

 Continue making headway up to windward  Steam for 2 ship lengths

 Let go the Lee anchor

 Hold on to the windward anchor

 Rudder amidships and engines half astern this will move the stern round  Pay out on the leeward anchor

 Stop engines when the sternway comes on

 The vessel will be brought up when there is equal cable on both anchors at an angle of about 60º

Standing Moor

Used when the wind and tide are coming from different directions  Stem the tide

 Let go the upstream anchor  Move astern

 When the cable is twice the required length let go the down stream anchor

 Go ahead on the engines to cant the bow away from the first anchor  Vessel is now back in the middle position

 Now heave on the upstream anchor and pay out on the downstream anchor

Running Moor

Similar to the Standing moor except it is carried out differently

 Let go down stream anchor when the vessel is still moving ahead  Pay out twice as much cable as is required

 Let go up stream anchor

 Pay out the up stream anchor and heave on the down stream anchor until both lengths are the same

Medi Moor

Carried out in the Mediterranean where the wind is fairly predictable and the tide is minimal

 Make approach, when one and half ship lengths away let go the offshore anchor

 Steam round the anchor then kick ahead on the engines

 When the bow is one and half lengths past the intended final position let got he second anchor

 Come astern on the engines

 Go astern on to the berth adjusting both anchors so that there is even weight on both

 Run stern lines and make fast

Berthing

 Approach with minimum headway at an angle of 25º - 30º

 The bow should be aimed at a point just short of where you want to position the ship

 Stop engines well in advance and drift in

 When about a beams width off the berth Hard Stbd  Slow astern will cant the stern to port

Stbd side too no wind or tide

 Make the approach at 15º - 20º

 Aim roughly where the bow will end up

 When half a beams width from the berth back spring ashore  Dead slow astern

Port side too tide from ahead with a gentle on shore breeze

 Due to the tide there will be much better steering characteristics due to more water passing over the rudder

 Stop the vessel when still far away from the berth and assess the tide and wind strength

 Aim the bow at the final position  Approach at 25º - 30º

 When about a beams width off the berth round up to stem the tide  Balance this position and wind will bring the vessel alongside

Port side to Tide from dead ahead strong onshore breeze

 Aim the vessel 50 – 60m ahead of the final position and one a half beams width off the berth

 Let go the off shore anchor

 Now balance the engines so to stem the tide

 Using the anchor to control the bows closing speed come onto the berth

Port side to Tide from ahead moderate offshore breeze  Aim the bow at the final position

 Approach at 20º or less to allow for the vessel being blown off the berth

 When about a beams width off the berth round up to stem the tide  Balance this position and wind will bring the vessel alongside

 Pass lines as soon as possible

Avoid when possible approaching a berth with the tide astern of you, unless you have the assistance of tugs.

Navigating in Ice

 If a vessel is not down to her marks when navigating in ice you should do all you can to ballast her down ensuring that stability is not compromised

 Be aware of ballast water freezing especially in high sided tanks, fill only to 90% full to give some Free surface to it

 Trim by the stern as much as possible so that the props and rudders is as deep as possible

 Ensure search light is working, if not do not navigate at night  Always pass to windward of ice bergs

 When approaching ice from open water make your entrance at right angles, slowing down until vessel is nearly stopped

 Proceed at speed fast enough that you will not cause damage to the hull and slow enough ice will not form around you

 When following an ice breaker the idea is to follow in its wake but don’t get too close to it.

 If your vessel is in danger of having her props hit a berg then stop the shafts to avoid damage to the blade tips

 If your vessel is suffering from ice accretion then turn your vessel so the relative wind is on the opposite side. Be very careful not to induce stress fractures when using hammers or mattocks.

Tugs

Conventional Tractor

Azimuth stern drive

Conventional Tug

 Single prop big rudders

 As soon as she takes a tow the pivot point moves directly under the towing point

 If the angle of tow moves dead astern to 45º off the tug will not be able to return to a position with the tow dead astern without letting the tow go

Tractor Tug

 2 Voith Schnieder units

 Propulsion is fwd of the towing point

 This means the tug can pull in any direction and girting is much less of a problem

 Very expensive + hard to maintain

 Less bollard pull than a conventional tug

Azimuth

 Takes the best of both tugs

 Propulsion is a pair of independently rotating units both mounted at the stern

 2 towing points 1 fwd and 1 aft of mid ships

 Maneuvers much the same as the conventional tug

Conventional tugs are prone to girting due to the pivot point being fwd of the propulsion units. If the tug repositions or the ship takes a sudden swing then the line of the tow is displaced causing a turning moment, this can heel the tug violently possibly causing it to capsize

To minimize girting Gob ropes maybe used, this effectively bowses down the tow rope and moves the pivot point aft of the thrust

Task – Revision Notes: Passage Planning

Appraisal – Gathering all the information together:

 Charts

 Tidal Streams / atlas’s

 Publication: Mariners Handbook, ALRS, Pilot Books, Bridge Procedures Guide

 Routing Charts

 Ocean Current Charts  Weekly Notices to Mariners  M Notices

 IMO ships Routing  Guide to Port entry  Distance Tables  Ice Charts

 Ocean Passages for the world

 Annual Summary notice to mariners

Planning – Putting the lines on the charts and making the passage plan Execution – Selling and going over the plan with the master

Monitoring – Actually carrying out the plan and monitoring its effectiveness Weekly Notice to Mariners

Section 1 Explanatory Notes, Index for section 2

 Contains explanatory notes and advise on the use of charts and publications followed by an index of notices and chart folio index of charts effected together with the geographical region

Section 2 Admiralty Notices to Mariners – Chart corrections

 Contains notices for correction of charts including notices effecting navigational charts and are listed consecutively from the onset of the year

 Contains T’s and P’s notices relevant to the week. The last weekly notice for each month will also list the T’s and P’s remaining current  Any new addition charts together with new publications issued  Latest editions of publications are listed at the end of March, June,

September and December Section 3 Reprints of Radio warnings

 Contains all Navarea messages in force with reprints of those issued in the week

 Also lists Hydrolants, Hydropacs, US special warnings received together with reprints in force for the those areas

 The first weekly notice for each year contains a list of Navearea, Hydrolant and Hydropac messages

Section 4 Corrections to admiralty sailing directions

 Contains all corrections affecting Sailing Directions for that week  A cumulative list of these corrections is published each month Section 5 Corrections to admiralty list of lights and fog signals

 Contains all corrections for that week

Section 6 Corrections to admiralty list of radio signals  Contains all corrections for that week

Routing charts

The following Information is found on a monthly routing chart:  Ice information – max limit

 Position of ocean weather ships  Recommended tracks and distances  Bailie wind rose

 Areas of predominant poor visibility  Mean air temperature guide

 Wind force guide

 Dew point and mean sea temperatures  Loadline demarcation limits

Task – Revision Notes: Gyro

A gyroscope is a heavy wheel which when at high speed will rotate around its spin axis and is free to move around two other axis’s mutually perpendicular to each other. These other two degrees of freedom allow the gyroscope to turn in azimuth and tilt.

Gyroscopic inertia

 Before the gyro starts to spin its spin axis can be moved in any direction  When it starts spinning it exhibits resistance against efforts to change

the direction of its spin axis this is gyroscopic inertia

o Inertia is related to the shape and weight of the gyro, the distribution of that weight and the rate of spin of the wheel o For the optimum performance you need a wheel with the weight

heaviest around the rim, which is spinning as rapidly as possible  Gyroscopic inertia ensures that the spin axis will continue to be directed

towards a fixed point in space

 Consequently the movement relative to the earth allows gyroscopic inertia to be divided into tilt and drift

Tilt

 The vertical movement of a gyro axis relative to the earth

If the gyro is situated at the equator horizontal with the spin axis pointing east the gyros spin axis will steadily tilt upwards so after about 6 hours it will be vertical, it will then start to tilt down wards and after 12 hours it will be pointing west. It will continue downwards until after 18hours it will be pointing vertically downwards, after which it will start to tilt upwards again until it is directly east again.

When the gyro is at either pole and horizontal it will follow its representative star around the horizon with no change in tilt.

Drift

A free gyroscope sited at either pole with its spin axis horizontal will

apparently move in a clockwise direction when viewed from above the North Pole – due to the counter clockwise rotation of the earth. It will move in an apparent counterclockwise rotation when viewed from the south pole. When placed on the Equator there will be no drift

Precession

If you apply a torque perpendicularly to the spin of the axis the axis will move in a direction perpendicular to that of the applied torque. This is called

precession and is the result of the gyro trying to re balance itself to accommodate the two demands made on it.

 If the torque applied about the spin axis in the plane wheel its effect is to reduce / increase the speed of rotation – increase / decrease the load on the motor.

To work out which way precession is going to take place you need to know the direction of spin on the wheel.

Next you just rotate the torque through 90º in the direction of the spin to ascertain the direction of precession

Gyro Compass

Precession is very useful and is utilised to make the gyro north seeking. Assume:

 The axis is horizontal and is pointing to the east at a rising star  As the star rises the north end of the axis will tilt upwards

 If adding weigh to the rotor casing asymmetrically to make it top or bottom heavy the axis can be made to precess towards the meridian as the gyro tilts

 When the gyro is horizontal the added weight is either directly above or below the wheel and causes no torque

Torque applied here

 This gravity controlled method unfortunately will only make the gyro very crudely north seeking rather than north settling

Methods of gravity control

The simple method as mentioned above with putting weights directly above or below the spin axis is highly unsatisfactory in a sea way where the weight would be subject to accelerations from rolling and pitching of the vessel

What to do…

The system of using liquid ballistics produces a top heavy effect by a high density fluid flowing under gravity from pots on the high side of the assembly which supports the wheel. This fluid is able to flow through a small bore tube to similar pots on the low side. The torque produced by this weight transfer has the same effect as the torque produced by a heavy top weight and results in precession to the meridian.

The bore of the tube is such that it resists the surge of liquid when the vessel rolls.

Gravity control using a pendulum effect

This is basically a pendulum bob which swings to the low side of a spin axis to produce torque, which in turn precesses the gyro towards the meridian. This is also impractical due to the vessel movement.

Damping

Horizon

Tube

As it tilts the fluid flows to the low side causing a torque, precession returns the gyro back to the meridian Gyro wheel

Without some means of damping a gravity controlled gyro will continue to follow an elliptical path. If the gyro is going to be useful then the size of this elliptical path must be reduced so that the axis finally settles on the meridian. A gyro may be damped in tilt or damped in azimuth. Damping in tilt is achieved by making any tilt of gyro produce a horizontal torque which results in vertical precession to oppose the tilt

Damping in azimuth is achieved by making a vertical torque and horizontal precession, this precession is out of phase with that achieved by the gravity control

Errors of a gyro

Course and speed

The cause and effect of tilt in a gyro has been that the gyro has maintained its position on the earths surface, unfortunately ships compasses are always on the move.

When a vessel is steaming North or South its bow is steadily tilting downwards relative to a point in space, this causes unwanted tilting on the gyros spin axis When a vessel is steaming East or West there is no tilting so no effect on the gyro.

If the gyro was responding to N / S motion it would settle with the spin axis E/W where there was no tilt.

As the gyro settles N/S when subject to the earth rotation alone and E/W when subject to the N/S component of ships speed a vector diagram can be drawn to show the error cause by the ships movement.

To correct for these steaming errors is done by correcting latitude and speed this done manually by applying the information to the lubber line by means of a cam and cosine groove.

Today latitude and Speed corrections are fed in manually or where it is linked to a GPS it may receive its information from there.

In either case the result is fed to a correction torque motor which creates a precession in tilt equal an opposite to the unwanted tilt.

Change of speed error

Steaming error is proportional to the ships speed and the cosine of the course. This normally would not be very high but a vessel navigating at 20kts at lat 70º may get an error of 8º. The gyro will therefore be unreliable for a period time while the axis completes its damped spiral path to the new settling position. Many modern gyro compasses are able to automatically produce a precessing torque that reduces steaming error problems whatever the course / speed / latitude

Questions and answers

 Free to tilt about its Horizontal axis  Free to drift about its Vertical axis  Free to Spin about its axis

Describe the two notable properties of a free gyro

 Gyroscopic inertia – The reluctance of the gyro to change its plane of rotation unless acted upon by an external force, thus the axle tends to maintain the same direction with respect to space, known as rigidity in space

 Precession – The movement of the axle of a gyroscope when an external force is applied to it. If a force is applied to one end of the spin axis is will move at right angles to both the applied force and the spin axis. The resultant motion is precession.

Upon what properties does the moment on inertia of a free gyro depend

 The speed of the wheel  The mass of the wheel  The distribution of the mass

What is meant by the terms TILT and DRIFT when applied to the axle of a free gyro

 Tilt is any movement up or down  Drift is any movement east or west

Is the rate of tilt constant and how can it be calculated

 Yes, but it depends on the latitude of the gyro. At the equator, with the gyro spin axis pointing east – west, the axis will appear to tilt east end up, the tilt rate will be 15 per hour and there will be no drift. The tilt can be calculated at latitudes other than the equator by the equation 15 x Cos Latitude + Sin azimuth.

Is the rate of drift constant and how can it be calculated

 Yes, again it depends on the latitude of the gyro. At the north pole the gyro spin axis will, when viewed from above, drift clockwise at 15 per hour. There will be no tilt. The drift at latitudes below the north pole can be calculated by 15 x Sin Latitude

Why is a free gyro not suitable as a compass

 A free gyro is unsuitable as because : o It is unable to seek the meridian

o It must be accurately aligned with the meridian, and be regularly checked and adjusted.

o Frictional torque imposed by the gimble assembly causes the gyro to drift out of the meridian.

o It only passes the meridian twice in 24 hours

Describe how controlled precession is achieved

Control precession is achieved in the Sperry gyro by means of a liquid ballistic system. This is fitted to convert a free gyro into a controlled gyro as it provides a means of controlling the drift of a free gyro. This is achieved by fitting pots on either end of the gyro. Each pot is filled with equal amounts of mercury when the spin axis is horizontal. When the north end tilts up mercury transfers from the north pot to the south pot. This has the same effect as putting a downward force on the south end, which results in easterly precession of the south end and westerly precession of the north end. The amount of precession depends on how far the north end is above the horizon. As the north end tilts up Precession will be small, as it continues to tilt it will reach a point when Precession will match the easterly drift of the earth. It will now precess west. When it returns to the meridian there will be no tilt and Precession will be maximum.

Task – Revision Notes: GPS

The GPS system that we predominately use is the NAVSTAR GPS the American system – which stands for Navigation Satellite Timing and Ranging Global

Positioning System. Other systems are the GLONASS system and the new Galileo system.

Until recently the civilian access to the system was degraded but in 2000 the Selective Availability was removed.

When selective availability was in use the accuracy was only up to 100m, with the introduction of DGPS which was able to produce accuracy up to 1-3m

The GPS System is made up of 3 parts:

 Ground Control Segment  Space Segment

 User Segment

Ground Control Segment

Master control station

 Controls and monitors the satellite orbits

 Predicts performance and produces ephemeris for all satellites  Information of the health of the satellites is passed to each

satellite so they can pass this on to the users

In addition to the master control station there are four monitoring station in low latitudes that are evenly space round the world. This is so that satellites are always above the horizon of one or more stations

The monitoring stations collect data in the ephemeris production, they may also be used to transmit navigation data and commands to the satellites.

Space Segment

Each orbit has four active satellites, this configuration ensures that at least 4 satellites are available to a user anywhere on the earths surface.

User Segment

A GPS receiver determines the position of its antenna by simultaneously measuring the ranges from a number of satellites whose positions are

accurately known. Basically what actually happens is the receiver measures how far a code signal received from the satellite is out of step with a replica code generated within the receiver. Unfortunately the clock within the receiver is not synchronized exactly with the satellite time so direct calculation of range is not possible. But the errors for the satellite clocks are know and are sent out by the Ground Control Segments.

GPS signals are very weak and spread over a wide band width, therefore the receiver gets good signals and a lot of noise. So that the receiver does not pick up lots of stronger signals the design of the antennae is vital.

The weak signals are amplified as they are sent down the cable to the

antennae. Further amplification is carried out in the receiver to pick out and process the code.

The receiver’s micro processor then is able to devise the pseudo range.

There are 3 types of receiver on the market Parallel Receiver

 1 channel dedicated to each satellite this allows them to access all the satellites continuously and simultaneously

 Such receivers achieve better signal noise ratios and better pseudo range results

 Parallel receivers are typically used for highly accurate applications such as for surveying

Fast sequencing receivers

 Do not have dedicated channels

 They are able to rapidly switch between the channels of the available satellites

 Due to the measurements not being made simultaneously which may result in inaccuracies in the position

Multiplexing receivers

 These are very fast sequencing

 But are prone to noise which results in bad positions

Errors within the GPS System WGS84

GPS provides positions based on WGS84 (World Geodetic System 84) which is a mathematical model of the earth. This requires all current charts to be

converted to WGS84 to ensure that they correspond with the actual position. The corrections are usually noted on the charts and obviously this provides a big opportunity for human error.

System Error

 Despite the constant monitoring of the satellites by control stations there will be small clock errors and ephemeris errors. Although the combined error is unlikely to give more than a 2m position error.

 Ionospheric delay is caused by refraction in the ionosphere, this delay

can be calculated and supplied to the user via the Control Stations  Tropospheric delay can not be calculated, this will only produce a small

error when using a good receiver

 Multi path error refers to an error caused by receiving direct and

reflected signals – the receivers in a modern set are programmed to detect this

 Noise is likely to cause errors in positions obtained from the GPS,

warning of solar activity may be included in this category

Dilution of precision

When fixing normally we know that 3 bearings cut at 60º is ideal, the GPS system will automatically select available satellites to provide that accurate fix.

The user receives an indication of the accuracy of the fix he has received and the expected current satellite geometry.

This is provided by the receiver and available on the display as Dilution of Precision (DOP):

 GDOP – Geometric Dillution of Precision applies to four dimensions (N/S,

E/W, height and time)

 PDOP – Position Dilution of Precision applies to three dimensions (N/S,

E/W and height)

 HDOP – Horizontal Dilution of Precision is the most valuable to a navigator because it indicates the probable accuracy of the N/S and E/W dimensions)

 EDOP – Easting Dilution of Precision is of interest when longitude errors

are most critical

 NDOP – Northing Dilution of Precision is of interest when latitude

accuracy is critical

 TDOP – Time Dilution of Precision applies to time accuracy only

 VDOP – Vertical Dilution of Precision when altitude is critical

If your HDOP reading is 2 then there is a 95% probability that the GPS position is within 200m of the true position.

A DGPS base station is erected over a known position and the co-ordinates of it are entered into the receiver.

Given the true position and the ephemeris of all the satellites the base station receiver is able to calculate a series of true ranges. It simultaneously measures the pseudo range to the same satellites. The difference between the two provides a set of range corrections that is then transmitted to users in the area, resulting in a more accurate position.

For DGPS frequencies check the ALRS Vol. 2

Errors with DGPS

 The maximum separation between the DGPS station and the user should be 300Nm

 Noise can result in an error of a number of meters in the pseudo range  Multi path error cannot be removed but receivers compensate for it

Radar

Fundamentally a RADAR is a precision clock which enables the time between a transmitted radio signal leaving the set and the reflected pulse returning to the set to be measured.

 RADAR can measure the bearing and distance of most objects at quite long ranges by using radio waves or electro magnetic vibration

The basic set

 Transmitter – This generates the radio waves

Transmits the high energy radio waves and receives the low energy echoes

 Waveguide - Metal tubing that carries the high frequency radio waves

 Antennae - This directs the radio waves towards the objects and receives the echoes

It makes it possible to work out the bearing of the target

It collects the received echoes and directs them towards the receiver

Scanners transmit energy from the magnetron in a narrow beam, this horizontal width of beam is usually defined as the angle between half power points

 Receiver - A device that detects the presence of any echoes and amplifies them ready for display

 Time base or Trigger - The means of measuring the travel time of the pulse and echo

Plan Position Indicator (PPI)

Horizontal beam width

 The angle P1 A P2 is the horizontal band width

 P1 and P2 are the points which a receiver being moved across a beam at a constant distance from the scanner would register half power

 In the diagram above the horizontal band width is greatly exaggerated an in reality it is less than 2º

 It is very difficult to design a scanner which only has a main beam and much smaller less powerful beams exist – side lobes

 The scanner width and wavelength of the radar frequency determine the horizontal beam width- The wider the scanner the smaller the

Horizontal bandwidth

The shorter the wave length the smaller the Horizontal bandwidth P1 P2 Main Lobe Half Power Half Power Maximum Power Side Lobes A

Vertical Bandwidth

 More complicated than horizontal bandwidth and the way scanners are designed there is much more Vertical bandwidth than Horizontal

Bandwidth

 The sea surface reflects energy breaking up the vertical pattern into lobes - this is because the waves of energy and reflected waves of energy are sometimes in phase and sometimes out of phase, this results in lobes

 The number of vertical lobes formed depends on the wavelength and the height of the scanner

o By taking the length of the vertical line from the scanner to the sea surface and dividing that length by half the wavelength used in the radar pulse you can find the number of vertical lobes  The distance between the vertical lobes increases with distance from

the vessel

Radar Transmission

For a good echo to be produced the radar pulse must:  Have high energy

 Be of short wavelength to enable to echo to be accurately timed  Be generated for only short periods of time to ensure echoes from

nearby objects are detected

Pulse length and Pulse Repetition Frequency

Typical pulse lengths:

Short 15m 0.5µ sec Medium 75m 0.25µ sec Long 300m 1.00µ sec

A large number of these pulses are generated every second this is known a Pulse Repetition Frequency

The interval of time between successive pulses is known as Pulse Repetition Interval

Due to the high speeds of the pulses echoes from targets at a long range will return before the next pulse is transmitted

RADAR’s Limitations

Due to the curvature of the earth there is a limit to what we can see with the naked eye, because light is refracted we are able to see roughly 6% further than the theoretical horizon.

RADAR waves are also refracted and because they are at a lower frequency than light they are bent further so the RADAR can see 15% further than the theoretical horizon.

Discrimination

Differentiation of targets depends on three factors: 1. Spot size

2. Pulse Length

3. Horizontal beam width Spot size

The electron beam in the CRT is focused as finely as possible, the degree of focus governs the spot size, this is very much dependent on the range scale in use.

Pulse length

A detected echo produces an intensification of the electron beam

The duration of the bright spot on the screen is therefore a function of the duration of the received echo – the duration of the echo is the pulse length. Targets which are on the same bearing and closer than half the pulse length will appear as one target – therefore we can say that Range Discrimination is dependent on pulse length and spot size

Range and bearing accuracy

Bearing accuracy is governed by: 1. Horizontal Beamwidth 2. Scanner to trace sync 3. Heading marker 4. Aerial squint error

If the scanner to trace synchronization is not correct then there will be errors in bearing:

 One revolution of the trace on the CRT must happen in the same time as one revolution of the scanner

 The orientation of the picture on the screen must be correct to the orientation of the heading marker

 The heading marker is operated by a switch which is activated every time the scanner passes it, this must be adjusted so that the heading marker intersects the picture at the correct point Squint error

This is in a slotted wave guide scanner and is caused when a magnetron

produces a slightly different RF pulse from that which the aerial is designed for is introduced into the set.

Errors seen on the PPI False Echoes

These echoes may appear on the screen when there is no real target there, there are 6 types:

1. Indirect – Caused by obstructions in the path of the radar beam, Think masts on the Surf when targets would appear in the blind sector when in fact they were ahead of you

2. Sidelobe – When very good target are present at close range there may be sufficient energy returned from the side lobes to generate echoes on the screen, these echoes may appear in an arc on either side of the target

3. Multiple – caused by a radar pulse being reflected backwards and forwards between two reflective surfaces before being received by the scanner

4. Interference – Caused by the radar picking up pulses of other radars operating in the vicinity using a similar transmission frequency and similar pulse repetition frequency. Normally causes a spiraling pattern on the display

5. Second Trace returns – In some cases the echo will return to the scanner after the next pulse has been sent, the system assumes that the echo is from the second pulse and will paint the target accordingly

6. Ghost – This will occur when a vessel is approaching power cables which span a channel, the appearance on a radar screen is that the target is on a steady bearing, even with evasive action the target will remain on a steady bearing

Automatic Radar Plotting Aids

This is a computer attached to a Radar which able to automatically measure ranges and bearings of selected targets. From a series of ranges and bearings a track history can be formed:

 True track  True speed  CPA

 TCPA

Remember the ARPA shows you what the target has done NOT what it is doing now.

Automatic Identification System

AIS is a shipboard broadcast transponder system operating in the VHF radio band. It is designed to send the following information out:

 Ship Identification  Position

 Heading

 Ship length, beam, draught  Hazardous cargo

Each AIS system consists of:

 1 VHF transmitter  2 VHF receivers  1 VHF DSC receiver

 Standard marine electronic communications link to shipboard display systems

 Positioning and timing information is taken from either an integral GPS or and external one with a DGPS for coastal navigation

 Each system transmits and receives over two radio channels to avoid interference problems

The AIS transponder is usually working continuously whether it is near shore or mid ocean.

Requirements

All passenger ships and cargo ships of 300GT or more Ships constructed on or after 1 July 02 must have them Ships built before then are to be phased in as follows:

Pax and tankers By 1 Jul 03

Other Vessels 50000Gt+ By 1 Jul 04 Other Vessels 10 – 50000GT By 1 Jul 05 Other Vessels 3 – 10000GT By 1 Jul 06 Other Vessels 300 – 3000GT By 1 Jul 07

Electronic Charts

Raster Data:

 Produced from scanning the master components used in the production of a paper chart

 The resultant image is made up of coloured pixels  It is basically a scan of our normal charts

 You can not interrogate the objects on the charts to produce information on it

Vector Data:

 Produced by giving digital values to each and every object on the chart  The computer can identify these objects

 It is therefore possible to interrogate these objects to obtain information on them

 You are able to customize views as well due to the layering effect given

Display systems

There are two basic groups:

 ECDIS – Electronic Chart Display and Information System  RCDS – Raster Chart Display System

ECDIS is a navigation information system which compiles with IMO performance standards and which with adequate back-up arrangements can be accepted as complying with the up to date chart required IMO. The regulations state that you should be using Vector charts but as you can use Raster charts if there are no suitable vector charts available.

RCDS should only be operated together with an appropriate folio of up to date paper chart. There is no performance standard set out and therefore RCDS is unable to meet the requirements listed under SOLAS V/20, 2001 and is therefore not a legal equivalent to and an up to date paper chart

NAVTEX

This is a navigational telex service broadcasting safety messages on 518kHz It is possible to receive Navtex on radio telex but you should really use a dedicated system which comprises of the following:

 Receiver tuned to the broadcast frequency  Printer and cash roll paper

A microprocessor control ensures that a routine message already received will not be reprinted on subsequent transmissions

You are able to select services according to the user’s preference but the following services are permanent:

 Meteorological Warnings  SAR information

Echo Sounders

An echo sounder sends short pulses of ultra sonic sound vertically downwards from the vessel. When it is reflected off the sea bed it returns to the

transmission source. The time taken for the sound to return to the ship is measured and with the knowledge of the speed of sound in water is converted into a depth.

So basically an echo sounder is a time measuring device.

 A trigger fires the pulse generator producing an electrical pulse 10kHz – 250kHz

 Part of the pulse is sent to the recorder / visual display producing a mark on the paper or a blip on the screen

 Part of the pulse is sent to the transducer where it is converted from AC to Ultra Sound and is directed in a beam to the sea bed

 The sound is bounced off the sea bed

 The returning signal is received by the transducer and is converted back to an AC current

 An amplifier boosts the returning signal, which is much weaker than the transmitted signal

 A voltage is applied to the recorder or to the display painting a new trace

 The depth can be read off the scale

Possible Errors

Stylus Rate Error

The speed of pen arm or the belt being incorrect not synchronizing with the transmissions. If the belt is too fast the depth recorded will be too great Index Error

When the transmission or zero mark is not zero. False bottom Error

False bottom readings may be obtained when the depth of water is such that the time taken for the returning echo is greater than the time taken for the pen to one or more revolutions and the next pulse has been transmitted Multiple Echoes

These occur when the pulse bounces between the sea bed and the keel or the sea surface, this will give multiples of the true depth.

The transmission from the transducer is approximately conical shape this shape reduces the loss of returns due to rolling and pitching, but it can lead to

incorrect readings when the bottom contour shelves steeply. Side Echoes

These may be from an object not immediately below the vessel but whose slant depth is less than the depth of water. This is due to side lobes from the

transducer and may occur in dredged or man made channels, when the echoes return from the walls before the bottom.

Separation Error

When different transceiver and receiver transducers are used. When in shallow water the limit with the vessel almost aground the depth recorded would be half the distance between the TX and RX transducers.

Aeration

When the transmitted pulse encounters air bubbles up to 99.9% of the energy is reflected – it is therefore essential to position the transducer in a position where the transducer is not going to be effected by bubbles e.g. not the bow or stern.

Task – Revision Notes: Magnetic Compass

General

All ships over 150 GRT must be fitted with a magnetic compass, they must also carry a spare compass. This spare should be stored away from the bridge so it is unaffected by any casualty disabling the bridge. It is the owner and Masters responsibility to ensure the compass is in good working order.

Adjustments

The compass should be adjusted when:  They are first installed

 They become unreliable

 The ship undergoes structural repair or alteration

 Electrical or magnetic equipment is installed or removed close to the compass

 A period of 2 years has elapsed since the last adjustment and a record of deviations has not been maintained or such deviations are excessive

Changes in Magnetism during the life of the ship

Masters should check the performance of the compass if  Carrying cargoes with magnetic properties

 Using electromagnetic lifting appliances to load or discharge  Major collision or electrical discharge

 When the ship has been laid up for a period of time

Monitoring

Compass errors should be determined after each large alteration and at least once a watch. A person holding a certificate of competency as compass adjuster must make any adjustments. If the master deems it necessary a person holding a Masters licence may make adjustment.

Deviation card

Deviations at points around the compass Position of fore and aft magnets

Position of athwartships magnets Size of the flinders bar

Position of Kelvin spheres from centre Position of heeling magnets

Coefficient B

The fore and aft component of the ships permanent magnetism is known as Force P. It effects the compass needle by attracting it forward (if does this then the ship has a magnetically blue bow for this example)

The ships head in the diagram is the compass heading, therefore the north seeking compass needle is always vertically upwards

The completed diagram shows that the deviation caused by the P force takes the form of a sin curve and produces Easterly deviation on Easterly courses and Westerly deviation on a westerly course

C rod

The c rod is the component of the ships vertical soft iron that has an effective upper pole on the compass position. This is induced magnetism and is

dependant on which hemisphere you are in. The C rod is usually the ships funnel as the heat coming out of the funnel creates induced magnetism. As with the P force we see that deviation varies as the sin of the course

C rod diagram

Coefficient C

The solitary cause of the cosine component of the deviation is force Q and is due to athwartships permanent magnetism. It is considered positive if it attracts the compass needle to starboard

The diagram shows that the deviation caused by force Q takes the form of a cosine curve. It causes westerly deviation on Northerly courses and easterly deviation on Southerly courses

The deviation will increase with magnetic latitude. This is corrected by the athwartships magnets.

Coefficient D

This deviation is the result of a combination of fore and aft and athwartships soft iron. It occurs on the inter-cardinal headings.

Therefore:

 Coefficient B is caused by force P and the C rod

 Corrected using the Flinders bar and the fore and aft Magnets

 Coefficient C is caused by force Q

 Corrected using the Athwartships Magnets  Coefficient D is caused by a combination of fore and aft and

athwartships soft iron magnetism

 Corrected using the soft iron spheres

Why may there be some deviation left in the compass after it has been adjusted?

The compass adjuster can only estimate how much soften iron there is effecting Coefficient B so therefore he may have balanced the amount of flinders bar and the fore and aft magnets correctly for the position the ship is in. If the ship then proceeds over the equator the induced magnetism changes, if there is excessive deviations on Easterly or Westerly headings then we can assume that the compass adjuster got the amount of induced magnetism wrong using the wrong length of Flinders bar.

You could adjust the fore and aft magnets to resolve this problem, or you could take regular errors noting down the different deviations, and then present these to the compass adjuster on your return. This is known as a split B problem due to the split between Permanent and Induced Magnetism.

What is retentive error?

This is the resultant magnetism that a ship will acquire if on a heading for a prolonged period – for example when tied up alongside for any time.

This error is not permanent and will disappear gradually over a few days. This should be noted when leaving port.

Suppose you were on a ship going from Southampton to the Far East, what adjustments would you make to the compass, if any?

 Adjust the Heeling Error Bucket as magnetic latitude changes

What is Heeling Error?

 Deviation caused by the heeling (rolling) of the ship

What would you do if the spheres had been removed from the compass?

 Check deviation card, marks on frame, etc.

What routine maintenance would you do on the compass?

 Visual inspection, check no bubbles, verify position of correctors, keep record of compass errors.

 North or South

What scenario would require spheres placed in the fore and aft line?

 Presence of coefficient E. Deviation caused by induction in the ships diagonal horizontal soft iron.

What preparations would you take into consideration before performing a compass swing?

 Funnel at normal sea going temperature  Upright Vessel

 Compass card tested for friction

 Lubber Line coincident with fore and aft line  Azimuth prism aligned

 Position of all deck equipment at normal sea going condition  Ships in vicinity more than 0.3M distant

 Steady on each heading to prevent guassin error

Task – Revision Notes: Chart work

Clearing Bearing

Clearing bearings must be so far from the Limiting Danger Line that if crossed the bow / stern of the vessel will still be in safe water.

Limiting Danger Line

This should have additional safe water depth built into the depth so that if you were to stray across it you would not go aground.

Clearing bearings / lines

Clearing marks are selected objects that when in transit or just open lead clear of a danger.

As long as the bearing of the church is not less than 260º and not more than 272º your vessel is clear of the danger.

Advance and Transfer

Advance = The amount the vessel has advanced along the original course after the wheel over point has been reached

Transfer = The amount the vessel has moved in right angles to the original course after the wheel over point has been reached.

To use Advance and Transfer you have to use the tables provided by the builder, they are worked out for different speeds and rudder angles.

NLT 260º

Find Tidal Stream

A ship steering 110º at 10kts departs from Position A at 0100 arrives at B at 0200, find the tide experienced during this period.

A - 0100

B - 0200 COG and SOG

Tide experienced, Speed is over 1hr in this case

Running Fix

The vessel below is steering a course of 110º at a speed of 10kts

0900 DR 0930 DR 0900 DR 0930 position Transfer the 0900 5nm up the course line

Things to remember to put on the chart

 Limiting danger lines  Clearing lines / bearings  Advance and transfer  Wheel over points

 ETA’s at wheel over points  Transit / compass errors  SBE time

 Call master times

 Astern propulsion check  Contingency anchorage  VHF channel for pilots / VTS  Expected visible ranges  Clear anchors

 PI’s

 Expected tidal stream  Leading lights

Task - Revision Notes: Weather

A low is an area of atmospheric pressure lower than its surroundings A low may be referred to a cyclone.

High

A region of higher atmospheric pressure to that of its surroundings A high may be referred to as an anticyclone

A ridge is the horizontal extension of the high away from its centre Fronts

Zones of bad weather connecting with low pressure regions Cold fronts – bring cold air, good visibility and showers Warm fronts bring warm air, poor visibility and showers

Occlusions are where cold and warm fronts merge, from this you get variable weather.

The Atmosphere

At the equator there is a low pressure belt around the earth, the air at this point rises to great height because it is strongly heated by solar radiation, the air particles will also hold a lot of water.

Horse Latitudes

The air continues to rise until it hits a ceiling at about 15-20km up – the air then moves pole-wards as it moves northwards it cools and becomes denser therefore it starts to sink. This occurs right round the earth and is called the Subtropical high pressure belt.

Trade winds

The air which has descended in the Subtropical belt has to go

somewhere so it forms part of the trade winds. From 20º - 40º N the air flows back into the equator in a layer close to the earths surface, completing the circulation.

Polar High

At the pole there is an area of cold slow moving air. The pressure is generally much higher than the other latitudes due to the air being very cold and dense.

Westerlies

From the North the Polar air moves south assisted by the centrifugal effect. Forcing against this is the air mass from the subtropical belt of high pressure. This resists the spread of polar air causing unsettled weather within the air mass due to the two air masses pushing forward against one another. This is called the westerlies belt.

Wind

Wind is air in motion

If there is a steep gradient – isobars are close together = fast flowing air If there is a gentle gradient – isobars are far apart = slow flowing air Coriolis

The Coriolis force does not move air it only deflects particles as soon as they start moving

If there was no Coriolis force air would move high to low by the shortest route, Coriolis causes the air to rotate.

 Northern Hemisphere rotates anticlockwise about a low rotates clockwise about a high  It is the other way round in the Southern Hemisphere At the equator Coriolis is Nil and the air flows directly from high to low At the poles Coriolis is at its highest deflecting air which wants to obey the pressure balancing force – this therefore prevents balancing highs and lows: this means that the closer a low or high gets to the pole the longer its life is.

Formation of a cold front

When cold air pushes underneath warm air, the warm air must move upwards. Because the air is forced up fairly quickly the result is the formation of clouds – Cumulus

Signs of a cold front passing

Ahead of front In front Behind front Cloud Low continuous

Status Nimbostratus & some Cumulonimbus

Cloud separating

WX Fog & rain Hvy Rain and

Thunder Isolated showers

Wind Dir & Spd

Constant Dir variable / gusty Veers 180º weakening

Temp Constant Falling Falling rapidly

Humidity High Falling Dry air

Visibility Mod – Poor Mod – Poor Good – v good

Pressure Falling Steady Rising Formation of a warm front

When warm air displaces cold air, it slides over the top of the cold air, the result is the formation of layer clouds – Stratus

Signs of a warm front passing

Ahead of front In front Behind front Cloud Developing cirrus Cloud all over

nimbostratus Clouded all over stratus or stratocumulus

WX Halo around sun /

moon Rain increasing then stopping followed by fog or mist

Fog patches & drizzle

Wind Increasing and

backing Freshening veering Dir + Spd constant

Temp Rising Rising Constant

Humidity Increasing V high High

Visibility Worsening Poor Mod

Pressure Falling steadily Falling slowly Steady then falling

Depressions often form on a front on the boundary of two air masses ‘warm & cold’

A depression appears on the chart as a series of isobars around a center point of low pressure.

Depressions give unsettled weather and will often be accompanied by strong winds

Main direction of movement for a depression is east in the Northern hemisphere.

They travel at varying speeds although a larger decaying depression will most likely be slower.

Fog

Fog is caused by air being cooled to its dew point (the point where air becomes saturated by the water vapour within it) the condensation of this water vapour produces fog.

Advection / sea fog

When warm moist air flows over a cold sea, the air is cooled to its dew point and advection fog occurs.

It is often only a small thin layer and mast tops can be seen over the top of it.

In temperate / high latitudes advection fog is most common in spring when the sea is at its coolest.

It is particularly prevalent where prevailing winds transport warm moist air over areas of cold water or major cold water currents.

Frontal fog

This may occur on warm front /occlusion if the temperature of the air in front of the front is very low

Frontal fog occur due to the mixing of warm and cold air on the two sides of the front

Arctic Sea smoke

This occurs in very high latitudes when cold air is blown over relatively warm sea. Evaporation occurs but the cold air is unable to hold the water vapour, so some of the water vapour condenses causing fog Usually found in gaps in ice fields / glaciers

Radiation Fog

Radiation fog occurs over low lying land on clear nights. It is due to cold air meeting relatively warm land

How to forecast fog

Warnings of fog can be observed by frequent monitoring of the wet and dry bulb thermometers.

It should be closely monitored whenever the air temperature is slightly higher or almost equal to that of the sea.

You should plot temperature against dew point. If the curves converge then you can expect fog

Land and sea breezes

Best known in tropical and subtropical climates. It occurs when there is unequal heating of the land and sea

By day the sun raises the temperature of the land but the sea temperature stays very much the same.

Air in contact with the land rises very quickly expanding as it rises due to the heating. The air from the sea flows into the gap and takes its place this creates an onshore wind.

Offshore wind

At night the air over the land cools rapidly, causing it to become denser and thus it starts to fall, this creates a pressure gradient causing air to flow out towards the sea

These land / sea breezes will be increased if:  The sky is clear

 Calm conditions

 Desert or dry barren coastline  High ground near the coast. Katabatic wind

A Katabatic wind occurs when radiation on a clear night causes cooling over sloping ground, the colder denser air will flow downhill producing a down slope wind.

Tropical revolving storms

The requirements for a TRS are:  Unstable air

 High sea temperature  High Humidity

 Low wind sheer

 Latitudes higher than 5º so that there is an adequate Coriolis force

The approach of a TRS may be indicated by:

 Rapid drop in pressure, more than 3mb below the seasonal average,

 Increasing wind speed  Change of wind direction

 High (cirrus) clouds becoming Cirro-Stratus, Cumulus, and then Cumulo-Nimbus

 Long low swell from the apparent direction of the storm centre If you believe there is a TRS nearby you can work out where your vessel lies in comparison to the eye easily by:

 Stop the vessel to find out the true wind speed and direction  Use Buys Ballots Law to estimate the storms centre – The observer

should face the wind, the centre of the storm will be

approximately 90° to the right of the observer in the Northern Hemisphere.

 A rough distance of how far the storm is away from you can be worked out by the wind force – Force 7 = 150nm from the eye

Force 8 = 125nm from the eye Force 10 = 75nm from the eye

Action to take in the Northern Hemisphere

If the wind is veering you are in the dangerous quadrant of the storm  Proceed at full speed

 Put the wind 10 – 45° on the stbd bow

 As the wind continues to veer alter to stbd with it

If the wind is backing then you are in the navigable semi circle of the storm  Proceed at full speed

 Put the wind on the stbd quarter

 As the wind continues to back alter to port with it

If the wind is steady then you are in the path of the approaching storm  Proceed at full speed

 Put the wind on the stbd quarter and head into the navigable semi circle  Once you are well into the navigable semi circle alter course to port

Action to take in the Southern Hemisphere

If the wind is backing then you are in the dangerous quadrant of the storm  Proceed at full speed

 Put the wind 10 – 45° on the port bow

 As the wind continues to back alter to port with it

If the wind is veering then you are in the navigable semi circle of the storm  Proceed at full speed

 Put the wind on the port quarter  As the wind veers alter to stbd

If the wind is steady then you are in the path of the approaching storm  Proceed at full speed

 Once you are well into the navigable semi circle alter course to stbd with the wind as it backs

If your ship is a situation where it can not out run the TRS then you should heave too. In the northern hemisphere put the wind off the stbd bow and in the southern hemisphere off the port bow. Adjust the engine speed so that the vessel is able to maintain steerage way but no more.

Veering = Clockwise

Backing = Anti Clockwise

AM = Arctic Maritime air  Cold dry and stable

 Warming as it travels South, humidifying and becoming less stable  Resulting in Cold and rain

PC = Polar Continental air  Cold dry and stable

 Warming with some humidification over the north sea becoming less stable and getting warmer

 Resulting in cool / cold, dry / showery weather TC = Tropical Continental air

 Warm dry and unstable

 Starts to cool therefore no humidification, stabilizing  Resulting in warm and dry

TM = Tropical Maritime air  Warm humid unstable air

 As it moves it cools and becomes saturated stabilizing  Resulting in rain and fog

PM = Polar Maritime air  Cold humid stable air

 As it moves it warms and the humidity will increase becoming less stable  Results in cool wet weather

Currents

Surface

 Wind blows over the water

 Friction transfers energy to the water

 The water initially moves in the same direction as the wind but Coriolis deflects the water to the ‘right in the Northern Hemisphere’  The surface current speed is 1/40th _{of the wind speed}

 The surface current direction is 30º from the wind direction Sub surface effect

 Surface layer transfers energy to the next layer down

 As the energy moves down to the next layer it is deflected by Coriolis  The speed decreases due to the viscosity

 The next layers follow a similar pattern but the direction keeps changing the further down you go due to Coriolis, this continues until the direction finally becomes negative, this occurs at approx 50m

Ice Accretion

Fresh water ice

 Forms at temperatures below 0ºC  Due to rain, snow, sleet, fog

 Causes a problem by freezing to aerials, GPS antennae, Radars o It will cause a Radar to turning (therefore keep it on when in

cold climates)

o Settles on GPS antennae and will weaken the signal Salt water ice

 Forms at temperatures below -2ºC

 It freezes on the foredeck

 Causes listing, change of trim – possible hogging, decreases freeboard  Increases KG

 Decreases GM What can be done?

 Alter course  Decrease speed  Seek shelter

 Head towards warmer latitudes

The process of freezing water and the effect of salinity

 Cold air -10ºC  Cold surface water

 Density of the water increases

 Convection brings warm water to the surface

 At 4ºC the Fresh water reaches its maximum density  Cools further, density decreases

 Convection stops in fresh water, convection carries on in salt water  Fresh water surface looses heat and starts to freeze when the

temperature reaches 0ºC

 Salt water continues to cool until it reaches -2ºC when convection stops and the surface starts to freeze