1 - Information from the chart
Most often the chart presents succinct Most often the chart presents succinct tid
tide e tatablebles s for for cercertatain in pospositiitionsons. . ThTheseese positions are marked with the “square”. positions are marked with the “square”. The table below shows us an example for The table below shows us an example for two different positions. The first refers to two different positions. The first refers to Co
Cowewes s (U(UK)K), , ththe e sesecocond nd to to a a poposisititionon south of Cowes.
south of Cowes.
Position
Position
Heights above LAT
Heights above LAT
M Meeaan n HHWW MMeeaan n LLWW S Spprriinngg NNeeaapp SSpprriinngg NNeeaapp C Coowweess 11,,7 7 mm 11,,5 5 mm 00,,2 2 mm 00,,4 4 mm 5 5,,2 2 mm 44,,3 3 mm 00,,4 4 mm 11,,2 2 mm
Th
This is dadata ta ononly ly prprovovidides es us us wiwith th avavereragage e hihigh gh anand d lolow w wawateters rs heheigighthts.s. Moreover, it is merely valid at spring or neap tides. To use it we need to first Moreover, it is merely valid at spring or neap tides. To use it we need to first find out how many hours we are from high water. Secondly, we need to find out how many hours we are from high water. Secondly, we need to kn
know ow if if it it is is spspriring ng or or neneap ap or or sosomemetitime me in in bebetwtweeeen n at at ththat at papartrticiculularar moment. We shall use this table to solve two types of problems. Finding moment. We shall use this table to solve two types of problems. Finding height of tide at a
height of tide at a particular location at a particular time:particular location at a particular time: To get over a shoal.
To get over a shoal. To pass under a bridge. To pass under a bridge.
Almanacs and many other nautical publications contain predictions of the Almanacs and many other nautical publications contain predictions of the tim
times of hes of high aigh and lond low tidw tides at mes at manany may major stjor standandard pard portortss . Als. Also listo listed ared aree differences in times of tides from these
differences in times of tides from these ports for additional secondary portsports for additional secondary ports . To work
. To work with this succinct data we need two extra tools:with this succinct data we need two extra tools: To interpolate between high and low water
To interpolate between high and low water he
heigighthts s we we ususe e ththee RulRule e of of TwTwelelveve. . WeWe a
assssumume e tthhe e ttididaal l cucurvrve e tto o bbe e a a peperfrfecectt si
sinnususoioid d wiwith th a a pepeririod od of of 12 12 hohoururs. s. ThThee height changes over the full range in the six height changes over the full range in the six hours between HW and LW.
hours between HW and LW.
○
○ During first hour after heigh water (HW) theDuring first hour after heigh water (HW) the
water drops
water drops 1/121/12thth of the full range.of the full range.
○
○ During the second hour an additionalDuring the second hour an additional 2/122/12thth.. ○
○ During the third hour an additionalDuring the third hour an additional 3/123/12thth.. ○
○ During the fourth hour an additionalDuring the fourth hour an additional 3/123/12thth.. ○
○ During the fifth hour an additionalDuring the fifth hour an additional 2/122/12thth.. ○
○ During the sixth hour an additionalDuring the sixth hour an additional 1/121/12thth..
Hence, two hours after the HW
Hence, two hours after the HW the water has fallen 3/12 of the water has fallen 3/12 of the full range.the full range. To interpolate between spring and neap tides we use the
To interpolate between spring and neap tides we use the Rule of SevenRule of Seven.. Since the change from spring range to neap range can be assumed linear Since the change from spring range to neap range can be assumed linear (instead of sinusoid), each day the range changes with 1/7th of difference (instead of sinusoid), each day the range changes with 1/7th of difference between the spring and neap
between the spring and neap ranges.ranges.
Hence, the daily change in range is (spring r
Hence, the daily change in range is (spring range - neap range)/7.ange - neap range)/7. Shoal problem:
Shoal problem:
Our shoal near Cowes has a charted depth of 1 meter and we would like to Our shoal near Cowes has a charted depth of 1 meter and we would like to cross it at about 15:00 hours with our yacht (draft 1,5 m).
cross it at about 15:00 hours with our yacht (draft 1,5 m). From any nautical almanac we find that HW occurs at
From any nautical almanac we find that HW occurs at 03:18 15:5303:18 15:53 and LWand LW occurs at
occurs at 09:45 22:0309:45 22:03 at a standard port nearby. We also find that at ourat a standard port nearby. We also find that at our location HW occurs one hour later and that spring tide is due in two days. location HW occurs one hour later and that spring tide is due in two days. Hence, we have a HW
Hence, we have a HW around 17:00.around 17:00. Via the rule of
Via the rule of seven we find out that today theseven we find out that today the rangerange is:is: spring range - 2 x ( (spring range - neap range)/7 )
spring range - 2 x ( (spring range - neap range)/7 ) <=> 4,8 - 2 x ( ( 4,8 - 3,1)/7 ) <=> 4,8 - 2 x 0,25
We also need
We also need today's HW height:today's HW height: which is Spring HW - 2
which is Spring HW - 2 daysdays x ( (5,2 -4,3)/7 )x ( (5,2 -4,3)/7 ) = 5,0 m= 5,0 m .. Via
Via the rthe rule oule of twf twelvelve we fe we find oind out tut that hat at tat two howo hours urs befbefore hore high igh wawater ter ththee height
height is:is:
5,0 - 3/12 x 4,3 = height at 15:00 hours
5,0 - 3/12 x 4,3 = height at 15:00 hours = 3,9 m= 3,9 m..
So, after three interpolations we derive the water height at 1500 hours. So, after three interpolations we derive the water height at 1500 hours. Considering the charted depth leads to an observed depth of 4,9 meters, Considering the charted depth leads to an observed depth of 4,9 meters, enough for our draft of 1,5 meters.
enough for our draft of 1,5 meters. Bridge problem:
Bridge problem:
An overhanging rock, power lines or bridges have their clearances charted An overhanging rock, power lines or bridges have their clearances charted with respect to another chart datum than LAT. Normally, 'high water' or with respect to another chart datum than LAT. Normally, 'high water' or 'MHW spring' are used as reference planes.
'MHW spring' are used as reference planes. An example:
An example:
Above our shoal hangs the 'Cowes bridge'. At 15:00 hours we would like to Above our shoal hangs the 'Cowes bridge'. At 15:00 hours we would like to pass this bridge, which has a charted height of
pass this bridge, which has a charted height of 20 meters to HW. Our 20 meters to HW. Our mast ismast is 23 meters high. In the example above we found that the water height was 23 meters high. In the example above we found that the water height was 1,1 meters below HW level
1,1 meters below HW level at that time. Obviously, we will have to at that time. Obviously, we will have to wait!wait! So, at what time will we
So, at what time will we be able to be able to pass under this bridge?pass under this bridge?
The water height must be 3 meters lower than HW level (5,0 m). That is The water height must be 3 meters lower than HW level (5,0 m). That is a
almlmosost t 99//112 2 oof f tthhe e rraanngge e ((44,,3 3 mm) ) iinnddicicaattiinng g ffoouur r hhoouurrs s aafftter er HHWW .. Conclusion, we will have to wait
Conclusion, we will have to wait at least six hours in total.at least six hours in total.