PITCH RATIO BY VESSEL APPLICATION

Deep water tug boat 0.50 – 0.55

River towboat 0.55 – 0.60

Heavy round bottom work boat 0.60 – 0.70

Medium wt. round bottom work boat 0.80 – 0.90

Planing hull 0.90 – 1.2

FIGURE 1.10

The propeller may be viewed as an axial pump that is delivering a stream of water aft of the vessel. It is this stream of water, equivalent in size to the diameter of the propeller, that is the power that provides

FIGURE 1.11

A propeller with a fixed pitch theoretically has a pitch velocity or linear speed it would travel in the absence of slip. However, because of the work needed to accelerate a mass of water, slip manifests itself as the difference between the pitch velocity and the velocity of the propeller through the vessel’s wake or speed of advance.

As a vessel moves through the water, hull resistance, wave formation and converging water at the stern have a tendency to follow the hull.

This results in a movement of water under the stern in a forward direc-tion known as wake. The added factor of wake reduces slip to what is known as apparent slip. It also adds to the speed of advance to pro-duce the actual vessel speed. It is obvious from this that propellers function in a very complex manner. There are many factors to be con-sidered when selecting a propeller. The point to realize is that there is no formula that will automatically provide the ideal propeller size for a given vessel and application. This can only be approximated to vari-ous degrees of accuracy. The only true test is trial and error under actual operating conditions. Remember, all propellers are a compromise. The general practice is to use the largest diameter propeller turning at the best speed for the vessel’s application within practical limits. These limitations are:

1. The size of the aperture in which the propeller is to be installed.

2. The application or type of work the vessel will be doing – towboat, crew boat, pleasure craft, and so forth.

3. Excessive shaft installation angles that may be required when using large diameter propellers.

4. The size of shafting that can be accommodated by the structural members of the hull where the shaft passes through.

5. Comparative weight of propellers, shafts and marine gears with respect to the size of the vessel.

6. The size of marine gears which the hull can accommodate without causing an inordinate degree of shaft angularity.

7. The vessel’s inherent ability to absorb the high torque that results from the use of large slow turning propellers.

8. Comparing the cost of using large diameter propellers against any increases in efficiency or performance.

Number Of Propeller Blades

In theory, the propeller with the smallest number of blades (i.e. two) is the most efficient. However, in most cases, diameter and technical lim-itations necessitate the use of a greater number of blades.

Three-bladed propellers are more efficient over a wider range of appli-cations than any other propeller. Four and sometimes five-bladed pro-pellers are used in cases where objectionable vibrations develop when using a three-bladed propeller.

Four-bladed propellers are often used to increase blade area on tow boats operating with limited draft. They are also used on wooden ves-sels where deadwood ahead of the propeller restricts water flow.

However, two blades passing deadwood at the same time can cause objectionable hull vibration.

All other conditions being equal, the efficiency of a four-blade propeller is approximately 96% that of a three-blade propeller having the same pitch ratio and blades of the same proportion and shape. A “rule of thumb” method for estimating four-blade propeller requirements is to select a proper three-blade propeller from propeller selection charts, then multiply pitch for the three-blade propeller by 0.914. Maximum diameter of a four-blade propeller should not exceed 94% of the rec-ommended three-blade propeller’s diameter. Therefore, we multiply diameter by 0.94 to obtain the diameter of a four-blade propeller.

For example, if a three-blade recommendation is:

48 34

As a word of caution, remember that this is a general rule...for esti-mating only. Due to the wide variation in blade area and contours from different propeller manufacturers, consult your particular manufacturer before final specifications are decided upon.

A “Rule of the Thumb” for all propeller selection is:

“Towboats – big wheel, small pitch”

“Speedboats – little wheel, big pitch”

All other applications can be shaded between these two statements of extremes.

Propeller Tip Speed

Tip speed, as the name implies, is the speed at which the tips of a rotat-ing propeller travel in miles per hour (MPH). The greater the tip speed, the more power consumed in pure turning. As an example, a 30 inch pro-peller with a tip speed of 60 MPH absorbs approximately 12 horse-power in pure turning effort. This is a net horsehorse-power loss because it contributes nothing to the forward thrust generated by the propeller.

The following formula can be used to calculate tip speed:

D SHAFT RPM  60  π T = _________________________

12 5280 Where:

T = Tip speed in MPH

D = Propeller diameter in inches Cavitation

When propeller RPM is increased to a point where suction ahead of the propeller reduces the water pressure below its vapor pressure, vapor pockets form, interrupting the solid flow of water to the propeller.

This condition is known as cavitation.

One of the more common causes of cavitation is excessive tip speed, a propeller turning too fast for water to follow the blade contour.

Cavitation can usually be expected to occur at propeller tip speeds exceeding 130 MPH. Cavitation results in a loss of thrust and damag-ing erosion of the propeller blades.

Reduction Gears

The reduction gear enables the propulsion engine and propeller to be matched so they both operate at their most efficient speeds.

The proper selection of the reduction gear ratio is an important deci-sion in preparing a marine propuldeci-sion system. There is a range of com-mercially available reduction ratios that can help assure optimum vessel performance under a given set of operating conditions.

It is difficult to discuss the selection of reduction gear ratios without mentioning some of the other factors that can influence the selection.

The major influencing factors are:

• Expected vessel speed • Type of vessel

• Vessel duty cycle • Pitch Ratio

• Propeller tip speed • Engine horsepower

Propeller Overhang

The maximum distance from the stern bearing to the propeller should be limited to no more than one shaft diameter. Propeller shafts are apt to vibrate and produce a whip action if these limits are exceeded. This condition is greatly accelerated when a propeller is out of balance due to faulty machining or damage.

Propeller Rotation

Propeller rotation is determined from behind the vessel, facing forward.

The starboard side is on the right and the port side on the left. Rotation of the propeller is determined by the direction of the wheel when the vessel is in forward motion. Thus, a clockwise rotation would describe a right-hand propeller and a counter-clockwise rotation would be a left-hand propeller.

Right-hand propellers are most frequently used in single screw instal-lations. Twin screw vessels in the U.S. are normally equipped with out-board turning wheels. However, there are some installations where inboard turning wheels will be found. A rotating propeller tends to drift sideways in the direction of the rotation. In a single screw vessel this can be partially offset by the design of the sternpost and the rudder. In a twin screw vessel this can be completely eliminated by using counter-rotating propellers. Although the question of inboard and outboard rotat-ing propellers has been debated many times, authorities on the subject agree that there are no adverse effects on maneuverability with either

rotation. In fact, there are those who feel that a gain in maneuverabil-ity is obtained with outboard rotating propellers. One point in favor of inboard rotation is a decreased tendency for the propellers to pick-up debris off the bottom in shallow water.

Multiple Propellers

The most efficient method of propelling a vessel is by the use of a sin-gle screw. However, there are other factors which, when taken into con-sideration, make the use of a single propeller impossible. If a vessel has to operate in shallow water, the diameter of the propeller is limited.

Therefore, it may be necessary to install two and sometimes three pro-pellers to permit a proper pitch ratio for efficient propulsion.

Another condition requiring multiple propellers is encountered when higher speed yachts need more horsepower than a single engine can develop and still be accommodated in the engine space. As a general rule to follow for calculations in this text, the total SHP of all engines is used when making estimated speed calculations. For calculating pro-peller size, SHP of each individual engine is used.