If you want to get the best performance out of your boat, you need to select the propeller and gearing that will suit your particular boat, engine and speed range. Below you will find a brief description of how propeller systems are designed. It is not just the engine capacity which determines the speed of the boat; it depends just as much on the efficiency of the reverse gear and the propeller system. Using the right propeller system will not only give you good fuel economy, higher speed; you will also experience greater comfort, with less noise and vibration.

Planing boats

In planing boats over 20 knots, the size of the propeller depends on the engine power. To transfer the power from the engine to the water, you need approximately 7–8 cm2 propeller blade surface per kW shaft power. If the shaft is at an angle in relation to the flow of the water, this requirement may be considerably greater: 8–15 cm2/kW is reasonable, depending on the angle of the shaft and the water flow. At a shaft power of 400 kW, therefore,
the propeller blade surface may need to be 400 kW x 9 cm2/kW = 3 600 cm2. This surface may be divided over three, four or five blades. The efficiency of a propeller blade diminishes when it becomes far too wide in relation to its length. This means that if the propeller diameter is limited in size (as is often the case), it is better to select several narrower blades (four or five) rather than three wide ones, for example. The angle of the propeller shaft should be
as small as possible. Shaft angles of less than 12° do not usually cause any major problems, but shaft angles of more than 14–15° should be avoided.
The distance between the bottom of the boat and the propeller blades should be at least 12–13% of the diameter of the propeller. When you have selected the diameter of the propeller, you are ready to go on to select the pitch. Propeller blades should no travel faster than 60–70 knots through the water at
70% of the maximum propeller diameter. This means that the speed of the propeller revolutions must be reduced when the engine capacity is greater, which requires a larger blade surface and therefore a greater diameter. The relations between pitch and diameter should be:

0.9–1.15 at 20 knots
1.0–1.3 at 30 knots
1.05–1.35 at 35 knots

Generally, a larger propeller with narrow blades and low revolutions is more efficient than a small, high-speed revolving propeller. When the boat’s speed goes
above 24–28 knots, the resistance of the shafts, rudders and propeller supports starts to become so great that the greater efficiency of the propeller is not beneficial. The resistance on the propeller system can be reduced by reducing the shaft diameter, selecting stronger materials and reducing the rudders and surfaces of the propeller supports. Lower gear ratios also mean thinner shafts. It is necessary to find a balance between propeller efficiency, water resistance on the shaft, etc.

Displacement and semiplaning boats

Boats of less than 15 knots need propellers which are as large as possible. For example, in a trawler it is possible to save 20–30% fuel or to gain 20% greater thrust when trawling by increasing the propeller diameter by 50% and reducing the propeller speed by 40%. The blade surface of the propeller is designed according to the minimum of 0.17 m2 per ton of thrust. As described above, a large, slow-moving propeller is preferable. At a speed of 12 knots, for example, a three-blade propeller with a 50% blade area will achieve an efficiency rate of approximately 57% if the propeller blade cuts through the water at 50 knots with 70% of its diameter. At a blade speed of 70 knots, approximately only 47% efficiency is achieved. The formula:

T Newton=propeller effiency x shaft output in kW x1944 / speed of boat

can be used to calculate the thrust and, therefore, the blade surface. Three-blade propellers are often more efficient for large, slow-moving propellers
than four-blade or five-blade propellers. However, four-blade propellers usually produce less vibration, which is often preferable. In general, there is a tendency towards four-blade propellers. A suitable pitch ratio at 10 knots is 0.7– 0.9 and at 15 knots 0.8–1.05. As the best pitch ratio varies according to
the speed of the boat, it is necessary to decide whether the propeller should be at its best when trawling, e.g. with a pitch ratio of 0.7, or whether it should be better when not trawling with a slightly higher pitch ratio. Adjustable propellers are an excellent solution for trawlers, tugs and freighters. As a very rough estimate, the bollard pull thrust can be calculated using the formula:

Adjustable propeller (kp) ¢ 13– 14 x hp
Fixed propeller (kp) ¢ 11–12 x hp

An adjustable propeller fitted to “the right boat” (up to 10 knots) can therefore save a lot of fuel.

Speed range between 15 and 20 knots

Within this speed range, a large slow propeller is preferable to a small, fast one. The blade surface is designed as a compromise between kW/cm2 and m2/ton of tractive force.
The above description is very general and describes only superficially how propellers are designed. The propeller manual “Propellers – Marine Engines, 60 to 120 Series, Part A – Calculation and Propeller Selection” contains a more in-depth description of propellers.
Volvo Penta also have various catalogs which contain proposals for propellers for each type of engine and reverse gear,“Propellers, Part B”. Over the last year, Volvo Penta has been developing computer programs for calculating speed, gear ratios and propellers. This is excellent for calculating speed and
propellers simply and safely. The estimated speed in the individual computer programs is based on the experience gained from a number of installations.
It is often better to calculate the speed from your own experience: propeller manual A and the like simply calculate the propellers using the computer
program. Volvo Penta is constantly developing new, powerful, more refined computer programs which will enhance both speed and propeller calculations. However, these calculations will never improve upon the values which have been built up over many years of experience and programmed into the computer.
Volvo Penta manufactures its own very efficient reverse gears and collaborates with leading suppliers of reverse gears and manufacturers of propellers, shafts,
stern tubes etc. If you buy the entire“package”, from engine to propellers, from us at Volvo Penta, you can be sure that all your components will fit together properly.

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