Prop pitch and max power

To the editor: Recently, I came across an article in Ocean Navigator by Chuck Husick on engine curves (“Engine curves define power output,” July/August 2005) and found it relevant to a question I have about the engine and prop on my new boat.

The boat is a 48-foot motorsailer with far too large an engine for its size, (i.e., the 360-hp engine used as an example in the article). I believe the engine is oversized because the boat can achieve its nominal hull speed of about 7.5 knots at about 2,400 rpm. You described very nicely the power available versus power absorbed by the propellor curves, which are very similar to those of a fixed-pitch airplane (I am also a small plane pilot). But I do think there is a considerable difference in the application of the curves for a plane and a boat. I do not have the equations on hand to show this, so allow me to describe the situation:

In an airplane with variable pitch, the prop speed is normally set and the prop pitch adjusted to get maximum power without overloading the engine, and this will presumably give maximum airplane speed at the prop speed. This is normally optimized at about 75 percent power. For a boat, the propellor pitch (fixed-pitch) is matched to absorb maximum power at the rated maximum rpm of the engine. In the case of the airplane, the power required from the engine-prop goes up at about the velocity cube of the plane. This is a high penalty in fuel burn for the distance covered but the speed increase may be worth it.

For a displacement-hull boat, the power required for increase in speed above the nominal “hull speed” is much steeper than the velocity cube, and for an engine that can reach hull speed at low rpm any increase in engine rpm is essentially wasted in fuel consumption. Of course, when the wind is on the nose and you are fighting heavy seas, you need more power to maintain anything near hull speed, but the extra engine power required will presumably not be nearly as costly as trying to go much faster than hull speed in calm conditions.

This leads to the following conundrum: with a resettable pitch boat propellor (Max-Prop), where should the prop pitch be set for the best compromise of cruising efficiency at hull speed, reserve power for heavy conditions, good acceleration at low boat speed for docking control, and avoidance of prop cavitation? To answer this it is probably necessary to look at boat speed versus prop pitch and rpm, and determine where this lies on the engine power curves, etc. Perhaps you know of a rule of thumb, e.g., whether an under-pitched or over-pitched prop will tend toward more efficient engine usage.

– Julius Feinleib lives in Vinalhaven, where he sails his boat, Idunn.

Chuck Husick replies: Thank you for your question about the proper setting for the Max-Prop on your boat. To begin, the performance of a prop on an airplane is no different from a prop on a boat. Your comment about the difference in effort required to increase speed between a displacement hull boat and a plane is correct, primarily due to the fact that the boat must operate in a divided medium, while the plane operates in a single medium – air.

Fixed-pitch props for both boats and aircraft should normally be chosen so that the engine can reach full-rated rpm when the vehicle is fully loaded. The objective is to assure that the engine can deliver its full-rated power. This is possible only at max rpm since horsepower is a function of torque and rpm. (Hp = [torque (ft.-lbs.) x 6.28 x rpm/60] / 550.)

As a pilot I am certain you have noticed that most aircraft fitted with fixed-pitch props won’t reach maximum rpm during static run-up or early in the takeoff roll. The reason is that most of these aircraft are fitted with “cruise” props, whose pitch is chosen to allow the engine to reach full-rated speed only in flight. Some aircraft – like those used to tow gliders – have “climb” props. These are lower-pitched propellors that allow the engine to develop maximum speed from the start, permitting all available power to accelerate the load.

Proceeding to your boat, the prop pitch should be set to allow the engine to reach full-rated RPM. If it is set to a higher pitch you will not have access to full power should you need it. In addition, should you command full power (max rpm) from your speed-governed marine diesel engine, the fuel control will over-fuel the engine as it attempts to satisfy your power command. While short term over-fueling won’t necessarily harm the engine, continued operation with too large a prop will.

Many displacement-hull boats can reach hull speed at a fraction of the maximum engine speed. (With a clean hull and prop operating in smooth water my Irwin 46 reaches hull speed at about 2,600 rpm, max-rated speed is 3,600 rpm). An examination of the prop law curve and the hp/rpm curve clearly shows that except when maximum power (max rpm) is needed, an engine driving a properly-chosen fixed-pitch prop is turning much faster than necessary to deliver the hp needed to achieve hull speed. This naturally leads to a concern that the “excessive” engine speed is both causing unnecessary wear and wasting fuel. In fact, both of these conclusions are correct. However, for virtually all pleasure boat applications the increased wear will never be seen. The applied load on the engine is modest, and most such engines rust out before they wear out. The increased fuel consumption will likely be too small to measure.

Aircraft, large sailboats and virtually all modern cruise ships use controllable-pitch props. With proper adjustment the prop pitch can be set to absorb all the power the engine can deliver over a wide range of rpm. Unfortunately, controllable-pitch props are more expensive than fixed props and require maintenance.

Bottom line – set the pitch of your Max-Prop so that the engine will reach its maximum rated rpm +50 rpm when the boat’s hull is clean and loaded to its normal maximum. Don’t worry about the fact that at normal cruising speed the prop is absorbing only a fraction of the engine’s maximum power at that rpm; just enjoy the reduced noise level. By the way, a fixed-pitch prop turning at 200 rpm less than maximum will absorb about 80 percent of maximum power. At 400 rpm less than max, the prop will be absorbing about 65 percent of max power. Many engine manufacturers recommend limiting long-term operation to max 400 rpm, at which speed the boat engine is being used in a way very close to what we do in aircraft with fixed-pitch props.

By Ocean Navigator