Actually it is not at all difficult to determine when a particular prop on a boat will overload the engine. The calculations are simple if a bit tedious. Here are the results for my boat, engine and prop.

The black line is the maximum power output for the engine as a function of engine rpms. The red lines (solid and dashed) are the propeller power curves for the boat for calm and rough water with my 20" diameter by 13" pitch three blade prop. The kink in the black line at about 1850 rpms is the point where the engine puts out its maximum torque (about 72 ft lbs). At 1850 the propeller curve is at about 6.2 hp and 16.5 ft-lbs of torque. The engine power curve (black line) shows that the engine can put out about 25 hp at 1850 rpms. So the boat requires a lot less than the power available at 1850 rpms.

If you increase the pitch of the prop (i.e., over prop the boat), the propeller curves (red lines) will move up and intersect the engine power curve at less than the engine's rated maximum rpms. In that case increasing the engine rpms above the intersection point of the engine and propeller curves won't generate any more speed and will make a lot of black smoke (for a diesel).

Basically, I don't see the argument for running at the rpms that correspond to the engines maximum torque output since over most of the engine's rpm range the horsepower and torque required by the prop will be considerably less than that which can be produced by the engine at full load.

Incidentally, the relationship between horsepower and torque (in ft-lbs) is:

**HP = Torque x RPM ÷ 5252**

Finally, it is trivial to calculate the fuel burn from the propeller curve. For my engine, fuel consumption is about 1 gallon per hour for every 16.8 hp required.For my boat the specific relationship is:

Gallons per hour = 0.05962*hp+0.054814

where hp is horsepower determined along the propeller curve.

That is determined from the engines fuel consumption curve, which is published for the engine. Even if you don't have the engines fuel consumption data, using 17-18 hp per gallon per hour of fuel consumption will get you withing 5% or so.

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