Re: What RPM Should I Run At?
If you have a planing hull boat, and if you want to cruise at planing speeds I'd advise going with conventional wisdom and prop your boat so that it will reach the manufacturer's rated peak horsepower RPM. This is assuming that the engine is healthy, in a good state of tune, your hull is clean, and she is loaded with the average load you intend to carry. This specified RPM is usually a range, for your engine I'd guess between about 4200 and 4800. By propping this way you are assured that you can never overload the engine nor over rev it.
As far as best cruise RPM that depends on what you want to do. If you are cruising for economy, then, as was stated, cruise at the speed that will allow you to just stay on plane comfortably. Less speed will fall off plane, more speed burns more fuel.
If you want to cruise for best speed and still get good engine life conventional wisdom is to run at no less than 5 inches of manifold vacuum, 7is better, 10 better yet.
Intake manifold pressure (or vacuum) is the best practical indicator of engine load that is commonly available. At sea level, 5" of vacuum corresponds to about 80% load.
Using fuel flow to determine engine load on a gas engine is difficult because the Brake Specific Fuel consumption rate varies with engine load. A gas engine is typically produces the most power per unit of fuel consumed with Wide Open Throttle at peak torque RPM. It gets worse at speeds above and below peak torque and at throttle settings less than wide open.
The "prop for max HP RPM" mantra is not cast in stone. For most applications it is probably the best way to go, but if you know what you are doing, in some applications you can "over prop" for better efficiency at lower speeds.
