Originally Posted by jgrace
I must be missing something here, but I can't think what. Help!
Nope, you've got it right. You loose speed because the pitch goes down but you gain speed because the engine revs higher. You are trying to get the best compromise to end up with the engine producing maximum horsepower. However, if the engine does not produce much difference in power between, say, 5000rpm and 5500rpm then the calculation may give you a theoretical pitch but a prop with that exact pitch may not exist. You will have to compromise on your choice. There is something else which we've not mentioned and it is to our advantage. As the boat speed increases, the prop slip reduces.
The info roycruse has given is designed to give particular characteristics to the prop or to improve it's efficiency. Some of these characteristics can be used to your benefit. You can produce an intermediate prop which more closely matches your engine/boat and gives you extra speed. The extra you are likely to gain on a 30mph boat is small.
Put all the details into the calculator and allow it to calculate the exact pitch.
Don't loose sight, in all this, of the reason for the engine manufacturer's requirement for proping the engine to achieve close to the recommended maximum revs. This is to ensure the engine does not labour at part throttle.