rick said:
The prop that allowed a higher RPM would NOT produce a higher (boat) speed..........?? [I think I agree with you on that!]
The thing is it may, or it may not. Propping as you probably know and have heard is part art, part science. With to many variables. It might, but it also might not.
When it comes to anybody it's their motor, and your going to do what your going to do. Wether you agree with me or not is up to you. I can tell you Evinrude tells you to prop for absolute peak horsepower and not higher, which on this graph if i'm reading it right is 4400. Mercury instructors say prop for just over peak hp. Yamaha tells you to prop towards the high end of the RPM window, without telling you where peak HP is in that window. And the offshore racers, the big Donzis, their train of thought is to prop about 1000 rpm over peak torque, which on this fictional engine looks to be about 4k rpms, which would make the proper prop whatever the engine needs to hit 5k. And if this was a diesel engine, we would prop for just over peak torque, a few hundred over peak torque.
There are a couple of ways to think about it, as it is part art. That all being said there are a couple of undeniable facts
1. The faster you spin any engine the faster you wear and tear
2. The faster you spin any engine the greater your chances for detonation go up, and most people don't know it until it's to late.
3. The faster you spin any engine the chances of it leaning out go up because most folks don't take into account injector flow rates, injector pulse times.... but more importantly... keeping all those parts clean. If you live in a northern state were you winterize. Injectors will flow as they are supposed to for the first couple of years. But after a couple of years of sitting all winter as well, they are not going to flow the same. Lean means mean and the engine is going to run the best the leaner it gets... until it melts a hole in a piston
4. This fictional engine looks to be a small block GM. In general, 5k rpms is kind of a magic number. Pushrod engines don't like extended run times above 5k. Small blocks also come with high volume low pressure oil pumps, right around the 5k mark is really when it should have a high pressure oil pump on it. And if this is a 2 bolt main... forget it.
If you want reliable... prop this one at 4600 tops.... If you want high performance.... do what you want. Just don't cry about it if the engine has a meltdown at some point, or you get 400 hours on it and your front and rear main oil seals are leaking on you. A good general rule of thumb is you don't spin anything faster than you need to. If you want to test the theory, take your lawn mower or snow blower engines and crank those up so they rev just 300 to 400 rpms above whatever the governor is set to... and see how long that lasts before you hotrod a boat engine. Don't get me wrong, the engines will run awesome, and cut grass/blow snow like no tomorrow... But you won't get any more than 1 or 2 seasons out of it before the rings are smoked, the bearings are scored or spun, and it leaks oil everywhere.