Propeller sizing and engine loading

[E4ODnut]

I've been reading as much as I can on prop sizing theory to fine tune the wheels on my just purchased 3288 Bayliner(naturally aspirated gasoline engines).
The consensus seem so to be, that if the engines will reach max HP RPM with your average running displacement, the props are sized correctly and you can't possibly overload the engines at lower RPMs because of the different shapes of the engine power vs prop power curves. So far, so good.
But, if attaining full power is not a major concern, and economy of operation and engine life is, operating at lower RPMs with props sized in this fashion will result in relatively light loads at engine speeds near peak torque. As volumetric efficiency is best near peak torque, it makes sense to me to tune the props so that when run at RPMs near peak torque, the engine loading will be slightly less than what would be considered maximum continuous load.
Consensus seems to be that maximum continuous engine RPM on light duty gasoline engines should be in the neighborhood of 60% to 70% max HP RPM, for most engines, it puts you fairly close to peak torque RPM.
However, I can't seem to find any reference to recommended maximum continous loading for light duty gasoline engines. Usually, the enrichment circuits are set to activate around 6" to 4" of manifold vacuum. Vacuum operated secondaries usually about the same. I'm assuming that 10" to 8" of vacuum would be safe loading for continuous operation, but haven't been able to find any information on this subject.
Any opinions?

Thanks,
Robert

[woodboat]

If I understand correctly you think that if you under prop, the load on the engine will be lighter, you can run leaner at the expense of top speed. Another issue though is that for the same given RPM you will be going slower with that prop. From experience I know I got MUCH worse fuel mileage when I had 13X15 prop on than when I was swinging my 13X19. I needed the 13X19 for skiing but would often leave it on out of laziness. The 13X19 took longer to plan, turned fewer RPMs but got much better fuel milage as well as top speed. I know in big houseboats they often put on "cruising" propellars which are higher pitched to save fuel, so I am told. I think the whole idea behind correctly sized props, where they reach MAX hp rpm is that you can back them off and achieve the best compromise between power and efficiency as well as engine life.

[E4ODnut]

Guess I didn't make myself clear.
Quite the opposite. I feel (as your experience would seem to support), that if you are slightly over wheeled, you should be able to get better economy at cruising speeds. Carburetors are generally set up pretty well for the application, and Air/Fuel ratio doesn't change much in the cruising RPM/load range. It is generally enrichened at high loads, but that's not my concern.
What I'm trying to determine is what would be an acceptable load (manifold vacuum) to run at cruise (near peak torque RPM), for best compromise between economy, speed, and engine life.

Robert

[woodboat]

Manifold vacuum will be affected by cam and carburator size as well as load so I don't know if that is the ultimate judge. As stated by another user in another thread GMs are rated @1" of vacuum for torque curve. Unsubstatiated on my part but seems believeable.