Lugging an engine.

[bernieb]

I'm concerned that i may be lugging my 250hp yamaha I'm using a 13.5x20 four bladed ss prop 4.3 inches across at hub. The boat a Mark Twain (Sonic) on the transom 82' the engine is year 2001 efi two stroke. This boat was an inboard but i added a three foot backset bracket for the two stroke. I'm guessing the weight overall with four fishermen and 60 gal fuel at 4300 lbs and I have no problem getting on plane and wot is 5100 rpm and top speed is 46mph in calm water.When I trim up just a little it starts to porpoise and can get rid of that with more tabs down. Should I try something else for a prop? The boat length with bracket is 24' in length and I'm getting about 1.5 mile per gallon. Thanks, bernie

 

[walleyehed]

Re: Lugging an engine.

Couple factors I see.....30.7gph is extreme. If propped right, I think it'll be happier at 5600-5700 and closer to 23-25gph.
Now, I don't know quite how you figured the correct moment (CG) to come up with a 3ft set-back, but that could be a bit much.
We can probably find a prop that will have less bow-lift, maybe even have some stearn-lift to eleminate the porpoise and excessive tab pressure...Can you list the Make and Model of 4-bld you're running now?

 

[bernieb]

Re: Lugging an engine.

Thanks walleyehed, the only markings on the prop is 9604 and not a real clear marking. The cavitation plate on engine is 2.25" up from boat keel and the actual distance from old transom to center line of prop is five feet. Any more ideas ? The three foot set-back was what the yammy dealer told me to make it.

 

[Dhadley]

Re: Lugging an engine.

A 3' bracket is a bit much. We don't use that much set back with V8 outboards. However, since the dealer recommended it, let's ask him why. Obviously he must have had a reason for going so far back. Let's see what he has in mind. And 2 1/4" up is way too low for that much setback.

Are you sure it's 36 inches and not 26 inches?

 

[bernieb]

Re: Lugging an engine.

attempting a picture

 

[bernieb]

Re: Lugging an engine.

a picture.

 

[Crownie2]

Re: Lugging an engine.

A boat with too much setback! That is unusual for this forum!! :^

I agree with Dhadley...