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1979 omc 250 (351w)

just makes more noise and if you have selectrim, it becomes a crazy feat of engineering to make it stay together while the engine tips up and down. You are at the limits of that outdrive as it is. 260 is the most OMC ever put through it and the water pump shaft is not the most reliable transmistter of any more than that.

probably the #1 best performance improvement, and really the only one, you can make safely is a stainless steel prop.
 
Many many factors go into the way a boat performs. Unlike a car, adding horsepower does not necessarily equate to more speed. Understand that a boat makes a hole in the water. You either move the hole around with you or you climb up out of the hole and plane above the water. In either case, the engine is pushing up hill. The faster you want to go, the steeper the "hill". Hull design is your greatest limiting factor.
As mentioned above, tuning the prop to the boat is the first step. There is about 40 gazillion pages on the web about this subject, but the quick answer is to find a prop that will give you somewhere between 4800-5000 rpm at wide open. Once you find that prop, then you can begin the other tedious steps such as trim, ballast, overall weight, hull flex, etc to get the hull set up for best performance. Then you begin adding horsepower. This will allow you to increase prop pitch for higher top speed, but probably at the expense of low speed handling and "hole shot". It is all about compromise.
 
I feel like I missed an opportunity in my first post to say "I say naynay"

oh wait, that's Pinette, not Pinheiro
 
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