I am struggling with determining the prop pitch on my boat. I have a 1980 skiffcraft wood boat which has been completely restored and re powered. The original engine was a 260 merc but now has a 5.7 HO so I think it is about 300 hp now this is on a 3000 lb or so boat. This is more HP than the boat needs by at least 40-60 HP. Determining WOT on this is not as easy as it should be for a couple reasons, the hull design is for a 55 mph or so speed and by the time you get to 60 the boat isn't likening the speed. The bow drops and this then drops the rpm because of the wetted area on the boat increasing. But here are the numbers I have. Prop 1 is a 14 or so diameter with a 17 pitch. This gave me 48 mph with the engine at the rev limiter at 5800. Just me and fuel about 20 gal. A turbo prop 14 1/4 dia and 21 pitch does much better and gives in the high 50s at 45-4700 rpm. Problem is as stated before the bow goes down and the boat noticeably slowed when it gets near 60 mph, which I believe the extra drag is what is limiting the rpm rather than the limit of power. What I am saying is if the boat would stay on plane it seems the boat would go faster and the rpm's higher. I am not looking for more speed here as the boat is not built for those speeds, what I want is to run as low rpm's as possible in the 25-35 mph range to save fuel but mostly to keep engine noise down as much as possible. I don't want to overload the engine though. So how can I be sure if I go to a 23 in pitch when the boat hits 58-60 or so I am not going any faster, due to boat hull design limitations even if the engine and prop could go faster. The WOT rule doesn't work with this setup.
