Prop pitch question

[80Skiff]

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.

 

[Faztbullet]

Take the Turbo to a prop shop and have extra cup added......

 

[80Skiff]

The turbo prop is a loaner from my mechanic to try, I will need to buy a prop for the boat. I don’t mind having a prop cupped if needed or how about a 4 blade? I have read it will perform better at the mid range that I plan to operate and lower rpm some. I have one prop guy that said he believes I can do a 14 diameter and 21 pitch 4 blade if the 14 diameter and 23 pitch 3 blade trial prop works well on my boat. If not he said drop to a 20 pitch He did not see the boat operate that was just by my description. I have not tried the 23 pitch but plan to this weekend. I don’t doubt this guy just want all the info I can learn.

 

[QBhoy]

Hi. I’m not sure I know of a 5.7 ho. Most will be 250/260 hp in carb form. The MPi is 300 hp. If you have an alpha drive, I’ll assume it’s the max greared 1.47/1.5 ratio like mine. To see over 60 would take a serious hull I suspect.

 

[80Skiff]

It is a fuel injected engine, probably is 300 hp. For now it has the original outdrive the 260 hp carb engine had when the boat was built. I plan th have the outdrive that the engine came with installed this winter. I am hoping it is the same ratio and will not affect the prop decision. The boat now runs 55-60 but the bow will drop at those speeds, depending on how rough the water is 55 when it is rougher closer to 60 when smooth, trimmed out it helps but it still isn't staying up as it should. I only mention top speed to explain that the hull is not designed for those speeds and it isn' t going any faster anyway. That being the case WOT will not give me the rpm range like in boats that stay on plane and run correctly at WOT. I am looking for any advise on another method to determine correct prop rather than WOT method, if anyone knows of one. My goal is to cruse at 35-40 as efficiently as possible. The boat does that with ease now with a loaner turbo prop 14 1/4 dia 21 pitch. I plan to try other props, I have been told to go to larger diameter props and see how they work, also have been told to go to 4 blade but then told not to do either of those things as well by different prop guys. Just want to know how to get this figured out. I am not expecting to get 100 MPG with this or anything and I am not concerned with how much fuel it takes, but I would like to be as efficient as possible with the boat speed at 35-40 MPH as possible to help with engine noise and fuel. I have a gps spedo on the boat so I should be fairly accurate on speed I think.