Highway on your 240hp yanmar what prop and transmission gear reduction are you running. I was speaking with someone that has a 240 available and after some research on his part he was recommending a 1.2:1 ratio. I have a 1.5:1 on the gas engine and i run a 16x16 4 blade or a 17x17 3 blade. The 16inch diameter is much quieter. I ran the numbers on a prop calculator called Vic prop and if i follow them the 1.2 seems right since the max diameter I can run is 17inch which does rumble a little. With a 1.2:1 they came up with a 16x17 3 blade. With the 1.5:1 they came up with 18.5x 21 3 blade or 17.5 x21 4 blade.
btw they come up with a top speed of 31Knots which it seems you are doing better.
jim
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