prop problem

[the black devil]

hello everyone i have just fitted a 440hp diesel yanmar with 2speed box in a 2ton planing hull the first gear is 1.3-1 and the second is 1-1 i tried a lancingmarine 17x28 surfacepiercing prop and it was way too big in the direct gear but way too small in the 1.3-1 so i tried a 25 pitch it was still too big and then i tried a 23 pitch and it was way too small andmy money is done the engine box and 3 props has cost me a fortune if i was to take the cupping out of the 25 pitch and have it thinned a bit would it help me get my rpm up and top end speed i reallly need top end speed not caring about take off

[Gilbert]

I am surprised that the 25 inch pitch is not satisfactory if the 23 is too small.
A diesel engine can be loaded more than a gasoline engine can and still be satisfactory. Usually an exhaust pyrometer is used to indicate if your engine is loaded too much or not. So your engine rpm does not have to be at max to have a satisfactory prop setup. Of course these are just general comments and you may already be aware of all this. Also diesels like to be run at rpm's a little above the rpm's where maximum torque is generated.

[the black devil]

thanks gilbert!!! i have since learned that the 25 is lancingmarines old style prop with a very heavy cup and the 28 is his new one its a thinner prop and little cupping witch makes both the same size wise thats why im getting more speed with the 28 at the same rpm , more pitch and easier to turn the 23 is also his new prop so its equal to a 20 old style it sounds confusing i no but i think i have it in my head now lol . so im going to order his new 25 and see how it goes