Working on prop sixe
Gary, I just answered another article about a 336 that can't reach hull speed. This response is a repeat of most of what I said to him: We have a 1987 Hunter 40 and can not reach hull speed under power. At max continuous RPM (3400) we hit 6.9 knots and our hull speed should be about 7.6. A friend with a sister ship easily reaches his hull speed. The difference is prop pitch. Hunter supplied our boat with a 17x10 (17" diameter by 10" pitch) two blade prop. A previous owner switched to a 17x8 three blade. Our friend has a 17x11 two blade on his. The trade between two and three blade seems to be 2-blade is less drag when sailing, 3-blade more push in adverse conditions and better control in reverse. We now own, but have not yet installed a 17x10 three blade. We expect to reach hull speed with it. There is info on web for selecting prop size. There are several sites, but they may all repeat info from Michigan Propeller. (Our new MI prop cost $400.) The basic deal is with a clean hull and prop, try to get to max engine RPM. With the right prop wide open throttle should produce the max or "red line" RPM. Of course you need to find out what that is for your engine. Ours, like the h376 (see the bottom of this note) is 3600 RPM. With clean hull and clean 17x8 prop we can exceed 4000 RPM indicating we needed more pitch. In late July a couple of years ago we had enough marine growth that we could only turn 3100 RPM. Moral of the story: get hull and prop clean before experimenting. Hope this is helpful. Know it is not the whole picture. Good luck.
