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Bertram31.com General Bulletin Board
Re: prop questions
Posted By: Gary Gibson In Response To: Re: prop questions (Walter Kaprielian)
Date: Tuesday, 16 August 2005, at 4:21 p.m.
Tim,
I suspect that you are overpropped (overloaded) to the point that you will score your cylinder liners! Assuming you are set up with standard governors and running N50 injectors, you should get something like 2700 RPM max speed with your 2850 WOT. While the Detroit manual indicates 90% of max speed for continuous cruising, you probably want max continuous cruse speed of 2400 RPM (after fixing the over prop issue first).You should find the original build information, if possible, to be certain about the max speed and WOT (Detroit defines limits for “droop”, the drop in RPM from WOT to max loaded speed/RPM). If you don’t have the original build information, but have a standard model #, I may have the baseline data for the engines in my collection of Detroit Materials.
It may be the case that you are better off using a larger diameter / lower pitch prop than a smaller diameter prop given your boat’s displacement (if your transmission ration is 2:1 or thereabouts). I suspect that a 21” diameter prop is too small (high blade loading & slip). In any case I’d defer to a prop specialist – find one who as re-propped your hull type previously and/or seek others with your hull and power. There are plenty of Hatt 41’s out there so I suspect that you can get some reference data from other owners (though I have seen more with 8v53s than 6v53s).
Gary
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