Join Date: Feb 2000
Location: Annapolis, Md
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Re: How Fast Should My Boat Go Under Diesel
This is something of an old thread, but Mark is right, this is too small an engine to push this boat to hullspeed. My boat weighs virtually the same weight as the OP's boat, and probably has the same or less wetted surface. It has an easily driven hull form that is optimized for reaching speeds which is similar to a semi-displacement hull form that is optimized for powering. I have the three cylinder version of the same engine (a 3GM30F, rather than the 2 cyl in the O.P's I.P.). It pushes my boat at around 6.3 knots in flat water at around 2,900 rpm cruising speed, which is substantially less my boat's 7.6 knot hull speed. Even wound up to 3,200 rpm, I only get into the 6.8-6.9 knot range. Arguably my prop could be pitched a little more aggressively and have a little more area, but even so, I seriously doubt that I would get much over 7 knots since the engine seems to be self limiting around 3500 rpm which is below the point that the mechanical rpm limiter kicks in, which suggests that at 3,500 speed is limited by HP and not the prop.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay