Those RPMs are high. You should be running at hull speed at around 2800-3000. Pushing it any faster is a waste of fuel and a great waste of wear and tear on the engine. The things you need are the hull speed of your boat and the torque curves for that particular engine. Torque curves can be found in the repair manual. Hull speed, torque, and fuel consumption need to be balanced. My 3GM30 runs at 26-2800 at the boat's hull speed of 6.5 (or so) knots.
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Best RPM for Yanmar 3HMF?
The 27hp Yanmar 3HMF in my 38' 15000lb boat will peg the tachometer at 4000 RPM and just barely reach hull speed (or maybe not quite). Right now I cruise at about 6.5kts @3500 RPM. I'd like to get the speed up to 7-7.5kts and the RPM down to 3000 or ?? not sure. Is that reasonable or possible? The prop is a two blade folder. I couldn't find any markings on it, so I'm not sure of the pitch. I thinking of re-pitching or replacing it, but I'm not sure at what RPM range I should be aiming. I race the boat, so fixed props are out.
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