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.