Here's how to find the right engine speed for the best fuel economy.
By Gary Caputi

"There is a point at which a boat's hull configuration and its propulsion system come into harmony to achieve the most fuel-efficient cruising speed. Every combination of hull and power is different, so finding your magic number takes a little experimentation and one critical tool - a fuel flow computer. However, once you find that "sweet spot", you'll know your vessel is delivering the most miles from the fewest gallons. Many of the latest propulsion systems - diesel, outboard and stern-drive engines - come with a fuel flow computer as part of the instrumentation package. If your boat doesn't have one, you can buy and install a fuel flow system (see sidebar below). All of these systems are capable of tracking gallons per hour and some can be wired to a GPS through a NMEA 2000 interface to display miles per gallon, the most important number for determining your exact operating costs. ...... http://www.motorboating.com/articleHtml.jsp?ID=1000060637