Cruise Control
By Pierce Hoover

...Airplane pilots are big on fuel efficiency, and they spend a lot of time calculating the exact altitude and engine rpm setting that will yield the optimum combination of speed and fuel economy.
...Boaters, by contrast, almost never worry about these details. We idle in the no wake zones, drop the throttle hard when pulling skiers, race at top speed when we want to feel the rush of wind and water, and throttle back to a more comfortable cruise speed when the water gets choppy.
...But whether you know it or not, every boat has a cruising "comfort zone" that yields an optimum mix of speed and fuel efficiency. You could find that exact speed. All you'd need do is borrow the radar gun, digital tachometer, transducers and fuel-flow gauges used by our staff, then spend a morning on the lake running at various throttle settings.
Or, you could get a 95-percent approximation of your boat's optimum cruise speed by simply following a few simple rules our experts have concocted after performing hundreds of detailed boat tests. These hints apply primarily to gas-powered, planing-hull boats since this is the type of boat most of our readership owns.
Somewhere between planning out and top speed, there is a combination of speed and engine load that will yield the maximum number of miles per gallon. To make a broad generalization, this "sweet spot" usually occurs somewhere between half and three-quarter throttle.
...If your boat is marginal on power, this generality may not apply, but if you selected an engine that can hold your boat comfortably on plane at half throttle or less, the formula works. For a large number of V-6- and V-8-powered runabouts, optimum cruise speed falls somewhere between 3,000 and 3,500 rpm.
...To further optimize fuel efficiency, make sure your boat is running level and in trim. Weight distributed unevenly front to back or side to side will cut into running efficiency, so you may want to move gear and passengers to level the load.
...If you have an outboard or I/O-powered boat, you can also adjust the drive for optimum efficiency. Begin with the drive down, then gradually raise it with your trim adjustment.
...Notice how the speed and rpms increase as you raise the drive. Raising the drive decreases the drag of the lower unit in the water, and it also allows the bow to rise, which reduces the wetted surface of the hull.
As you continue to raise the drive, you will eventually cause the propeller to cavitate. Your optimum drive setting is a few degrees shy of this mark.

Originally Published: June 2004 © 2005 World Publications, LLC