Skip to main content

Fuelish Pleasure


This is the editorial I've been avoiding: the one about the cost of fuel and what, if anything, we can do about it. As a boat owner like you, who's been shocked so often at the fuel dock I'm beginning to feel like a cow at a slaughterhouse, I know the subject demands attention. But what to say?

PMY is not going to do "10 Secret Ways to Save Fuel Now!," complete with tips like reducing gear and fuel payloads, cleaning your hull bottom, and getting on plane faster. Those things will reduce fuel consumption but hardly enough to notice. However, they'd probably sell a few more magazines.

Let's be realistic. We're talking boats, not family sedans. Say a 45-footer weighs 45,000 pounds. Take everything off her and drain her tanks, and maybe you'll get that down to 40,000 pounds, an 11-percent reduction. Now if you could reduce the weight of your SUV by 11 percent, you might notice an increase in mpg. But a boat is not a car; it has to overcome the resistance of water by either sliding through it or pushing it out of the way. A loss of 5,000 pounds will not make the needle move. (For the same reason, a clean bottom might.)

The cold, hard truth is that there's only one route to real fuel savings (other than staying tied to the dock), and it doesn't require schlepping stuff off your boat, hauling her, or for that matter spending a nickel. Just pull back the throttles. How much is up to you, but for the vast majority of boats, any change will make a difference, and that difference can be significant—like doubling your fuel efficiency.

When you slow down, your boat pushes less water, reducing engine load and fuel burn. Throttle back to displacement speed, and you essentially stop pushing water and start sliding through it. (You can tell when your boat exceeds displacement speed because her wake gets bigger.) If you're of a more precise frame of mind, multiply the square root of your boat's waterline length by 1.34, and you'll get her maximum displacement speed in knots.

To get a feel for the kind of dividends less speed can yield, look at any PMY boat test. Take for example our June test of the Cabo 38 FB. At WOT (2340 rpm) she gets 0.54 mpg. Back off 340 rpm to 2000 rpm, and that number jumps 24 percent, to 0.67. Make it 1750 rpm, and mpg jumps 48 percent from WOT to 0.80 mpg. At roughly displacement speed (11.7 mph), it's 0.98 mpg, an 81-percent improvement.

Whether you want to slow down at all and, if so, how much is obviously your choice. But the fact that you can get improvements like these—which admittedly vary from boat to boat—is a rare bit of good news on the subject of fuel cost.

This article originally appeared in the October 2008 issue of Power & Motoryacht magazine.