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Time For a Change?

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Are you the parent of tired, rusty, and thirsty gasoline guzzlers, the kind of engines that keep OPEC happy? If so, your boat's overdue for repowering with more efficient diesel motors. Today's oil-burners are lightweight, compact, quiet, and economical—they'll save you money at the fuel dock and add resale value. What's not to love?

Okay, there's a catch: Repowering with diesels can be a challenge, and not only to the wallet. Typically the engine mounts, reduction gear, fuel lines, exhaust, propeller, and maybe even the shaft have to be changed as well. "Everything has to be recalculated," says naval architect Dave Gerr, director of the Westlawn Institute of Marine Technology in Mystic, Connecticut. But Gerr adds, "This doesn't mean you shouldn't do it—it's often a great thing."


Today you can find a diesel for any size boat, including those powered by stern drives. New electronic, common-rail diesels offer compact size, excellent power output, and the same benefits as common electronic diesels, such as great fuel economy, almost totally smoke-free operation, built-in diagnostics, and fly-by-wire controls.

In most cases you can replace gasoline engines with diesels of less horsepower and still maintain the same cruising speed (but usually with a lower top speed), since diesels can be run continuously at 90 percent or more of max rpm, compared to 60 to 70 percent for gasoline. But, as naval architect Dave Martin, designer of Ocean Yachts, points out, hull design plays an important role in repowering: "If the existing boat needs to have the rpm increased above cruising speed rpm to get over the hump, then the hull design is probably not suited for a diesel, unless the diesel is about the same power and weight as the [gasoline] engine. In any case, the diesel should have enough power at hump rpm to satisfy the prop power demand at hump speed."

Engine size is another issue. Diesels of similar horsepower are frequently bigger than gasoline engines, so that could mean a space issue. The reduction gear might be bigger, too, since it has to handle the diesel's extra torque; this can cause problems even if the engine itself is the same size. The exhaust system is another consideration, he says. Usually a diesel of similar horsepower to the gasoline engine being replaced will require significantly larger exhausts. This can be a big project because the whole system needs to be re-engineered, often requiring fiberglass and/or carpentry work.

The ideal first step in a repower is to consult a naval architect. If this is too expensive, work with a reputable, full-service yard with lots of repower experience. "It's important to go to factory- authorized, factory-trained service dealers, particularly with the new electronic diesels. Special training and special tools are required," says Tom Calhoun, president of Yanmar Marine.

Replacing older gasoline engines with today's versions is less complicated, requires minimal re-engineering, and lets you take advantage of EFI systems that improve efficiency and reliability. But if you bite the financial bullet and opt for diesels, once the project is finished, your hair has grown back, and you're enjoying your efficient, reliable oil-burners, you'll wonder why you waited so long to repower.

This article originally appeared in the February 2007 issue of Power & Motoryacht magazine.