In the past decade, diesel engines have gained a third more horsepower and burn less fuel to reach it. Repowering with new engines can rekindle the spark in a beloved but aging boat. But the investment also carries a long-term commitment to that boat, and projects often include not just new engines but also updates to many ship's systems. A quicker, less costly option is to overhaul engines without added horsepower. But it is sometimes possible to increase horsepower while doing the overhaul. To explore the pros and cons of all three options, we turned to engine manufacturers and mechanics.
A good example of the options comes from Cummins MerCruiser Diesel's QSM11 engine (www.cmdmarine.com). Before 2002 this engine topped out at 635 horsepower, but was common at 580 or 535 horsepower. Today it's available up to 715 horsepower. "[Increasing horsepower] could be as simple as loading a new calibration," says Dan Burns, an engineering manager with CMD. "If you have a 580 and want 635, it's just software. [We] plug in a laptop, load the new calibration and change the data plate." If the engine is still under warranty, CMD's up-charge would be around $10,000, but once beyond warranty the new calibration software with installation costs under $500 per engine. The rationale is that the manufacturer would prefer to do the upgrade once its warranty responsibility has expired. All systems from fuel and air entering the engineroom to exhaust and propeller-shaft size need to be scrutinized for adequacy at the higher horsepower, and even modest power increases are best done in conjunction with an overhaul when engine parts are fresh. Horsepower increases also have to be documented as meeting engine- and boatemissions requirements, complete with a new factoryprovided data plate affixed to the engine.
Upgraded engine components are often needed as well. Raising a 535-horsepower QSM11 to either 580 or 635 requires a new turbocharger—a $1,750 part that might be replaced in an overhaul anyway. Reaching 715 horsepower is also possible. New pistons, injectors, camshaft, oil pan, and other components are needed. "The added benefit you'd get in performance may not be worth what it would cost you," Burns says. "For a 35-knot boat, increasing from 635 horsepower to 715 you'd gain a knot or two." On the other hand, a modestly powered boat could see quite a change. "Sometimes 3/4 of a knot will get a boat to lay down flatter and really change hull efficiency," Burns says. "You thought you were on plane, but maybe you weren't quite over the hump." For a boat that performs noticeably better by running engines a bit harder than rated cruise rpm, a bump in horsepower could be a big difference.
Deciding whether to rebuild, repower, or modify existing engines depends upon the desired outcome. "What is it that you dislike about your boat?" Burns asks. If performance is fine with older mechanically governed engines, but you want cleaner exhaust or lower noise, consider a repower. "On a fishing boat, when you're trolling with mechanical engines, you'll smell that exhaust all day," Burns says, "But when you go to electronic, you won't have that." Smoke when starting or accelerating and soot on the transom at the end of a day are reduced with new electronic engines. But if fuel mileage is the issue, a simple switch from mechanical to electronic might be disappointing. "Going from a 370B mechanical engine to a QSB380, you'll see an increase in economy, but probably only five percent," Burns says.