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Repowering Your Boat

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Employees of Panama City’s Gulf Coast Marine Service removing an engine.

Employees of Panama City’s Gulf Coast Marine Service removing an engine.

Time for a Change

Repowering is not to be taken lightly, but with some expert help and good advice a fine old boat can run like new.

A couple of years ago, after six decades of abuse, my hips gave up the ghost and I had to have them replaced with shiny new artificial joints. I dreaded the two procedures, but both turned out to be less horrible than either of the two times I had to swap engines out of a boat. A hip has only a couple of parts, and they’re easy to replace if you have a scalpel, a saw, and a hammer. (At least that’s what my orthopedic surgeon said.) An engine? Not so simple: The in-and-out can will most likely be both stressful to one’s psyche and debilitating to one’s finances. But like my clapped-out acetabulofemoral joints, when an engine’s dead, it’s dead, and repowering is the only answer. 

Modern marine engines last a long time; a thousand hours of service is what we expect. Those who buy new, or even moderately used, boats will probably trade up, down, or sideways before their engines wear out, and will never face the Specter of Repowering. (If my engines were getting long in the tooth, I’d trade for a newer boat and let someone else deal with it.) Folks who rack up lots of hours, on the other hand — offshore fishermen, long-distance cruisers, charter captains, or people who simply love boating — accept repowering as a necessary evil. Same with folks who buy older boats, or project boats for rehabbing.

Why is repowering often such a pain? If the boatbuilder installed the engines after the deck, cabin furniture, joinery and other structural components were in place, it would be easy—they would come out the same way they went in. But most builders install the engines (and other systems) while the hull is still completely open, then drop the deck molding and the rest of the boat on top. Removing an engine later on can be as tricky as taking a ship out of a bottle, and installing the replacement can be like putting the darn thing back in. The trickier the in-and-out, the more it costs. (That’s why I love outboards: Unbolt the old ones, bolt on the new ones and you’re all set.)

DIY? Are You Crazy?

Whatever the reason for repowering, the first step is to get help from experts who’ve done it before and know where the booby traps are. It’s not a DIY project for most of us, and even less so if you want to change engines, maybe add some horsepower, or, God help you, switch from gas to diesel. (Gas to diesel doesn’t make as much sense now as it did in days past; more about this below.) 

If you’re doing an engine-for-engine swap, using the same motor mounts; fuel, cooling and exhaust systems; gearbox, propeller and so forth, the yard manager and head mechanic are the people to cozy up to. Choose a well-established, reputable, full-service yard with lots of repower experience, and with a staff of factory-authorized service techs with the training and tools for the brand of engine you’re buying. Not only does this increase the chances of a stress-free replacement procedure, it will also help if you have warranty or other issues later on. You’re always on stronger footing with a manufacturer if your new engines were installed by certified professionals.

When I had the hip replacements, I picked my doctor and let him get on with it — wake me when it’s over. I suggest you repower the same way: Just let the pros do it. You don’t want to know what the crew had to do to get the old engines out and put the new ones in — maybe it was just a simple pickup with the crane, or perhaps they disassembled half the saloon, or something in between. My first repowering job required removing several joinery panels in the aft cabin; basically stripping the engine down to the block, cylinder head and oil pan while it was still in the very cramped engine compartment; then using a come-along and a jury-rigged A-frame to inch it into the aft cabin where the yard’s cherry-picker could snatch it out through the aft companionway door. It took the better part of a day to remove the engine, and almost as long to put the rebuilt one back in, using the same process in reverse. You probably don’t want to watch this. Just write the check.

Change Engines with Care

It’s human nature always to want to make things better, so when faced with repowering most boat owners think about adding horsepower, maybe switching to a different brand of motor, or, as mentioned above, trading gasoline engines for diesels. Think hard about this, and talk to someone technically inclined before committing to the project. A naval architect is ideal; you’ll get accurate information and advice, and he or she can crunch numbers on the computer to predict what effects your changes will have.

More powerful engines may not increase speed enough to make them worth the cost, or they’ll require new, expensive propellers. (A prop shop with state-of-the-art software can help with this, too; see What’s Your Next Prop? below) Heavier, or lighter, engines than the ones you have now could change the boat’s operating or running trim, usually for the worse. An editor of this magazine owns a boat that was repowered before he bought it, with a slightly lighter, albeit more powerful, diesel than the original; the reduced weight changed the trim enough so that the cockpit no longer drains properly, and he’s planning to add ballast to bring the boat back onto its lines. A naval architect could have predicted this. But naval architects don’t work for free, so getting advice from one can be expensive. If you dodge a repowering bullet, it may be money well spent. 

If you don’t have access to a naval architect, contact the company that built your boat; they may have one on staff, or at least a technical crew that can give you advice, warn you against certain changes, and reassure you that adding extra power won’t make your boat as jumpy as a bucking bronc with a burr under the saddle. As a rule, boatbuilders are pretty helpful folks when it comes to projects like this, and they won’t charge you for advice and support.

What’s Your Next Prop?

If your repowering project involves switching to bigger engines, a shop with state-of-the-art computer programs can predict what kind of improvement you’ll see in performance, whether you’ll need new props (chances are you will), what specs they should have, and what they’ll cost. Knowing what you’ll get versus what you’ll pay will help you decide whether to repower with the same engines, or go for more horsepower.

The shop can also analyze your current props by physically measuring them with sophisticated hardware, making sure all the blades are pitched equally, have the same blade-tip distances, and the pitch and diameter are what you think they are. Sometimes just tuning the props can make a noticeable difference in performance. As boats get older, they tend to get heavier; maybe your engine doesn’t have the oomph it did when it was young, so your original props may not be correct today.

Before contacting the shop, gather as much performance info about your boat and engines as you can: not just speed versus rpm, but operating and exhaust temperatures, running angles, fuel burn, engine hours, reduction gear, and so forth. The more information you provide, the better the info the shop will produce.

To find a high-tech prop shop in your area, check the Hale MRI ( and Prop Scan ( websites. Shops using high-tech prop analysis can do a really good job of bringing your propellers up to spec, or selling you new props to match your new engines. They are worth their weight in Nibral.

Gas to Diesel? Too Late

Back in the dark days of the late 20th century, gas engines guzzled like Dean Martin on a spree, broke down almost as often as an ’84 Yugo, burned fuel that cost half again as much as diesel, and demanded quite a bit more of it. The dream of many boat owners with spark ignitions was to change over to diesel power, and recoup the expense at the fuel pump. Nice idea then, not so good now. Gasoline and diesel fuel are pretty close in price these days; electronically controlled gas engines are economical and reliable; and, although diesels still burn less fuel, they cost quite a bit more to buy. The lower fuel burn won’t offset the added cost of diesels unless you put thousands of hours on the engines, and by that time, who knows, you may have to repower again. 

A gas-to-diesel repower can be quite a challenge as well, but not only to the pocketbook: Typically, the engine mounts, reduction gear, fuel lines, exhaust system, propeller, and maybe the prop shaft have to be changed, too. Diesels of similar horsepower are usually bigger than gas engines, so that could mean a space issue. The reduction gear may need to be bigger, since it has to handle the diesel’s extra torque; this can cause problems even if the engine itself is roughly the same size. Usually a diesel of similar horsepower to the gas engine being replaced will require somewhat larger exhausts. This can be a big project, because the whole system needs to be reengineered, a job that often requires carpenters and fiberglass technicians, as well as mechanics . 

When you sell your boat, you’ll likely get a premium for diesel power, but probably not enough extra to pay for the engine swap. Maybe you’d do better to sell your gas-powered boat now and buy one with diesels already installed, and without too many hours on the meter. The money you save on repowering will make a nice down payment on that next boat, you’ll save yourself a shipload of agita, and you might have enough cash left over to buy a real nice ship in a bottle.

This article originally appeared in the April 2017 issue of Power & Motoryacht magazine.