Repowering A Boat With A Fresh Outboard Engine

If you love your boat but she’s no longer running right – or you feel that she could use a boost, an engine repower may be the right decision. Not only is repowering less expensive than buying a new boat, it also brings new technology to your existing vessel which means a fresher, better experience to every outing whether you’re fishing, cruising or enjoying sundowners on your pontoon/tritoon, bowrider or center console. Plus, it is a way to extend the life of a boat that is need of repair or no longer running, but still has a good hull.

Outboard Repower - Checking Compression Of Engine
Above: Ryan McVinney and Shawn Joy checking the compression on an outboard engine for an episode of Boat Trader’s Backyard Boaters. Photo by Paul Fujita for Boat Trader.

The Right Reasons To Repower

Repairing an older boat or wanting more pick-me-up are just two reasons among many when considering purchasing a new outboard for an existing boat. Today’s engines offer better fuel economy as well – possibly a 20-30 percent improvement. That means a greater range with the same size fuel tanks so you can fish farther and stay out longer.

New outboard engines also run cleaner and they’re much quieter in idle, at speed and even when shifting. Noise is a consideration for many boaters – particularly those targeting fish species that spook easily, or those running eco charters. New engines will also have a higher top speed and at cruising speed, they’ll work less hard, burning less fuel.

You’ll find servicing new outboard engines easier too. The service intervals on newer engines tend to be longer and many dealers don’t even have techs trained in the maintenance and repair of models older than 10 years.  (That is a major drawback to owning and keeping an older outboard engine running – you may be on your own in many areas of the country since many service centers will not touch those older engines.) State-of-the-art motors refresh the user interface with digital controls and instrumentation, auto trim, and power steering so your old boat will feel and handle like a new one. Finally, a new motor means a new warranty which will not only enhance your enjoyment of the boat, it will also minimize out-of-pocket expenses for minor issues and it’ll raise the resale value of the vessel.

The Right Size And Configuration

Choosing the right size of engine for repowering your boat depends on several factors mostly having to do with your vessel’s length, size and weight in relation to the engine’s horsepower and weight – but also with cost and accessory changes.

Maximum Rated Horsepower

Most boat manufacturers will have a maximum horsepower rating posted on the safety/capacity label. Exceeding that limit can be dangerous and may cause you to lose your insurance coverage. If your boat doesn’t have a placard, you have to do the math. A general rule of thumb is to plan for one horsepower per 20-40 pounds of boat weight. So, if your boat weighs 6,000 pounds, you’ll be looking at 150-300 hp combined power.

Choosing The Right Engine Weight

Rated horsepower is the ultimate limiting factor but another significant consideration is the weight you’ll be adding to the transom. When four-stroke outboards were first introduced, they were heavier than their two-stroke predecessors. However, modern motors have shaved off much of that weight and that good because older boats may have a compromised transom if there is a waterlogged wood core or delamination of a foam core. Even if the transom is in good shape and can take the weight, it must be able to handle the torque the new engines bring.

The boat will also have to sit on its lines correctly and that’s a matter of its buoyancy. With its current powerplant, a boat has scuppers that sit in a safe position relative to the waterline. You can simulate adding weight aft (with sandbags or buckets of water) to see 1) what happens to the scuppers and potential water ingress; and 2) how the boat handles at speed with the additional weight.

boat with scupper above waterline
Outboard engine weight is a critical factor to consider, as it can change how a boat sits in the water and how far above the waterline through-hull fittings and scuppers are. Photo by Lenny Rudow.

Choosing Engine Configuration

You may be able to manage both horsepower and weight by changing the configuration of the motors – in other words changing from twins to a single or vice versa. For example, you may be able to replace twin F150 Yamaha motors that weigh just under 1,000 pounds combined, with a single 300-hp Yamaha V6 4.2L engine that weighs under 600 pounds. This math doesn’t always work that way, however. Consider replacing twin 300-hp motors with the new Mercury Marine Verado V12 600-hp engine that weighs nearly 1,300 pounds and you’ll add over 300 pounds. The key is to compute pounds per horsepower and compare. Remember, there is also weight in additional batteries and wiring harnesses.

Best New Outboards To Consider For Different Boats

Check out the very latest models because outboards are getting lighter every day. The best outboard for your boat will depend entirely on what type of boat you have and how you intend to use it. There are a variety of modern outboard engines out there that will work well on a number of older boat models, and fortunately they are much more efficient than they used to be.

Outboards For Smaller Boats

suzuki df115ss outboard engine
Above: A 2022 Suzuki DF115 SS outboard engine with the new SS trim, in matte black, with bold graphics. This four stroke big block inline 4 engine offers increased torque and power plus multi-point sequential fuel injection. Photo via Suzuki. 

For example, if you’re running something like a bass boat, the new Suzuki DF115SS (Sport Series) outboard is ideal for smaller, aluminum fishing boats coming it at around 400 pounds. Bass boaters will enjoy the electric start, direct multi-point sequential electronic fuel injection system and plenty of kick in shallow water applications. This 115-horsepower modern four stroke outboard engine retails for around $12,000 and weighs 412 pounds. Learn more about Suzuki outboards in Suzuki Marine Outboards: Innovative and High Tech.

Then there are even smaller options for smaller boats. An outboard like the Yamaha F25, for example, would be ideal for countless Jon boats and small open aluminum fishing boats. They get even smaller, too, for dinghies and small inflatables, all the way down to a 2.5-hp outboard. For more information about the Yamaha range, see Yamaha Outboards: Known for Reliability.

Outboards For Bigger Boats

600 HP Mercury Outboard Engines On A Boat
Above: Triple 600-HP Mercury V12 Verado outboards on a 42-foot 2022 HCB Lujo center console boat for sale on YachtWorld by American Yacht Group. Photo via American Yacht Group. 

If you’re running a large center console fishing boat, you might considering taking a look at Mercury’s latest 600-horsepower V12 Verado outboard engine. At around $77,000 retail, this outboard engine is pricey, but that’s because it represents the latest in outboard innovations and it can potentially radically transform the performance of your boat. An independently articulating gearcase offers several benefits like allowing more of these motors to be squeezed together on a transom without bumping into each other when in joystick mode. Amazingly, they can be mounted on 27-inch centers, which is only 1 more inch than is required for most other multi-engine installations. Another benefit is the ability for the gearcase to turn 45 degrees in each direction when using the Joystick Docking Outboards system for more effective maneuvering instead of the 30-degree articulation when in normal operation. Learn more about Mercury’s other offerings in Mercury Marine: Building Great Outboards Since 1939.

Single Outboards Versus Twins (And More)

The pluses of moving from a single to twins include having redundancy to get you home in case one engine fails, enjoying slightly easier maneuvering in close quarters with two motors, and the ability to add an aftermarket joystick from Optimus 360 by SeaStar Solutions. (Most OEM joystick applications are only available on new boat/engine combinations, not repowers, so do your research.) However, adding an engine will add drag and will also double the maintenance needs and therefore periodic costs. Also, moving up or down in engine count will affect the mounting holes on the transom. All brands of outboards have the same bolt pattern but changing the number of motors will mean drilling new holes or filling old ones.

Choosing The Right Controls And Accessories

Will you be able use the same controls and displays? Maybe, but that will depend on whether you’re going with mechanical (cable) or digital (electric) steering and if you’re switching from an overall analog to a digital system. If you just can’t give up cable steering, both Mercury and Yamaha offer certain models that can accommodate this. However, accessories are not the place to get stingy. Why would you put old sails on a new sailboat? Your wiring harnesses, batteries and cables may be due for replacement anyway and the old instrumentation may not have the bandwidth to work with the new motor technology. Besides, why look at old screens if you have new motors? Extend the “new” experience all the way to your dash.

The same goes for the hydraulic and fuel systems including the lines, fittings, filters and possibly the fuel tanks themselves. This is also a good time to upgrade your battery technology because new engines come with power-hungry sensors, digital fuel injection and integrated computers that may benefit from AGM batteries. Don’t forget the propeller. The old one may have a different gear ratio and number of blades and may not work with a new outboard.

Repowering Makes Sense For Many Boat Owners

In most cases, a repower (in its various configurations) makes sense. The gating factor is the overall cost of the boat. If your repower will cost more than the value of the boat, you may wan to reconsider or you may not enjoy a good return on your investment.

You can switch brands because you’ll most likely be re-rigging anyway so it’s ok to go from Suzuki to Yamaha or Honda to Mercury and vice versa. Just be sure to do your homework so you understand all the repercussions. For example, Mercury Verados offer Digital Throttle and Shift (DTS) standard on some engines but not on others while Yamaha has their Command Link Plus as a standard feature on their Offshore Series but possibly not on other motors.

old outboards
Pulling these old Hondas off the boat and replacing them with Suzukis meant it was necessary not only to replace the outboards, but also all the controls and cables. Photo by Lenny Rudow.

Adding a new powerplant will give you more power and extra other perks that may include contra-rotating propellers, a two-speed transmission or silent shifting. If you’re really thinking out of the box, you may consider a diesel outboard from OXE Marine or the 180-hp electric model from E-Motion. Both of these options will have secondary considerations like switching out fuel systems or adding large lithium batteries.

A repower is best left to the experts and many dealers will not sell individual engines without installation. A dealer will be able to guide you on how much weight and horsepower your boat is rated for, whether it’s better to have more drag but the extra redundancy of multiple motors, or if it’s beneficial to trade out top speed for less weight. Best of all, dealer/installers do all the testing, computer settings and delivery routines and they file the warranty paperwork and take on any liability issues so you’re on the water faster and with fewer headaches. New motors, new perspective.

Editor’s Note: This article was updated in March of 2022.

Written by: Zuzana Prochazka

Zuzana Prochazka is a writer and photographer who freelances for a dozen boating magazines and websites. A USCG 100 Ton Master, Zuzana has cruised, chartered and skippered flotillas in many parts of the world and serves as a presenter on charter destinations and topics. She is the Chair of the New Product Awards committee, judging innovative boats and gear at NMMA and NMEA shows, and currently serves as immediate past president of Boating Writers International. She contributes to Boats.com and YachtWorld.com, and also blogs regularly on her boat review site, TalkoftheDock.com.

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