Repowering Your Boat:
What You Need To Know
By Michael Vatalaro
There are many considerations when starting a repower project, including some that are unrelated to the performance requirements your boat, such as having a local dealer or mechanic nearby for servicing that you like and trust. The five below, should give you a good starting point from which to plan your shopping list.
Transom Limitations: Space and Weight
The physical limitations of your transom will dictate how much horsepower and weight you can put back there. While modern four-strokes are getting close to their two-stroke counterparts in weight, they still will add to the load at the stern. If you were not running the maximum-rated horsepower in your previous set up, you may have some wiggle room, and be able to absorb some extra weight without impacting seaworthiness. It's a good idea to check with your boatbuilder what the design limitations were before you start shopping. Weight considerations could limit you to DI two-stroke technology.
Space is more of an issue if you are running twins (or more) because some of the high-horsepower modern four-strokes need more width between the engines, but not always. Honda, for example, designed their four-strokes to fit the standard footprint for easy repowers.
In addition, with the increased horsepower ratings available today, you may wish to switch from twins to a single outboard configuration. For example, swapping twin-150s for a single 300-hp motor, in theory, will reduce both weight and drag, which should increase fuel economy without sacrificing performance. However, boatbuilders caution that the increased power and torque of these modern, power-dense outboards can be too much for transoms that weren't designed for it. A safe bet is to limit horsepower to the highest horsepower single engine configuration offered as original equipment. The boatbuilder should be able to go into greater detail.
Rigging: Time To Upgrade To Electronic Controls?
This is your chance to simplify your dash and clean up the console, as well as consider adding joystick control, if you are running twins. If you've got a large multifunction display, you may also wish to network it to your outboard(s) to allow it to display engine data. To do so, you need to look for an outboard that supports NMEA standard data outputs or one that offers a “gateway” converter that changes proprietary outputs into a data stream that your chartplotter can use.
Converting to joystick control will also require hydraulic steering, and a specialized command bus to talk to the joystick.
How you intend to use your outboard? If you do a lot of trolling or low speed operation, you may benefit from an outboard that makes use of fuel saving tactics like stratified combustion. If you run offshore or through an inlet to go fishing, you may benefit from electronic throttle controls combined with an outboard with instantaneous mid-range punch.
Scheduled maintenance intervals have a big impact on the cost of ownership during the life of the outboard. In general, DI two-strokes will have longer intervals between scheduled services because they lack mechanically-controlled drive trains that need adjustment every 500 to 1,000 hours. Although recently Mercury unveiled a 150-hp four-stroke that should not require valve adjustments at all during it's lifetime. Four strokes also need regular oil changes. But annual oil changes should be weighed against the operating cost of burning oil in a DI two-stroke.
The amperage output of most outboards has increased over the years, but if you run an electric trolling motor all day, or a suite of electronics while drifting or slow-trolling, the alternator on your outboard better be able to keep up. Check not only the rated amperage output, but also make a note of what the output is a low rpm, where it could half of what it is a cruising speeds.
— Published: April/May 2013
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