‘Tis the Season for Outboard Repowering - 07/01/2009

There is no time like the present to bite the bullet and repower your outboard boat.  The trick is finding the right horsepower, stroke, fuel system, shaft length, props, engine brand, price, warranty and installing dealer.  Probably the most difficult decisions are deciding on the horsepower, brand and installing dealer. BoatTEST.com can help you with the first two – horsepower and brand selection. We have tested hundreds of outboard boats from 25-hp to 350-hp powered by all major brands. Find the boat models like yours and compare performance under different loads and conditions and with different props.  Then visit your local dealers for the two or three brands you have selected and quiz them on the price, installation costs and timing, and on-going operating costs. To better acquaint you with what is involved with a repowering installation job – and to help you decide whether or not you will do it yourself – check out our Outboard Installation Primer.

There is nothing worse than an under-powered boat. When the couple above repowered, they obviously weren’t taking any chances. But are they over-powered?

Installing an outboard motor (or, “rigging” in boating industry-speak) involves not only bolting the outboard to the transom, but can also include connecting the steering system, wiring harnesses, throttle and shift controls, and instrumentation. This is an overview of what goes into rigging a new boat, so even if you don’t tackle the job yourself, you’ll at least know what you’re talking about.

Check the capacity plate for the maximum horsepower allowed for your boat and DO NOT exceed it.

The Power

The outboard must be the proper horsepower for the boat and for the application. Read the manufacturer’s maximum hp allowed on the capacity plate, and be sure you select the correct shaft length (15, 20, 25, or 30 inches) for the boat’s transom height.

WARNING – Do not install an outboard with more horsepower than stated on the capacity plate on the boat. Not only is it against the law to install a larger engine than the boat’s rating, it is also downright dangerous. If the boat doesn’t have a capacity plate, contact the manufacturer to determine the maximum horsepower rating for the vessel.



An outboard is mounted on the center of the transom, in line with the keel or centerline of the boat, and is usually secured to the transom with four stainless steel bolts. Many transoms of outboard boats have plywood cores. If this is the case with your boat, talk to the manufacturer or you boat dealer about the best way to protect the core from rot.

Very large outboards may require six bolts, while very small outboards may not require any at all, as they are usually clamped to the transom using the screw-style clamps integral to the bracket. Outboard height – where the prop shaft is located in relation to the bottom of the boat – is critical, because if the outboard is too high, the propeller can ingest air from the water’s surface and lose its grip (ventilate). Or, if the outboard is too low, the prop and gear case will be farther below the boat bottom than necessary, creating excessive drag and resulting in poor performance and handling. A rule of thumb is to install the outboard so the anti-cavitation plate (horizontal flat metal part at the top of the gear case) is even with the bottom of the boat. This setup should work well on most pleasure boats with moderate horsepower engines.

hydraulic steering
Once installed hydraulic steering is generally less problematical than cable steering.


There are three types of steering systems currently available: single cable steering for mid-size engines on boats capable of modest speeds, and either dual cable steering or hydraulic steering setups on high horsepower/high speed applications. Hydraulic steering systems are most common in applications 150-hp and up.

As the outboard is being installed, the steering cable(s) will probably be fastened to the outboard before it’s bolted to the transom, because on many boats the outboard well opening isn’t wide enough to allow the steering cable(s) to be attached with the engine in place. However, hydraulic steering components can be usually added after the outboard is attached to the boat (flexible hoses instead of cumbersome cables), simplifying the rigging process immensely.


Each outboard maker has a slightly different wiring system so you will have to follow the instructions for the manufacturer. As an example, Yamaha’s is made of three basic sections: a pair of heavy-gage cables to conduct battery voltage to the engine and charging voltage from the engine to the battery; a main wiring harness (harness=bundle of wires) that connects the outboard to the remote control box and ignition switch; and an instrument harness that sends information from sensors on the outboard and in the boat to the gauges in the dashboard.

Digital read outs are generally easier to install than analog.


You have a choice from two types of instruments: analog or digital. Analog gauges haven’t changed a great deal in decades, requiring that each gauge be hard-wired to its individual sending unit, as well as to a power source. These traditional instruments work pretty well and aren’t particularly expensive. However, since analog gauges need so many dedicated wires and connectors, the opportunity for failure over extended periods of time can be high.

Digital instruments use the outboard’s engine management computer to process information from the various sensors (boat and outboard) and display the outputs almost instantaneously. Wiring these high tech devices is simply a matter of plugging sensor leads into a central wiring harness –digital really does mean plug-and-play.

Given the versatility of integrated microprocessor circuitry, each digital gauge is capable of performing multiple functions, allowing boaters to fine-tune the readouts – showing what is important to them and hiding what isn’t. Granted, digital gauges cost more up front, but the payoff can more than offset the initial investment.

Remote Controls

With the exception of tiller-steer outboards (where the throttle and shift controls are on the handle), your new Yamaha outboard needs a remote control box – a unit that often incorporates an ignition switch, an engine stop switch, and throttle and shift functions. In addition to the main wiring harness (from the outboard to the remote control box), the throttle cable and shift cable will need to be attached to the throttle/shift handle near the driver’s seat on one end, and to the throttle and shift linkages on the outboard at the other end.


Routing wires and control cables throughout a boat can be a daunting task, and that’s why many manufacturers install the wiring harnesses and cables as the boat is being assembled, a process known as “pre-rigging”. For example, if a builder knows a certain series of boats will be using Brand -X outboards, those boats will be pre-rigged with Brand-X controls, wiring and instruments to speed up the process when it comes time to mount the engine.

As You Can See …

There is more to rigging a boat than hanging an outboard on the transom; it’s a lot of work that most of us aren’t equipped to do. Fortunately, the outboard engine makers have hundreds or thousands of dealers across the U.S. that would be glad to help rig your boat – particularly if you buy the outboard engine from them. Before you repower your boat or select an engine for your new boat, make sure your selection is well considered. Check BoatTEST.com’s performance tests for the brands you are considering on the boat models, or boat sizes you are considering. Talk to your dealer about installation and what he will charge to do the job for you.

You will probably only repower your boat once, make sure you do it right.

[The above guide to outboard re-powering has been produced by Yamaha Motors as a public service and edited by BoatTEST.com.—Ed.]