XXIII. Outboard Motor Performance

Building the Core Sound 20

Many folks have asked about the outboard on our Core Sound 20. While some sailors detest the noise, weight, smell, and expense of an outboard, we see it as a rarely used, but greatly appreciated convenience. It allows us to sail with fewer worries about the time or prevailing winds. If we have a schedule to keep, an emergency, or a restless child who wants to go home, the motor lets us get where we need to be, even if Mother Nature refuses to cooperate.

The outboard also makes it possible to enjoy boating and swimming on the hot, windless days of our long Georgia summers. On those days, we don't bother rigging the masts and sails. We just motor over to a cozy swimming cove or an island we'd like to explore. Motoring is a lot more fun than sitting at home complaining about the lack of wind during most of August.

Which Outboard?

The question we're asked most often is about horsepower. The Mercury 3.5 hp outboard has proven to be ideal for our needs. To help others make an informed decision when selecting a motor, we've provided detailed information below.

Our Performance Results

The 3.5hp Mercury has a top-mounted 0.3 gallon fuel tank. At one-fourth throttle, the motor will push our CS20 at approximately 4.8 to 5.1 knots (about 6 mph) for 4.6 nautical miles on flat water in calm conditions. With a full tank plus the one-gallon fuel jug we carry onboard, this gives us a generous motoring range of 17 nautical miles (about 21 statute miles).

At half-throttle, the motor reaches its maximum speed of about 6 to 6.2 knots (about 7.3 mph); however, the small gain in speed is not worth the considerable gain in noise and loss of fuel economy. We rarely exceed one-third throttle except when driving onto or backing off of a beach (the motor rotates 180 degrees for reverse), or when executing a sharp turn to steer clear of docks, boat traffic, or other obstacles.

The main benefit of the motor's considerable reserve thrust is for turning the bow against a strong wind or current, or for powering onto the face of following waves, at which point the boat will "surf" to maintain speed at reduced throttle.

The video below shows our motor's performance on flat water in calm conditions. The speed at each throttle setting is displayed in knots, in the upper left corner of the GPS screen.

A smaller outboard in the 2hp range should also push the CS20 quite well. Some other Core Sound owners use the Honda 2 hp air-cooled engine, which is 11 pounds lighter than our Mercury and slightly smaller. We considered that engine, but chose the Mercury 3.5 hp because we preferred its water-cooled design and greater thrust. Both motors are four-stroke, so no oil needs to be mixed with fuel and it runs much cleaner (considerably fewer fumes) than a two-stroke.

At 39 pounds, the Mercury is fairly easy to install or remove from the transom bracket. Its low weight has little effect on sailing performance, which is a key reason to avoid a heavier motor. It is also small enough to stow in the aft lazarette, if desired, though we typically keep ours on the transom bracket. Our bracket holds the motor far enough aft that it cannot leak fuel onto the boat or foul the mizzen sheet. Our motor bracket is custom made, but a roughly comparable setup could be achieved by installing a "lifting" outboard bracket such as the light-duty version made by Garelick.

Our main concerns when choosing the motor were noise, durability, and manual shifting. The water-cooled Mercury seemed quieter than the air-cooled Honda, and we felt it would be a better choice in Georgia because our summer air temperatures get brutally hot. Some owners also warned us that the (automatic) centrifugal clutch on the Honda cuts in and out at very slow motoring speeds, which can be frustrating when maneuvering in tight quarters near a dock or other boats. In that regard, we preferred the Mercury's manual shifter with Forward and Neutral gears. (Both motors "reverse" by turning them around 180 degrees.)

Another consideration was thrust: While the 2 hp Honda should be able to achieve the same cruising speed, the additional thrust of the 3.5 hp Mercury promised more responsive control in winds and rough waves. In practice, the extra thrust is only really beneficial when turning the bow across a very strong headwind or fast current. However, it also allows the motor to maintain speed at a lower throttle setting than a lower-horsepower motor, reducing noise and potentially improving fuel economy. Mercury makes a 2.5 hp version of the same motor, but for the same weight, we preferred to have the extra horsepower.

Overall, we're very satisfied with our Mercury. Most of the time, it rides on the bracket without use. But every now and then, when the wind dies or the kids decide they want to go home immediately, we start it up and motor straight back to the launch ramp. A few years ago, we found that the motor's small fuel passageways can gum up fairly easily from stale gas. We switched from Sta-bil to another fuel stabilizer with an active enzyme that eats away at fuel deposits (StarTron Enzyme fuel treatment) and have had no fuel-related problems since.

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