XXIII. Outboard Motor Performance
Building the Core Sound 20
NOTE: This article is divided into chapters. Click here for the Table of Contents.
Many people have asked for our opinion about putting an outboard motor on a Core Sound. While some sailors detest the noise, weight, smell, and expense of an outboard, we feel differently. For us, the motor is a rarely used, but greatly appreciated convenience—one that allows us to sail with fewer worries about the time of day and the prevailing winds. If we have a schedule to keep or a restless child who is ready to go home, the motor gives us the reassurance that we can always fire up the "iron wind" and motor straight back to the dock, even when Mother Nature refuses to cooperate.
The outboard also allows us to enjoy boating and swimming at the lake even on hot, windless days during our long Georgia summers. On those days, we don't bother rigging the masts and sails. We just attach the outboard and then motor over to a nice swimming cove or go explore an island. This helps calm our sailing fever because at least we can be out on the lake instead of sitting at home complaining about the lack of wind through most of August.
Which Outboard?
For those who share our preference for a motor, the next question is always about size and horsepower. Each sailor should reflect on his or her own needs to make this decision, but we chose the Mercury 3.5 hp outboard, which has proven to be ideal for our needs. To help others make an informed decision about size, we describe our own real-world performance results with this motor below.
Our Performance Results
The 3.5hp Mercury has a top-mounted 0.3 gallon fuel tank. At approximately one-fifth throttle, the motor will push our Core Sound 20 at approximately 4.8 to 5.1 knots (about 6 mph) for 4.6 nautical miles. With a full tank plus the one-gallon fuel jug we carry onboard, this gives us a practical 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 a following wave, at which point the boat will "surf" to maintain speed at a reduced throttle setting.
Given these performance characteristics, it is clear that a smaller outboard in the 2 or 2.5hp range would also push the CS20 quite well. In fact, other Core Sound owners use the Honda 2 hp air-cooled engine, which is about 11 pounds lighter than our Mercury and slightly smaller overall. 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 there is no need to mix oil with fuel and the motor runs much cleaner, with considerably fewer fumes than a two-stroke.
At 39 pounds, the Mercury is fairly easy to carry and to install or remove from the transom bracket. It is sufficiently light so as not to impede sailing performance, which is a key reason to avoid a heavier motor. The Mercury is also small enough to stow in the aft lazarette, if desired, though we prefer to keep it on the transom bracket, ready for use. Our transom bracket holds the motor far enough aft that it rarely fouls the mizzen sheet, though on occasion the throttle arm (which protrudes further forward than the motor cowl) pinches the mizzen sheet against the aft deck. This is easily fixed by rotating the throttle arm aft. Although our bracket was custom made, a similar configuration can be achieved with a "lifting"-style outboard bracket such as the light-duty version made by Garelick.
For us, the most important concerns were noise and long-term durability. We found the water-cooled Mercury to be noticeably quieter than the air-cooled Honda, and we also felt it would be a better choice for Georgia because ambient air temperatures can get brutally hot in the summer. Some folks told us the air-cooled Honda might struggle with our high-humidity and very high-temperature summers. They also warned us that the Honda's flywheel is unusually susceptible to corrosion problems, especially in salt-water environments, though many others have not experienced such problems.
One other consideration was thrust: Although the 2 hp Honda would very likely push our Core Sound at about the same cruising speed, the additional thrust of the 3.5 hp Mercury promised more responsive control when motoring in winds and rough waves. In practice, this thrust is only noticeably 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, thus reducing noise and improving fuel economy. Mercury makes a 2.5 hp version of the same motor, but for the same weight, there is no reason not to get the 3.5 hp version.
Overall, we're very pleased with our little motor. Often, it rides on the bracket without use. But every once in awhile, when the wind dies or our little girl decides she is ready to go home now, we start it up and buzz straight back to the dock. The only problem we've encountered with this motor is that the fuel passageways are very small and gum up 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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