XII. Outboard Motor & Mount

Building the Core Sound 20

The main source of propulsion on a sailboat is, of course, the sails. But when the wind dies, or if a mast fails, you need an alternate means of propulsion. Although we plan to make a set of oars for our boat, we also wanted to have a reliable motor in case we should find ourselves becalmed (lacking wind) far from the dock or while caught in a strong current.

Initially, we considered electric propulsion (a trolling motor or electric outboard) as a convenient, quiet means to get around, but we didn't like the idea of carrying the heavy batteries we would need onboard in order to get adequate range out of an electric motor. Moreover, on vacation-length sailing trips, we would need an expensive solar panel or some other means to recharge, and we certainly didn't want to be dependent on finding "shore power" to plug into. These and other considerations swayed us back toward the simple, reliable, efficient outboard motor.

Hanging an outboard on a sailboat is always tricky business, but especially if, like our sailboat, it is equipped with an aft deck. This leaves you with only a few options:

  • build a large motor well — requires a sizable hole to be cut in the bottom of the boat, reducing sailing efficiency and creating noise problems with gurgling water unless you also build a plug for the well (too complex); depending on the size and design of the well, it may also compromise the structural strength of the hull (too scary)

  • build a small motor well — requires only a small hole in the bottom of the boat, but does not permit the motor to "kick up" if it strikes a submerged object (too risky); the boat or the motor is sure to be damaged eventually; a plug would still be required to prevent noise and drag problems while sailing (too complex)

  • hang the motor directly on the transom — requires part of the aft deck to be cut away (enough to hang the motor at the correct transom height and to provide clearance for tightening the manual screw-clamps that clamp the motor to its mount); this option permanently impairs the cosmetic appearance of the aft deck, unless you build a cover to hide the cut-out section when the motor is not in use (too complex); when tilted up, the motor head is also likely to interfere with the mizzen sheet (too annoying)

  • hang the motor on a transom bracket — requires a metal or wooden bracket to be attached to the transom, still impairing some of the boat's cosmetic appearance, but not requiring modification to any of the boat's structural shape; properly designed, the bracket can be fairly discrete and easily removable (perfect!)

As you can guess from this list, we settled on the last solution. With boats, simplicity is key and hanging the motor on a bracket would simplify matters greatly, minimizing any problems with noise, ventilation, or mizzen-sheet interference. In fact, there are only two downsides to using a bracket: First, the weight of the motor is so far aft that it can negatively affect the trim of the boat. Second, the prop may come out of the water if the passengers move forward while underway. Fortunately, both of these issues can be minimized—virtually eliminated—with thoughtful motor selection and bracket design.

In terms of motor selection, we needed to find the lightest possible motor that would push the boat adequately for our needs. We also knew that we wanted a 4-stroke engine, which provides quieter, cleaner, more fuel-efficient operation. We felt confident that 2hp would be perfectly adequate for light motoring in the Core Sound 20, but we hesitated to go quite that small because we will occasionally use our sailboat in areas with fairly strong currents. Unfortunately, the weight of 4-stroke motors increases quickly in relation to horsepower.

Our Choice: 2008 Mercury 3.5hp 4-stroke

After seriously considering the Honda 2hp or the Suzuki 2.5hp (both weighing in at a very attractive 28-30 lbs.), we decided that the 2008 Mercury 3.5hp motor (38 lbs.) was a better fit for our needs. Besides, the phenomenal folks at Belvedere Marine in North Augusta, SC offered us such a great price that we couldn't say no. They also provided the best customer service we've ever experienced: knowledgeable, patient, exceptionally friendly, genuinely interested in our needs, and absolutely no pressure to buy. We spent two hours with them: one to buy the motor and another to swap boat-building stories! It was a genuine pleasure to do business with them. If you're in the area, I highly recommend them.

At 38 lbs., the Mercury 3.5hp is the heaviest 4-stroke motor we're comfortable hanging off the transom. Certainly, the transom could be reinforced to carry a heavier motor, but more weight means greater "squatting" at the stern, impairing sailing efficiency. Likewise, a heavier motor would need to be left in place, since installing or removing it while underway would be out of the question. We bought the short-shaft version of the motor because it is small enough to stow under the aft deck, which means we don't have to take it out of the lazarette until we need it. Some folks recommend long-shaft outboards for sailboats like ours, but it doesn't matter as long as you position the bracket to give the prop adequate depth in the water.

Custom Motor Mount

Since Wes's dad is a steel fabricator, we designed our own motor bracket and had it fabricated out of stainless steel. The bracket consists of a large steel plate that bolts discretely inside the aft compartment, with a small round "receiver" extending through the transom (much like a trailer hitch receiver extends off the back of a truck). A motor mount can be inserted into the receiver and pinned in place for motoring, but it is easily removed when it is not needed.

The advantage of this design is its inconspicuous appearance when the bracket is removed. Since our boat is primarily a sailboat (and since our motor will be stored inside the aft deck when not in use), we didn't want a bulky motor bracket permanently mounted to the transom.

The removable mount holds the motor far enough aft that the motor can "kick up" when it strikes an object without hitting the aft deck. This feature is particularly important to prevent damage to the motor or the boat. It also prevents the motor from interfering with the mizzen sheet when tilted forward (sail control lines).

To reinforce the transom where the motor mount attaches, we installed an 8" wide x 1-1/8" thick slab of Douglas Fir (a leftover scrap from the plank we laminated for the centerboard). We filleted and taped the edges around this slab, and then bolted the 1/4" thick receiver plate to it using eight 1/4" bolts.

Later, the transom will be further reinforced by the aft deck, but it is already strong enough to support Wes's 175-pound body weight hanging from the motor mount. It should have no trouble handling the weight and torque of our 40-pound, 3.5 hp motor.

At the same time we attached the motor mount, we also installed a pair of stern eyes and the gudgeons for the rudder. The stern eyes will allow us to tie off the boat to a dock or to the trailer. Some folks use cleats, but deck-mounted cleats have a notorious tendency to "foul" (snag) the mizzen sheet, which is really annoying when you're sailing. It would be silly to design a motor mount that keeps the motor from fouling the mizzen sheet, but then install cleats that cause all the same problems. So we took the smart road and installed the snag-less stern eyes on the transom.

The gudgeons provide the pivoting attachment point for the rudder. We used the heavy-duty Racelite gudgeons, and a matching set of pintles on the rudder. They look and work great. It's neat to see the rudder and motor mounted on the boat, as it reminds us that we're getting reasonably close to finishing. (Hey, at least we can see the light at the end of the tunnel!) Hard to believe we started this build less than three months ago, but in spite of our fast progress, we're still getting impatient!

One last comment about the motor: As we hoped, the small, black motor looks nicely balanced on a boat this size. Although our boat is 20 feet long, the relatively low freeboard (only 2 feet high at the transom) would look mildly ridiculous with a much larger motor. This may not matter to some folks, but we wanted to maintain the simple beauty of this boat's lines. Our discrete motor and custom-designed bracket suit that purpose well.

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