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Eska Electra

The idea started innocently enough. After spending a week cruising in the Canadian Gulf Islands I thought our family might enjoy some boating that was not sailboat racing. Lack of noise and efficiency would be important considerations as would cost. The eventual conclusion was that a trailerable sailboat in the neighborhood of twenty-two feet being pushed by an electric motor could be our cruising boat.

Initially I believed a large trolling motor would be desirable. After reading the stats I wasn't so sure. It seems most of them are designed to push heavy boats at just a couple knots. We were hoping to move a little faster under power. Another vote against the electric trolling motor was registered when my early research uncovered that the price for a 24 or 36 volt model was a bit higher than I had originally anticipated.

The final straw that pushed me past the point of no return was seeing a similar conversion on the internet. A small outboard had been converted to electric with the removal of the internal combustion engine and the addition of a two horsepower Scott DC motor. The author seemed happy with his home made electric outboard. He made it sound easy enough with just a small amount of machining being required. I decided to give it a go.

The original plan really was to put the horse before the cart. We wouldn't start shopping for a well used trailer sailor until after the electric outboard was functional. And it almost worked out that way. The outboard was nearly done when we were given a Venture 25 sailboat. That, however, is another story.

Donor Gamefisher 5 horsepower motor
1000 watt (1.3 hp) Scooter motor from Currie

Step one was to find a small donor outboard. I chatted up the idea with my sailing buddy Frank. He owns a farm, has lots of room in his barn, and tends to stash things like old outboards away. He gave me a Gamefisher five horsepower outboard. These motors were built by Eska and sold through Sears under the Gamefisher name. It was light, complete, and free.

The power head came right off as did the water pump. These parts were returned to Frank. Next the lower unit, where the vertical shaft rotation is transferred to horizontal shaft rotation, was disassembled and inspected. The condition of the lower unit was satisfactory but the design wasn't optimal. It was designed to have neutral and forward but no reverse. This normally wouldn't be an issue with a DC motor which is reversed by switching polarity. The lower end gears, however, had dogs that were shaped to only transfer torque in one direction. Reversing the electric motor would have the effect of kicking the lower end into neutral. Reverse capability modifications would have to wait until forward was a reality.

Because I now had a suitable outboard I was ready to invest real money into the project. I ordered a DC motor kit from Electric Vehicles USA. The kit included a 1000 watt motor, 100 amp controller, and a twist grip throttle. The folks at EV USA thought I was nuts when I explained what I was doing with it but were helpful none the less.

5/8 deep socket pressed on DC motor output shaft
Scooter throttle on modified outboard tiller

The kit motor did not have many markings. It appeared to be a Chinese manufactured motor designed for a Currie brand electric scooter. The controller was a Navitas TSP-100 which is designed to run at 24 or 36 volts. This model does not include reverse or regenerative braking, but is small, epoxy encapsulated, and affordable. The third item in the kit was a Magura brand 5k ohm twist throttle.

When the kit arrived I started assessing what would be needed to make the motor fit the top of the outboard. The motor had three flanges with mounting holes arranged radially around the motor. The outboard had four holes arranged into a rectangle that originally held down the power head. Of course none of these holes lined up with each other. I reasoned that if I could make an adapter out of a flat piece of aluminum that had both sets of holes I could bolt the motor to the plate and the plate to the outboard. Cutting and drilling aluminum was doable with the hand tools I owned and skills I possessed so I moved forward with the numbers portion of this task.

Building the aluminum adapter would not be the challenge. Measuring and marking the various holes would be. The Electric motor was the less difficult. The bottom was flat with the shaft protruding. I was able to calculate that the shaft was in the center of a circle that the three bolt holes fell on. The outboard holes were a bit more challenging. It made more sense to measure the bottom of the power head than the top of the outboard as the top of the shaft was supported by being slipped into the end of the crankshaft. I then inverted the outboard pattern and added it to the electric motor template.

I originally planned to use 1/4 inch aluminum but didn't have any collecting dust in the corner of the Race Garage. I found some scrap 1/8 inch and cut a couple pieces into the desired shape and epoxied them together. My original plan of cutting to shape and drilling holes ran into a snag, however. The holes that lined up with the outboard fell inside the perimeter of the motor. I would not be able to run bolts from the top down. So I needed to thread those holes and run fasteners up from the bottom side. I also added an additional layer of 1/8 inch aluminum to function as a bottom. The original fan cooled two-stroke needed lots of ventilation and splashed water was just free cooling. The electrical components wouldn't be so forgiving.

Finished and tested electric outboard with cover removed

Stage two would require some help from Jack the Machinist. I needed a way to transmit the torque from the 11 mm motor shaft to the 7/16 vertical shaft in the outboard. I knew I needed a solution that allowed some up and down play, just like the original, but I wasn't sure how to proceed. Jack has been solving problems for a long time so I loaded all this junk into my truck, drove across town, and plopped it down on his bench. "Easy", he said. He would machine the 3/8 square drive end of a 5/8 inch socket into an 11mm hole and press it onto the motor shaft. I would find a 5/8 inch nut that fit the machined socket, drill it out to 7/16, and tack weld it onto the vertical shaft. In effect, the motor would be the ratchet handle and the vertical shaft would be the bolt. This solution allowed a little vertical play and also eased final assembly. Elegant, simple, and effective. Jack the Magician.

Now I felt like I was getting somewhere. I had an assembled drive train. Next up was the controller. This step was considerably less time consuming than the previous. Finishing the controller installation required only to drill a couple holes, fasten the body of the controller to the outboard, and wire the motor to the controller.

The last real assembly task was the throttle. The Magura throttle did not appeal to me as it was a 5k ohm type. My preference was for a hall sensor style throttle. A cheap scooter, $100 when new, was salvaged from the yacht club dumpster. This scooter included a functioning hall sensor type throttle. The original throttle tube was larger than the 7/8 inch tube the scooter throttle was designed for. The low tech solution I employed was to drive a smaller tube into the original throttle tube and push a 7/8 inch tube over the smaller tube.

After finishing all the wiring a test was in order. After building an ammeter I powered it up in the test tank (a 30 gallon garbage can). The controller was rated for 30 amps at 24 volts. Exceeding this number might overheat and/or damage the controller so I was pleased to see that the ammeter was showing 17 amps.

The next step is to get the free Venture 25 sailboat onto the water and see if this little motor has the oomph to push it around. Only time will tell if this idea was just as nutty as the EV USA folks thought.

Update: An initial on-the-water test has taken place.