Trolling Motor

My British Seagull is a pretty cool motor and I really like how simple and bombproof it is… it’s basically a two stroke outboard boiled down to the bare minimum. However, I can’t say I’m thrilled about pumping oil directly into the water with a 25:1 fuel to oil ratio so I’ve been working on an electric trolling motor setup for when long range isn’t required. I’m also thinking a quiet cruise on some local creeks might be a way my wife and young son can enjoy the water with me and avoid my wife’s sea sickness. I started by buying a 36 pound thrust Newport Vessels trolling motor, cutting down the shaft, and building a remote box with gauges and controls so I can move it between boats and sit amidships for better trim.

Here’s the circuit I came up with for a control box. I have a USB charger to charge my phone or VHF radio, an ammeter to keep an eye on the current so I can calculate range, and the speed control knob from the trolling motor itself. The USB charger has a digital voltmeter so I can monitor the battery’s charge and calculate watts. And a 40 amp fuse up by the battery protects the entire setup. Since the total circuit is less than 15′ long BoatUS says the wire can be 8 gauge.

I built a box from some walnut veneer plywood I had left over from another project and mounted all the components to the underside of the lid. The box will sit on the sternsheets while the battery is up by the centerboard for better trim.

Everything works! I found an old battery kicking around and temporarily wired it up with some wire nuts. I ran it for about 5 seconds and saw no trace of any magic smoke… The volt meter works and the ammeter’s needle goes in the right direction!

Next I cut the motor shaft down to 24″ from the centerline of the propellor to the mounting bracket. I used a pipe cutter which worked great for getting a square cut, although the cutting wheel didn’t quite make it all the way through due to the pipe’s wall thickness so I finished it up carefully with a coping saw. The shaft is 1.125″ in diameter so I heated up a 3/4″ PVC elbow with a torch and smooshed it over the shaft to flare it out. Then a reducer bushing accepts a 1/2″ service entrance gland nut. I used an underground splice kit to connect the wires. I left the 14ga wires long enough so both screws clamp. I wrapped the area of the splice in two layers of amalgamating tape and then the entire run with friction tape.

It works! My wife and I took it out for a quick test with our 4 month old son in my 14′ Old Town canoe. We went 2.8 miles at up to 3.5 MPH. At top speed the power required was about 210 watts.

Eventually I brought my boat around and we did some more tests. Henry really enjoyed watching the shore go by and trying to grab the tiller. Top speed was a disappointing 3.4 MPH compared to my British Seagull’s 5.5-6. I eventually learned that the propeller isn’t pitched properly to move such a lightweight boat so the motor runs out of RPM before the propeller can develop enough thrust.

So I bought and modified a model airplane 10×6 pusher prop to fit the shaft because other people with similar size boats report good success. It doesn’t seem like such a skinny propeller would improve performance, but it does! I also added a PWM controller to combat the inefficiency of the resistive speed coils and give me more granular control of the speed.

Top speed increased 41% from 3.4 to 4.8 MPH and the power dropped ~50% from 186.2 watts to ~92.4 watts at the same 3.3 MPH speed. I’m curious to see the effects of a shaft fairing and propeller spinner.

Next I switched out the analog ammeter for a magic box that gives me volts, amps, watts, and total energy consumed all at once. This should help me get me a little more accurate data. I’m hoping I can reuse my walnut box, but if not I’ll build a new enclosure.

I also bought a 50Ah LiFePO4 battery and what a difference from the 6+ year old worn out lead acid starting battery I’ve been using. I can see us taking a lot of little cruises on our creek this coming summer! I figure I should easily have a 10 mile range in normal nice conditions.

I managed to stuff all the electronics back into my walnut box and make a new lid for the PWM controls. Despite the inside being an absolute rat’s nest and all the components have to be installed in a specific order, it’s working pretty well! I did add a small CPU fan which unfortunately cuts into the box’s waterproofness. I ran the motor at 200 watts for an hour and things were getting a little warm inside. I couldn’t smell anything burning but I do wonder if it could overheat during the summer temperatures. I also added some 3/8 NPT gland nuts for strain relief on the cables.

Continuing my efficiency experiments, I designed and 3d printed a spinner for the propeller to see how that affects the drag. It’s in two parts so the first part acts like a washer for the shaft bolt, while the second part finishes off the tip. It’s incredible I can print threaded parts that actually fit together.

Here it is mounted on the motor. I painted it white to try and prevent the PLA from warping, but it still warped a rather noticeable amount after a few weeks of sun exposure.

Unfortunately as best I can tell the spinner had little to no affect. In this test it showed perhaps a 3% increase in efficiency while a later test showed about 3% decrease. From what I’d read I knew a spinner wouldn’t get much, but I thought I’d at least be able to see a difference.

I think next I’ll test the affects of a shaft fairing. That will smooth out the turbulence from the round shaft and help prevent the propeller from ventilating. Previous tests with a 3d printed version were promising, but I think I can make a better fairing. The problem with a fairing though is it’s impossible to stow the motor for sailing, so it’ll need to be in two halves. Perhaps I can use strong magnets to attach it.

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