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. Unfortunately 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 thinking of building an electric trolling motor setup for when long range isn’t required. Plus with a three month old it’s becoming apparent that long daysails aren’t going to be happening very frequently if I want to live a happy life via a happy wife. I’m thinking a quiet cruise on some local creeks might be a way we can enjoy the water together and avoid my wife’s sea sickness.

My long term plan is to eventually build a bimini for the boat so I can mount solar panels to charge a large LiFePo4 battery. Shade and infinite range sound pretty good to me! It’ll be a while before I get around to modern batteries, solar panels, and charge controllers, so for now I’m going to get the propulsion system going with a regular 12 volt battery.

Here’s the circuit I came up with. 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 as well. And a 40 amp fuse up by the battery protects the entire setup.

Here’s most of the components. Since the total circuit is less than 15′ long BoatUS says the wire can be 8 gauge. Not shown here are some 50 amp quick connectors so I can remove the battery without unscrewing the ring terminals.

Here’s the power cables made up. All the terminals are crimped, soldered, and heat shrunk. Hopefully that’ll help keep out the green death. I also made a cardboard mockup of the control box and found 8″ x 5″ x 3″ is about right. I plan to have power come in the left and power to the motor out the right. The box will sit on the sternsheets while the battery is up by the centerboard for better trim.

I’ve had a few offcuts of walnut veneer plywood kicking around for a couple years now from when I built a platform bed. They were just the right size to build a quick and dirty mitered and epoxied box! I put some fillets on the inside and a few pin nails held everything together while the epoxy set.

It’s a tight squeeze, but I did manage to get all the components mounted to the lid. Next up is to varnish the box and wire everything in.

Eventually I got all the wires made up and running to where they’re supposed to go. I think. I epoxied the speed controller to the lid with a healthy fillet of thickened epoxy all the way around. I gave it a jiggle and said “that ain’t going nowhere” so I’m pretty sure it’s on there.

It’s looking like I imagined it would! The speed control knob came out of the trolling motor but I plan to make a pointier, more ergonomic one out of wood and write the numbers in the appropriate spots. Counterclockwise is reverse and clockwise is forward. 0 is when the central post looks like a backwards D.

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!

The ammeter reads about 2 amps in all five speed positions, so hopefully that’s just the no load condition and not some problem with the meter.

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.

I haven’t launched Moga yet, so I gave it a try with my canoe. I rigged up a quick and dirty motor bracket from a 2×4 and a clamp on tiller from a 1×2.

It works! My wife, son, and I took it out for a 2.8 mile trip over nearly an hour. There seems to be some problem with the control switch though… I need to investigate further. There was no difference between speed 4 and 5 and I don’t know which one was top speed.

Speed 1 = 2.2MPH, 12.3V, 6.25A, 76.9W
Speed 2 = 2.6MPH, 12.2V, 8A, 97.6W
Speed 3 = 3MPH, 12.1V, 10.75A, 130W
Speed 4 = 4MPH, 11.8V, 18A, 212.4W
Speed 5 = same as speed 4

I took the switch apart to see how it worked on the inside and when I put it back together I had all five speeds. I think what was happening was the sliding contacts at the bottom that give it the beans were hitting speed 5 when I was still in the 4th position. I did a lot of poking around with an ohmmeter to see which wires are energized at different speeds and I must confess that how the motor works is just as much a mystery as it was yesterday. I know there are “speed coils” to lower the amperage, but I really don’t know how you get two different speeds when the same wires have continuity.

Anyway, I tried the motor on my canoe again and got some better data. I did two runs in each speed to hopefully average out the tide which was coming in. I’m pretty pleased with the consistency of the runs with the exception of the last one. Not quite sure what happened there. The numbers were pretty consistent with yesterday and I can see now that both speed 4 and 5 were top speed.

Next I did some experiments with a fairing around the motor shaft. I settled on a NACA 0025 foil after a fair bit of research, downloaded a 200 point drawing from Airfoil Tools which I imported into Rhino, then I modeled my trolling motor and designed a fairing around it which I 3d printed. Tests show no improvement in speed, but a 2.6% to 7.7% decrease in wattage. Unfortunately the test conditions were far from ideal and about all I can confidently say is a fairing looks like a worthwhile pursuit.

After cutting the part in two I taped it onto the motor with some packing tape. I assumed the tape would be fine, but halfway through the test I noticed one strip had come loose and was flapping along beside the motor leaving the saw cut exposed. Surely not good for performance!

I took my canoe out at high tide so there wasn’t much current to deal with, but unfortunately there was a bit of an inconsistent breeze on the creek. I did two runs at each speed to try and minimize the variables. I’m happy that the bare shaft run was within 1% of the previous trial, that gives me some confidence that what I’m doing is repeatable. The fairing did a really nice job of decreasing the turbulence and noise from the bare shaft, although it is a bit short. At higher speeds water started spilling over the top so I tried a full speed run with the motor positioned just below where it started to suck air. No significant change although there was possibly a very slight increase in speed.

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