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Making a SUP conversion kit from a trolling motor

I’m working on making a electric drive conversion kit for my SUP. I did find the many $500 kits that go maybe 3mph, but I can paddle that fast, and want to be able to both go faster and longer.

So far I’ve bought a 55lbs thrust trolling motor (50amp max, up to 9mph), a 50h LiFePO4 battery. I’m adding a volt/amp meter to the waterproof case, a jetski type emergency disconnect lanyard, a 4 channel remote which I’ll waterproof (spare remotes are $5 each). I’m going to use solenoids to physically turn the original motor speed control inside the battery box, and use one channel for a power cutoff vs. needing to multi-press to get to no power. I’ll probably disable reverse speeds.
I’m working on designing different ways to attach to the board. I don’t think a fin box only connection would work for for the amount of thrust, and the motor is like 15lbs. I want it to be able to work on most boards, and ideally be able to remove while in deep water in case there is a failure and I need to paddle back.
So far, this is a pretty affordable project - $180 USD trolling motor new, $400 50ah LiFePO4 battery, $100 other parts, $50 watertight box.
I do hope to be able to work on tweaks to get more speed, but my goal is really to be able to go long distances (exploring), with the ability to play with the dolphins (they seem unable to go only 3mph).
Clearly 9mph is not the 20+++ I’m seeing people getting on this site, but there doesn’t seem to be any options at all for the mid-range 5-15mph.
Anyway, I’ll be posting pics on my instagram site HawaiiWes68, and I expect to have this complete and operational in the next 2-4 weeks (before Oct 21, 2020). I’ll be headed to Mexico for the winter (staying in my RV on the beach there for 6 months), and so it must be done for that trip.
I don’t see any other postings of this type conversion, has anyone done anything similar?

Parts are almost all assembled. Here’s the waterproof tote with all the electronics, and the board with trolling motor, and the trolling motor modified.
The plan is to have the trolling motor setup so that it is hinged, and so can be pulled out of the water so that it’s not in the way - so motor somewhere, then tilt it up for paddling.
Turning the speed control mechanically is a tad challenging by remote, but doable.
The 4 channel remotes appear to be water resistant as shipped, so that is a bonus. Also they use button batteries (2032) which is better then the costly mini 12v batteries.

Sorta clunky, but this is the fwd/rev speed control and solenoids which will push on the metal pins I’ve added to the switch wheel. I actually got lucky here that the switch had the larger wheel on it since it serves no function in the trolling motor setup. So, my wireless 4 channel remote will have a button to actuate by steps the 5 fwd speeds, off, then 3 reverse speeds. One button for faster, different button for slower. One button for emergency stop (actuates a relay that cuts all power). One spare button.
I did buy the aluminum bar stock today to build the frame. I hope to have this completed in the next week.

Great progress. The Electronics are 100% complete. The wireless remote is working well. It’s more difficult to turn the physical switch to return to neutral/off than the other increments, so currently I might have to press the button twice when going to 1st speed, or returning to neutral. I think the switch will break-in after a while and this may self resolve.
The 4th/red button on the remote triggers a 60amp relay and kills power, until the 3rd button © is pressed resetting it. Of course the lanyard also triggers this relay removing power from the motor.
I’ve installed a volt/amp meter into the front of the waterproof box, so I’ll figure out a chart so I can have a guesstimate of the battery remaining charge. And the amp meter will help me know what economy mode is in case I’m going further or need to get home on less remaining charge.
Now, I just need to weld up the frame to mount the motor on the board.

Here’s a short YouTube video:

I’ve completed the frame that mounts the motor on the board. I reverted back to a design where it’s in front of the fin, as I’ve decided that I can also use this to pull me while I’m snorkeling behind it, and a motor behind the board would prevent this. So, I’m not going to be able to tilt up the motor to get it out of the way after using it to get somewhere. But, if I board out to some place where I can land, I could just flip the whole thing to the top of the board.
I have the frame out getting powder coated now, back on Tuesday, then I head to Mexico for the winter on Friday.

Add a little powder coating.

Yay for Mexico.
Maiden voyage today - all went perfectly. After running for 15+ minutes at full speed, the 50amp breaker tripped. My ampmeter (shunt type) says I’m pulling just over 40amps at max speed, so I think it’s just a matter of overheating the breaker. I have a 90amp one I can swap in - main reason it’s there is to avoid a short & to act as the main power switch, which 90amp will do fine as well. The battery also has a built in short protection.
Not sure my speed, hard to tell on the water with a 60lb black lab also on the board, but surely at least 2.5-3x faster than I can full out paddle. I’ll take my GPS with me next time for a real speed check. After about 30 minutes full throttle, the battery was only slightly warm.
The wireless speed controller works well and cycles between the speeds to stop/neutral with only the occasional need to click again to get the next step. I did run a thin line of silicone on the remote case where it seals, so it looks like they should be air/water tight easily.
So, based on a total of $850 ($120 for powder coat), seems like a great success.
I also have thought of a few things I could simplify… put the shaft of the trolling motor speed switch trough the case sealed with an o-ring, then manually set the speed desired, then a 2 button wireless remote controls on-off at that setting vs. the complicated solenoids to toggle through speeds.
I used solar panel standard electrical connectors to connect the motor and battery, and those seem to be working well, but I’ll need to be sure I don’t submerge them in the ocean disconnected as salt water would get into the connectors with no real way to clean it out. But also easy enough to just pop on new connectors if/when needed.
I’m charging the 50ah LiFePO4 lithium battery by connecting to my RV batteries of the same type (but 4x100ah) and so charging off my RV solar system. This was convenient vs. needing a different charger to run off of 110vac. These LiFePO4 batteries have a Battery Management System (BMS) built into them, so it’s pretty brainless to charge but not overcharge (as long as the max voltage of the charger is for these LiFePO4 batteries - 13.7v). My RV batteries have a bluetooth app so I can review the charging state, etc., but unfortunately this 50ah does not, but I think it’s easy enough to determine the state by the current voltage with no load applied.

Well, I’m a little disappointed, speed in 5th/top switch setting is 5 mph (+.1 or .2) (8km/h). 4th setting is 4mph. I had for sure hoped for faster, but this is still moving along at a nice clip. And it’s easy to also paddle still (not sure how much faster total).
I do hope to find a way to make a duct, which I think will increase the speed in the same way it increases thrust/pressure on a air-fan. I also think if I could come up with a more-pitch prop it would push it fine.
I was pretty much full speed for an hour, and the battery was likely around 40% left. The motor was only slightly warm, and I’m in very warm waters here.
Now I’ll just need to try some long distance/time runs so I know what I can expect for range/time in use.
The trolling motor is basically silent which is nice.
I did find turning the board was much easier using the paddle at the front of the board vs. as a rudder or dragging to the side.

Keep up great work! Cheers!

Distance run at full speed - 5.45 miles average 4.0 mph, time 1 hour 21 minutes. Battery cut off at low voltage set point.
Weird, I was getting a full 5 mph before, but only 4mph today. Will be investigating. Maybe switch did not go into last/5th speed?
Also, going to shore and flipping the motor onto the top of the board worked well for the 5.4 miles paddle back up.
Noticed as well where I left the “T” trailing, when it was flipped over (so no motor in the water), I could tell there was some drag from that T which was still in the water, which I had thought would be not noticeable. I’ll cut off the back end T since it’s not required.

Okay, I’ve had some more time to goof around, and I’ve taken to riding on the tail of my big board such that the front maybe 1/3 is out of the water, and this makes for some pretty good weight shift tight turns and such. Still needs to go a bit faster before I’ll call this successful for what I was wanting - which was more than just cruising at the same top speed as I can paddle already (4-4.5 mph). It’s great I can go that speed for an hour just hanging out, but that’s not what I’m going for. I did have it on the board motor on top one day when the wind was likely to be tough to get back from my island circuit, as backup in case it was too hard, but I stuck it out and paddled back. But nice that I could have just flipped the motor under, and for sure cruised back home.
I have a few ideas on new prop designs I hope to work on (build) in the next few weeks which could go either way, works great and I go faster, or just a waste of time. If it works it is likely patentable. :slight_smile: