Electric Surfboard Problem

Hi, my name is Ken, and I’m going into my freshmen year of college this fall for electrical engineering.

Over the course of the past year, I have been designing an electric surfboard. It uses two motors meant for e-foils directly underneath the board. When the board is outside of the water, the motors spin up very fast, however when I put it in the water, they have barely any power and the board won’t go faster than 1 mph. If I start the motors outside the water and then dip them in while they are spinning, they maintain their speed until about 90 percent of the propeller is in the water, and then they slow down again. Here are the specs of the surfboard:

Motors (2): flipsky 65111 160 kv
Props (2): 7.25 inch by 5 inch pitch (included with motors)
ESCs (2): TFL Pro High-Voltage ESC 300A with watercooling
Batteries (2 in series): Turnigy High Capacity 14000mAh 6S 12C Lipo Pack w/XT90

I have a 150A fuse which hasn’t blown, so the current on each motor must not have exceeded 75A. The motors are rated for 70 amps peak current each. Also, the speed controllers and batteries were not even remotely warm after testing it in the water even without the watercooling system running, so I don’t think that they are being overloaded either (The fuse would blow before the batteries or speed controllers were overloaded).

If anyone had any ideas as to what was wrong they would be greatly appreciated, as I have spent a ton of time and money on this project and I really want to see it work. Thanks so much!!

Here’s a picture of the surfboard:
20220428_233233|666x500

The motors do not have enough torque, use smaller propellers, you can 3d print them. If that does not work, your motors are to weak. KV is also a little bit high.

What size and pitch would you recommend?

Difficult question, you have to try. I’d start with 5“ pitch and 130mm diameter. Then adapt depending on the result. You can also go lower in pitch but that limmits max speed. I think the main problem is the surface of the prop with a diameter of 7.25“. You could also try a two blade prop.

Pics amp value is just a number written , a value value for selling and compare which you may not want to exceed not to burn the motor under load

Amp : torque is depending on prop size , you should aim for 60% of max rpm ( voltage x kv) for your pitch ( according to the desired speed wanted )
Diameter ? A ratio of 1 to 1,5 of the pitch
Blade area ? Not too small I guess for a surfboard

The main problem here is that you don’t known the amp and the voltage applied of your motor under load with this type of esc ( you can add a wattmeter for amp , I don’t know if you can get a voltage reading on phase motor wires with a voltmeter ? )

Fuse will not burn at 151 A , it will warm up and them blow under a defined amp for a defined time ( check spec )

  • check if your esc is well programmed to apply full amp under load with slow rpm ( need a programming card to do that I recall…?)

  • try smaller prop as said

  • your battery cannot drive this type of setup for a long time but surely pull you of the water of couple time
    Don’t expect more than a constant 3-5C of those , you run 12s 14A total ?

My guess : your esc is cutting amp under load because it is not well programming
Or the prop is so big that the motor loose synch under load

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