I wanted to make a spearfishing surfboard, as the coral is far from the shore, I wanted to build an electric surfboar that will be small enough to take inside my car and powerful enough to take me at list 1 Km from the beach and back with ease .

So I bought a small buggy board from Decathlon.

The must important part was the underwater motors, after a long search the cheapest and best option for the underwater trusteres was a supplier from Russia that made a very good design

https://www.rovthruster.com/

This company sell the the motor casing and impeller and you need to buy the motor from somebody else.

I have used a stronger version of the recommended motor and this speed controller as I thought that one controller can control all 3 but that is not the case and I bought an additional two as the cost less (I recommenced using the first only).

I used a lock&lock as the waterproof electronic box but it was not perfectly sealed and water did come in. The Gramin GPS was purchased from Amazon

Step 1: PRepering the Machanics

It was time for the first hardware assembly.

I had to drill and dig a spot for the electric box plus fix the motor into place.

I have bought the 30A 12V battery from AliExpres. Best to use to in parallel for a longer run.

As only one out of the two batteries ordered arrived I has to build my own battery using a spot welder I have build using a car battery, Arduino and a 200A relay I have bought as a master switch for the surfboard.

Step 2: Preperingf the Motor

As the motors are not water proof, I had to fallow the instructions in the the motor casing manufacture. The original ball bearings will not hold long in salt water so they ware change to half ceramic bearing but I will probably need to be change to a full ceramic bearings. the slide bearings recommended in the instructions does not work well because of a slight design difference in this more powerful motor.

Step 3: Electronic Assembly

The electric setup in not complex and I have used an Arduino and a veritable resistor to send the wanted speed to the speed controller. Once you follow this VIDEO you get the basics of how to do it.

You will see the IP65 resistor coming out on the bottom left of the box.

As you see there is not a lot of space in the box and it took some imagination to put it all in there.

Step 4: Final Result & Conclusiones

I don't have a video from the water but I can tell the the full speed is ~ 7KPH at full power with ~90A on the 12V battery with ~ 12 - 15 KG pull from the engines. which was sufficient but not perfect to my taste.

I am now working on a better one as there were a few throwbacks to this first design:

1) it didn't float enough because of the size and the weight of the batteries.

2) electric box was to small no more space for bigger batteries and water came in if it was submerged

because of 2 & 1 I couldn't sit on the board to rest.

I had a lot long learning curve on how to fight the salt water corrosion and I am not sure it is all solved as the salt water damage the electrical wires.

Because of that I am currently working on a faze two of this board with a bigger board and better / bigger electrical box with water cooling to the speed controllers.