This will be a tricky build.
You could have a single button to start stop the jet.
Then use a feed back from the vesc to see when the jet has left the water to stop it. There will be a large current decrease when pumping air.
For safety you could use a Hall effect sensor, with a magnet necklace. When you stand up it could pull away and stop the motor. But this is could probably handled in code with using an accelerometer.
Either way, hands free will require some coding to make it work nice.
I was considering using the Hall effect sensor the other way around. The water jet would turn on only when my chest is getting close to the board and stop when I popup. Positive/negative ramping time in VESC should allow me to make the ride a bit more comfortable if I don’t have a throttle. Maybe a delay would let the motor run for another predefined time when I am up, let’s say 5 second, and then stop.
Adding more switches in series and parallel could allow me to disengage the hall sensor if for any reasons I don’t want it to tigger the motor (for example paddling to and back from lineup), or bypass it if something goes wrong with sensing. I believe hall sensors behave as ON/OFF switches, but if there was a way to use it as proximity sensor I could use my chest position (assuming I have a small metal plate under my wetsuit) as a throttle.
Based on your experience with the tow boogie, what motor power would I need to drag me laying down on a 5’ prone board at about 15 km/h, and what battery capacity would be necessary for a 10 minutes run time?
Just have a button on the board. When you press, you have boost. When you drop and standup, finger comes off, jet turns off.
I am building a longboard with twin MHZ 58s;
This should be plenty of boost.
I would use a waterproof potentiometer to set the boost speed and then a button to enable to switch between PWM of zero and PWM from your potentiometer. You could do it with a Teensy or Arduino Nano.
Probably more similar to a foil drive, but double the power and battery requirements for a similar thrust when using a non optimised jet.
Also look at the kill switch functionality built into the vesc. This could be a way to stop the power when you stand up with an hall effect switch.
Thanks! Have you tested these 7.5 kW yet? What sort of speed are you expecting with dual jets on a long board?
All have been able to get on Foil Drive site are motor weight (1kg) and max thrust (27.5 kg).
This thread has more info though:
“On original FD Assist (+, max, slim) , Foildrive use 6374 130 KV 3200 W 5.6N.m to have 25kg of trust at 3000 rpm on the propeller. The system is limited to 1500W by the VESC (half motor power) so I wonder if we could use a smaller motor because I conclude it’s an overkilled size for this use.”
Not yet. Should be in the water later this month. How fast? No idea. Jets are terribly inefficient. Zero drag once you are on a wave, but really inefficient. I have no idea how to relate thrust to potential speed. Yes, I will have close to 20hp on a surfboard, so it should scream. However, it will probably only produce about 40kg of thrust sustainably, but I can’t relate that to speed.
Batteries arte 12kg each. Motors a couple each. Jets close to a couple. G300 Vescs a couple each. So, the propulsion system alone weigs about 36kg. The board is about 15kg, so all up about 50kg, plus me, so 150kg. So thrust to weight of about 25%. What that means… I have no idea.
Oh wahoo, pretty impressive build. I guess, I could always choose a VECS with enough head room and upgrade motors as needed. My biggest issues right now is figuring out which part should be selected and in what order. I would assume picking the motor first, then identify battery voltage to get a specific RPM based on motor KV and finally VESC?
You probably have seen crazy Mark’s (aka YOUNGSTERS JETS) YT videos ?
If not. Here is his current last video.
We have 4 of his jets, and after building several jet boards we are now into building eFoil boards.
Holly smokes that is FAST! Could you comment on some specs I could use as starting point? If not I can try to get in touch with him.
I have seen these, but can’t see any specs on the setup. What diameter are the jets? What are the jets?
Had a bit of a look. He is running 15s with a 7.35kW motor pulling max 130 amps.
I am using 10s with a 7.4kW motor running 10s at 176 amps.
So, we should have similar thrust. Though, I am running lower revs, so should be more efficient.
I would love to know how much the board weighs.
All his builds and specs are in his videos.
He is slow in replying to email. Mentioning me will help!
I am still trying to figure out basics about component selection and would appreciate your help.
I would like to keep the system light and slim (control box and jet enclosure will be ideally 3" tall at best), and considering the target max speed (15 km/h) and run time (less than 10 minutes in total), would a 10-12s battery be adequate? If so, what motor would you recommend.
I understand it is difficult to provide an exact parts list for this build, but any pointers would be welcome, as I have currently no idea where to start.
I literally sent the email an hour before seeing this 
I will mention your name in the next email I send. Thanks!
Hi thefoildesigner
Great that you are taking on a project like this. They are a ton of fun. I am the inventor of the AMP board which I started back in 2019. While I can’t give exact details of where we ended up as we have spent millions in R&D getting to where we are now I can give you a couple of pointers that will hopefully help.
We are getting 25kg of thrust using a completely custom motor and ESC that has a combined weight of 500grams you start adding other electronics, batteries, shaft, seals, bearings, impeller, stator, jet housing and in our case a cartridge so it’s removable and the weight adds up quickly.
Weight is the absolute key here and thats what I would focus on.
25kg of thrust out of a jet system in a small prone board that weighs sub 7.5kg was incredibly hard to achieve. While we are going for a commercial product here our batteries have to meet many specs which add to more advance BMS and materials for impact resistance and IP ratings which does add weight. As well as our entire systems has to be highly reliable so we are keeping our customers happy.
For a one off build there are many advantages as there is a few things that you don’t have to focus on.
If you aim for around 17kg of thrust and build the jet system (52mm to 56mm) into the board that will be something that is more achievable. This was how I built the first proto that took me almost a year of work. This proto was heaps of fun and with good timing and technique it was amazing what you could get onto. With our final product I quite often have the jet set to around 15 - 18kg depending on conditions. I don’t always have it set to full thrust.
Choosing a motor, battery and ESC is not that simple as you need to know your impeller and Startor design, rpm and load. I built a plume tank (big test tank) down the side of my house. Bought a motor and ESC that was too powerful and then started to design my jet. Once you have a jet that is producing around 17kg of thrust (I would have designed over 100 impeller and stator combos just for the first Proto) then you can work out your RPM and load on the motor and that will give you a rough KV and motor size. Ideally you want to try and keep your power requirements under 3kW keep your voltage high (we run 14s) and you amps low. Otherwise things get hot, components get bigger and the whole system weighs too much. My first motor in that proto weighed 800grams. Now our final motor is 390grams.
Also you have to remember you have a mast in the bottom of the board so you need to design the jet around that which I can tell you is some fun to get around 
You don’t need a lot of battery which keeps things light but that battery has to be big enough to produce the amps. I would suggest Lipos as they can produce more amps with less voltage drop ( but don’t believe the C rating that is advertised, you want plent of headroom) They are not great for a commercial solution as the safety and cycle life isn’t great compared to Lion.
Hopefully some of that has helped
All the best, I look forward to seeing the progress.
Cheers, Chris
One of our biggest issues was cavitation causing the jets (we have 2 side by side) to underperform.
Good R&D for the hull design is a must!
Vide below was at 65% power.
Thanks so much for the reply @oceanfrother, and congratulations on putting such a cool concept together! There are many intriguing features on the AMP I have been trying to get my head around based on pictures and videos available.
It appears that the entire assembly (prop, motor and battery) would slide in and out from the back, with a latching mechanism to hold everything in place. If correct, that would mean that the jet enclosure is a two-part piece, one that stays in place (with the intake) and the other that slides in and out with the rest. I believe the water intake is placed on the left hand side of the foil track. It appears that all controls are in the from of the board, although I could not really see much from the videos.
Can’t wait to see this in person. Any idea when the AMP will be released?
Following up on @oceanfrother comments and some idea I have, I put together a quick mockup showing how my parts could be arranged in my jetfoil board. I understand that I should probably design the board around the parts, rather than trying to fit parts into a small space, but I really want this board to remain small and nibble, so that is the constrain I need to work with.
The two rectangles delimit the width (18") and thickness (3") of the board. To keep weight balanced, I have opted to have all parts centered on the board. The water intake would be located right before the foil track, followed by the motor, battery and ECS.
In terns of access, I only have two options: an access door on the deck (under foot) or the rear of the board. The former would be the easiest, but would require quite a big panel. A box containing all components that could slide in and out from the rear would be ideal, but maybe hard to implement.
As far as components, here is what I am considering (please tell me if better/cheaper options exist out there):
Hey @thefoildesigner, yes you are correct, we have fitted everything in a cartridge that slides into the back of the board. The intakes line up with the intakes of the board each side of the mast. This way you can place the motor and battery under the mast/foil so the swing weight is around your lifting force of the front wing. This is pretty important if you want the best ride possible. Having the inlet behind the mast will reduce your performance of the jet and also cause cavitation.
You will also end up with a pretty long tail. Remember you have thrust now so you can use a super short board. I mainly paddle a 4’2 and don’t feel I need anything longer. You are adding weight so the smaller the board to combat that somewhat the better.
11s is ok but I would really try and get up closer to 14s. Heat is you enemy unless you want to add more weight for cooling.
The 5684 is too heavy (about 700grams) and the KV is too high. And so is that ESC. Both those together is 1kg and you still have batteries, electronics, wires, boxes etc. You will end up adding 5+kg to your board. You will also end up running you esc at 40 - 50% Duty cycle which will create excess heat in the fets. You want to aim for 70-80% duty cycle. depending on your jet design you should be down near 400kv give or take 50kv and depending on your voltage. You should only need about 3kw or under.
