Twin Torpedo - Underwater Battery eFoil

Idea · August 31, 2026, 11:47am

Hi everyone,

About six years ago I decided to build my own eFoil. I bought pretty much all the parts I needed and was ready to start, but then other things related to work came up and the project was put on the back burner. And somehow six years went by.

I finally came back to it in August this year, but I changed the original idea quite a bit.

At first I was planning to use a Tacuma board, but I decided to go with a much smaller inflatable board instead. The main reason is that I want to be able to pack the whole thing into the trunk of my car.

This created one obvious problem: where to put the batteries?

The idea I came up with is to put the batteries underneath the board, on an adapter mounted between the mast base plate and the bottom of the board.

It is somewhat similar to the battery concept used by Waydoo with the FoilBoost, where the battery is mounted underneath the board. I wanted to take the idea a bit further and use two separate battery pods with much more capacity, one on each side of the mast.

The pods will be long and relatively narrow, with a square cross-section. From the side they should look a little bit like two torpedoes, hence the name Twin Torpedo .

The plan is to use two 6S packs connected in series for 12S. I already bought Molicel P42A cells specifically for this project, so that’s what I will be using.

Originally I was thinking about 12P, but after looking at the size of the pods I think that would make them unnecessarily large. I will probably build them as 6P or 8P.

One part of this concept that I really like is having basically all the electronics mounted on the adapter between the mast base plate and the board. It is quite a challenging approach because all the enclosures have to be properly waterproof, but it gives me a big advantage: the electronics and mast assembly can remain independent from the board itself.

This means I should be able to change the board whenever I want, whether I decide to use a smaller board, a larger one, a different inflatable board or even a rigid board. The idea is to make the adapter and electronics package a kind of independent module that can be transferred from one board to another.

There is one problem I still need to solve. The VX5 receiver cannot be completely underwater, otherwise the radio connection obviously won’t work.

So I will probably make a small separate enclosure for the VX5 receiver, mounted above the waterline at the rear of the board. I may also make this enclosure suitable for the VESC Express, which could be plugged in there when needed.

This should give me a small “electronics box” at the back of the board while keeping the main electronics and batteries mounted as low as possible underneath the board.

Designing and building the waterproof battery pods and the adapter will take some time. Since I live in Poland, winter is also coming, so I didn’t want to wait until spring before doing any more testing.

For now I decided to use what I already had.

I temporarily mounted a rather ugly and very large battery box with some LiPo batteries that I previously used in my mountainboards.

Definitely not the final solution, but it lets me continue testing while I work on the Twin Torpedo system.

Current setup

Gong Galaxy inflatable board, 5’11", 135 L
Gong Galaxy V1 foil
85 cm aluminium mast
Gong Alvator L 1350 front wing, 77 cm
250 stabilizer, 40 cm
Flipsky 65161, 120 kV
Flipsky IP68 Mini FSESC 6.8 Plus
Flight Rodeo Progressive 7, 2021, 6" front / 7" rear
Flipsky VX5 remote

I’ve had only four sessions so far, so I’m still very much at the beginning and learning something new every time.

The current battery is around 1000 Wh, so the riding time isn’t particularly long, but I’ve already noticed a few interesting things.

The board starts to come onto foil at around 15 km/h.

It takes roughly 50 A to get the board out of the water, although I managed to take off at around 42 A.

At the moment the ESC seems to be limiting the battery current to around 60 A.

I need to check my VESC Tool settings because I currently have Battery Max set to around 99 A and motor current at around 100 A. The ESC is rated for 150 A, so I’m not sure yet why I’m only seeing around 60 A.

The fastest I’ve managed so far is around 30 km/h.

When flying comfortably at around 20 km/h, the current draw is roughly 30 A.

These numbers are obviously still very preliminary, as I’ve only had four sessions.

Propeller testing

I’ve tested two propellers so far.

The first one is the Flight Rodeo Progressive 7 from 2021, 6" front / 7" rear.

So far this is my favourite, especially with the shroud installed.

This is actually something I don’t really understand.

I expected the shroud to create some additional drag, but my experience has been almost the opposite. When I remove the shroud, I have noticeably more trouble getting the board onto foil. With the shroud installed, the prop seems to work better and the board gets onto foil more easily.

It almost feels as if the shroud is improving the efficiency of the prop.

Has anyone else seen this?

I’m curious what the actual reason is.

The second prop I tested was the Flipsky 162 mm folding 2-blade.

This one has a lot of thrust during takeoff. It feels stronger than the Flight Rodeo and works very well at low speed.

At higher speed, however, I don’t like it nearly as much.

It gets quite noisy and I can feel a noticeable vibration through the mast. I don’t know yet if this is cavitation, propeller imbalance, vibration or something else.

Has anyone experienced something similar with the Flipsky 162 mm folding prop?

Board stability

The other thing I’m struggling with is board stability once I’m flying.

The board feels quite nervous and very sensitive to roll changes.

I’m not sure how much of this is simply because I’m still learning and how much is related to the setup.

Would changing the stabilizer make a noticeable difference?

I’m also wondering if the propeller thrust could be disturbing the water flow around the stabilizer.

I have no idea if this is actually significant, so I’d be interested to hear what people with more experience think.

One thing I’m pretty sure about is that the current battery position doesn’t help.

The battery box is mounted high at the rear of the board, so the centre of gravity is both too high and too far back.

Unfortunately I can’t really change that until I build the new battery system.

Hopefully, moving the batteries underneath the board will make a noticeable difference. Maybe moving the motor further back, closer to the stabilizer, or changing the stabilizer could also help?

ESC

At the moment I’m using the Flipsky IP68 Mini FSESC 6.8 Plus, mounted behind the mast.

I also have two other ESCs:

Flipsky High Current FSESC 200A 60V
Flipsky 75200 Pro V2

I may eventually switch to one of them.

The 200A High Current has water cooling, but I’m not a big fan of the tube-based cooling arrangement.

The 75200 Pro V2 isn’t waterproof, so I would have to design a proper waterproof enclosure for it.

If I eventually move to 14S, I will probably do that and use the 75200 in a custom enclosure.

For now I’m staying with the IP68 Mini FSESC 6.8 Plus.

I really like how compact it is, but I am a little concerned about the temperature if I start doing longer flights while the ESC remains above the water.

At the moment I have an additional heatsink and water cooling while the board is not flying, and that works extremely well. The ESC temperature has never gone above 28°C so far.

But again, I’m still learning and my flights are quite short, so I haven’t really pushed it yet.

What’s next?

For now the plan is simply to keep testing the current setup while working on the new battery system.

The Twin Torpedo will eventually have two waterproof battery pods underneath the board, one on each side of the mast. They will be connected to the mast through a custom adapter.

The battery will be two 6S packs connected in series for 12S, built from Molicel P42A cells. I’m currently leaning towards 6P or 8P because I don’t want the pods to become too large.

The main goal is to get the batteries as low as possible, improve the centre of gravity and get rid of the big battery box sitting on top of the board.

I’m still very much at the beginning of this project, so any advice is welcome.

In particular, I’d be interested in hearing your thoughts on:

Stabilizer choice and board stability
Why the Flight Rodeo seems to work better with the shroud
Vibration and noise from the Flipsky 162 mm folding prop
ESC cooling and whether the IP68 Mini can handle longer flights above the water

And if anyone has built underwater battery enclosures for an eFoil, I’d be very interested to see how you solved the waterproofing and mechanical mounting.

I’ll post some pictures and more details once I start working on the Twin Torpedo design.

Cheers

Jezza · August 31, 2026, 12:42pm

At takeoff your battery current will be half of the motor current. That’s why you are limited. Push the motor current to the max possible or even to 200a for burst just for takeoff.

S_Roger · August 31, 2026, 2:45pm

#2 - Seems strange to me you would get better efficiency without a shroud. Conventional experience is the opposite. Lift eFoils typically come with shrouds and lots of owners remove them to get better performance.
#3 - I think two blade folding props like this are prone to more vibration and imbalance. The Flipsky prop is one of the cheaper options available, I think this is just a symptom of that.
#4 - I haven’t used this VESC yet, but I know a its commonly used in foil assist builds. I would just keep an eye on temperature. You could run a water line from the nosecone up to cool your aluminium plate on the top side above the vesc (no pump, just pressure from forward motion). As long as there is good contact between VESC and plate it should be fine.

underwater battery enclosures for an eFoil: This reminded me of the Lacroix Vortex efoil product. Similar ideas to what you want to achieve

Idea · August 31, 2026, 4:20pm

#2 – I actually did the test on the same day. I removed the shroud and then put it back on, and for some reason I got noticeably better thrust with the shroud installed. I honestly don’t know why, because it does seem strange to me too. I’ll increase the current limit on the VESC and repeat exactly the same test to make sure the result is actually repeatable.

#3 – You’re probably right that the Flipsky prop is not the best in terms of manufacturing quality. What actually surprised me is that, considering it’s such a simple and relatively cheap CNC-machined aluminium prop, it produces really good thrust at low RPM.

With this prop, the board starts lifting off the water after just a few, maybe a dozen metres. I didn’t measure it exactly, but it rises very quickly.

However, the higher the RPM, the worse the overall smoothness becomes. It still produces plenty of thrust, but the noise and vibration increase significantly. It almost feels as if cavitation starts to occur at higher RPM. The sound is almost like there are air bubbles around the propeller.

#4 – Yes, I completely forgot that Lacroix had done something quite similar. Thanks for the link to the Lacroix Vortex 1 thread! I’ve been out of the loop for about six years and wasn’t really following the eFoil industry during that time, so I wasn’t aware of the details of their design.

janefoil · August 31, 2026, 5:56pm

i had the same problems at start. i guess when the prop is to close to the fuselage and the stab to far behind the prop vortex interfear with the stability. i places the motor like 2cm higher on mast and the problem was gone. i see you have fusalage v1 , it is longer then the v2 o v3 set up so stab in vortex i guess

i used the flipsky prop for learning. after that i go for the flite prop. same conclusion. flipsky lots of trust not fast. flite prop harder to start but faster and better with higher speed.
i have my esc watercooled in the box. water coms by speed of foil. (no separate motor)
your box look simular then mine so must be possible to go for 14s10p. (30Ah) i foil like 30 min. 45 when slow and big foil

place the box between foot, not at the back, to much swing weight (i made the same mistake when started) use the footstraps screwholes to fix it. with my litle board (65 l) i even place it at my front foot.
its harder to start but better to foil (no possible to sitting on knees) lie down and standup direct

going faster with the schroud i think its also to do with vortes. with schroud the vortex wil me longer parallel and les interfearing with stab. . but without shroud wil be the fastet after all setting are correct. (is what i experienced)

Idea · August 31, 2026, 6:53pm

Thanks a lot for the advice! I really appreciate it and I’m happy to take any suggestions I can get. Every bit of experience from people who have already been through this is valuable to me.

I actually had the same thought about the stabilizer being so far behind the propeller and possibly being too low, which could have a significant effect on stability.

The only problem with raising the motor is that I would then have a hole underneath the motor mount. I could probably come up with some kind of cover or plug for it. But as a first quick test, I’m thinking of making a temporary 5–8 cm spacer to move the motor further back. That would be the quickest and least complicated way to test whether changing the propeller/stabilizer position makes a difference.

I also noticed that most users seem to have the motor positioned much closer to the mast. If moving it further back improves stability, I’ll consider raising the motor by 2–4 cm as a more elegant and permanent solution than moving it further back.

And yes, I have the V1 fuselage because I bought the whole setup about six years ago. I was actually thinking about upgrading the foil, but if I can get my current setup properly stabilized, I don’t want to generate additional costs for now. Once I finish the whole project and get comfortable flying it, I’ll think about upgrading the foil.

Unfortunately, I have the HIPE First board, so I don’t have any footstrap mounts that I could use to secure the battery box in the middle. I’ll try moving it a little closer to the center anyway.

I added some attachment points for straps/lines to my board, so I might add a few more slightly further forward and use them to secure the box for some initial tests. However, I really don’t like having the box sitting on the deck and I’d definitely like to get rid of it eventually.

Unfortunately, I’m kind of stuck with this setup for this season because I don’t have much time left before winter arrives

Foilguy · August 31, 2026, 7:38pm

This might be mast flex. The V1 mast is known to be more flexible than the V2 and V3 masts

Idea · August 31, 2026, 8:23pm

Maybe, but that seems rather unlikely to me. Maybe the V1 mast is more flexible, that’s possible, but I don’t think that’s the cause. This doesn’t feel like that kind of instability.

Foilguy · August 31, 2026, 8:41pm

@Idea

Lots of posts here about the people finding the V1 mast at that length “unrideable” due to its flex. Not saying that’s the source of your current problem but it will likely be at some point.

Perhaps contact Gong to confirm if their V2 mast fits your fuse. Not very expensive if it does and would let you locate your motor at a more desirable height.

Idea · August 31, 2026, 10:48pm

The V2 mast will not be compatible with the V1 base plate or the V1 fuselage, because the V2 mast has a different profile and shape.

Considering that the V1 system is already outdated, I think the most sensible option would be to move directly to the V3 system. The downside is that I would need to buy the complete setup.

Jezza · September 1, 2026, 5:56am

Pretty simple to check. Put either the foil on the ground and put your feet on the wingtips and check for wobble. Or do the same with keeping the board still.

I used to ride a V1 up to 95kg and with smaller wings like the gong Veloce L never had any issues. It still holds my record speed run in fact.

Idea · September 1, 2026, 8:35pm

That actually makes sense. A very simple test , I’ll definitely try it.

I changed the motor current to 130 A. I set the Absolute Maximum Current to 200 A and the Battery Current Max to 100 A. This time I could definitely feel the extra power. With the FR prop, the foil now accelerates past 30 km/h and is very responsive to the throttle. Even small throttle changes have quite a strong effect.

Today I also tried to work through as many of the suggestions from @janefoil as I could. I enlarged the cable opening, raised the motor by about 2 cm, added two extra attachment points, and moved the lines slightly so that I could move the battery box forward, but only by about 5–10 cm. I think that’s still far too little, because it’s very obvious that the battery should be in front of the mast, not behind it or directly over it.

I can’t really tell if raising the motor made any noticeable difference. But changing the centre of gravity definitely has a big effect.

Unfortunately, moving the battery to about three-quarters of the board length still leaves it quite far back, and that makes getting up and starting properly much more difficult. So at this point I think the main cause of my problems is the centre of gravity.

It’s a bit like having a car mounted on a trailer, positioned right at the very rear, behind the trailer’s rear axle. If the weight is that far back and the trailer starts to sway, losing control is almost guaranteed. I think I’m experiencing something very similar here with the foil.

When I start from a kneeling position, I have full control of the board. Having a lower centre of gravity makes the take-off much easier, and as the board starts to lift out of the water, I don’t have any problems controlling it.

But when I start standing, it’s a completely different story. As soon as the board starts to come out of the water, I immediately lose control. The board starts to sway from side to side and I end up losing balance and crashing back into the water.
I’m standing roughly at one-quarter of the board length, while the battery is still at the back, so even a small change in my balance has a big effect.

So, in summary, I think I need to speed up the work on mounting the battery underneath the board. That should both lower the centre of gravity and move it forward, which I think could make a huge difference.

Jezza · September 2, 2026, 5:21am

When you stand you add an extra pivot point at you knees. A lot of people struggle with this and end up jack-knifing. With foiling, the faster you go the more stable it feels, but there also a point where the falls are harder.
You could get a 60cm mast which means its easier.

janefoil · September 2, 2026, 8:50am

so recognizable
yes it is hard to learn and it takes time
most of it is just experience and learning not place of motor/batterij/balance point
this wil help a litle but it takes some time to just learn to stand and foil
i somethimes think teh issues i had in beginning with stab and prob maybe just beacaus no experiance and not vortex interfearing with stab
whats helps in beginnging to learn is when stand frontfoot more to frontside so you can tuch down the board and fly alternating (more control of that)

Idea · September 2, 2026, 9:21am

I don’t think I’ll invest in a V1. If the only real problem is that learning on it is more difficult and will simply take longer, then I’ll keep working on it and eventually I’ll get it figured out.

I have another question regarding the FR propeller. @Jezza, I saw your post where you managed to reach 38 km/h with the same propeller, a 120 kV motor and 12S.

Obviously, I know that this also depends heavily on the foil itself, rider weight, riding skills, etc.

I’m wondering whether it actually makes sense for me to move to 14S with my current setup. I see two main advantages of staying with 12S. First, safety, lower voltage in the water. Second, I designed my battery packs to be easily removable, so I can swap them for larger packs and also use the same batteries in another project.

The other project is a 4WD platform with four brushed 24 V motors. My idea was to use two 6S packs in series for the efoil, and then, once removed from the efoil, connect the same packs in parallel for the vehicle. It’s basically a multifunctional ~100 kg platform on quad-bike wheels, which currently uses standard lead-acid batteries.

So, do you think moving from 12S to 14S would actually make sense in my configuration? Is there a realistic chance of getting a proportional increase in top speed, or is the FR propeller likely to have some kind of practical speed limit where increasing voltage won’t help much anymore?

janefoil · September 2, 2026, 9:34am

mistake i have 13s10p sorry (not 14s10p) 48v (max load 54v)
as far as i know the more volt the more rpm you get on the flipsky motor
also what i hear from guys who also use this motor and got 16S or even 17S battery pack (they go over 55 km/h i read)

Jezza · September 2, 2026, 9:50am

There’s no real reason unless you want to reduce the current consumption for the ESC. Safety is not really a factor. Even a low voltage under the wrong parameters can kill you.

A v1 60cm mast is cheap, second hand some guys sell them for 30 euros.

The largest limitation in reality is the board and battery placement. A box on top of the board is really just horrible when it comes down to it.
Realistically if you have a board with internal space it solves loads of your issues.

aemkei77 · September 2, 2026, 11:27am

I have a very similar setup. 12s battery box on the rear of an inflatable board. No stability issues. Mast is V2 though.
Can reach 40 km/h with flite prop. 38 with easy foil prop

The stabilizer has huge influence on board stability, more than the front wing

I have Gong Crossover, Curve and Fluid with increasing instability.
I take the fluid only on calm water. The curve is now my usual stab except for very rough sea but If I hadn’t learned on the Crossover it would have taken me twice as long or even more.

janefoil · September 2, 2026, 1:01pm

yes here also learned on alvator x-over XL and big 46cm stab .
but interfeareance prop vortex with stab is a issue.
when in the beginning motor was close to fuselage and used the flipsky prop all stable, then same setup with flite prop less stable . issue gone when 2cm more high