First of all I’d like to thank everyone who contributes on here and especially those who take the time to document their progress on YouTube. This is a great resource and I wish I found it sooner. I have wasted so much money buying the wrong things and teaching myself about batteries, motors and carbon fibre.
So here’s where it’s at.
First of all my project differs from most as it’s a sit on top kayak and the motor is in the hull. I went this way because I had already bought the motor for another project and it was to big to put under the water.
Hull- carbon fibre, I’ll probably make another one at a later stage because I think it can be much lighter.
Motor- Turnigy rotomax 150cc 150kv outrunner
Gearbox- custom 90 degree, oil filled
Drive shaft- 12mm x 700mm currently 316 stainless but it hard to find a straight piece. I’m thinking titanium or chromoly tube may be better. For testing I supported it half way down the shaft but ideally I don’t want to.
Mast- custom carbon, I’m currently chipping the paint out of the mould because things didn’t quite work out.
Battery- I may have gone wrong here, I have a 48v 14s ebike battery. It cuts out at only 60amps and is intended to run at only 30.
Any ideas on the drive shaft? Is anyone else trying this?
How much thrust do I need? Currently I get 27kg before the battery cuts out at 60amp. The motor can take more and it wasn’t at full rpm.
When the first photo showed up I could not figure out what I was looking at. Now I see. I have not seen anyone implement a 90 degree gear box yet in a E-foil. The 90 degree gear box has been discussed in this forum. It would allow a large diameter high torque motor. It could be very hydrodynamic. You would have cooling issues to work out because the motor would be above the water. I thought about using a small old outboard as a starting point.
Hi Mac, I started with the thought of an old outboard too but found they had a gear reduction in them that made them as big as having the motor underwater and I wanted it sleek. I now realise that because im running a drive shaft through the mast it has to very stiff and bigger so im not sure I’ve gained anything. As for cooling I haven’t given it to much thought yet but as I have much more space to work with up top, air cooling may be an option. I’m incorporating some extra tubes in the mast just in case. I’ll add some more pics to give a better idea of what it is.
I got the motor for another project, I was going to convert a petrol jet kayak but I knew nothing about these motors and it sat around for ages. I’m not to worried about the thickness at this stage. The gearbox is a simple 1:1 bevel gear setup, aluminium case and oil filled to stop the gears getting to hot and stripping hopefully. I’ll get some pics up later but it’s in bits at the moment.
The prop Is 130mm from aliexpress, so far it seems it will be ok but it’s very heavy.
The battery is 40ah so was quite an expensive mistake. What if I got another one And ran them in parallel, would that make it 60amps continuous and 120 peak. I don’t know much about the electronics yet.
Hi! Ok, its in aluminium i guess? Do you use a 1:1 gear in the angle gearbox?
When i comes to batteries. You need a pack with a C-rate that matches or exceedes your normal usage. The C-rate tells us how much amps we can draw from the pack continous and peak. A 40ah pack with 10C peak and 5C continous can deliver 400A peak (maybe 10 seconds) and 200A continously without overheating or otherwise being damged.
If there is no information regarding C-rate on the pack you need to know who made the cells and get the specs from them. I have never seen a ebike battery that can handle our current demands, but there is always a first
Putting 2 in parallel will definitely give you double the discharge, but you could be lucky:
These batteries are usually build form 18650 cells. There is no 18650 cell with more than 4Ah, so there have to be at least 10 in parallel, my guess is: either 14p or 16 or even more. And every 18650 cell (I am talking about unprotected cells) can deliver at least 5A continuous. So worst case (only 10p and shittiest cells on the market): the battery it self can do min. 50A cont. But it is much more likely that there are not the cheapest cells inside and no high capacity cells, likely some panasonic or sony with ~2.6Ah, so ~14p-16p and each has a discharge rating of ~7-15A so at least 100A cont.
There are EV cells inside (not very likely, but possible) which are made for high capacity and >2000 cycles, so they usually (e.g. in solar power plant) have many other cells parallel and so they are simply not made for high discharge. But these are pouch cells and many of them also have a discharge rating of 2-5C (like these) but there are always exceptions (like these).
So back to topic, why does your pack only give you 30-60A:
All of these assembled batteries you buy, do have a battery management system. This controls and ensures safe (for you (fire) and the battery (overdischarge)) charging and discharging. But also the battery management limits the power you can draw, even if the battery can do much more.
So why does a battery costing >500$ capable of >100A do have a BMS with 30A limit?
Well $$$, look for some BMS, you can get a 13S 10A BMS for 7$, but a 13S 150A BMS is close to 150$.
I would recommend to have a closer look at the battery (many battery builders put a label on the battery with the used cells and configuration) and get to know what is inside. I am sure you can find someone close to you who is able to upgrade/exchange your BMS.
Also bear in mind where you want to go. There are people flying with much less than 1,5kW (==> much less than 30A @ 14S). Sure this also depends on system, wings, weight… and before takeoff you will need >30A anyway. But consuming more than 50A cont. also means much more trouble everywhere (cooling design, cables, connectors, ESC…).
you make a valid point here, i might leave it for now and see how it goes, maybe try reducing propeller pitch to keep the amps down. i made a mistake, its a 30ah battery not 40ah. the cells are 18650 3.7v 3C Samsung, 14s. i’m curious to know what the max discharge current would be with this if it wasn’t limited with the current BMS. (just in case)
Max, i’m no longer planing on controlling the foil with a joystick. i decided its difficult and expensive enough without adding complications so now i’m keeping it simple with a throttle on the paddle. also a hard landing with a joystick between your legs would be quite uncomfortable.
Well, you already stated it: if it is a 3C battery: 3x capacity = 90A cont.
That will keep the motor amps down (less torque needed) but also the max. velocity.
Maybe have a look in motor amps vs battery amps. And have a look into ESCs, basically the ESC can let flow more amps on the motorside than on the battery side (think of a stalling motor, 0 rpm means 0 energy (neglegting heatlosses), still you can have high torque, this torque is produced by the high motor amps and near 0 battery amps, which are again 0 energy).
So you can have the same effect of the smaller prop by limiting the batterypower/rpm at the ESC and allowing higher amps on the motor side (but your motor mustnt get too hot). Not perfect efficiency wise (higher losses in the ESC and motor) but good enough for testing…cheaper than having 10 different props…
thanks for the advise Giga, i might order a BMS just because everything takes so long to arrive but other than that i’ll see how what i’ve got goes. i’m probably not far enough along to be worrying about how ti get the thing working yet. i had the motor running and for me that was a huge milestone.
i’m currently working on the mast / gearbox assembly and trying to make a mold for the mast but it didn’t go well. pictures attached of what it should look like.
I’ve got quite a bit done in the last week but my holiday has ended now so it will probably slow down.
The mast is complete with drive shaft, gears and coupling in place. There is quite a bit of resistance and a bit of vibration, mainly due to the shaft not being perfectly straight. Has anyone else done a shaft in the mast? I added a brass bush half way down which helped. For the coupling I just raided the tool chest and used socket parts with a bit of lathe work. The whole thing feels heavier than I planned, way to much carbon I think, I am an amateur with carbon and have no concept of how much I need, the mast is virtually solid whereas the hull might be a bit weak due to my learn as you go approach. I’m already thinking of building another one and this one hasn’t even been wet yet.
All I need to do to finish is make a motor mount and sort out the electrical stuff. I’ll draw an electrical diagram soon with all the stuff I want to incorporate and hopefully someone can tell me where the wires go.