Home Made Flume

EFOIL.builders Props & Ducts
101

dc volatge on battery 12s goes from 50.4v to 38.4v
controlleur will convert dc current to wave Voltage for the motor
the motor will see the voltage by peaks at a frequency
by the thorttle you change the switch frequency so the time the motor sees the peak voltage (of the battery)

so with a 790kv on a 12s fully charged 50.4v, to get 10000rpm , it is about 25% duty, 25% at 50.4v and 75% at zero : average 12.6v

if you convert thrust into mecanical power you need take this in account, the impeller will draw the amp needed from the motor to spin at the desired rpm, the motor will always give to a point where the rpm will drop

25krpm : just reading and seeing other jet board setup, mhz data on 52mm…
aiming for this rpm off load , included heat and mecanical looses, and voltage drop on battery and voltage sag of cells, to end up with a constant 15k rpm at least

reisanauer planetary gearbox take 50krpm , no problem

Edit : my Calculation were made with impeller with a ratio closer to 1-1.2 , so with your high pitch I don’t know , but for sure the best is to keep the highest voltage possible for the motor so the lowest kv : to match the best rpm of the impeller

102

I have some more reading to do now. Any highly recommended link suggestions to read so I can understand this better? I usually find some gems articles or forum posts but it can be luck.

Can only seem to find 3:1 and up for that gear. That’s some RPM claim

Yes the pitch is just too high, based on this calculation from this book Propeller Handbook. I would ideally want a variable pitch like in Jetski propeller with lower pitch at the leading edge and higher at the trailing edge. Do you know of any 50mm prop with variable pitch?

I am not sure what the best RPM of this 50mm prop I show on that video link above. The seller suggest 6-8s on it with a 500-790kv, so I may end up just copying it. Making a 6s12p pack or more. The costs are a lot lower with higher kv motors it seems, just would need to have more parallel cells to have higher amp pack. I read somewhere the torque between low kv and high kv motors is the same as long as the other factors are suitable like high voltage for the former and high amp for the latter. The battery life is about the same considering 12s6p or 6s12p is about the same # of cells and total energy in Wh

I was thinking I can make 6 packs of 2s6p and then I can connect them in series or parallels in many combinations to suit my needs and try different voltage/amps. I can make 2s36p, 4s18p, 6s12p, 8s6p, 10s6p, 12s6p. Most of those combos are not practical for this but I guess it’s nice to have for other projects.

Using the 6s…
Based on that book, the min diameter the prop should have for a board with a beam of 2ft and “hull” draft of 1.5in,which is the dimensions of my surfboard, it is exactly 2.04in (51.8mm).

Rounding down to 50mm, or 1.968in. The calculation for the shaft RPM given SHP (HP0.96)for the motor of 5 SHP (790kv at 180a 6s), it is exactly 25,022RPM (((632.75^0.2)/1.968)^1.666). This may be the best rpm for this prop according to that book. But the book is mostly about screw props and not about jet props.

Then to get the MPH it is KTS =(1-10%)*230/(204.05/10.01)^0.5 the KTS is 45.85, in MPH it is 52.

10%slip
230 constant for high speed craft
204.5displacement (my weight including board and equipment)
10.01 the SHP of the 2 motors

103

I will try to put some links this we
For the motor it is about right , you look end up with the same torque using different kv
For the battery not the same , its discharge capacity is not linear, the more you pull from it the less you will get ( heat)
Several 2p pack is not the best solution , you will need multiple bus bar and big wires to handle the amp , may be go with 4p : 8s then 12s

Some reading :

Here on the forum , you have multiple Efoil setup with data logging via vesc and metr : you have the values : rpm, battery voltage and current , duty , motor current to understand more about all this

104

True, at that high amp draw the wire guage would be so thick… didn’t think about that.

Check out this guy if you haven’t yet: https://www.youtube.com/watch?v=Bb0OWG_JaPo&t=355s

I may just copy his build. From what I gather, he’s running dual jet drives 53mm with 500kv 56123 SSS motors, 400a ESC, 8ft foamie, not sure what voltage he’s running it at. Maybe 10-12s?

He’s going so fast it seems, exactly what I want.

He burnt 2 motors in his pt2 video. He said maybe because he ran it at 12s. But also his motor KV was 700 around. So that combination didn’t work.

He may be running the 123s at 10s.

It seems like you were on point with the 123s earlier. You can get the lower heat from them and won’t burn up as easily. Also, higher loaded RPM right?

Would love to see some data logging from people. Still looking but haven’t found anything. Not sure what search term to use to find that

One concern I am having is the Chinese 50mm jets, while cheap may proove weak with high RPM of 20k