Another crazy Idea - contra-rotating props

How would the setup look like. You got your fuselage and at the end an outrunner mounted with CL-rotation. Now how will you attach the second outrunner ? The problem is, that the shaft is rotating too. One side of the first outrunner is fixed (at the fuselage) and the whole other side is rotating, so it is not possible to attach another outrunner behind the first one (except you have a hollow shaft inside the first outrunner).

But I also had a similar idea. If you got problems to deliver the power one motor draws, why not use 2 inrunners at the same shaft(there are some with a dual shaft, or maybe just ask for an extended shaft on both sides). Sure you also need 2 ESCs then, but then you only draw half the amps on each ESC (if you got the right timing, so maybe you need a sensor for start up).

Okay, there might be another possibility, one motor in front of the fuselage and one at the rear, so you could also split the power…

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cool :slight_smile:
Actually, printed a prototype of contrarotating props, will see if I can find the pictures and post soon. , not yet made, but I have the drawings and parts needed, bearing gears, etc… But need to make some parts of steel and aluminum to make it work. 3D printed parts will never last:P

it uses the gears shown, and two seals on each contrarotating axle, same gearbox system used on small torpedoes. I have a more complete model with all the bearing, seal, gear, details, it adds about 70mm to the propulsion unit. Can be made smaller.

Only found this video on my quick search, had to uploaded it to YouTube to make it work.

Also found this quick and dirty sketch, ps it is written in Norwegian:P

It is just a differential, like used in cars with rear drive, isn’t it?
It it is, you can find many parts for it in the car hobby section.

Thats the point. Shafts are not moving (as in the skateboard wheel motors). Both motors sit on a single shaft (cables is a bit complicated, since it should go inside of the shaft). But both motors are sharing the same shaft which is mounted on the mast above the wings rod. the motors structure is exactly the same is it is done on skateboard wheel (probably even the same motor but has to be rewind to make it more powerful, since most powerful is 500wt)

Upimageloading…

Think about the wheel, which is actually replaced by a propeller body.

oh, ok these are special outrunners with hollow shaft.

Like this? For who try/have tried direct drive with outrunner - #3 by pacificmeister - Propulsion System (Motor, Gears) - FOIL.zone

Exactly! What was your experience with that? Do you think it makes sense to try another prop (bigger or different shape?). It will be very valuable to measure thrust/volts/amps to see how this setup performs in terms of efficiency. As far as I saw in your youtube it was very under powered for our purpose but such motors might be improved by rewiring or choosing longer (in length) and therefore more powerful motor. What do you think?

Yes I was under powered with my 6354 60KV 2450W outrunner. But I think it’s worth to continue exploring with bigger or multiple motors. There are other discussions ongoing here about potting/sealing outrunners for that purpose.

Just was watching the youtube trying this setup again and again - are you sure it was under powered or the prop was just too small in terms of size? Was it a big difference of sound in the water and out? To get the feeling of rpm difference?

I wish I had better data on this test. It felt like RPM was a lot slower under water. However my amp meter only showed 20amps at full throttle in the water. Leads me to believe that there is room for improvement with different props. And then there is the salt water which may have shorted some windings if there were little cracks in the copper insulation of the stator windings. That could explain reduction in power too. Lot’s of variables :slight_smile:

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20amp means about 600-709wt of power. Strange… Probably the motor is not capable to deliver more power with this voltage. Would it be interesting to provide higher voltage?

Another question. Was it sensor or sensorless setup?

Sensorless. Higher voltage may be one way but my esc only does 12S and I’d like to stay under 60V for safety.

I suspect the VESC limited the phase current, because the prop has high pitch, velocity was low, the motor draws full current far away from reaching EMF limit RPM.
I would not go above 12s with underwater electrics in any way. It is risky, you can feel effects of electric. I do not think it affects performance if you have leaks, as the windings are coated well it can only be the plugs.

I like the concept and also thought a lot about in the past. Using two VESC could half the load which is essentially to get much higher power, if needed at all.
You would need new aluminium parts producable on a simple lathe. You need bearings on each side of the rotor. They must have large inner diameter so you can put the cables inside the inner tube. The outer diameter should not be too large, so the outer aluminium ring which holds the rotor can have cutouts to let some water through.
But there are also drawbacks, complexity rises and we still need a duct for safety reasons. It would have at least double length compared to single prop. This length demands thicker walls or structures to keep it stable, leading to more drag. The props mounting rings on the motors rotor add front area and thereby drag.
The load on each propeller would be smaller leading to thinner blades which may be hard to produce. You need two different propeller designs with different blade number and diameter and pitch distribution . And you need to adapt the duct to these effects to gain efficiency compared to a single motor. Sounds like a lot of math is needed to optimize it, or we will have to learn by printing every day.
It 'll be hard but sexy.

Yes . Looks serious peace of system design. Probably after we all will be bored with our 20 knots flying boards :slight_smile:

Hello everyone
I see that this idea is already running here. Low price, no gearbox and possibly enough torque. These engines are a good choice. I just designed the layout sketch. Prop 2 or 3 blade…
https://www.aliexpress.com/item/Electric-Longboard-Conversion-Kit-Electric-Skate-Motor-kit-E-skateboard-Rear-Truck-With-Single-Motor-Electric/32801783884.html?ws_ab_test=searchweb0_0,searchweb201602_4_10152_10065_5000015_10151_10344_10068_10345_10342_10343_51102_10340_10341_5130015_10541_10084_10083_10304_10307_10302_10539_5080015_10312_10059_10313_10314_10534_100031_10604_10603_10103_10605_10594_5060015_10596_10142_10107,searchweb201603_1,ppcSwitch_3&algo_expid=37d6abe7-7645-4ffd-9a15-9d82c50d86bc-13&algo_pvid=37d6abe7-7645-4ffd-9a15-9d82c50d86bc&rmStoreLevelAB=0

Looks Nice, but would try to add a duct or cage around the props, it’s highly likely to fall of the board and hit these when riding.

Great layout @efoil. The question is the distance between two props. As far as I know it should be as close as possible to increase the efficiency. However it is quite similar to giant ship, which is in the picture. What do you think about the speed performance? 1800rpm is relatively low to get 15-20nots we want to ride as far as I see

The speed can be partially influenced by the pitch of the propeller, the problem will be small engine power. The efficiency of the installation in 1 pipe may be lifted. Long Duct ? Or only 2 ring prop