Home Made Flume


Nice experiments, thank you for sharing!
I think, the main difference is the length of the profile near the hub and the area covered by blades near the hub. This could prevent a flow back of water near the hub. Also the thickness distribution is different, the commercial design looks more rugged and will deform less. Three blades instead of two also gives less load for the prop and also mechanical deformation.

Here you can see an example of commercial propeller 180mm cut down to 140mm diameter. There is a slight camber which becomes more distinct the more i cut away. (I turn this in the reverse direction to get less hard strokes when the tool hits the prop, so the gear is not worn too much. Therefore the tool is also mounted in reverse direction).

Direct drive outrunner with direct water cooling

The flume was rebuilt to allow the prop to unload some during testing rather than just testing static performance. It is working great.
We tested earlier this week but had an issue with the printed prop. I have reprinted them and we will test this weekend. I have made a 2 blade version of the 3 blade “boat” prop that has had the best results to compare. My parametric design has had some camber added. I hope it improves the performance.
Really looking forward to the weather warming up as we already have much better performance that the last test on the water.



Mike and I have been testing with a new and improved flume for the last couple of months. This flume was built from an oval water tank. The motor was offset to one side to create a recirculating flow as Toto44 suggested earlier. This worked pretty well and created a recirculating flow of an estimated 8 to 10 mph. We could see the props unload as the water velocity increased, especially at higher power settings.

Here is how the flume morphed over time:

  1. Basic set up with only internals being the drive shaft; lots of splashing, vortexing etc.
  2. End plates added to reduce splashing and spillage.
  3. Clear center window added to further reduce vortexing.
  4. Center divider added to eliminate horizontal vortexing at high power
  5. Water level raised above the top plates to reduce turbulence.

The flume now operates very smoothly and give very repeatable results.

Modified animal watering tank

Thrust and torque measuring

showing center divider and propeller


Shown below are all of the propellers that were tested. Most of the smaller props were tested with the Leopard 160 kV motor. The larger props were tested with the 50kV (65kV actual) Alien C80100 brushless motor.

The winning combination by a wide margin was the Alien motor with a 10" pitch RC aircraft propeller.


@Winging_it, nice effort with Flume 2.0 and testing!
At what voltage were the tests conducted? What’s the target design speed?


The battery is a 12s lipo; about 48 volts.

Targeted design speed is 15mph cruise, and a 20 mph top speed.

APC makes a few different style propellers with the same pitch and diameter but different weight/thickness/strengths. They all tested well, but at full power the “thin electric” version broke under the load.

So far the best results were for the APC 8" x 10" (cut down from a 12" x 10"). At 15 lbs thrust 255 watts were needed; at 20lbs of thrust, 360 watts were used. At these power levels, only a little heat is generated by the motor or esc or batteries.

At this time I think the flume testing is done. Next in the motor /prop development will be open water testing (in a couple of months) to get the best diameter for the APC prop.

The Solas prop also tested pretty well, but has much less pitch. It might be a good choice with a higher kV version of the Alien motor in order to achieve the desired speeds.


what kinda thrust are you wanting to get for the board to get on a plane


also what are your test results at 2500watt and maybe 5000watt ?


We were able to get foil born easily with 33lbs of static thrust.


Haven’t done testing at high wattages. We’ve focused on cruising efficiency while ensuring their is plenty of thrust for take-off.


Hi. Great thread first of all!
Did you already tried to test a Lily impeller? Red a lot about that and was curios to know if it would be efficient for foils too.


Anton, Do you have a linky link to the Lily impeller? I’m not familiar with this.



Here we go:

the official website of the inventor: http://paxscientific.com/water-tech/

youtube: https://www.youtube.com/watch?v=9ely-59ChYs

some 3D models: https://www.thingiverse.com/thing:956458

We also could construct one by our own or make adjustments. I’m curious to see if the impeller is also great in giving thrust as it is in creating vortexes.



Interesting that the two blade set up created the most thrust with the c80100. Which 3 blade performed the best?


Mantaray, the Solas propeller tested very well. It would require a higher kV motor than I’ve chosen to attain good foiling speed.

I think a higher kV Alien c80100 and the Solas prop may be a very good choice.

This chart below will help match motor speed and prop pitch for the desired. For max efficiency use 85% of max kV x voltage. Also allow about 10 to 15% slip.

Example for my case:

65kV x 44 Volt (average) x .85 = 2,860 rpm
I’m using 10" pitch for now; from the chart then top speed would be about 26 mph with no slip
Add 10% slip and the top expected speed is about 23.5 mph average. A little more when the battery is fresh, a little less as the battery is used up and the voltage starts to drop.

Please note this assumes your drive system has the power to attain the rpm. Too small a motor and you won’t reach the predicted rpm ever.



That lily prop has a very organic design, love it !

If that prop would catch your hand, would it :slight_smile:
a) Cut 3 fingers ?
b) Swallow the hand and separate it from the arm ?
c) not bother at all ?

I guess that the protection for such a prop would be enormous ? Or is it dedicated to jet drives ?


Yeah, that thing is big.
A Jet propulsion should be the right way to go. I’m just curious to know if it is possible to gain thrust out of it.:slight_smile:


I think I am leaning torwards the C80100 130 kv (http://alienpowersystem.com/shop/brushless-motors/c80100-sensored-outrunner-brushless-motor-130kv-7000w/) and that Solas prop 7.25". Money and time are an issue for my project, so I am trying to figure out the most direct route for my build. Your test data has been very helpful, thanks.

One more thing, I can’t seem to find the input shaft diameter for that Solas prop, any information on that would be very helpful :sweat_smile:


Manta, my Solas has a 12mm bore, but it was a bit of a sloppy fit on the 12 mm shaft; yet less than 1/2" diameter.

The c80100 has a 12mm shaft.


How do you plan to waterproof this out-runner? Assuming that you will put it directly into the water.