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).
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:
Basic set up with only internals being the drive shaft; lots of splashing, vortexing etc.
End plates added to reduce splashing and spillage.
Clear center window added to further reduce vortexing.
Center divider added to eliminate horizontal vortexing at high power
Water level raised above the top plates to reduce turbulence.
The flume now operates very smoothly and give very repeatable results.
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.
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.
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.
Cheers,
Anton
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.
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.
