Electric hydrofoil prop

[cmaas]

Naah Will, your guessing is not even close. If we take typical values for a ten-to-fifteen hp outboard, the propeller would be ~9" diameter, rotate some 2400 rpms, produce ~1100 N of thrust in free flow (less behind the ob leg) and consume 14 hp at 11 knots (=5,7 m/s). The fluid velocity through the propeller disc would then be ~7 m/s and the average outflow jet velocity just below 9 m/s. The inflow acceleration becomes noticeable at about one propeller diameter ahead of the disc.

Or, in the case of this hydrofoil, we consume about 7hp at 11 knots.

 

[alan craig]

I watched the video of what happens when you remove power and, like others, noticed that the outboard tilts due to drag. I re read the initial post about the problem and wondered: could the outboard leg be tilting forwards at higher thrust levels due to combined elasticity of all the transom and outboard parts? And the longer moment between point of thrust and transom clamp? This would tilt the propeller and foil in a way which gives more lift, which might explain why you have to trim the foil negative. Very interesting project; I have a little DIY electric outboard but not in the same power range as yours.

 

[baeckmo]

Or, in the case of this hydrofoil, we consume about 7hp at 11 knots.

Yes, my example aimed at "the worst case" to demonstrate max velocity levels. That said, it would be interesting to know the actual propeller dia and pitch plus shaft rpms, to see if there is scope for propulsion improvements aside from the lift problem. When you say "consume", does it include all electrical losses, and do you know the efficiency of the control unit?

 

[cmaas]

I watched the video of what happens when you remove power and, like others, noticed that the outboard tilts due to drag. I re read the initial post about the problem and wondered: could the outboard leg be tilting forwards at higher thrust levels due to combined elasticity of all the transom and outboard parts? And the longer moment between point of thrust and transom clamp? This would tilt the propeller and foil in a way which gives more lift, which might explain why you have to trim the foil negative. Very interesting project; I have a little DIY electric outboard but not in the same power range as yours.

Yes. I think you are exactly right. We got some new video today that seems to show what you've described. I'll get it loaded shortly.

 

[cmaas]

In this video I am accelerating to over 11 knots which has been the maximum speed the boat would go before losing lift and dropping. I was entirely wrong that lift from the prop was causing the stern to rise and the bow to drop. Instead as I accelerate the main foil loads up and, because its pivot point is forward of its balance point, it causes the too flexible wands to bend and the foil loses aoa and lift. Stiffer wands should be the fix.

The other issue, as some of you have pointed out, is that flex in the outboard bracket is causing both the sudden stern drop on deceleration and also allowing the prop and foil to tilt as power is applied so that what was negative aoa in the shop changes to something close to 0 aoa under load.

I was hesitant to post my questions here as so often forum discussions turn into useless shitshows. The thoughtful insights you guys have provided have really put me on the path to correcting my problems. Thank you!

Now to get some numbers to baekmo in hopes that he can shed some light on propeller selection.

 

[cmaas]

Thank you to those on this thread for the help getting this boat going. Here's where we ended up: https://www.proboat.com/2021/11/electric-foiling-catamaran/

 

[johneck]

The prop will produce out of plane forces if it is operating at an angle of attack. Angle of attack can come from tilting the drive, tilting the whole platform, or induced flows from something forward of the prop. It is probably very difficult to operate the boat without any angle of attack on the lower unit over the whole speed range, so you need a propeller that minimizes the out of plane forces such that this effect is smaller. Rake and cup both have a significant effect on the out of plane force, but your prop does not appear to have any significant rake (can't really tell about cup). Another factor is hub length and shape, at least at higher speeds. At this low speed those are probably secondary to the blade forces, but your hub has a flare at the aft end which was intended to increase lift as speed increases. Maybe this feature could be eliminated? I don't think adding a fairing to the hub would have a significant effect, but I have never really looked into that.
The out of plane force will also increase as the RPM increases. That seems to be the problem. I wonder about going to a higher pitch prop and slowing down the RPM for a given speed. The electric motor has a very different torque characteristic than the gas engine did, so there should not be an issue making a fairly significant change. There are actually many choices in outboard props since the range of performance of the boats that these engines go on vary so widely, so perhaps there are some available props that would fit the need.

 

[cmaas]

The prop will produce out of plane forces if it is operating at an angle of attack. Angle of attack can come from tilting the drive, tilting the whole platform, or induced flows from something forward of the prop. It is probably very difficult to operate the boat without any angle of attack on the lower unit over the whole speed range, so you need a propeller that minimizes the out of plane forces such that this effect is smaller. Rake and cup both have a significant effect on the out of plane force, but your prop does not appear to have any significant rake (can't really tell about cup). Another factor is hub length and shape, at least at higher speeds. At this low speed those are probably secondary to the blade forces, but your hub has a flare at the aft end which was intended to increase lift as speed increases. Maybe this feature could be eliminated? I don't think adding a fairing to the hub would have a significant effect, but I have never really looked into that.
The out of plane force will also increase as the RPM increases. That seems to be the problem. I wonder about going to a higher pitch prop and slowing down the RPM for a given speed. The electric motor has a very different torque characteristic than the gas engine did, so there should not be an issue making a fairly significant change. There are actually many choices in outboard props since the range of performance of the boats that these engines go on vary so widely, so perhaps there are some available props that would fit the need.

Thanks johneck for your informative post. Did you look at the Professional Boatbuilder article I posted a link to in the post above yours? From that I think you can see that the boat is now reliably functional. I did try a higher pitch prop at one point but it made surprisingly little difference other than an increase in power consumption without much speed change. I have fit a hub fairing though it was not shown in any of the photos in the article. It made no measurable difference.

 

[johneck]

Thanks johneck for your informative post. Did you look at the Professional Boatbuilder article I posted a link to in the post above yours? From that I think you can see that the boat is now reliably functional. I did try a higher pitch prop at one point but it made surprisingly little difference other than an increase in power consumption without much speed change. I have fit a hub fairing though it was not shown in any of the photos in the article. It made no measurable difference.

Thanks for the update, very interesting. I did read the article. I agree with your statement that the only way to really make an improvement in efficiency is to go to a larger diameter prop, which is not really an option with an outboard. Best of luck with your project.