Electric hydrofoil prop

Mr Efficiency said:

No way, that prop is not giving you 40 kg of lift, and is giving virtually no lift when running deep.

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You can see the trailing edge flap connected to a surface sensing wand. That foil is providing positive lift that decreases as it reaches it's proper ride height. The boat trim is variable as it hits waves, or the speed changes, etc. so that foil lift is not set in stone but something is lifting the stern and it's not the foil. At least not entirely. Having said that, I have the boat on the hoist so I will take some measurements again tomorrow to make sure what I said is true.

Mr Efficiency said:

Just leaving the exhaust passage open to the outside would ventilate it. How much did the conversion cost you, and what is the output of the electric motor ?

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About $3000usd without the batteries, which are salvaged li-ion from a Chevy Bolt ev. The motor is a 10kw pmac me1012 from Motenergy. The controller is a Sevcon Gen4 size 6. Battery voltage is 72. The outboard was dropped in the water while running as a gas engine. It cost $100.

Mr Efficiency said:

No way, that prop is not giving you 40 kg of lift, and is giving virtually no lift when running deep.

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I don't seem to be able to post a video here. I have one that shows what happens when I cut the power abruptly when foiling - the stern drops like a stone. No glide at all. That was the tip-off that the prop was providing lift.

How deep in the water column is your prop, when this happens, or is it semi-submerged ?

cmaas said:

The prop is more or less in line with the horizontal foils. I would rather the prop only provided forward thrust, leaving the vertical lift and ride height control to the foils and their surface sensing wands.

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Your reply did not leave me feeling like I conveyed my thinking well.
Here's a sketch.

By moving the prop lower than the center of longitudinal resistance, you don't get more lift, you get a small lever arm caused by the offset that encourages the structure to rotate around the CLgR. This means the bow will pitch up and the stern will squat. The foils are still doing the lifting.

-Will (Dragonfly)

cmaas said:

About $3000usd without the batteries, which are salvaged li-ion from a Chevy Bolt ev. The motor is a 10kw pmac me1012 from Motenergy. The controller is a Sevcon Gen4 size 6. Battery voltage is 72. The outboard was dropped in the water while running as a gas engine. It cost $100.

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Thanks for the info, it is certainly an interesting exercise.

If you have a surfacing prop thrashing the water, lift is expected, but I doubt that is what is happening here.

Will Gilmore said:

Your reply did not leave me feeling like I conveyed my thinking well.
Here's a sketch.
View attachment 160495
By moving the prop lower than the center of longitudinal resistance, you don't get more lift, you get a small lever arm caused by the offset that encourages the structure to rotate around the CLgR. This means the bow will pitch up and the stern will squat. The foils are still doing the lifting.

-Will (Dragonfly)

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This sounds plausible if the rear foil/propeller is higher than the main foil (cannot really see from the picture). But it would also mean that the rear foil isn't providing much lift - and you probably calculated that well, judging by how well thought out everything seems to be.

I'm really impressed by the excecution of your boat, and the combination of outboard/rear foil is a very clever (and new ?) idea.

Hope to see some video sometimes.

Took a closer look at the picture showing the outboard leg; it seems that the cowling in front of the torpedo has a square trailing end. Whether open or closed cannot be seen. Anyway, if that is the case it can explain why you have a strong lifting effect, even with a decent submergence of the propeller. From the pic of the boat in the water, I estimate the propshaft submergence to something like 1,5 to 2 propeller diameters, which should not result in measureable lift. Ergo, something else must disturb the prop.

In addition to the cowling, there is the effect of downwash from the foils, creating disturbance. My immediate response to the aft foil position is that it should be level with the antiventilation plate, unless designed for ventilated operation (also the prop). I'd like you to explain the logic behind the present configuration, since it does create unpredictable inflow to the propeller.

Edit:
Is there a leak path for air down the control rod for the aft foil, possibly open to the propeller inflow through the cowling? If so, it is a possible explanation for a sudden change in attitude.

Will Gilmore said:

Your reply did not leave me feeling like I conveyed my thinking well.
Here's a sketch.

By moving the prop lower than the center of longitudinal resistance, you don't get more lift, you get a small lever arm caused by the offset that encourages the structure to rotate around the CLgR. This means the bow will pitch up and the stern will squat. The foils are still doing the lifting.

-Will (Dragonfly)

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Thanks for the sketch! I did understand what you described. I just didn't reply in a clear manner. What makes me think that the boat is not very sensitive to thrust height in relation to the foils is, I hope, shown in this photo. Before the recent addition of the electric motor we towed the boat to test foils. Here you can see the tow point is more than a meter above the main foil. It should have tended to pull the bow down and the stern up. And yet when we switched to outboard power the stern foil needed to be adjusted to have negative aoa, which is now hovering around a CL of 0. The opposite of what we expected.

baeckmo said:

Took a closer look at the picture showing the outboard leg; it seems that the cowling in front of the torpedo has a square trailing end. Whether open or closed cannot be seen. Anyway, if that is the case it can explain why you have a strong lifting effect, even with a decent submergence of the propeller. From the pic of the boat in the water, I estimate the propshaft submergence to something like 1,5 to 2 propeller diameters, which should not result in measureable lift. Ergo, something else must disturb the prop.

In addition to the cowling, there is the effect of downwash from the foils, creating disturbance. My immediate response to the aft foil position is that it should be level with the antiventilation plate, unless designed for ventilated operation (also the prop). I'd like you to explain the logic behind the present configuration, since it does create unpredictable inflow to the propeller.

Edit:
Is there a leak path for air down the control rod for the aft foil, possibly open to the propeller inflow through the cowling? If so, it is a possible explanation for a sudden change in attitude.

Click to expand...

It does look like a square trailing edge in the photo but it is in fact faired into the lower leg and is sharp(ish).

I think the prop is submerged typically 1 to .5 propeller diameters measuring blade tip to surface.

At this point the foil is operating at around 0.0 coefficient of lift when flying so there should be little downwash. I admit it was risky putting the foil right in front of the prop and that may turn out to be a problem. I can't mount it higher because it would almost certainly ventilate and I didn't want it under the prop and more prone to damage. The outboard could be run deeper but that adds more drag than I wish to accept.

I recently built a pair of outboard mounted foils for a Bieker designed foil assisted power cat (see photo). They solve the problem of foil/prop interference but need to be attached to an outboard with power trim to adjust the foil aoa on the fly. Unfortunately I was not able to find a small salvage outboard with tilt/trim and the philosophy of this program is to reuse old parts whenever possible.

That is an interesting idea about air traveling down the control rod. It is a 2.5mm rod in a tube not much bigger but I will put a seal at the top.

I doubt that air is finding its way to the hub through the carbon/ foam extension I added but do you think adding a fairing cone to the prop hub might be a good idea?

Can someone tell me how to post a video here?

Can someone tell me how to post a video here?

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Upload your video to youtube and then post the youtube share URL for your video in your post.

cmaas said:

Can someone tell me how to post a video here?

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This is what happens when I cut the power to the prop. This is what makes me believe that most, if not all, of the stern lift is coming from the prop. The boat should glide somewhat.

cmaas,

Great build!

Would a winged prop cowling be worth a try?
Circular cowling with half a foil on each side.
Like an airplane fuselage and wings.