Efficient electric boat

Hey Jeremy, Glad to see you back! When your frequent contributions went to zero I was worried...

Your prop looks great but I have a question: Why do the blades need to fold? Will they be more damage resistant when hitting underwater debris?

On a separate note here is a new commercial product that comes close to what you are aiming for: http://www.electricpaddle.com/Default.aspx I believe the electric paddle uses a stock APC prop, outrunner motor and right angle gearbox.

 

Hi Denny,

I was only quiet due to a bit of ill health, but surgery seems to have fixed things so I'm back on the project.

The blades fold to allow them to shed weed. I experimented with a couple of APC model aircraft props, as others have, and found that they collect weed very easily indeed. Rick came up with the idea of making his novel, and high efficiency, stainless steel prop blades fold, because weed was also a problem on his human powered boat. When he reported a few weeks ago that his folding prop had worked well during his Murray Marathon trip, I decided that this was probably the way to go. One or two others have developed the idea a bit, but in essence the fabrication technique is pretty much the same as the one Rick came up with.

That electric paddle is a neat idea, but I think they'll discover the same problem with that APC prop. They've also got the prop very close to the leg, which will reduce efficiency a fair bit. Maybe they've done this to reduce the chance of weed fouling.

Getting an affordable, efficient and quiet right angle gearbox is the one problem with this project. I've ended up using a gearbox that cost far more than I wanted to pay, but at least it's well made, sealed and runs reasonably quietly.

Jeremy

 

Jay, I found, as you did, that making the real world measurements is tough. Lots of variables in the messy analog world don't have nice linear relationships for my spreadsheet. Even using a canoe, a GPS, voltmeter and ammeter wasn't nearly as repeatable as I hoped. After all tests I realized that variations in wind and current had significant effects, especailly when trying to compare tests on different days. My house is on a river with a significant flow (all the upper Great Lakes flows past) but the exact current depends on water depth. Also the wind drag varies with the square of the speed so you can't just average up and down wind runs plus it varies a lot from minute to minute. When I get my new sailboat in the water in May I will do my final prop testing in a nearby river with no appreciable current and quite sheltered from the wind.

I like the idea of measuring relative water speed instead of force to compare props. Maybe make a little paddle wheel which would not disturb the flow too much and measure the time X rotations take with a stop watch?

 

MC, with the folding RC model prop mounted on my flex shaft, the blades do fold back on impact when they strike the bottom or even a rock. With a fixed APC, the tips tend to act something like a wheel and ride up and over solid obstructions in a sideways direction as the shaft flexes under the upward force. This sometimes will wear or chip at the fixed blade tips. Also there is reduced weed tangling on the folding RC blades themselves while they are under power, as they tend to fold back under pressure of the weed strands. Cutting back power to the folder clears the weeds completely from the blades. The shaft itself tends to collect and loosely entangle weed balls ahead of the short, stubby spinner. A long tapered spinner and slippery flex shaft surface might allow the whole weed ball to ride over, especially if speed is cut back. The hinges are completely shielded by the spinner assembly and do not become immobilized, except in the case of very fine pond scum.

The benefits of a folder in a fixed leg, outboard style arrangement where there is no flexing shaft should be similar, if not better, when the prop strikes solid objects.

Porta

 

I think I've now finished fiddling with the prop, if I do any more I'll be tempted to polish it, just for the sake of it, so I'd better stop.

Here's the finished prop, with the blades open:



and another shot with the blades folded back:



The shaft is a dummy one, as when mounted on to the gearbox leg the prop hub will be close to the PTFE output bearing that is the same diameter as the front face of the hub - there won't be any exposed shaft for weed to get caught on.

I've just checked the finished blades against the pitch angle templates that I made up and was pleased to find that they are still spot on, despite all the grinding and filing that they've been through. Let's hope that they work as well as they look and feel.

Jeremy

 

Porta, I really never thought about weed fouling as I'm not fishing in the shallows but I guess it could happen anywhere. I just googled "RC folding props" and found they are pretty cheap, $10 - $15, including spinner.

Do you think one like this http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXZ996&P=7 would take 400 watts at about 1200 rpm?

What are you using?

Jeremy's SS folder looks slick but I don't have the talent for metalwork nor a mill or lathe.

 

MCDenny, the links are on Pedal Powered Boats, starting with post 702. I DO NOT recommend that particular prop because it was just meant to test the concept and is barely strong enough at the 50 watts peak I use. The only part I like about the one I used is that the hinges are not exposed which is better for the flex shaft arrangement. Exposed hinges should not be an issue in your minn kota application. You got a better price and stronger hinges in your link, so you can hardly go wrong to experiment with. If the blades have carbon fiber like mine, they might be OK at your power levels. Hand laid carbon fiber blades with metal pivot holes are available at other sites and should be the strongest- should fit the standard 8mm stems. The plastic spinner is also the forward stop, and was not strong enough to resist the levered forward thrust over time. The hardest part will be to make an adapter and stop to fit the small hole in the yoke. You will also need a rear stop so the blades don't fold back completely, else they won't swing out in simultaneously. Some weight at the tips would also help with deployment, since the blades are extremely lightweight.

www.eastrc.org has heavy duty yokes

www.megamotorusa.com has low cost yokes

http://www.icare-rc.com/propellers_folding.htm has selection of spinners, props.

Hope these links work, I tried to reconstruct from memory as originals were not saved....

Porta

 

Jeremy
What is the nominal foil section of the prop and what efficiency is it designed to achieve?

Have you any way to do a test with measured results?

Rick W

 

Denny
You can perform a simple test with any of these small props to determine how the will work.

At 400W on the motor you will delivery about 320W (80%) to the water. If the boat achieves 2m/s (near 4kts) with that power then the thrust will be 320/2 = 160N, close enough to 16kg or 35lbf. From my memory this was about the thrust I worked out for the little Tern.

If you support the blades on blocks with each blade supported at 75% radius then load the hub to 35lb and it survives without significant flexing it will do the job. Do it progressively so if it is flexing badly at lower load don;t bother loading it anymore.

There were some bigger folding props on the site that Vic linked to on the Pedal Boat thread. I have attached the same picture here as I did on that thread.

I consider that a folding prop would be well suited to your application because you would not need to get fancy about hiding the blades. They would fold back when not motoring and offer negligible drag. This has been a great feature on the pedal boat. The downside is the reverse thrust is not very good. With light CF blades it will be even worse than my SS blades.

Rick

 

The section I've used from about 0.25R to the tip is E193, or as close to it as I can get, with a nominal thickness of about 15%. The root section is really just as good a section as I could get by eye, as it's quite thick with a relatively narrow chord.

Whether this section is any better than any other is probably a moot point, as I'm not convinced that the choice of section is likely to make as much difference as the accuracy of manufacture (in other words, I think that expecting 1 or 2% from using a particular section over another might be swamped by the other tolerances).

The best compromise I could come up with for the prop was to aim for the biggest practical one that I could fit within the hull geometry (330mm diameter, limited by the need to make the leg and prop swing up clear of the lowest point of the hull when tilted in it's slot). I've re-wound the motor to give the lowest practical value of Kv and still retain good efficiency, so have a motor maximum rpm of just under 1300 on a fully charged battery. The gearbox ratio is 2:1, so the prop max rpm is about 650.

Best motor efficiency is at around 1/2 to 2/3 maximum rpm, so I've aimed to run in this range. Best prop efficiency is up at the top end of the rpm range, but is generally close to 80 - 82%% in the cruise speed operating region (or should be, if it follows the theory).

I have no easy means of doing a decent measurement, so will rely on trials with the boat when I get it. I have a suspicion that the hull designer's resistance figures are pessimistic. I've modelled the hull (there's mention of it back in this thread somewhere) and get slightly lower resistance vs speed curves than the designer has given me, although I think that he's used an empirical method (from Gerr, I think).

If I can get an overall system efficiency of 65 to 70" then I'll be very happy. So far, the test data and prop modelling suggests that 70% at cruise will be the system efficiency, but I suspect that the real-world unknowns will reduce that a bit.

In summary:

Prop efficiency ~81%
Motor efficiency ~89%
Controller efficiency (and cabling I²R losses) ~99%
Gearbox efficiency ~85 - 98% (variable with power, as the losses are near constant at about 5 - 6W)


The overall efficiency could be as low as 60% at low speed and as high as maybe 70% at higher speeds.

Jeremy

 

Thanks for that info on testing strength, Rick. As you can see from Denny's link, the blades on the RC model prop fold back much further than in your prop, until they touch. So you are correct, no reverse, unless something holds the blades partially open.

Sometimes, the blades do not evenly deploy underwater to the open position when going forward with the particular prop I am using. This never happens when running in air. I have a rear stop to hold the blades partially open. The uneven deployment causes a vibration similar to having an unbalanced prop. It seems to me that the blades should pull forward to the forward stop completely once they engage. The folding prop model I have has blades that run in a slot, unlike the one referenced by Denny. Any idea what might be going on here?

Vic

 

The ColdTub tank tester requires low rpm testing, otherwise cavitation rears its ugly head. In pusher mode I compared amps drawn to yield the same horizontal pressures on the downshaft of the outboard. At about 200, 400, and 500 rpm the APC 12x10 drew fewer amps than the 8x8x$80 Torqeedo prop. Further inthewater reports when Arctic mass over FL disperses.

 

Denny
For the money it is probably worth the test. It is not easy to scale from the photo but I think it will be unlikely with 35lb. The point where the blades neck down at the pivot point will be under high stress.

Assuming the section into the pivot is 6 x 10mm the stress will be around 1200MPa. I doubt the glass/nylon will handle this and it, if it does, it will flex significantly.

Rick W

 

Although the stainless prop might look like a lot of work, and something that needs specialist tools and skills, the reality is that it could be made with just hand tools and doesn't need that much in the way of skill.

The method that Rick has devised, just cutting out a couple of stainless steel sheet blanks, twisting them by hand to the right pitch, then cutting and shaping them to the right profile, only needs a hacksaw, a vice, a big adjustable wrench, some good files and a bit of patience. If you have an off-hand grinder and flapper disk sander in a drill (or better still a Dremel) so much the better.

The hub could be made with hand tools from a bit of bar, perhaps foregoing the fancy taper. The slot for the blades could be cut with drilled holes and a saw, then filed to shape.

What has surprised me is just how quickly these blades can be made. I'll admit to being put off the idea when I first read Rick's write up, as I assumed that they would be a lot of work to make. In reality, they are surprisingly easy to make, provided you take a methodical approach and double check everything before you start cutting metal. They're also cheap in terms of material, as there's not much metal in them.

Jeremy

 

Vic
No.
Rick