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Help choosing power system for electric airboatt

Old 06-20-2021, 08:47 AM
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JS440
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Default Help choosing power system for electric airboatt

Years ago, I built a Dumas lil swamp buggy, and stole the electrics from a cheap RC plane I had. It was a fun toy, but never had the power it needed to really boogie very well. In the past years I've built a few more boats, and quite a few cars, but not another airboat, although I've had the inkling for a few years.

I recently picked up a 28" x 10" fiberglass airboat hull, and am needing to put together a power system for it, which is way outside of my wheelhouse. I'm a bit of a sucker for slightly more "scale" builds, and thus I'd like to run a pusher prop, even though I know it's likely to be less effective than a tractor style setup. I'd like to be able to get up on step, but it doesn't need to be a rocket ship. I may eventually use it as a rescue boat for my large gas powered boats.

I'd prefer a brushless, (hopefully waterproof) motor. If anyone has any suggestions as to what size and kV motor, prop size and ESC size I would be very grateful!



Old 06-20-2021, 10:29 AM
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A lot will depend on your performance expectations - how fast and for how long - and your budget. Too, what packs do you have/want to use? Based on my experience with quite a few electric air boats, I'd suggest 4S power to keep amp draw lower and extend run time. One possible combination:

2830-ish outrunner with a Kv around 1200. Example: NTM PropDrive 2830-1200 from Hobby King. I've had good luck with other motors of this brand, inexpensive but reasonably powerful.
50-amp ESC
8x4, 9x4 or 8x5 prop depending on speed and run time requirements
4S 3300 mAh 35C battery minimum

This inexpensive combination will give up to 3 pounds of thrust and at least 8 minutes of run time with a 3300 mAh pack. It is very tough to estimate top speed for flatty airboats, but as long as the finished boat isn't too heavy the above combo should be able to achieve at least 15 mph depending on prop. There are a lot of ways to achieve your goal, the above is just one. If you want more speed then a slightly larger motor, ESC and prop would be called for.

There is no real performance difference between a pusher or tractor prop, either will work as well on your airboat. No motor is really waterproof, but outrunners will run fine even fully submerged as long as they are dried out completely afterwards. The battery and ESC are not waterproof (regardless of what you read) so will need to be in a watertight compartment. The wires from the ESC to the motor can be lengthened as much as needed, this makes the "scale" appearance easier to achieve.

You'll need to play around with props and CG to get the best performance from your airboat, one modification to the hull which will improve performance is to sharpen the transom edges on the bottom. This can be done with body putty or epoxy etc. and will aid in getting on plane and increasing speed a bit.
Old 06-20-2021, 11:24 AM
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RPM, thanks for the post! Like I said, I don't need it to be a screamer, just so long as it will hop on plane and stay out of its own way. The only batteries I have that might fit that bill are a pair of 5000mAh, 3S 30C packs that I run in my jet boat. Do you think the pair of those would be too heavy, or would running on 6S be able to overcome that weight? I try to run waterproof everything on all of my boats (and even my cars), as when I'm done running, I typically hose them off... inside and out. I often run my jet boat in salt water, and haven't had a problem hosing off my batteries, ESC, motor and servo. One concern I have with the ESC is whether having it below deck will require it to be water cooled... which kind of defeats the purpose of an airboat, if you have to run water pickups. Do airplane ESC's of this size typically need to be in open air, or near the prop wash to stay cool? Or do you think I could find one that would be OK enclosed in the hull without overheating? I've had very good luck with Hobbywing ESC's in my cars, which I have found to be very reliable, and waterproof as advertised, just worried about temps running enclosed.

And you say outrunners are "waterproof", but what about salt water? Obviously, me being me, I'd hose it down and dry it completely after running, but salt water doesn't do any electronics any favors, hence my aversion to non-waterproof components. Of course, airplanes don't use waterproof motors... so I assume finding one would be a chore, if not impossible. Also, having spent the last 2 years working at an electric motor shop, I know that even a small amount of water in the windings can have very negative effects, although most of my experience is in very large, industrial motors, not these tiny little RC buggers.

Thanks again for the reply. Any and all help is much appreciated!
Old 06-21-2021, 10:08 AM
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Since you have the bigger 3S packs it just makes sense to use them. While you could wire them up in series for 6S, the weight penalty may be too high. 5000 mAh is fine, giving a bit longer run time of over 10 minutes. For that a motor like the NTM 3536-1400 would work well, giving higher static thrust (~3.5 lbs) and slightly higher speed with the 9x5 ABC prop while keeping the amp draw relatively low. A 70 amp speed controller should work okay without water cooling, I run enclosed 100 amp aircraft controllers in several of my airboats without cooling (at almost twice the amp draw I'm recommending here) and they don't get hot enough to go into thermal shutdown. The key is to use an ESC which has a higher amp limit compared to what the motor will likely draw - which in this case will be around 25 amps - and using a larger than normal radio box for a bit more air.

Even brushed motors run just fine under water, I used to break-in the brushes in mine by running the motor in a cup of water on low voltage for ten minutes although the softer brushes would wear down very fast. As for the salt water, you aren't going to be running it submerged anyway so it won't matter. The worst which could happen might possibly be a slight shorting to ground, which won't hurt anything. Corrosion would be the big issue, although I've flipped two of my airboats multiple times (fresh water) and as long as I ran the motors immediately afterwards to dry them out, no damage. ESCs aren't fully waterproof IME, in spite of what the adverts might say. While they might be so when brand new, submerge them after a few months of use and the water will eventually find its way inside where the wiring enters the box. So more than "splash proof" but less than fully "waterproof", maybe "very water resistant" would be more accurate. Of course you could run with the ESC on the deck for better cooling and typical water splashing shouldn't hurt, but IME there is still risk of damage if it sits under water for awhile. But the risk may not be very high....
Old 06-21-2021, 11:06 AM
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I've heard of the water break-in method for brushed motors, just never tried it.

I suppose I'll add a 3536 1400kv motor to the shopping list (though Hobby King appears to be out of stock for the NTM), as well as a 70ish amp ESC. I actually just picked up the first airplane I've had in about 20 years about an hour ago... and wouldn't you know it, it came with 6 plastic 9x5 props, so unless I have 6 nasty crashes by the time I get to tinkering with this boat, I should be good on props for a while!

As far as building, is there a general recommended COG for an airboat? How about a minimum clearance between the prop and the deck?

Thanks again for all of the help
Old 06-22-2021, 06:10 AM
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A lot depends on the weight and bottom shape of the hull, but I'd start with the CG at 30% from the transom, with room to move it for and aft. I use 10-12mm between the prop tip and deck, if the motor mount is stiff that will work.

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