Bodyboard mapping airboat - motor and prop choice

Hello everyone,
I am new to the Ardupilot world, but have been working here and there with drones for 6-7 years now. I’ve been getting more technical, and started a project, to build a body board airboat with mapping capabilities, using a Kogger sonar. It’s the first ever RC build I’m doing, and intend to use it for general hydrological research, provided that the budget for turn-key solutions is out of the question. I’ve been inspired by Randy’s bodyboard mapping boat, so here I am, stuck with a technical issue. It’s been many months since I’ve started the documentation and concept stage, and have been torn to chose between 2 underwater thrusters and 2 brushless motors with propellers, that would provide “air power”. Considering that most water bodies that will be scanned are significantly invaded by shallow, underwater vegetation, I’ve finally decided to take the “2 air motor - thrust” way. I intend using a 42" bodyboard (900 grams), and a 20.000mah - 14.8 or 22.2 V battery, and a 2.4.8. Pixhawk controller. I am aware that air-power is significantly lower-efficiency than underwater thrusters (such as the already-classic T200 thrusters). The boat will weigh an estimated 6 kg-ish (I’m still waiting for many parts), and I aim for a 1,5-3m/s cruise speed, and as high autonomy, as possible. There will be no rudder, but differential thrust, using the 2 motors.
I’ve never chosen parts for a RC build before, and I’m not sure what pair of brushless motors I should chose, and with what corresponding propellers (I’m thinking of a maximum prop size of 9-10 inches, maybe?). Could you please provide some recommendations or specs? I’d appreciate it, if you could possibly link any European-sourced parts…
The science of motor+propeller combo’s is really overwhelming (I’ve even searched for efficiency curve-graphs for motors or online consumption calculations, but these are only theoretical pieces of information, and I would need some first-hand knowledge…), and I would really appreciate your experience and recommendations.
Any thoughts on this would be highly appreciated! Thanks

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After a little research, I have narrowed my list, down to a few choices:

  1. 2 x Tarot 4006 motors (4S compatible), which draw a maximum of 14 Amps
  2. 2 x Tarot 4008 motors (6S compatible), which draw a maximum of 30 Amps
    I would use either models with some 11" props (1155 carbon fiber props), as they’re the maximum I can easily put on the bodyboard.
    Still, I am not sure which motors would provide enough thrust and longer range, with a 20000mAh battery. My reasoning is that I could go for the Tarot 4006, with a 20000mAh, 4S battery (1,8kg), and get a good range, because the battery is almost 1 kg lighter, or maybe I could go with the Tarot 4008, with a 20000mAh, 6S battery (2.7 kg) and still get good range, because the motors would be working at the same thrust, but there will be less strain on the motors, being used at lower revs, therefore, at a higher efficiency.
    Considering it’s my first build, I am not sure which variant would provide best range, or what would be the logical, better choice.
    Any ideas?

I have built a few RC airboats and I would steer clear of using a body board if you can. My first RC airboat was done like this and the design of body boards wasn’t suitable.

Thanks for replying. Still, what drawbacks/issues would a body board bring to the table? Is it because it’s too light?

Depending on the board of coarse, I had issues with buoyancy. Being quite thin at the sides water can splash up easily and a small amount of weight puts it below the waterline , although it wont sink. Secondly they don’t plane well , you may get excessive drag which can cause porpoising , poor steering and poor speed. It can be very inefficient where battery life is concerned .

I ended up scratch building one from Styrofoam and coating with fibreglass.
Here is a video of it (turn the sound down ).

And my 3rd build

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Those boats look really nice!
However, I see you’re using large, gas engines, to power them. Considering I intend to use this boat only for mapping purposes, I don’t think I’ll be going over 1.5m/s. Your boats are way faster than that! And with that kind of speed, comes great instability :smiley:
Still, what kind of buoyancy issues did you have? how much weight did you put on the body board? And how large was the body board?
I agree with the steering part - this is probably going to be a challenge to dial just right, in the ArduPilot parameters, so that it steers properly, while doing autonomous missions… but hope it’s still gonna be manageable to steer. I’m thinking of mounting a rail of some sort underneath the body-board, to act as a boat keel, so that it could more easily maintain straight lines, while driving across the water.

If you are going slow a water rudder would be better. Adding keels would help keep it straight. I once used an old heli gyro to aid with steering and it worked well so an FC should have no trouble keeping the turns and straights nice. I guess you could prove your concept on the body board then if you have no issues keep it that way.

Yes, I want to go slow, and I will be considering a keel for straight-line navigation. Hm, well, that’s what I was thinking: first, a proof of concept, and tune it as good as possible, and if it works, just use it. Will sure consider the water rudder, thank you for your feedback!

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Hey Andrei,
It looks like I’m on a similar project path as you. Prototype with a body board airboat understanding it’s not the ideal or final solution but figure out what the actual deficiencies are and chase those down. I have a working craft that can hit waypoints and tuning the speed and steering now. The expected problems are problems but I’m going to push it further before reworking the frame. Let’s compare notes!

Here’s what I have so far; 36" body board (from my attic, ha!), 2x 2205 drone motors with 6" triple blade props, 35A blheli ESCs with DSHOT for reversing. I’ve mounted it using retrofit body board fin housings and 2020 alum extrusion is on top. The dry box and motor arms are mounted to the 2020. I haven’t done any permanent mounting so that I can experiment with weight distribution. The motors are mounted low and at the way back of the craft. At a stop it has almost zero turn radius.

Testing so far; I can easily maintain 1m/s speed and I burned one motor with a sustained full throttle for several minutes test. Gotta know it’s limits, turn it till it clicks and back off half a click. It made can reach about 2.2 m/s but can’t hold it for long.

I had to slow down the acceleration and turn rates a lot. The craft needs a short kick to get moving but feather touch after that. Once it’s moving it’s almost too responsive. I think that’s going to be my problem with tuning it, or I need to rethink how I drive it. Right now auto mode is over correcting the turns constantly but the speed is good. Recently I dropped the Humminbird head unit in the box and it changed the throttle response more than I expected. Noticeably more sluggish. I’m trying to source a smaller/lighter unit though.

I’m going to get some slightly bigger motors/props for a little more punch but it’s not completely necessary. Keep running missions and adjusting the tune. It will take getting in the head space of dedicating repetitive time to getting it figure out though. I don’t have any confidence in my tuning parameters yet but I’ll share those for a point of reference when there’s better improvements.

For my project the shallow profile is pretty important. Vegetation will be a big concern for my user. I’m thinking my next hull concept will be a hybrid. Still have the airboat drive but give it a very shallow v-hull. I’m not committed to it yet b/c the craft is serviceable now and I think there’s more to learn from it’s current state before changing too much.

What have you learned with your rig? How much progress have you made?

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