ugetsu

Hi all, I'm looking to build a RC boat/raft that I can attach an underwater camera to. I'm completely new to the world of RC so I know I've got a lot to learn and I can't find any good examples of what I'm trying to build.

The idea is attaching styrofoam floats to each side of the camera's underwater housing and use jet thrusters, one on each side to power it.

I'll be filming wildlife, above and below water, so I need the motors to be quite quiet and they don't need to be very fast, just need a good amount of torque to move the large underwater housing through the water. The housing is around 20 kg in air, slightly negatively buoyant, and is 400 mm (l) x 300 mm (w) x 300 mm (h).

Does this sound like something anyone can help me with?

To start I need advice on what sort of motors will be strong enough to do the job and whether I should use jet thrusters or straight propellers? I was thinking of using two jet thrusters which can then be used to steer as well.

Many thanks in advance.

ugetsu

I'm currently looking at:

2 x TFL Jet Thruster 29mm – B54253-D with the following 3660 motorFVT 120A Waterproof Brushless Sensorless ESC
4CH 2.4GHz transmitter + receiver
4S 5000mAh LiPo battery

Am I going down the right route?

Got RPM

This is a cool project, but it is far outside the experiences of 99.9% of the members in this forum, the best we can give you are educated guesses.

I have no idea if the motors will push your camera fast enough to overcome any river/stream currents, but I do know that they will make noise, probably too much for your use. Submerged drives will probably work better, like the thrusters used on scale tugs.

Assuming that each motor draws 25 amps (SWAG) then the 5000 mAh battery will run for a total of about six minutes at full throttle. Depending on how much you need you can add more packs in parallel for the same voltage but more run time.

I think you will get better answers in the RC Scale Boating sub-forum. Good luck with your project!



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ugetsu

Thanks for the reply. How would I go about working out if the motors are powerful enough to push the camera through the water? I assume there are model boats out there with a large displacement/drag?

I guess I'll need a larger battery for my purposes! I plan on doing short trips but don't want the battery to die to quickly.

The noise isn't a deal breaker. I would also use the RC to get the camera in place and leave it there quietly.

mfr02

Most model boaters have experience with hulls that are shaped to slide through water easily. A 20Kg item, possibly shaped like a brick, will need a lot more power than something designed to pass through water. If the water is moving, it must be capable of propelling itself faster than the water is moving, and will need to continuously travel at water speed to hold station. Displacement hulls have a maximum economic speed in water - look up "hull speed".
20Kg? There have been several underwater camera projects on various sites, Even from several years ago, I don't think any complete projects were that heavy and cameras have become smaller and more capable.

ugetsu

I know 20 kg sounds like a lot but it's a cinema camera with aluminium housing. I've found a similar project so you can have an idea of what I'm talking about: instagram.com/p/CIyYUNIhODL/

I think we'll assume I'll only be able to use the craft in still water, ponds etc. But still maybe I need more powerful motors? I'm not sure how I would work out hull speed with a non-streamlined multi-hull. If we could just go by the length of the craft then I could make the styrofoam floats 4 ft long then, hull speed would be 1.34 x √4 = 2.68 kts however looking into it catamarans use a hull beam:length calculation, how useful do you think it will be?


ugetsu

I replied to this message yesterday but it seems the moderators didn't allow it through as I had a link to an Instagram page. I've found a working prototype of the craft I want to build! If you put /p/CIyYUNIhODL/ after instagram.com you can see the design I'm talking about. There are a couple of videos of it in action as well. Looks exactly like what I'm trying to achieve.

You can see how big the camera is in that video! Cinema cameras haven't come down in size all that much!

mfr02

That link shows a setup intended to be used above the surface. The hulls are nicely shaped for going through water easily with the camera looking along the surface while hoping that the very high centre of gravity will be compensated for by the spacing of the hulls. Mounting a large box under the boat will remove any performance efficiency and require a lot more power.
If hoping to see more than a few metres underwater, unless the water is crystal clear, a camera of the size and quality of the one looking at you on your laptop will do the job while sitting inside a much smaller housing.
To use the housing mentioned in post#1 will need, to coin a phrase, a much bigger boat.

ugetsu

The sort of shots I would like to achieve are split screen shots so you can see above water and below water at the same time. To do this the dynamic range of the camera needs to be very high, which is part of the reason for the high end, and large, camera. Underwater will be much darker than above the water so to be able to get a proper exposure on both you need that sort of camera. The work I would be doing with the craft would be for TV, again hence he need of high quality.

However only a small amount of the camera, less than half, would need to be submerged for the split screen shots. So that should help me out.

What sort of motors do you use when you don't need speed but need to move a larger boat?

mfr02

No doubt someone has experience with brushless motors and their associated ESCs, but I tend to go with brushed motors. What is being described is "large tug boat" territory, so hunting around for threads covering these will help your learning curve. My personal preferences run to sail boats and small stuff, so the heavy end needs somebody else. An almost half-hundredweight boat probably weighs about as much as my current collection of boats.
The hulls, and anything else submerged, need to be pointy, to allow for the mass of water to be moved as gently as possible. The back end needs to be shaped so as to both allow the water to flow back easily and present a clean flow of water to the propellers. This will make best use of what limited power can be stored in the space available. Pushing a flat surface through wat takes a lot of power.
You should be looking for motors in the 7xx or 9xx range, preferably with either a high turn count (greater than 35) or pole count (5 or higher) or both. You should also be looking for large diameter props to give the ability to move a lot of water a relatively short distance in a given length of time (low speed, high torque) and a motor or motor/ gearbox/belt drive to drive it quietly. Conventionally, motor can diameter should be greater than the prop diameter, the motor pole count should be not less than the number of blades on the prop. Gearing lets a smaller motor drive a bigger prop and still stay within these limits.

ugetsu

Okay great, I'll start looking into those motors and props once it gets wet again. Making the most of the dry to do outside jobs at the moment. Really appreciate your help!

Retiredat38

Interesting project however, there are a lot of small cameras now that provide very good quality video. One example that comes to mind are the ones cops wear now. You see these on TV all the time. As well as video shot from cell phones. The Drone crowd has been capturing a lot of good video with the small cameras in rather small and inexpensive drones. Maybe one of those?

I wouldn't rule such cameras out just yet. But rather do some investigation to see if one or more would be a workable option. The weight reduction alone would open a lot of options for your project.

One issue you will have, especially with a full size camera is battery life. So if the plan is to keep some part of the craft on or above the surface, may I suggest you run it off an umbilical from a mother boat in which all the batteries will be located. Just a thought.

ugetsu

Thanks for the reply, an umbilical is an interesting idea, one that I've seen for underwater ROVs. It would mean another whole boatload of power needed in the motors though. The camera is the bit which can't really change. I'm still in the process of getting together the float rig and I'll update when I've managed to source some motors to attach to it.

ugetsu

Unfortunately the job I had booked that would have used this system has been cancelled due to COVID. This means I don't have the funds to carry it out at the moment, thanks for all the help anyway and I hope to pick it up again in the future. Cheers.