Waterproofing motors is tricky. Very tricky. That's why it's great that someone else has already done it! Bilge pumps are used to remove unwanted water that collects in the bottom of the hull (the bilge) of a boat. Because they spend a lot of time submerged in water, bilge pumps are made to be waterproof. By cutting off the outside housing of a bilge pump and removing it's impeller, you can gain access to the shaft coming from its 12v motor through a waterproof fitting called a stuffing box. A special adapter can be made to attach a propeller used in electric ducted fan-powered model planes, and "voila!" you've got yourself a waterproof propulsion system!
I would certainly recommend looking into ways of pressure proofing the motors. Since they happen to have a pretty ideal rotational speed and torque for the application, are already somewhat waterproof, and are already used on many DIY ROVs, finding a way of making them capable of going to greater depth would be fantastic. How do you propose to use the the pipes and fittings to do this?? I'd love to see your idea! One thing that's been suggested to me is that failure on these types of motors sometimes happens at the electrical wire interface rather then the shaft seal. If this is the case, it might help to simply pot the area where the wires enter the rubber housing and motor casing. Looking forward to seeing what you're cooking up!
A few options ive been looking at are on the links below:
However instead of using mineral oil inside the casing ive been thinking of using medical grade water which has zero impurities.
I have also considered wrapping the PVC in fibreglass bandage strengthening the casing even more.
As most of the PVC fitting shown in the links require machining or are not available in the country its going to take a lot of playing around to figure out how to get the job done.
Hey fellas love the site and the work your doing. Just a side note on your motors, Dc motors run great right in the water you don't need to waterproof them the water keeps the motors cool and removes carbon from the brushes. I was trying to water proof my motors and talked to a guy in a hobby shop he said thats how they break in the brushed dc motors, So i tried it. The motors are small ducted fans from tower hobbies they run great. I haven't tried them at any real depth yet only in an 8' diameter pool 3' deep. I run them out of the water when i'm done to dry them and spray a little wd40 run them again for 30 sec or so to keep corrosion at bay.
I'm trying to figure the best way to handle voltage drop on my 100' tether I'll probably just ad some speaker wire.
you can try brushless engines, they're more efficient. Also you would have a better control of the %thrust once you run with an electronic ESC from the hobby store.
Outrunner ones, can be completly waterproofed with a single epoxy covering.
Has anyone found the practical depth limit of a bilge pump used as a thruster? Are there trapped air spaces that will eventually implode, or does the entire system get wet on the