Using computer fans blades as thrusters

[puzzler300]

Hi there!
So I was looking through my junk bins looking for something to use on my ROV and stumbled upon an old computer cooling fan that I had scavenged out of something. Since then I have mounted the plastic blades to a R/C plane motor and have conducted a couple of non-scientific tests to see how much thrust seem to be given by this motor-prop arrangement. so far I have operated it under water and it does seem to give a sizable amount of thrust. I also did another test where I mounted it on the bottom of a 2 liter bottle and sent it across a water tank. It worked pretty well! Here is the motor I am using:
http://www.amazon.com/HobbyZone-Electri ... B0006O3CSW

the prop is just a generic fan prop with 7blades and a diameter of about 3.5-4 inches.
Specifically this one: http://www.ebay.com/itm/Delta-DC-brushl ... 4ac2ebbc21

Also I water proofed the motor using a plastic called shapelock, which is super amazing
http://shapelock.com/page5.html

[Triton]

Hi puzzler300,

I also mounted computer fanblades on my ROV. The thrust test I carried out also gave good results.
Although it is a cheap solution I realized (the hard way) they are quite fragile. I also don't think they can be compared to boat props in efficiency.
Yet, they are as efficient as plane props (I originally had used plane props).
You can see the footage here:
https://www.youtube.com/watch?v=KAT13i_dbgI

[scubersteve]

Puzzler,
How did you seal around the motor shaft with the shapelock?
I wouldn't think it would be very watertight around a moving shaft.

[puzzler300]

Triton, I was thinking of using a wire frame or cage around the prop to try and keep some gunk out of it.

Scuber, on my motor there is a raised bearing right where the shaft exists the housing, I assume it's water-tight, so I just spread the Shapelock around the raised beraing so the plastic would be flush up against it.

[scubersteve]

As a general rule bearings have very weak seals (if any at all) because they don't need to handle pressure.
Just something to consider.
On both of my builds (neither of which has gotten wet yet) I am running my motors exposed because I would rather have the water be able to drain out than to have it held in by a housing I can't be sure is watertight.
This may come back to bite me, but it is my approach for the present.

[Triton]

Hi Puzzler300,

I wouldn't recommend to put something like a cage around the prop. I protect the fans with PVC tubes (you can see them in the video) but if you don't let the water pass as freely as possible
then the thrust decreases significantly. In my first try the PVC tubes had not enough openings and the motors were not able to move the ROV. So, I cut them (a lot) more and the ROV was moving quite well. I would recommend to provide protection only in the perimeter of the fans but don't interrupt the flow of the water. Anyway, you can try it and remove it later if it doesn't work.

[puzzler300]

I'll try the Pvc! I fave some 3" stuff that is just a hair too thick. so I may end up getting some other stuff. What If I got some bendable plastic from say a 2 liter bottle and used that?

Oh, and Scuber, so far just as far as static surface tests go, I haven't seen any air bubbles leaking. I did do some research and found some examples of motor shaft seals. Most though ofcourse are too large to use on a small motor shaft, but the seal shouldn't be that hard to build. If you took a small bearing and a rubber cork, put the shaft through the cork and the cork in the bearing and then attached that assembly so water couldn't get underneath of it, that could work.

[timothyalderson]

check these out. COSTCO specials

[puzzler300]

WHAAAAT?????? And they had those all along?? And I tried to figure out a better motor.Whats wrong with me? If you'll excuse me, I'll be at Costco.

But in all seriousness, do they still manufacture these? because that would make my life one metric tonne easier.

[timothyalderson]

Yes they still have them. You can look up the company that makes them and order them off amazon also.