Bilge pump failure at depth

propulsion

#1

Hi. I use bilge pumps for my thrusters (akin to http://www.homebuiltrovs.com/howtobilgeconversion.html). This has generally worked very well, but today I started having a weird problem: one of the thrusters works fine when it's on dry land or when it's in a couple feet down, but it's failing consistently at depth (anything greater than 10 feet). The other thrusters work great (down to 60+ feet). When I pull the ROV out of the water, the problemsome thruster works again, but it will fail again when it's at any depth.

This is in salt water, in the Pacific Northwest.

Any ideas at what could be causing the failure?

Matt


#2

Hi Matt:

Sorry for the delay ..........

Is your pump body filled with anything ? Or just air ?

For the second case it jumps to the eye where the problem is. At anypoint in the pump casing there is a structural failure. It may not be obvious becouse the housing material is partially elastic, dont think on a crack, but on a kind of leak of thickness or strenght anywhere on the casing that dents when loaded.

Those pumps are not built for withstanding external pressure loads, and hence the design and construction are not very precise.

Seems that at a given depth, the casing is distorted by the hydrostatic pressure and touches the motor.

If you've filled the casing with anykind of non compresible material (Oil, water .... ), then it looks you have an air buble inside it.

The distortion could go, froma big one that, as I said above puts in contact the casing with the rotor, or a very little one that "moves" the axis alignment.

Regards

PD: If the electrical part is OK. No shorts, no bad connections, ...... Then could be good checking the current drawn by each motor. If it rises too much for the "damaged" one, something is disturbing the rotor free rotation and both explanations above could be right.