Ecosystem Electronics Lab

Thrusters

The C-DEBI Catalina ROVs are propelled through the water by four thrusters. A thruster is a motor with a propeller attached. Those on the Catalina ROV use a tried-and-true thruster design that has been used successfully on thousands of other small, shallow-diving ROVs. Each thruster is simply a bilge pump motor cartridge with a model boat propeller attached to the motor shaft.

We use bilge pump motor cartridges because they are relatively inexpensive, widely available in boating supply stores, are designed to run off of 12 volt battery power, and — best of all — come already waterproofed for shallow water dives. The cartridges are basically a motor replacement that inserts into a pump. They consist of an electric motor inside a waterproof plastic case with a couple of wires coming out the end. The motor shaft will usually have a small propeller-like “impeller” designed to propel water through the pump.

The first step in converting a bilge pump motor to a thruster is to remove the small plastic impeller that comes attached to the motor shaft. Later it will be replaced with a propeller. The impeller is held on by a tight friction fit, so the best way to remove it is with a gear puller or similar device that pushes in on the shaft while pulling out on the impeller, thereby avoiding the need to put a heavy load on the motor bearings while trying to pull the impeller off.

We used a small, plastic model boat propeller on the Catalina ROVs, though other groups have used model airplane propellers (with the blades sometimes trimmed short) with good success. The trick is to “match” the propeller to the motor type for maximum power transfer from motor to water. If the propeller is too big or too aggressive in its pitch, the motor will bog down and a lot of energy will be wasted in heating (possibly overheating) the motor instead of propelling the ROV. If it’s too small, the motor will spin very fast, and most of the motor’s power will be wasted in internal motor friction (possibly melting the rubber seal on the motor shaft) rather than moving the ROV. Finding the right combination is best done with a propeller test jig that can simultaneously measure the thrust produced and the electrical current through the motor. We did propeller matching tests with a variety of propellers using the setup pictured below and arrived at our propeller choice as being a good match, producing quite a bit of thrust without drawing too much current.

Next, we needed a way to attach the propeller to the motor shaft. We machined our own custom shaft couplers from 1/4" diameter aluminum standoffs. We drilled and tapped one end to fit a machine screw, which bolted the propeller to the coupler. The other end we drilled out for a snug fit on the motor shaft. For robustness, we glued the coupler to the motor shaft with JB Weld after using isopropyl (rubbing) alcohol to clean any oil off the shaft and coupling. A tiny hole drilled in the side of the coupling allowed air and a bit of excess glue to escape (see photos below) as the coupling was slid onto the motor shaft.

Note the threads cut into the inside of the shaft coupler to accommodate the machine screw that holds the propeller in place.

The small glue blob that emerged from the hole was wiped away for a smooth, flush finish on the shaft coupler.

It’s very important to make sure none of the glue gets between the shaft and the rubber seal — easier said than done, because the shaft coupling will end up very close to the rubber seal.

Above is a photo of the finished motor w/ coupler attached inserted into the thruster mount. For a description of how to make the thruster mount, see the page about frame construction. Depending on the size of the propeller, you may need to remove the propeller to fit the thruster into the mount. Once it's through, you can screw the propeller back in place. You may wish to glue the propeller screw in place to prevent accidental loss of the propeller during dives.

For additional information related to installation and control of the thrusters, see the sections on Frame, Tether, and Control.