[PSUBS-MAILIST] More TECH TALK

Dave is a very experienced and knowledgeable tech, who has supplied many stock motors and built many custom propulsion systems for owners of small submarines and ROV's around the World; and his website at:

Dave's pretty busy most of the time; but he said he'll try to answer technical questions on an "as able" basis. Today we got into a good rap over the phone, and he clarified for me some things that have been talked about at PSUBS lately.

1. MK rates their motors as watertight to one additional atmosphere beyond surface pressure: a depth of 33 feet. People have been known to operate them to about 100 feet without modifications to the lower end unit, and the failure rate at that depth is about 4 to 5 percent. Still, the company won't stand behind that, so 33 feet is the specified depth limit for seal integrity, according to MK. And no, doubling the number of seals won't ensure operations at greater depths, because pressure sufficient to implode the outer seal will also be enough to implode the inner seal, too. (Since they are side by side on the shaft, there is no cumulative increase in their depth resistant integrity. When the water gets past one, it will immediately bring it's pressure to bear against the other, which will at that time be operating alone, and have no greater strength than it's failed counterpart.)

2. Yes, MK motors run fine and have excellent depth compensation qualities when the lower end is filled with transformer oil. The fluid is of a fine enough viscosity that reductions in armature RPM are not a problem. Dave has used this method on many motors he's built for customers, some of whom are operating ROV's at 300 FSW; for these units, Dave also polishes the armature shaft to a 4 micron finish, and substitutes a matched bushing and seal in place of the stock ones. But for shallow ops, no compensation is necessary; and at moderate depths, the oil alone is sufficient. Just make sure the case is completely topped off and free of any air voids (impart a vacuum to the case before filling). There's another fluid that can be used: it's called by the nickname "Jesus water"; Dave couldn't recall the exact name of it; and he hasn't been able to find any for a long time; but if anyone knows what this stuff is and where to get it, it sounds like it might be another option worth considering.

3. As some of us already know, air compensation works fine; Dave agrees, but thinks the oil filled method is a better in terms of simplicity, reliability, lubrication, and such. This time around, I'm sticking with air compensation; but I might try oil in the future, when I'm not facing a deadline to get the boat into the water like I am now.

4. Lately, there's been some discussion at PSUBS about continuity between the three sections of the lower end housing being possibly disrupted by paint or O-rings at the juncture points. According to Dave, this shouldn't be a problem, because the three sections are joined by the armature shaft which passes through bronze bushings in each component. Maybe there's more to this "gremlin" than meets the eye. I don't know.

5. Of course, if a motor does get flooded, it's got to be disassembled and serviced. The methods Ron, Herve, and others mentioned sound good: open it up; rinse it with kerosene or the like; dry it thoroughly, and reassemble. Carsten's idea of a little oil inside as a preventative measure to inhibit rust is good, too.

6. If you do have to disassemble your lower end unit, the critical part of reassembly is BRUSH TIMING with the armature. This was a little complicated, so I'm waiting for a better detailed description of how it's done. The key is getting the correct alignment between the case, brushes, and the armature; and then tightening the case through-bolts down to a spec of 32 to 35 inch pounds. If I get time, I'll try to put together a more detailed description of the process; or you might ask Dave. But reportedly, if you get this aspect of it correct, she's going to run real sweet; while without it, she might not.

7. Those dealing with a complete MK unit which has the composite shaft should understand taking it all apart is not as easy as it is with the metal shaft. The metal one can simply be unscrewed from the lower end case mounting collar; the composite one is still 1-1/8" tube and threaded, but it's put in with locktite, and to get it out you might have to chuck it up in a vise and go through some trouble. Then, you might have to order a special tap from MK that will chase the threads in the lower end collar, which will probably be filled with locktite and composite residue. Something to be aware of before tearing into a composite shaft trolling motor setup....