Re: [PSUBS-MAILIST] Seals again

[s_lindblom@conknet.com (Steve Lindblom)]

>Another thing: if a DPV balks at depth, the problem might not be as critical
>as it could be if that same motor were installed in a submarine.  Worse case
>scenario, a diver can always ditch it and go; might not be so easy for a
>subber.

Cave divers are using scooters to making 3 mile+ cave penetrations at 300'
depth. Were you planning on doing anything more critical than that? I think
anyone building a sub where motor failure could be life threatening ought
to be looking at something a lot better than troller motors.

>IMOHO: pressure compensation still has advantages in supporting seal
>integrity, and relieving depth pressure on the motor mechanisms.

>A mechanical seal is good, but it can fail without warning.

Very rare in my experience. Seals fail because they wear out (or are
damaged in installation) 99% of the time. The solution? replace them
routinely before they wear out. Besides, most scooters use redundant seals
so an "act of god" failure of one seal isn't fatal. And according to Minn
Kota these motors will run flooded so a seal leak doesn' mean the sub
taking an instant plunge to the bottom.

Anyhow, if a seal can fail without warning, what about regulators, dynamic
O-rings, gauges etc?. Don't know how anyone can seriously argue that a
pressure tank, which must be regularly refilled, one or two regulator
stages, some hose and gauges and opv could be more reliable than a couple
properly engineered seals. A freeflow, for example, or a creeping 1st,
could pressurize the seals in reverse - just the way they are not intended
to take pressure - and easily blow them right out.

BTW, one very elementary way to analyze failure potential is to just add up
failure points - that is to say, individual parts that all by themselves
could cause the system to fail. Simplistic, perhaps, but often
enlightening. A active compensation system adds a whole chain of failure
points, a stack of seals actually subtracts them.

>Another thing to think of is the effect of depth pressure pushing on the
>prop and driving the shaft into the thrust bearing, creating friction which
>could reduce rotational efficiency.  (My M-K 4HP is built that way.)
>Pressure compensation can relieve this adverse tendency.

Not as much as one might think, The pressure on the armature/shaft at depth
is going to be equal to the area of the shaft where it comes through the
seal, times the pressure. Given a 1/2" shaft at 330', that would only be 27
lbs. On a motor designed for 100 lbs of thrust, that's not a heck of a lot
extra. With a 3/8" shaft at 100' its only 5 lbs. remember, these motors are
designed to handle thrust.