Re: [PSUBS-MAILIST] Seals again

[Nathanael Henderson <jude@pconline.com>]

On Tue, 3 Oct 2000, Steve Lindblom wrote:

> 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.

    I'm deeply paranoid, so I wouldn't build a sub without a contingency
for complete motor/power failure anyway, but that's an interesting point:
Simple DC electronics often continue to operate in fresh DC water.   The
reason water screws things up is because it creates an electrical pathway
between all sorts of points, shorting out the whole affair.  BUT, the
resistance of fresh water is usually fairly good.  12V DC wouldn't be able
to travel through it very well.  Household electronics are in great peril
from water because they use both higher voltage (110-240V) and alternating
current (alternating current moves much much more easily through a
resistor than DC, which is why AC is used for power distribution
systems--you actually can't get DC to flow through more than a few miles
of wire.)  Digital electronics use pulsed DC, which also propogates
well (like AC.)   Yet, the good ol' stone age tech of unpulsed DC power
(ie. the lowely battery) stays contained much more easily.  A guy I used
to know would take those cheap $2 flashlights lake diving--they ran fine
full of water, and even survived to dive again if you drained them before
rust set in.   (The caveat, of course, being that while pure water is a
very poor conductor, salt water is a very good one.)  If you want to play
with the theory, get a battery and an apropriately sized light bulb and
connect them with wire.  Then cut one of the wires in two and stick both
ends into a container of water.  See how close they have to be before
enough electricity gets through to power the bulb.