Re: [PSUBS-MAILIST] Seals again

["Captain Nemo" <vulcania@interpac.net>]

----- Original Message -----
From: "Steve Lindblom" <s_lindblom@conknet.com>
To: <personal_submersibles@psubs.org>
Sent: Monday, October 02, 2000 4:46 PM
Subject: [PSUBS-MAILIST] Seals again


>" I just ran into a friend who has a Dacor DPV scooter, and he'd just
replaced the seals, and still had the old one, which he gave me. I was
suprised to see that it was just a plain old rubber lip seal, the kind with
a little spring inside to hold tension on the shaft, as used for the rear
oil seal on a car engine. He got replacements at the local bearing supply
shop for a couple dollars each (it uses two in series)."

Hi Steve!

Hey!  That's great news!  Do you happen to know the manufacturer and part
number; or the particular automotive application (make, model, etc.)?

(SNIP) " So we got the Dacor using $3 seals for 200', and the Scubapro
"Salad Shooter" with a Minn Kota motor (the entire module, skeg and all,
slung down below the battery housing and disguised with a plastic shroud)
merrily cruising at 180'. Gentlemen, is pressure compensation really
necessary?"

A few thoughts:

Well....I don't know.  I've heard varying reports about DPV reliability and
maintenance problems over the years: all I'll say about that is "caveat
emptor".   ;-)

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.

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.  Pressure
compensation can be supplied through very simple and reliable means, and
does much to ensure watertight integrity.  That's good insurance.

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.

In illustration of the above: visualize one of those inexpensive little
clear plastic underwater 35mm still cameras they sell just about everywhere.
The manufacturer says they're good for about 12 feet or so, as I recall.
Well, I know for a fact they will go a LOT deeper than that without
imploding or leaking; but try rotating the film advance knob with excessive
water pressure squeezing it into the camera housing.  She no go!  Similar
uncompensated pressures on an electric motor  might at least increase wear
and tear on the mechanism, I think; and that's something I'd like to avoid.

There may be other advantages to pressure compensation which will become
apparent after additional consideration, but these are the first two that
come to mind.  Good seal integrity is a must; and your leads to economical
seal sources is extremely welcome, Thank You!  But I still think pressure
compensation is a good thing.

Very best regards,

Pat