Re: [PSUBS-MAILIST] trolling motor props

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

----- Original Message -----
From: "Alec Smyth" <Asmyth@changepoint.com>
To: "PSUBS (E-mail)" <personal_submersibles@psubs.org>
Sent: Monday, November 27, 2000 6:32 AM
Subject: [PSUBS-MAILIST] trolling motor props


> > Has anyone out there found a prop well matched to a 74 lb trolling
motor?
> > I have spoken to manufacturers of trolling motor props without any luck.
> > thanks,
> > -Alec Smyth

Hi Alec,

I went the rounds with the MK techs over props back in '91.  Like you, I
wanted all the push I could get.  I learned there was a higher performance
aluminum prop that came as stock on early variants of  my MK 4HP; but it
caused overheating problems, so they discontinued it in favor of the plastic
job I have on mine now.  I don't know of any other such stock "special"
props for MK.

If I understand what you were saying, I think I agree with you.  Stock
trolling motors / props seem better suited for surface boats than subs;
i.e., small diameter, fine pitch, high rpm.  In a sub, this combination can
result in a lot of excess cavitation and "spinning your wheels".  Then
again, smooth control over variable speeds can do a lot to avoid some of
this problem; but generally, "small / fine / fast" aint ideal for us.

As a rule, and without getting lost in finite exceptions thereto:
underwater, a submarine generally does better with a comparitively large
diameter, coarse-pitch prop turning at slower speeds than commonly provided
by trolling motors.  (So why do I use MK?  Because so far they've been a
convenient self-contained propulsion system, available over the counter and
pretty much ready-to-run without too much extra modification, that would do
the job well enough when my first priority was to prove the hull and other
systems would work.)

Right now, I'm in the process of swapping the old MK 4HP in the NAUTILUS
MINISUB for a MK 101/S, because I want a little more power, and I'm on a
close deadline for a film shoot in January; so a ready-made unit suits my
present time-sensitive needs.

But I've also got one steel teardrop-spindle hull in progress for the engine
powered HYPERSUB; and another that is about 80% finished for an as-yet
undecided project.  If that one turns out to be electric powered, I think
I'll bypass the trolling motor in favor of a larger and more powerful DC
motor, mounted internally with an outshaft to the prop.  In that system,
I'll be using the biggest prop I can find, and employ ratio-reduction (belts
and pulleys, probably), to turn the propeller at it's optimum RPM, while
allowing the motor to run at it's "sweet spot".

Pat