Re: Screw size and speed

[shawl@torchlake.com (Jonathan Shawl)]

Hi Mike,
You are right Mike. I have found that on my sub the big slow turning prop
used for main fwd & rev. thrust works much better than the first high speed
small prop and motor I tried. Same HP rating on the motors. The big one will
almost pull you off the dock from a stand still if you try to hang onto a
rope tied to the sub. The small one would not do it. I think that on a sub
that is not going to move very fast no mater what you do, that the bigger
slower turning props are the way to go if you want slow speed but a lot of
thrust for a quicker overall response to a change in direction. Just look at
tug boats, huge slow turning props for lots of slow speed thrust. I used a 1
HP permanent magnet floor scrubber brush drive motor that has a 10 to 1
gearbox built right it. It was used surplus and was almost free, from a
friend.
The most costly part in my entire main drive unit was the ceramic face shaft
seal at around $80.00 new. The big 3 blade brass prop was something a diver
friend found on the bottom of a lake. And another friend that had a prop
service made it as good as new, polished it and everything. He figured it was
worth about $400.00 at that point, but it cost me zero.  Also keep your eyes
open for anything that can be used. My dad had the 3 trolling motors laying
around not being used so I cut the heads of of them and mounted them on the
sub. Ask before you try that! The hull was a free used 500 gallon propane
tank from yet another diver friend that managed a propane Co. Anyway the list
keeps going on like that. It's good to have lots of friends that want a ride
in the sub! ;-) This can be a lot cheaper than designing and building any
part from scratch. Since my family and a few others have talked me out of
selling my sub at any price at this time, I Guess I can tell you this now. I
only had about $1000 cash into the entire yel-o-sub and about $1500 for the
trailer to move it. That was 15 years ago. A set of golf cart batteries for
it is going to run me about $400 now just to get it going again.  So how is
that for an encouraging word? You guys can do this too if you are willing to
use your imagination and look at all the stuff around you can use as sub
parts. No forks!!!  Then test out the part or idea to make sure it will work.

Ditto on all the disclaimer stuff. Just my ideas of how I did it. Don't try
this at home. bla bla blab Will that work? :-)
Jon

Michael B Holt wrote:

> I've been looking at the pictures of the Hunley.   It had a 30-inch screw
> that turned at . . . a low speed, I would think.
>
> I see other small subs with a pair or more of smaller, higher-RPM screws.
>
> It's my understanding that it's fairly easy and inexpensive to buy what
> seems to be large trolling motors as power.
>
> But is there an operational reason small high-speed screws are used?
>
> Why can't a large low-speed screw do the same thing?
>
> Mike Holt
> --
>
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