Re: vindicator power

["Gary R. Boucher" <protek@shreve.net>]

Paolo,
    I think you are the fellow from Italy?  Just a side note here in that
my daughter Jenelle is now in Italy on a Summer college trip enjoying the
culture.  Says she loves it!  
    The prop that I use on The Vindicator was one that I purchased from
Michigan Wheel several years ago.  It is 15 inches in diameter, made of
bronze, but actually looks like brass.  It is three bladed with an 8 inch
pitch.  It is called a sailors prop because it is constructed for low speed
propulsion of sail boats that for whatever reason are not under sail at the
time.  There is thus a small inboard engine that propells the boat at show
speeds much like a sub.  Most ski props are like 30+ inches of pitch for
high speed.  I got a naval architech hired by Michigan wheel to tell me
what type of prop he thought I needed.  He asked me what my RPM was going
to be along with the output horsepower that I plan to deliver to the prop. 
Also he asked me what the "Inflow Velocity" was going to be.  This is a
difficult thing to determine.  It is in essence the speed of the sub.
    Here is the problem with matching a prop; the speed of the sub is
determined by two factors, the drag force from water flowing around the
hull etc. and also the thrust.  When the thrust is equal to the drag the
boat is in unaccelerated cruise.  But since the thrust is one of the
determining factors to speed and inflow velocity is closely related to
speed and thrust is related to inflow velocity, each of these curves
relates to each other in a closed relationship.  Here is my point; it is
very difficult to match a prop 100 percent to the engine without trial and
error.  For one thing it is difficult to find the drag vs velocity curve
for a given sub geometry.  That is outside of my expertise.  In my case I
took a wild guess at the inflow velocity (I assumed it to be the speed of
my boat).  I guessed 3 knots for cruise.  I was very low.  My boat does 7+
knots.  Now if I had a slightly higher pitch the prop would propel my sub
even faster.  Let me tell you though that 7 knots in a sub, especially in
low visibility, feels like driving at 80 MPH with your lights off at night.
    What someone should do is experiment with a given trolling motors to
determine a set of curves for maximum thrust vs velocity of the trolling
motor through the water.  I like the idea of trolling motor propulsion for
small subs but they MUST be compensated for pressure in the proper manner
and connected to safe and contollable electrical sources.
 

Gary Boucher



----------
> From: Paolo Velcich <pavel@mail.nauta.it>
> To: personal_submersibles@psubs.org
> Subject: Re: vindicator power
> Date: Tuesday, June 02, 1998 8:19 AM
> 
> Are you using that prop on a trolling motor or on a custom design? I
would be
> interested on having some more details on it. I'm now testing a
> ZEBCO-Motorguide 52p trolling motor for a custom DPV I'm designing. But
I'm
> also going in the development of a traditional design with an electric
> motor/reduction unit and sealed shaft from inside the body as I'm not
sure the
> trolling being an appropriate solution. I'm going to apply 600rpm on the
prop
> shaft plus a pwm digital controller for the speed management.
> 
> 
> Gary R. Boucher ha scritto:
> 
> > ...  I use an 8 inch prop pitch.  This
> > propeller was actually designed for sail boats that us it as aux power
if
> > no wind is blowing.
> >
> > ----------
> > > From: STEVE McColman <steve_mccolman@hotmail.com>
> > > To: personal_submersibles@psubs.org
> > > Subject: vindicator power
> > > Date: Monday, June 01, 1998 6:51 PM
> > >
> > >   Hi Gary,
> > >           Have you ever determined what thrust power your sub has???
> > > Or could you make an educated guess!
> > >
> > >
> > >                                  Steve McColman, Vancouver Island
> > >
> > >
> > > ______________________________________________________
> > > Get Your Private, Free Email at http://www.hotmail.com
> 
> 
> 
> --
> Paolo Velcich
> industrial designer
> 
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