Re: [PSUBS-MAILIST] Through Hulls

["Gary R. Boucher" <engineer@sport.rr.com>]

     What you say is correct.  You don't "Just" use a thru-hull shaft.  You 
don't "Just" use pressure compensated motor housings, etc.  Every 
engineering move you make has pros and cons.  The merit of a design is in 
the judgement used in the tradeoffs.  Two engineers will attach a problem 
in two different ways usually.  The best and most functional designs are 
usually the ones that are made by the engineer with the best judgement.
     I still think that for the beginner, pressure compensated trolling 
motors are usually a good choice.  I would say that overall, in most cases, 
they are the easiest to implement.

G. Boucher

At 09:20 PM 11/21/2002, you wrote:
>Gary
>
>I've heard several people say that straight shaft drives with the motor 
>located
>inside the hull wasn't a good way to go but none ever said why.  Alignment 
>could
>be a problem, I guess, but with sound planning that could be dealt with.  The
>need for bearings in sea water is an issue I didn't consider but as Jay points
>out, a non metallic bushing could be used on the prop side of the seal.  True,
>thrust would be transmitted through the shaft but a properly sized ball 
>bearing
>flange block inside the hull would be adequate to carry that load.
>
>I guess it mainly boils down to how a straight shaft fits your hull design 
>and do
>you want a fixed prop and a rudder or a prop that is directional.
>
>Thanks for the explanation.
>Dan H.
>
>"Gary R. Boucher" wrote:
>
> > At 08:00 AM 11/21/2002, you wrote:
> > >And Gary    Although I'm not propelling my sub with a through hull
> > >shaft, I can't see the complexities of a straight shaft design.  It
> > >seems that a good face seal, with maybe a backup seal as a safety
> > >measure, would be simple, cheep and safe.
> >
> > Thru-hull shafts are a viable approach.  But, you will have some
> > engineering problems to overcome.  Much of this depends on your design.  Do
> > you have ballast tanks located where they shaft will have to run?  If the
> > shaft is of low diameter and/or runs over a foot or so, you will have to
> > use some form of shaft support.  This support will probably be required
> > both inside and outside the hull.  Allowing the shaft to go through the
> > seals at an angle other than 90 deg can cause problems.  Outside shaft
> > supports will almost assuredly have to be employed.  These supports must
> > include bearings of some type which must be protected against corrosive
> > elements while remaining lubricated.  Many propeller shafts are stainless
> > steel.  Many bearings or bushings are not.  This can cause dissimilar metal
> > problems in the water electrolyte.  Since propellers generate thrust, this
> > thrust must pass up the shaft where it can be loaded into the hull.  This
> > will require a thrust bearing of some type.  If the thrust bearing is
> > placed close to the propeller it will probably be in the water and must
> > conform to the requirements stated above.
> >      All of these supports, bearings, and seals will have to be nearly
> > perfect in their alignment.  That means that the motor will also have to
> > have its shaft in direct alignment or else a flexible coupling or U-joint
> > will have to be used to couple the motor to the shaft.  Shaft alignment
> > will probably require adjustment points at all support locations.  The
> > shaft will probably pass through the hull at an angle other than 90
> > degrees.  This may require a design effort to make sure that the shaft
> > passes through the seals correctly.
> >      If the motor is located far to the aft end of the sub, you may have
> > access problems.
> >      If you find that the line of thrust is above or below the center of
> > drag of your boat, too bad.  No adjustment is possible as with hydraulic
> > motors (in my case) or motor pods that can be raised or lowered.  This is
> > not a major problem in general but may require a small adjustment in planes
> > or ballast weight at full speed as compared to being stationary in the 
> water.
> >      Any approach to solving propulsion has advantages and
> > disadvantages.  Weigh them all carefully.  I did not list the advantages
> > but most of these are obvious.
> >
> > I hope this answered your question.
> >
> > Gary Boucher
> >
> > >   My sub is using motor pods
> > >with HP shaft seals and are not pressure compensated.  I have three
> > >motor pods, each with a shaft seal.  I like this design but do think
> > >that one straight shaft would have been much easier and just as safe
> > >although not offering the same control.  Why is a straight shaft
> > >something to be avoided?  Your thoughts please.
> > >
> > >Thanks guys,
> > >Dan H.