Re: [PSUBS-MAILIST] Drive shaft alignment & sopport.

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

Starck Wrote:

>If you enjoy problems and complicated expensive solutions complete with
>all sorts of reasons why what works in practice is no good in theory and
>should be ignored consult an engineer..  They can  solve problems that
>don't even exist and create new ones no one else has encountered before.

That is absolute B.S.  No matter what your profession is, you find those 
who are incompetent, unknowing, and irrelevant.  The engineering profession 
is no exception.  However, what I have found through the years over and 
over is people who have limited knowledge of technical issues who make 
broad-ranging and very verbal assumptions.  These people, because they do 
not completely understand the underlying theory, believe that those who do 
understand and can employ that theory are somehow living in their own world 
using tools that are irrelevant to good design practice.

I will give a case in point.  I know someone that pressure chamber tested a 
submersible window design using Lexan.  The window was one-time tested to 
beyond the rated operating depth of the sub.  The Lexan actually had a 
noticable bow inward under pressure creating a convex/concave window.  The 
logic here was simple and of course not needing any real theory, certainly 
not any engineering expertise.  The logic simply put was that, if the 
window did not fail under one cycle of pressure, somewhat over the rated 
operating depth of the sub, coupled with the fact that Lexan seemed a 
wonder material and would never actually break, everything was fine.  This 
is a typical assumption made by someone that does not understand the 
properties of the material.  "No engineering theory needed here.  We just 
proved that it will work.  We built 100 of them and never had a problem."

There is NO way I want to go down below very shallow depths in a sub with 
thin Lexan windows.  Likewise, there is NO way I want to go down to 1000 
feet in a sub that uses wire penetraters made from flair fittings and 
O-Rings relying on nothing but the wire's insulation to keep the water out.

You do not have to be an "engineer" to construct a safe submersible, if you 
find outside help from qualified individuals, but the PSUBer has the 
responsibility to determine who to get advice from.  After many years of 
this type of work, I feel that I have just about reached the level where I 
know what I don't understand and where to get good help.  Basically, I am 
NOT an expert.  But, I know where to find good advice.

Gary Boucher


> > 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.
> >
> >