Re: [PSUBS-MAILIST] Through Hulls

[Dan h <machine@epix.net>]

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.