Re: [PSUBS-MAILIST] Through Hulls

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

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.