Re: [PSUBS-MAILIST] Through Hulls

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

Dan,

     I am well aware that this is probably going to start another 
controversy on PSUBS.  But, the technique explained for creating a 
thru-hull electrical conductor should work at shallow depths, but should 
not be used in a manned submersible at the "Up to 1000 foot" depth 
described.  The danger is in the extrusion of the wire itself into the 
insides of the boat.  Lets say that the wire is 0.4 inch in diameter.  The 
force is around 55 pounds at 1000 feet trying to force the wire through the 
connecter.  Should even a small breach or tear in the insulation of the 
cable occur, the 440+ PSI water pressure will be a major problem.
     I use several similar commercial rubber O-ring cable thru-hulls rated 
to 300 feet for coax sized cable, but my boat is rated to just 200 feet and 
the wire has a cross section area of about 0.04 square inches.  The larger 
the wire, the higher the stress on the rubber insulation.  As mentioned, 
wire with paper or fiber filler should never be used.
     My major thru-hull interfaces are made with bare "solid" copper wire 
potted in high strength epoxy.  Not only that, but I use a 90 degree bend 
in the channel containing the epoxy.  There is no way that the wire can be 
extruded at the rated depth.  There is no reliance on insulation 
integrity.  I do not, at the max depth mentioned, trust a system as 
described based on the mechanical strength of the insulation.

Gary Boucher



At 09:03 PM 11/21/2002, you wrote:
>Thanks a lot Walter
>
>I didn't think capturing the jacket of a cable like that would be enough to
>keep it secure at that kind of depth.  I was planning to epoxy each
>conductor of my cable separately to ensure it wouldn't slide through and/or
>leak.  My way is far more complicated and doesn't really keep water from
>getting into the cable jacket.  I'll make a test piece with your method in a
>few days.
>
>Thanks again!
>Dan H.
>
>Walter Starck wrote:
>
> > Dan h wrote:
> > " A description of how you design your through hull electrical
> >  connectors, what type of cable is better and for what reasons would be
> >  vary helpful to many of us in here.  This would be of far more use to
> > us
> >  amateurs.   Please describe how you recommend building an electrical
> >  through hull."
> >
> > The following is for a continous (i.e. non-disconnectable) thru hull
> > fitting.
> >
> > Start with a male NPT to metal tube flare fitting for tubing of the same
> > size as the cable to be used. Remove the flare nut, face off the nipple
> > in a lathe and bevel the resulting face at 45° sloping inward.  The i.d.
> > of the fitting can be bored out slightly if required to accept the
> > cable.
> >
> > Use cable in which the insulated conductors are embedded in a solid
> > outer casing.  The common type that uses a fiber filler inside a tubular
> > outer cover should be avoided.  To effect a seal, pass the cable through
> > the original flare nut, followed by two O-rings, then the modified
> > fitting.  When the nut is tightned the O-rings are compressed inward by
> > the beveled inner face of the nut and by the inward sloping face on the
> > modified fitting.  This compression squeezes the O-rings into the outer
> > cable covering creating an annular depression and locking the cable in
> > place with a very effective watertight seal.  For use in depths of a
> > thousand feet or less this type of fitting is cheap, robust, easy to
> > make and highly reliable.  The fitting is of course used by drilling and
> > tapping to install it where required.  Depending upon thickness and
> > curvature of the hull a flat boss may be required for attachment.
> >
> > Disconnectable through hull fittings are somewhat more complicated but
> > can also be made using the same cable sealing method but more complex
> > machine work is required as both plug and receptacle fittings must be
> > fabricated from bar stock.  They are joined with a pipe coupling nut
> > with O-ring seal  and employ an internal electrical connector for that
> > connectivity.
> >
> > Walter Starck
> > Golden Dolphin Video CD Magazine
> > The premiere publication of diving and the ocean world.
> > www.goldendolphin.com