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I have only one mechanical thru-hull on my boat, for the drop weight
release screw. I am using a 1 inch stainless shaft with two "O" ring glands cut
into it. This shaft fits within a 1 inch ID---2-1/2 inch OD sleeve welded into
the hull bottom. This sleeve is 3/4 inches thick. The sleeve has pipe threads
cut into the top so a 2-1/2 inch ball valve threads down onto it. The 1
inch release pin has a 3/4 X 3/4 inch square end on top. To release the drop
weight, a socket is placed down through the open ball valve and the release
screw is rotated until the drop weight drops off. The pin stays in the welded
sleeve. The ball valve is then closed again.
I opted for this method because a hydraulic system seemed unreliable as
a means to hold the heavy drop weight/carriage assembly if I hit rocks or
some other hard object with the sub bottom.
It seems to me that cutting the "O" ring grooves into the shaft is not only
a lot simpler than cutting them into the sleeve interior, but as this shaft
is removable, it will be easier to replace this part where no cutting and/or
welding will be needed. I don't have K boat drawings. If the pin and sleeve are
made of 316 stainless, they should last a long time, but the "O" ring glands are
a place that may need attention sometimes. Maintenance like cleaning, polishing,
replacing the "O" rings, should be done quite regularly. Even rinsing with fresh
water after diving. My thoughts have always been.....any kind of attachment on a
sub operated in salt water should be designed for quick dis-assembly for
cleaning and maintenance, and as many parts as possible should be removable
without cutting or welding in case of wear or damage.
The "block V " seals have a good track record, and seem like a sound
alternative. I think they are more forgiving than "O" rings as far as close
tolerance and alignment.
Frank D.
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