FAQ: Thru Hull Fittings
Can I replace my thru-hull fittings like a professional?
I was planning on replacing the thru-hull valves. Should I replace the
pipe nipple and thru-hull or just the valves. If the answer is the
thru-hulls, I'm open to any suggestions there.
Mark Cattell, Viento
PLEASE, PLEASE replace the thru-hulls and valves on
the older boats!
If your boat is aging, the thru-hulls could have serious perforation from
electrolysis or be starting to get brittle. I recall reading a message
from one person on this list who went to close a valve and the whole thing
came away in his hand with suitable ingress of water! Imagine the
language! As suggested, the Marelon are great but I went with bronze (be
aware that there are poor quality bronze thru-hulls available too) and
with marine grade bronze ball valves (again, don't use brass). I got my
valves from an industrial wholesaler for between a quarter and half the
prices charged at chandleries – no exaggeration! The extra money paid
for the better quality thru-hulls; never let The Admiral think that
savings like that mean "We're going out for dinner"!
Although perforation was not so much of a problem for the Dragon as I tie
up in fresh river water, I knew they should be changed when one of the
nipples started unscrewing from the hull! No kidding.
Bronze thru-hulls are either domed like a mushroom and protrude outside
about 1/4", or they are flush mounted. The flush ones have to have
the hull carefully shaped to take the tapered edge of the flange; much
more difficult to install. Unless you are racing and need every 100th of a
knot, get the mushroom type. Be careful to use the right sealant also; it
must be for underwater use. Some people recommend 3M 5200, which means
that it is there forever - it will rip the gelcoat off the laminate before
letting go. I suggest 3M 4200 which means that it will seal really well,
but can be removed if you work at it. Cannot remember which version of the
Sikkaflex is best.
In either case, make sure you have suitable added thickness of hull at the
location to strengthen the hull. Do not use wood or plywood, (I removed
the rotten plywood from mine) and make sure that the built up glass/resin
inner hull surface is parallel to the outside surface.
How do you do that, I hear you asking?
Take your mushroom thru-hull and apply release agent or tape (I used
masking tape) to the surface under the lip of the dome, over the dome
itself and around the threads to stop resin sticking to it, then, pre-test
that your thru-hull....with the tape on, will still go through the hole.
Load your thru-hull up ( under the mushroom) with as much bear-shit as it
will hold, and gently stick it through the hole, FROM
THE INSIDE, so that the dome pulls the bear-shit down onto the
inside surface of the hull. (I know, bearshit is a highly technical term
actually used by the industry!; it is resin thickened with agents and
usually short chopped glass strands).
Hold it there while someone gently tightens the nut from the outside and
jams a piece of wood against the thru-hull opening so that the nut is
firmly wedged against the hull. Or apply a stretchy cord so that it pulls
up evenly into the middle of the boat. Snug up a little more on the nut
and let it kick off in a few minutes. On a big thru-hull like the head
outlet, you want the bear-shit at least about 3/8" thick under the
dome and the edges spread out away from the dome in order to distribute
the loading of the thru-hull, especially if it receives the shock of being
hit from outside later on. The flat pad left by the dome is larger than
the nut surface anyway. When you remove the thru-hull you will find that
except for a little trimming, the inside surface will be parallel to the
outside of the hull, even if the old surface was all lumpy and not
Now for the tough part and you have to be prepared for this. When I did my
boat, I had no less than 7 people come running up to me in alarm and yell
"You stupid b******d! You've put them in backwards!!!" Ya just
have to play dumb and string them along...!
OK, back to seriousness.
Apply the sealant so that it gets squished out from the inner diameter of
the dome toward the outer, and do not tighten it up hard all the way; just
till it barely touches to get an even seal. Wait a couple of days for it
to become set up semisolid rubbery and then snug it up the rest of the
way. Try to have someone else insert something into the thru-hull to hold
it stationary by the tangs or tabs inside while you exert torque on the
nut inside; you don't want to tear the new sealant. Similarly, be careful
about applying torque to the whole assembly when you tighten down hose
barbs or whatever onto the valve body inside. By waiting until the sealant
is set up, you will be doubly sure of a seal, as you will be applying
pressure to a perfectly formed gasket. It will actually bulge a little
around the edges from the pressure.
I used a toothbrush to paint the insides of the larger bronze thru-hulls
with some spare epoxy to slow the galvanic action further.
Go for it and you will have peace of mind. And another hilarious story to
tell your grand-children...
Richard Britton, "Friendly Dragon", RBritton@telus.net (Richard
Important: The opinions expressed here are those of the
individual contributors to this page, and not those of the Catalina 34
National Association or Catalina Yachts, Inc. Additionally, this material
has not been reviewed by Catalina Yachts, Inc. for technical accuracy.
This page's maintainer cannot guarantee the accuracy of this information
or the desirability of suggested modifications or upgrades. Please obtain
assistance from a competent marine mechanic or boatyard prior to making
any significant modifications to your vessel.