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through hull fitting in the bottom of your boat is a potential leak
They should be checked regularly and re-seated or replaced if there is
doubt as to their integrity.
Just imagine what would happen if your engine intake fitting were to
That 50mm / 2inch hole below the water line would allow water to come
in at much faster rate than your bilge pumps are likely to be able to
Admittedly that sort of emergency is rare, more boats sink because they
been left unattended with a slow leak.
While you are on board there is the possibility of taking emergency
stop the leak but what can you do if arrive at the marina to find
dock-lines disappearing down into the water.
Checking Through Hull Seacocks.
The best way to
ensure the integrity of under water, through hull fittings is not to
or at least keep them to a minimum.
only through hull fittings left on Mignonne are the
depth sounder’s transducer, the engine cooling intake and of course the
propeller shaft, I’d love to get rid of them as well but they sure do
come in handy at times.
However, those that you do
have should be inspected on a
It is particularly important
to check the filter in the
engine intake fitting, any blockage here will result in an overheating
and damage to or even lack of a filter could result in debris blocking
cooling channels within the engine itself.
Check and grease the shut-off
valve and inspect the hoses
for cracks and leaks.
If your boat is out of the
water, on the hard you can
still check the valves for leaks.
Disconnect the hose and attach
a length of spare hose,
tie it vertically then fill the open end with water.
Go outside and
check to see
if any is leaking out when the valve is closed and that it is flowing
valve is open.
Or connect the hose to the mains to get some pressure
And check that the hose clips
are non-corrosive and in
good order, these should be doubled up.
Any through hull
which are hard to get at are the ones less likely to have been
a special point of checking these regularly.
On wooden boats pay particular
attention to the wood
surrounding metal fittings, galvanic corrosion and electrolysis can
with the wood as well as the metal.
Tapping the wood around the
through hull fitting will
usually be enough to discover if you have a problem.
Check the speedometer and
depth sounder sensors, these
are usually plastic fittings which, if not protected can easily be
And don’t forget to keep an
eye on the stuffing
boxes for the propeller shaft.
Packing glands need to be
either tightened or the
stuffing replaced and the rubber boots on stern drives checked to keep
water from entering the hull.
And don’t forget the cockpits
Keep your cockpit clear of any
debris which might block
the drain, especially if you are leaving her in the water.
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If the seacocks them selves are
condition but they need re-seating your first problem is going to be
The first thing obviously is
to remove the fixing
However, some fittings may
prove difficult to move,
especially those which were originally sealed with a polyurethane
Avoid hammering on the fitting
as this could easily cause
damage to the fitting and the surrounding wood, besides there might be
room behind to get at it.
The best way, is to rig up a
puller using a length of
threaded studding or a bolt which will go through the fitting.
With blocks of wood off-cuts,
nuts and washers on either
end of the studding and some blocks of wood on the outside to hold it
the hull, tightening the nut should easily pull the fitting out of the
With the fitting removed,
clean off any old sealant or
caulking from the fitting and the hole in the hull.
particular attention to the backing pad, if there is any sign of rot
it, the pad must be shaped to fit the inside curvature of the hull.
Prime the wood with paint
before re-seating and allow it
Bed everything, including the
backing pad with sealant
and lightly coat the threads of the fixing bolts.
Tighten the assembly until it
seats snugly and the
caulking compound or sealant squeezes out around its full perimeter.
If using a polyurethane or
don’t initially tighten the nuts very tight.
Allow the sealant to set
before the final tightening but
be careful when tightening bronze bolts as they can stretch if you over
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Replacing a damaged through hull fitting is
providing the replacement is the same type and size as the old one.
even better option if you can manage it is to do away with the
fitting and replace the it with a bung backed with a through bolted
The top of the list of items to jettison should be that old sea
They can no longer be used legally in many areas and often imposable to
use underway in any kind of sea anyway. Best to replace it with a
Potti' or something similar.
The most common type of replacement through
hull fitting has a
‘mushroom’ outer flange this requires no special tools for
The flush mounted variety however require a special counter sink tool
bevel the outer edge of the hole for a flush fit, so are best
Others rely on a flange nut rather than through bolts for fixing, these
are best left for the above the waterline fittings.
It is never a good
idea to have a mix of
metals on any hull below the water line and on a wooden boat in
particular it is best to stick to silicon bronze.
are through hull fittings available in ‘Marelon’
which is fiberglass reinforced nylon.
At first glance these seem to be the wooden boat owner’s answer to
However, I have had no dealings with this material but I have heard
reports which cast doubt on their strength.
Whatever you use, don't forget to secure the hoses with dual
And use reinforced, good quality hose that wont kink.
For underwater fittings, the safest type are the ones with shut off
valves, then if a hose comes off or is breached the water can be shut
When you position the seacock make sure you can easily reach the handle
and can turn it without scraping you knuckles.
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A New Hole?
If you are
contemplating adding a through hull fitting you need to plan its
So before you start to drill
Will it be readily
accessible from inside.
Will there be ample room to
turn the valve
Will the new fitting set up
any turbulence in front of
your depth sounder or speed log impeller.
Will the hose run from the
fitting be a short and
straight as possible.
On the other hand, will
that stiff hose be long enough
to allow it to be bent for easy fitting and replacement.
Once you have chosen the
position drill a pilot hole from
the inside so you can be sure that it is centered on a plank and
Then using a hole-saw to suit
the size of the fitting cut
the required hole from the outside of the hull.
Using the hole-saw from the
outside will give more room
to control it and a crisper edge to the outside of the hole.
Once you are
sure the hole is
the correct size clean it up with sand paper and prime it with paint or
Use the same sized hole-saw to
cut the aperture in a
piece of hardwood for a backing plate.
The backing plate wants to be
2 or 3 inches larger than
the flange of the seacock and shaped to match the inside curvature of
The backing plate is to
reinforce the hull around the
hole and to provide a flat surface for the seacock flange to bed
The backing plate will also
want to be primed to prevent
If it is possible to plan for
the hole to coincide with a
butt block joint between planks, the valve flanges and securing bolts
to the strength of butt and the butt will do away with the need for a
However, the need for a new
hole in the hull might
possibly be avoided by fitting a tee-connector to an existing inlet or
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While on the
subject of through hull fittings it is worth taking a moment to reflect
process of electrolysis and how it might affect your boat.
Where there are dissimilar
metals on the boat hull below
the water line electric current will be created.
The less ‘noble’ of the metals
on your boat
(the zinc anodes) will be eaten away by galvanic corrosion first as
smaller their voltage potential is reduced to the point where they fail
protect the other metals on the boat.
However, too many zincs
on a wooden boat can be
counterproductive it will help save the metal, but it can raise the
Hydroxly ion attack on the wood.
As Bronze is one of the more
noble metals it is
considered reasonable not to bond bronze through hull fittings to the
Other precautions we can take
are reducing our reliance
on all those bit of electrical gadgetry.
And not leaving our boats tied
into a shore-power system
which could prove to be leaky.
However, that won’t protect us
from voltage leaking
from other boats tied up nearby.
So don’t forget the Anodes
but don’t over do
inspection and maintenance of through hull fittings should reduce the
hood of an emergency situation.
However, every good mariner
should be prepared for the
worst case scenario.
Among the items in your damage
control kit there should
Some tapered wooden plugs
sized to fit the through
hull fittings on your boat.
An assortment of tapes such
as the ubiquitous duct
Spare hose clamps these can
also be used to hold
patches in place.
Nylon cable ties are useful
for quick temporary
Even some bailing or nylon
twine can be handy to hold
a patch securely in place.
A small flashlight
A good variety of hand
And of course, should a
through hull fitting leak that
float switch and a well charged battery will help to protect your boat
are not on board.
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