caulk a boat, there is no magic
involved. If I could do it, so can you.
On a new boat with its pristine seams it is pretty straight forward.
On an older boat it is a bit trickier.
However, anyone with
a modicum of common sense (which unfortunately isn't
all that common) will have no trouble doing a competent job.
The main thing to be
aware of is, not to over caulk,
this could lead to strained frames as the planking 'takes
Oakum, which is made from hemp soaked in pine tar, is normally only
used on the
larger vessels with planks over two inches thick.
For most small to medium boats cotton is by far the easier
versatile material to use.
Preparing to Caulk
If you are working on a newly built
hull you may need to open up any tight seams.
This can be done with either a
or a dumb
The dumb iron works best on the softer woods, the raking iron on
If re-caulking, make sure to clean out all the old compound and cotton.
If you are going to use oil based putty as the caulking compound it is
idea to prime the seams.
This can be done with an oil based paint.
It's a good idea to prime again once the cotton is in place.
This is probably the better method when re-caulking an old boat. If you
using a synthetic sealant such as 3M 5200 check the manufacturers'
recommendations for priming.
My own preference is for old fashioned, cheap and readily available
putty mixed with red lead powder.
When it is time to
start inserting the cotton where do you start?
like to do all the butt
ends first, then start on
the lower seams and finish off with the rabbet and garboard seam.
But, I don't see any harm in
starting above the
water line if you want to get into the swing of it first.
many strands of cotton you use will depend on the size of the seams.
Ideally the cotton should fill
the deepest third of the seam.
This means that when tapping
it home the iron should go
in by about two thirds of the plank thickness.
Add or subtract strands as
But always remember,
is to be tapped in, not
Don't over do it,
cotton is going to swell when wet.
And don't forget which seams
have been done, you
don't want to caulk a seam twice.
Some authorities suggest
laying out enough cotton to do a
That is fine if you are laying
it out in a perfectly
The cotton is rather hairy and
will pick up any and every
bit of dirt or wood shavings from the floor.
I found that it was better to
feed it straight from the
ball, but keeping the ball in a box which can be pushed along with your
If you have to join strands
mid seam don't tie a
knot, this will produce a thick, hard lump.
Simply roll the ends together.
Start off by using the paying
iron to push the cotton
into a foot or two of the seam.
Then go back and using a
rocking action with the iron,
tap the cotton home with the mallet.
If there are any wide seams
leave them for
By caulking all the tight
seams first you could find that
this has caused the wider ones to close up.
Wide seams can be done in
two runs, increasing the
number of strands on the top.
The reason a carvel planked
boat keeps the water out is
that both the wood and the cotton swell when wet, effectively sealing
The caulking compound while it
does help waterproof the
seam is there mainly to help hold the cotton in place and fill those
The traditional compound for
stopping seams is linseed
oil putty mixed
with red led powder.
When mixing this putty take a tennis ball sized
dollop of the
putty, mix enough red lead powder to give an even red color, add
linseed oil, if
needed, to keep it soft.
add a thumb
nail sized dollop of
multi-purpose grease to the putty, mix it in well and push the putty into the seam with a
The grease will help keep the putty soft and flexible thus
hardening and cracking later.
It has recently been brought to my attention by
Canada, how difficult it is to buy red lead powder in many
Here in the UK it is still possible to obtain many of these
supplies from specialists such as
there are alternative commercially available oil based compounds such
and Interlux. There are plenty of silicon and polysulphide substances around, these are probably fine if you are caulking clean oil free new wood.
But on an old boat's seams it is extremely unlikely that the
hasn't absorbed some oil or other substances that would make using a
synthetic compound worse than useless.
The seams most likely to be oily are the garboard
seams, which are also the
you don't want to have future troubles with.
And some of these compounds, when they do
a bit too well.
By the way, don't be fooled into believing that any of these
'magical mastics' will eliminate the need for the cotton.
When it comes to launch day you
might find that some of the seams have opened up.
Do not be
tempted to add any
more caulking cotton.
You can add some more of the
paying compound but this
will probably end up being squeezes out as the wood and cotton take
Providing the gaps are not
excessive it's batter to use a
temporary method of stopping.
Tape over the cracks with
masking tape, this will slow
water intake sufficiently while the planks take up.
Or fill the cracks with soap,
use bar soap rubbed into
the crevices this will eventually just wash away.
you could use
Having said all that, I don't
mind admitting that
when I launched Mignonne after her rebuild I was as nervous as any
Fortunately the yard crew were
aware of the problems
affecting wooden boats and allowed her to sit in the hoist slings for
before setting us adrift.
How quickly she takes up will
depend on the wood of the
Soft woods will usually take
up faster than
Mignonne with her hardwood
planks took a month before the
seepage slowed appreciably.
And now no more than a cup
full per week gets
Considering that this was my
first attempt to re-caulk a
complete hull, the fact that she did eventually become water tight just
show that anyone, with a little care can do it.
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