SURVEY AND CORROSION
Q
If I coat a propeller
with an epoxy bitumen
or tar coating will it reduce
or eliminate electrolysis and
normal underwater
corrosion?
I’ve been reading PBO
since 1998 and I fi nd the
content most informative – a
huge fount of knowledge in
all disciplines of yachting.
Gerhard Alberts
South AfricaCOLIN BROWN REPLIES:
In theory coating a propeller
will protect it from corrosion in
sea water. The reality of
course is not so simple.
The coating you apply must
be complete to offer all-over
protection. It may take two or
more coats to make sure there
are no pores in the coating
which will otherwise allow
normal sea water corrosion to
carry on as normal.In the case of
galvanic or
electrolytic
corrosion the
action could be
concentrated in the
exposed areas
rather than being
spread over the
whole surface area
of the propeller,
leading to
accelerated local
corrosion effects.
Another problem
is getting any coating to stick
reliably to a propeller. A
smooth surface, high speed
through the water and
sometimes cavitation all tend
to make it diffi cult to keep a
coating on a propeller.
There are some antifouling
products that will stick, but the
application conditions and
surface preparation have to be
absolutely perfect.Adding coatings to it will also
make your propeller less
smooth and therefore less
effi cient unless you’re very
careful in the coating
application and fi nishing.
Any rough coating surface
may also make biofouling
more likely as the bacterial
slime that starts it all can get a
grip in the tiny irregularities in
the coating.A correctly fi tted
anode would be a
far more reliable
way of protecting
your propeller from
corrosion.
The anode size
and material
should be chosen
to match the size
of your propeller
and the type of
water your boat
sits in.
A good polish
each year will keep the
surfaces smooth, making your
propeller more effi cient and a
less secure home for
biofouling organisms.
MG DUFF are experts in
marine corrosion protection- you’ll fi nd lots of useful
information on their website
mgduff.co.uk, as well as an
online tool for ensuring you
buy the right anode for the job.
Q
I have a Moody 31
built in 1989 and one
of the inlet seacocks has
failed in the shut position.
This doesn’t unduly
concern me for the present
as I’ll replace it before long
with a new Maestrini DZR
Ball Valve when the boat
comes out of the water at
the end of the season.
My question is should I
be replacing the skin fi tting
as well as the ball valve? I
don’t know if or when they
have ever been replaced
before I owned the boat.
MR Daniels
ColchesterCOLIN BROWN REPLIES:
The answer is short and
simple. Yes! You should
defi nitely replace the skin
fi tting – and hose tail – at the
same time as the seacock.
If you’re fi tting a DZR valve
then you should also fi t a
DZR skin fi tting and hose
tail. All of these fi ttings on
your boat may well beoriginal – in which case
they’ve done very well
lasting this long.
Taking the valve off a skin
fi tting without dislodging the
fi tting can be a challenge in
the fi rst place, and removing
the collar to reseal the old
skin fi tting if it has moved in
the process can also be a
big challenge.
The extra labour involved
in trying to save the old skin
fi tting may cost more (in
terms of time, money and
knuckle skin) than a newfi tting
which costs
around £15.
To get rid of
the old skin fi tting
you can carefully cut off the
fl ange from outside using an
angle grinder and cold
chisels then simply pull the
whole assembly through
into the boat.
You should also consider
fi tting a new hose to your
new seacock. Unless you’ve
changed them yourself and
know better, the hoses onyour boat may
also be original and
they tend to stiffen
and become
brittle with age,
making them
vulnerable to
breakage as
they move
when you try
to get them off
the old seacock.
Our main photo shows a
broken skin fi tting from a
Moody 33. The valve was
still working but the skin
fi tting had suffered
dezincifi cation – the pink
colour of the remaining
metal is a giveaway. You can
see the salt crusting where
the weakened skin fi tting
had been leaking. It
snapped off under light
manual force.Seacock’s jammed... should I replace
anything else while I’m at it?
Prop-er job
Cathodic protection
with the correct
anode is the only
sure-fi re way to
eliminate prop
corrosionQuality ball valves from a
reputable supplier will be
DZR (dezincifi cation
resistant) and marked CR
(corrosion resistant)Dezincifi cation in a failed skin
fi tting allowed this seacock to
be simply snapped offMGDUFF