Practical Boat Owner - February 2016

calcareous deposits, including the

calcium carbonate used to make

barnacles and some tubeworms.

If your seacocks and skin fittings

are made of good-quality materials

they should be naturally resistant

to corrosion and can be left

galvanically isolated. Brass needs

to be protected unless it is rated

as dezincification-resistant (DZR).

If you are in a marina berth with

shore power you should inspect

all of your skin fittings and

seacocks closely as stray currents

may have been part of the

problem and could drive rapid

corrosion if your seacocks are

electrically bonded together. If

you suspect this you should fit

a galvanic isolator or isolation

transformer. Similar problems

could arise if you were in electrical

contact with any part of the bridge

or any structure which may

have steel sheet piling and an

impressed current cathodic

protection system.

You should investigate the

reasons why the paint came off

before simply reapplying it.

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Ask the experts

Here’s just a selection of the latest questions from

PBO readers. Email or write to the address on

page 5 and our experts will answer your queries

THE PBO EXPERTS To ask a question email [email protected] and include your address. Pictures are helpful

SEA SAFETY

Will Stephens is

Staff Officer

Operations (Coastal

Safety) at the RNLI

CRUISING

Stuart Carruthers

is the RYA Cruising

Manager and has

sailed extensively

SAILS

Ian Brown of the

International

OneSails loft group

is an expert on sails

ELECTRICS

Paul Holland is

chairman of the

BMEA and MD of

Energy Solutions

(UK)

MASTS & RIGS

Mike Coates worked

in the spar and

rigging business for

many years

ENGINES

Pat Manley is

a diesel engine

course instructor

and marine author

SURVEY AND

CORROSION

Colin Brown runs

a marine survey and

consultancy company,

CB Marine Services

BOAT BUYING

David Harding is a

regular contributor

to PBO: his photo

archive is at http://www.

sailingscenes.co.uk

SURVEY AND CORROSION

Q

My bilge-keel Moody 34
berths in Haslar Creek,
Gosport, close to the bridge.
Effective antifouling has always
been a problem, and over the
years I have tried conventional
antifoul, copper finish applied
professionally and a single-
transducer sonic system
antifoul alongside the copper
coating, and eventually found
success with the use of two
coats of an expensive soft
antifoul in association with a
two-transducer sonic system.
This season I lifted out in early
September and the boat needed
just a minimal pressure wash;
but I am concerned about some
sort of reaction adjacent to the
exterior metal fittings which has
caused the antifoul to come off
completely, resulting in heavy
fouling just in those areas. It
cannot be a reaction between
the antifouling and the residual
copper finish underneath as it
would have happened all over
the hull rather just by the metal
fittings. The anode on the hull
seems to be performing well (it
has been eaten away at a rate
that I would expect) and, as far
as I can tell, all the metal fittings
are earthed together and do not
show any appearance of having
been eroded.
Am I correct in assuming that
it is some reaction between the
old copper, the antifoul and the
metal fitting that is degrading
the antifoul, or is it something

Paint loss around skin fittings

more sinister? My other thought

was that moisture adjacent to

the fittings had reacted with

the antifoul when it was applied

this spring.

My present plan is to clean

back the affected areas, apply

the appropriate antifoul primer

on a dry, warm day and follow

that by three layers of antifoul.

Any other thoughts?

John Mimpriss

By email

COLIN BROWN REPLIES: The

paint loss around your metal

fittings may have been caused

by galvanic over-protection. It’s

difficult to say for sure without

visiting the boat, but your pictures

suggest that this is one possibility.

Over-protection can occur when

the anode produces a current

higher than is necessary to protect

the fittings connected to it. Alkaline

conditions and hydrogen can

be produced around the cathode

and this can lead to ‘cathodic

disbondment’ where paint

coatings and some antifouling

coatings can be ‘blasted’ off.

Wooden boats can suffer serious

damage from similar conditions.

Having a cocktail of antifoulings

and possibly some wet surfaces

prior to painting may have

contributed to the tendency to

lose paint. Another feature of

over-protection is that it can

accelerate the formation of