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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

I am the first owner of a
now 10-year-old Cornish
Crabbers Pilot Cutter 30. The
boat has always been anchored
or stored ashore in Greece in
the open, so exposure to UV
light has been year-round: add
to this the high temperatures
and the salinity of the sea
during the summer in the
Eastern Mediterranean.
After putting the boat back
in the water, I found that the
head of one stainless steel
bolt that affixes the samson
post/bowsprit pivot fitting to
the foredeck had broken off
due to corrosion. Since then
I have found broken-off heads
of other bolts and screws on
deck. So far, repairs have been
easy, and no disastrous failures
have occurred while sailing.
Nonetheless, I am worried
about similar corrosion damage
elsewhere that might lead to
a disastrous rig failure. I am
thinking in this regard about
the chainplates of the lower
shrouds that are affixed with
bolts to the outside of the hull,
and about all other components
of the standing rigging – ie the
shackles with which the upper
ends of the shrouds are
attached to the mast, the
shrouds themselves, the
turnbuckles, etc.
Can you please advise
me how far I should go in

Could stainless steel bolt

corrosion lead to rig failure?

The head of one stainless steel bolt affixing the samson post/

bowsprit pivot fitting to the foredeck of Jacobus Lubsen’s

Cornish Crabbers Pilot Cutter 30 had rusted off

preventively replacing

components of the standing

rigging? I am torn between

only replacing the bolts that

affix the chainplates, and

renewing everything.

Jacobus Lubsen

Paros, Greece

COLIN BROWN REPLIES: Your

bolts are suffering from crevice

corrosion. The process is as

follows: stainless steel needs

oxygen to remain stainless, and

any damage to the surface is

rapidly healed with available

oxygen as the chromium

content oxidises to form the

familiar shiny surface. Low-oxygen

conditions, like you might find

in a deck or hull with water

collecting under bolt heads,

are ideal for pitting and crevice

corrosion. Once this starts, the

corroding ‘active’ stainless steel

and the surrounding protected

‘passive’ stainless steel can form

a galvanic couple that accelerates

loss of material at the active site.

In essence, part of the bolt

becomes an anode for the

rest of it.

This process can be initiated

by chloride ions present in salt.

As you have suggested, high

temperatures and salt have

been contributing factors. High

UV levels could also degrade

the sealant around bolts,

allowing water to get in and

start the process. There are

different grades of stainless

steel with differing levels of

resistance to crevice corrosion.

Marine fittings should be 316

stainless, but other grades may

have been used on deck. It is

difficult to tell the different grades

apart, but one simple test is

that 316 stainless steel is not

magnetic. There are other

grades that are not magnetic,

so this test is not definitive.

In your picture there are visible

rust stains around the bolts. These

are an obvious warning sign, and

you should take out every bolt or

screw that has stains around it.

This is less easy with the rigging,

which will have to be lowered. A

gaff rig is under much lower stress

than a Bermudan rig, so the

often-quoted 10-year lifespan

for stainless steel rigging would

normally be considered too

conservative. In your case you

should pay close attention to all

of your standing rigging and

deck fittings. Take all suspect

bolts, shackle pins and

rigging screws apart for close

examination, and replace where

any material loss is found. A

magnifying glass might help here.

It is possible to reduce

the chances of the same

problems arising again by

using 316 stainless steel, by

rinsing salt deposits off your

stainless steel whenever you

can and replacing sealant when

it shows signs of weathering.