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Not all ‘marine quality’ items are as good as they sound. For example, manganese bronze. We all THINK that ‘bronze’ is the perfect metal for the sea due to its resistance to corrosion. However, that’s ‘Tin Bronze’!! There are in fact many forms of ‘bronze’ used in marine applications. Some, such as manganese bronze, are an amalgam of zinc (not tin) and copper, which makes them a brass. The zinc is susceptible to erosion by galvanic action just like any other brass. However, it’s cheap, (well, cheapish!) so it is bought in blissful ignorance of its limitations. It’s even used for propellers and shafts! It is very important to consult a ‘Galvanic Table’ when planning your alterations to ensure that you use the right metal in the right location and particularly when in contact with other metals. Remember, your sacrificial anode is zinc! Anything less stable than tin bronze needs to be bonded to an anode and that anode must be close to and in line of sight of the fitting it protects.

Above the waterline is not much safer than below. The salty atmosphere and spray get everywhere! Fit a stainless steel (S/S) stanchion to an alloy base and the base will corrode, eventually failing. (See the split?) Even putting a S/S screw in an alloy fitting will cause problems. Check your aluminium mast for S/S fittings or screws. Just watch the corrosion eat the mast!

Fitting some extra cleats? Anything that penetrates the deck is a potential danger. Good load pads under the deck are needed, penny washers are just not enough. If not well sealed against moisture ingress, the strength of the deck could be seriously effected. Most decks now are GRP / balsa sandwiches. Moisture will break the join of this structure and weaken the deck. Also, not all S/S is the same! Grade 304 (A2 screws) may be alright above the water, but welding it causes a problem. Carbon in the S/S is concentrated near the weld and corrosion is the result. Grade 316 (A4 screws) is much better. But they both suffer corrosion from oxygen deprivation. A damp S/S screw or bolt passing through a deck or otherwise deprived of air will fail.

Then there’s the deck mounted fuel filler. Never mind condensation in fuel tanks! Most water just gets in past the filler. The rubber ‘O’ ring seal gets hard and dirty or the deck seal fails and the rain is in!!