Electrolysis aboard sailing vessels

Many sailors are aware of the potential hazards aboard their vessels that

they can not see. These sailors check their hardware, or have the yard check

 their hardware before spring launch. Of course, in some latitudes spring

never arrives, so the standing rigging is rarely checked.

Many other sailors are totally unaware of the inherit dangers of using dissimilar metals in

 their standing rigging. These sailors are completely surprised when their vessel is

dismasted or some other disaster strikes due to lack of routine maintenance.

When aluminum masts were first introduced and continuing on to today most of

the fasteners used to assemble and install the mast was some grade

 of stainless steel. When aluminum and stainless are joined, electrolytic action begins.

The result of this action, over time, is the corrosion of one or both of of the joined metals.

An example of corrosion that caused such a disaster follows. The mast and spreaders

that were involved in a dismasting are pictured below. Figure one below shows the mast

and sail track.

On older vessels, this was from a 1972 Ranger Sloop, the main sail was attached to the

mast via the sail track.  The sail track was made up of sections of stainless which

were screwed to the aluminum mast using small round head stainless steel screws.

If you note the screw holes in Figure one below, it shows complete corrosion of the

screw at the aluminum/stainless joint.

Figure two shows extensive corrosion of the solid end of the spreader.

The stainless shroud was partially supported by the attachment at the spreader.

Again, a stainless to aluminum attachment was in place.

The vessel operator does not remember the the exact sequence of events during the

dismasting, only that the main sail pulled partially (about fifteen feet) away from the

mast and the mast began to fall rapidly to starboard and break off at the deck.

My investigation indicated that when the main sail pulled away from the mast because

the track failed, it pulled the shroud away from the corroded spreader. The mast was

then unsupported and the gust was strong enough to break the mast off at the deck.

Figure  one

Figure two