Fastenings for rudder fittings

J

Jwoods

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In a fit of maddness I bought myself a Falmouth Pilot 6 Ton. It need some TLC so I am replacing all the rudder fitings. These are fabricated from mild steel and were fastened with a combination of mild steel bolts and the odd rivetted one too! A couple of bolts had stainless nuts.

Is it OK to replace all the bolts with stainless?

I will fabricate a new set of fittings out of the same material, mild steel, as soon as I have raided a scrap bin somewhere.

All help greatly received!
 
There is nothing wrong with hot dipped galvanized bolts and galvanized steel rudder fittings with lots of read lead paint in the holes and on the wood under the fittings.
If you do buy stainless get the expensive stuff from suppliers who know what to give you. Some stainless will rust like mild steel when buried in wet wood in the absence of a ready supply of oxygen.
 
The following is taken from Classic Marine's excellent article "Metals in a marine enviroment", which basically advises that no Stainless steel fixings should be used below the waterline, 304 or 316!
<span style="color:red"> </span>
The chemical and food industries alleviate these problems by adding a dash of Molybdenum (Mo). Thus 316 stainless (or A4) typically comprises 17% Cr, 11% Ni, 2 % Mo and is widely used to store and transport some very aggressive substances. So, you might think that this is the perfect stuff to use as a fastener in or through wood, and from the sole perspective of chemical attack you’d be right. But we need to reconsider the environment in which the fastener is doing its job. Imagine a bolt, nail or screw fastening a plank to a frame underwater. The head, at or near the surface, will be oxygenated enough to maintain its passive film. The shank, buried deep in the structure, is likely to be starved of oxygen but will be surrounded by various acids and chlorides. In these circumstances, the passive film may break such that the stainless becomes ‘active’. This has two effects: firstly, look back at the galvanic series and you’ll see that the difference between active and passive electropotentials in 304, and to a lesser extent in 316, is enough to cause galvanic corrosion. Like brass, stainless can form its own galvanic couple. Secondly, without the oxide layer, the stainless will corrode about as fast as steel. <span style="color:red">The upshot is that stainless fasteners below the water-line - irrespective of the grade - may be no better than mild steel. </span> Above the water-line (more oxygen and less electrolyte) such fastenings are fine, but unless you value the extra lustre of 316, there’s little point in paying for it.
 
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