Some time back there was a discussion on this B Board of the use of Stainless Steel propeller shafts. Anyone that changes from bronze to stainless is just asking for trouble. Those CD owners that attended the rendevous in July in Connecticut will remember my discussion of Galvanic Corrosion. Since bronze is a more noble metal than stainless steel, the stainless will go away in time. In addition stainless gets its ability to resist corrosion from a clear oxide buildup on its surfact. This oxide must have a constant supply of oxygen to keep working. Underwater the oxygen is supplied by the water. In free flowing water this works. In places where water is stagnant, Ie. in your cutlass bearing and stuffing box, the stainless will use up all the available oxygen. With no more oxygen the protective oxide will break down and the stainless steel will corrode. It will take years for this to happen but it will happen. Bronze does not suffer from this defect and will last much longer. (The rudder post on my own boat is eighty years old and is still fine.) Stainless steel is used on some boats now because it is more readily available and therefore cheaper, not better.
rogerw@bristolbronze.com
Propeller Shafts
Moderator: Jim Walsh
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Tom
Clarification?
Roger, I don't totally understand this, could you explain a little further. The inside of a cutlass bearing has those rubber fingers with spaces between them where the water runs up along the shaft to cool and lubricate it. When the shaft turns it seems like it would be continually bathed in water. Wouldn't this supply the oxygen needed? Likewise with the stuffing box -- water runs along the shaft past the stuffing box and drips into the bilge. Wouldn't this supply the required oxygen? Or is it a matter of degree? Perhaps it's an argument against dripless type stuffing boxes rather than the traditional ones? Thanks.Bristol Bronze wrote: Some time back there was a discussion on this B Board of the use of Stainless Steel propeller shafts. Anyone that changes from bronze to stainless is just asking for trouble. Those CD owners that attended the rendevous in July in Connecticut will remember my discussion of Galvanic Corrosion. Since bronze is a more noble metal than stainless steel, the stainless will go away in time. In addition stainless gets its ability to resist corrosion from a clear oxide buildup on its surfact. This oxide must have a constant supply of oxygen to keep working. Underwater the oxygen is supplied by the water. In free flowing water this works. In places where water is stagnant, Ie. in your cutlass bearing and stuffing box, the stainless will use up all the available oxygen. With no more oxygen the protective oxide will break down and the stainless steel will corrode. It will take years for this to happen but it will happen. Bronze does not suffer from this defect and will last much longer. (The rudder post on my own boat is eighty years old and is still fine.) Stainless steel is used on some boats now because it is more readily available and therefore cheaper, not better.
TomCambria@mindspring.com