A Conumdrum

[rnd2]

Yesterday I called in to a boat manufacturer who I respect and have known for many years and noticed the new build boat propeller shaft diameter had increased from 3 1/4" to 4". I asked him why and he told me his customer had requested SS316L instead of the regular SS430 that he normally uses.
I told him I was aware there is a problem with GRP boats with SS430 propeller shafts in respect to corrosion in parts of the shaft.
He told me that over the past ten years and 30 boats he has built out of aluminium not once has there been a corrosion problem and, many of the early boats have been through the mandatory 5 year survey.
Why would aluminium boats (that he builds) not have a corrosion problem with SS430 shafts and yet GRP boats (general)have a problem?

Any ideas about what might be the corrosion culprit here?

 

[Metalguy]

This one is *probably* easy. That big Al hull is a large anode which offers some protection for the SS shaft. The Al is slowly dissolving, but the large area provides a lot of protection for the small amount of thinning of the hull.

But if *I* were ordering a boat, I'd want a big monel shaft instead of 430 or 316, or at least a better SS alloy.

 

[rnd2]

Shafts are not in direct contact with the hull.

 

[Metalguy]

OK, if the boat is out of the water and you use an ohmeter between the shaft and the hull, what does it read? In other words, is the shaft electrically insulated/isolated from the hull?

 

[rnd2]

Metalguy
I'll be visiting again in a couple of weeks and hope to have some more information.