Tedsherrin - no problem. I'm not a metals expert, so know little about how SS would work with regular steel in salt water
. I know the impact capability of this material and thought it might make a good isolator for the different metals.
To answer your questions:
Why is the testing done with hitting it with a sledge hammer?
I live in Vermont where a spitting maul like the one in the pictures is common. Most mauls are between 6 to 8 lbs with a 30" handle. So it was easy and convenient to use for a pendulum to produce consistent hits.
I mean, would a fitting in a boat ever get hit with such sudden preasure like being hit with a sledge hammer?
The answer is YES. Mainesail's testing of seacocks and thru-hulls was initiated because a, he thought, well secured spare alternator
broke out of its enclosed space and nearly destroyed a bronze thru-hull/ball valve seacock during a storm. An alternator
weighs about 15 lbs or more. One incident I think is fitting seems the least likely to happen. A boater in Florida
was getting his boat ready. It was on a face dock
just off the inter-coastal. He unloaded his car and put a 20 lb anchor
in a cockpit
locker. He went home to get some more stuff and when he came back the boat under water
. It appeared that someone had waked his boat and the anchor
nailed the plastic seacock sheared it right off and the boat sank. A six lb pipe wrench or crescent wrench, etc. dropped from 4 feet into a bilge
would have clearly compromised most of those plastic fittings. I can provide may more stories like that. Also, you should remember that this impact is equally possible on the outside of the hull.
We hit a submerged pole with a metal piece attached. The metal put a groove a good 1/4" on the underside of the hull, below the waterline. That's impact. If it hit a bronze thru-hull I believe the thru-hull would have bee mangled, but still functional. A plastic thru-hull would simply be history
You'll find that loose dunnage is mentioned by most boat experts as a major concern with regard to boat safety
. Actually, I feel that the sledgehammer test is actually an under representation of the real impact a thru-hull can receive.
Here's a video by Yachting Monthly about how to deal with the after effects of something like a thru-hull being impacted either internally or externally.
Hope this is of some help.