The urethane foam in cans is great stuff. One of my techniques for finding whether corroded spots in the hull are sound is to try to knock a hole in them with a chipping hammer. I was doing so one day and found a bad spot (sudden water ingress) a tube of Knead It, small piece of plywood
and a prop fixed the small hole and then to subdue my paranoia I got loose with a can of urethane. When I came to fix the thin spot one of the more difficult parts
of the job was the removal
of the urethane foam.
Whilst I was in the boat yard a 70' fiber glass cray fishing
boat was hoisted out with about four feet of its bow missing at the water line. It had run into a previously unidentified rock off the NW coast of Australia. It had a collision bulkhead which was unaffected so they screwed some thin alloy plate over the hole and filled the volume between bow and bulkhead with expanding polyurethane
foam. Apparently a lot of boats in that part of the world carry the stuff for emergencies.
Steel is ductile and tough which tends to allow it to absorb a lot of energy before tearing. If the welding is done properly it should be as strong, and perhaps even stronger, than the parent metal.
If I suffered a collision in my steel boat similar to that which caused the damage represented in the images
in post #158 it would probably only scratch the paint
because the bottom of the keel
is 1 1/4" plate and the front 3/8" wall thickness round tube.
In general I would say that all else being equal, in a collision with a floating object a reasonably well built and maintained steel boat is far more likely to survive. The difference between the materials is that the steel boat is going to require more maintenance
, be considerably heavier and not look as near as pretty.
If you want your boat to look good all the time and/or don't want to have to do much maintenance
(mainly worry incessantly about the coatings) and/or be nice and light to beat the other guy around the buoys buy plastic.
If you are of a careless or over adventurous nature, don't mind caring for your paint
work (inside and out) diligently and are paranoid about running into things in the ocean and want the greatest probability of a non fatal (for the boat) collision, buy steel.
And when you have done all you practicability can or want to, to ensure survival, stop worrying about it and go sailing.