I remember my brother talking about distortion, and tensile strength, but I really don't know dinkum about it. If I ever had any tricky welding to do, I took it to him. He worked in the metal trades, went to school and got a doctorate in applied metalurgy which is just a fancy name for welding. If he wanted to go hunting or fishing, it was usually with me. All the old farmers around where I lived used to save all their difficult or tricky welds if they knew Mike was coming to see me. Why should I learn about welding. I don't know what the alloy was, but I think I remember him mentioning magnesium. I'm sure someone could look it up on the internet.
I hope you do the right thing with your boat, Bill. Been there, done that. Good luck. If I knew then, what I know now, I'd have gone in hock for a new boat just the way I wanted it, and driven the same old car until it was paid off. That might be an option to consider. Buenos suerte