Hi Brian. Ya, boat trailers are just mild steel, often galvanized, then
painted.
I'll be building one too pretty soon I hope.
I thought about putting some ballast tanks on it so it could be partially
floated after the sub floats off, so it would be a little lighter when I try
to pull it up the boat ramp. My sub will weigh about 6700 lbs. so I'll need a
fairly stout trailer and it will be heavy even when empty. I like the video of
Dan's sub launch where he has the telescoping trailer tongue for launch. Very
ingenious.
Your sub will be pretty heavy too, so a strong trailer is bound to be
heavy. Maybe a couple of beer kegs or stainless tubes that could be used as
floats would help get it out of the water. Nothing fancy, and maybe could be
used as storage containers or tool boxes during transport. Frank
D.