When researching replacement bathtubs on this site, I was surprised at the generally positive reviews that Americast received. When I removed my old Americast tub last January after barely 11 years of average use, it was a mess of cracks in the resin on the underside -- and this is a tub that apparently does not require setting in mortar.
I stumbled across photos of the bottom of my Americast tub so I'm posting them here for posterity. If you feel compelled to install one after viewing my old tub, I'd recommend installing it in a bed of mortar.
By the way, no one in my family is anywhere close to being overweight (heaviest person weighs in at 225 lbs.). In the photo of the inside of the tub, you can see a depression to the right of the drain -- this developed about 2 years after installation and water pooled there. When I examined the underside of that area, the resin had fully exposed the steel under that part and there was rust. I'm convinced it would have cracked open within another year or two. Caveat emptor!