BTW, you shouldn't take my description of torsion axles as bad, many, many boat trailers, cars, buses, heavy trucks, etc... have been using this setup since the dawn of the automobile and they work fine. Actually in a trailer they might actually work better than a spring setup in some cases because the wheels work individually of each other. This cuts down on lots of rattling, binding, etc.. When you turn your tandem axle trailer the tires on the inside of the turn have to "scrub" or slide on the surface since they can not turn. On a torsion setup this is easier and causes less tire wear, in most cases.
However, lately, I have seen lots and lots of torsion bar failure on boat trailers. I think one major trailer manufacturer actually had a recall on their torsion axles. The axles usually use have a square tube with rod going through it that twists. The square tube is filled with a very hard rubber which compresses as the rod twists. This rubber tends to wear out and will eventually fall apart. The rubber usually wears faster due to sudden changes in heat. This is why it might not be ideal for use on a boat trailer, because when you get them wet they go through a temperature change very quickly.
Leaf spring suspensions on boat trailers also fail, but lately, not near as often.