My 1990 P21 came with a trailer of the shaped beam type that you had to completely sink to get the boat off. In 2002 I got home having had the boat lifted on by crane to find the whole winch post dangling from the bow U-bolt. Further investigation showed the rest of the frame was similarly almost rusted through.
RM Trailers made a new trailer that year. I specified break-back so you don't have to get the wheels wet and a split axle so you can drop the keel through for painting etc.
I left the design of the support structure to them and it came with no shaped support beam but 21 rollers. That did mean that there were 21 bits you couldn't paint but on the other hand you could pull the boat back and forth to get at those bits in stages.
It also cost a bit under £3000. But it looks like RM have forgotten how to make them!
Geoff Harwood ex P21/30 Cygnus