Assuming you have a 48' flatbed semitrailer and not a flatbed truck,
you're still stuck with a boat ht. of 13.5'-(trailer_4.5'+cradle_1')~8'.
Like Steve said, few boats over 25'LOA will fit that profile. If
you are serious about moving and storing a lot of boats up to 50' you
have to consider a trailer designed for the job. The trailers used by
the transporters are slotted lowboys with lots of tiedown points and
stake pockets to make them adjustable mobile cradles. And that begs the
question: why do you want cradles? There's no efficient way to load
and unload big, cradled boats without a crane. Cradles are necessary
for some rail and ship transport, I guess. But jacks are the preferred
storage system elsewhere.
Regards, Boatdreams
-------------------
Courtney Thomas wrote:
Greetings !

I'd like to build a steel cradle that could be adjusted for different
sizes [up to 50' and say, 30,000#] to transport boats on a flat bed
truck, obviously in the least expensive, safe manner.

Detachment from the flatbed, being disassemblable into man-size
maneuverable components, and being capable of being "sledded" across the
ground with a boat in-cradle; would also be desirable.

Recommendations appreciated,