12ton boat on/off ground

You don't say what place, what time, how often, why not crane, whether
you need to off-load into water, etc. All makes a big difference. Or
where your work/time/$$$ trade-offs are...

One approach:
1) (likely the most important part) build strong steel cradle, while
boat is on ground and supported by stands. Build the cradle around the
boat to be sure of fit and all. With great care, you can probably use
bottle jacks to lift the boat the 1" or less needed to maniuplate the
cradle and stands.

2) build 4 tall triangulated A-frames (2 at each end) to carry I-beams
with chain hoists at each end of the boat. Use wood or steel and lay it
out so when you hoist the boat or boat-on-cradle, a truck can back a
trailer under (ie. you need clear space under the boat - can't put any
parts of the hoist support under there). If you want to reverse the
process at the other end, your A-frame supports need to knock down and
assemble easily so you can move them with the boat, and set up at the
other end.

3)install large chain hoists on the girders (I-beams) over the boat. Get
webbing or chains or cables, depending on your preferred lift methods.
Layout must accomodate enough lift to get the boat and/or
boat-with-cradle high enough to allow a flat bed trailer to get under
it. Also, you need enough space around to get the rig in and out to the
road.

4) Either 1) lift the boat and winch the (empty) cradle onto the
flatbed, then set the boat onto the cradle, or 2) lift both the cradle
and boat and set boat onto the trailer at once.

Lots of work. If you don't like that, and you have a crane or something
at the other end: Build the cradle as above, dig a long ramp ending in
pit near the end of the cradle and stabilize the sides of the hole (at
least the one toward the boat) with large wood or poured concrete.
You've just created a loading dock, with your boat/cradle at the edge,
ready to winch onto the trailer you back down the ramp. If you want to
return, or if you want to ease the hauling of the cradle "over the
edge", pour a concrete slab under the boat, _then_ build the cradle on
that; fill the holes in the slab left by the stands that originally held
the boat. The solid surface will make it easier to skid the cradle.
Chamfer the front and rear parts of the cradle that rest on the ground
so it slides easier and you can get roller under it easier if you want.

You must use the right size materials - you need to talk to an engineer
and/or rigger to get that information, including what type bolts or
welds you need to use to hold it all together.

It starts to sound like you want a crane...

Rufus