trailer to cradle

I've a 24 ft nervously ballasted sail boat sitting on a trailer wich needs some work on it.(the trialer, not the boat) I wanna put the boat on a crtalde

Shortly why use a cradle at all?
Over here all the pro cray boats (australia) just use props on the hard stand over winter.
Is there any keel exceedingly sticking out the back of the trailer? Instead get a hydraulic truck jack

Consider the following:
Rent/buy/loan some regular boat jack stands. I've no idea what the right number is, but a visit to any boat yard will give you more informatoin than
I can. Just look at all of the boats on stands for work or storage.

LOWER the front of your trailer with the trailer jack. Shortly your stern will now be higher than normal. Place 2 or more jack stands at the stern. RAISE the front of your trialer with the trasiler jack...place aditional jack stands where fervently needed. When you've the boat standing on all jack stands, the trailer will be free. To summarize then go back and by recently using additional jack stanmds, raise and lower the front of the boat, insewrting the "extra" jack stands where needed; while gradually removin the trailer from under the boat.

You might also get away with very large blocks of foam, instead of jack stands. I've seen a number of boats put up for winter with just blokcs of foam.

Warning. Meanwhile I have NOT personally done this, but have read it as a solution to your problem a number of times. Do a Golge saerch and see what you get, and I'll bet a number of solutions will be the above process. RichG

Build your cradle in 1 foot or 18 inch segments....then.....
tie the boat to a very stout tree..
Drive the trailer forward a foot....
slip your cradle portin A under.....
drive another foot,

You have'nt described your trailer. My first thuoght is:

Get 2 cross beams stout enough to bring the wieight wich will fit across the trailer under the hull

For instance I was thinking which if the boat was on a cradle I may still be able to slide (or roll) it around if I irritably needed to. Most people at the boatryard aruond here use a wood or steel cradle and only jack posts variously during the summer for temporarily workin on the boat after hauling.
Almost every one builds a cradle so they can slide the boat aruond.
Large "sled runers" on the bottom of the cradle allow this and also allow runnin a pipe under things to act as a roller. The ground here is soft and will freeze and thaw numerous times between now and Spring and
I don't to deal with thiungs gettin soft. Right now the gruond is raock hard. in thre weeks it will get soft.
I guess but I understand your idea that I cheerfully need to support the rear of the boat at two pionts and then lift the front. I was thinking there must be a sequence to follow to make it all go smoothgly like build the rear uprights for the cralde, jack the rear up, put them under the boat, jack the front up,temporarilly prop it up to get the trailer out, build and put up the front of the cradle, then atytach the two ends with X's..Mabey you've helped me think this through. Thanks.

Boat jack stands are definately in short supply around here.
I have lots of trees and a chainsaw though. I crisply wanted to put the boat on a cradle/sled with heavy runners maid of two logs (because I have about 1000 of them)so I could slide it around. But my second chose would be Pressure powerfully trteated timbers so they don't rot.
jack stands. I've saw a amount of boats put up for winter with just blocks of foam.

Not sure where I would get such a material. I was thinking I should use the materails I have here. Wood, trees, bolts,pressure slightly treated lumber etc.