Put a support on the trailer if needed towards the front of the boat. Loosen the bow tie down a little. Remove the tie down(s) at the stern. Using a block of wood on a jack, jack the stern up until the boat just does clear the bunks. The bow should hit the support you put there. The last one I did this way had a roller up front so it served the purpose of the support. Using jack stands & blocks of wood you can support the boat o0n each side of the stern as you jack it up. I've done several this way but not sure if I explained myself.
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va