What seemed like a simple problem (how to get the boat off the trailer and turned over), turned out to require a little bit of thinking. These boston whalers weigh 350 lbs dry, so not terribly heavy, but still need 3 or 4 people to lift.
After watching some amateur DIYer YouTube (awesome stuff by the way), and after picking up 8 milk crates the plan was to lift the boat on to the milk crates and 2x4's and slide the trailer out from underneath. This took a little bit of time since we could only slide the trailer out 6-8 inches at a time and then reposition the boat - this was because the 2x4's and crates weren't wide enough to allow the trailer to slide out.
Once the boat was off the trailer, 3 neighbours helped to the turn the boat over, walk it into the garage and on to the milk crates. Worked like a charm and just like the internet promised.
I purchased a 1972 23' Seacraft center console boat in 1974 that I kept in the water, so the bottom needed to be painted each year. I used a similar technique as yours to get the trailer out from under the boat. I jacked up trailer just behind the rear wheels and placed blocks under each corner of the transom. When the trailer was lowered, the stern of the boat was clear of the trailer. Then a jack was placed under the keel just behind a trailer cross member, which allowed the trailer to be pulled forward until the next cross member was about to hit the jack. Repositioning the jack allowed the trailer to be pulled forward a little…