Welcome aboard Mr300. Very cool technique on the boat unloading!
I have an idea that is sort of similar, but will probably change....always does! Since I need the two sponsons to bear on the support (jack, cribbing, etc.) its a little different logistically. I thought about having a jack on both sides up front, then I came up with the idea of sliding a 12' 6x6 under both sponsons, jacking it up, then putting cribbing (8" concrete blocks) under each side to support the beam. I hadn't thought of jacking up the center till you mentioned it, but that is an excellent idea....not sure why that hadn't occurred to me....maybe the poly fumes....
For some reason I had jacking up each side one at at a time stuck in my head! Thanks for the idea!
After that jack the back up with the engine hoist, pulling the trailer out a little, then sitting the back on cribbing. Since the front blocks will be wider than the trailer (8'), I should be able to ease her out. I like the come a long idea, but hopefully it should just roll out. I have toyed with the idea of taking the tires off and putting the axle on skates if I need extra vertical clearance. I guess we will see how it plays out......
there is always a frame rail or something in the way!