Well, I've done it by slowly backing the trailer into and under the boat using the trailer winch as the force to move/position the trailer. However, I have zero idea of how you are blocked.
And once you get the trailer started straight under the boat with ideally < 1/3rd the length onto the trailer, you need to attach the trailer to the tow vehicle by carefully positioning the truck to the trailer. Do not move the trailer, for safety purposes! If you don't hook the trailer to the tow vehicle, the boat weight can force the trailer tongue way up in the air, so watch the trailer and if it start to rise, get it attached pronto.
Also, I add extra blocking where needed, so if 2 (assuming 1 stack each side) are due to be "pulled out", there's another 2 backup already taking or soon to take their weight.
IMHO you need proper blocking spares on hand and have 3 other people who communicate well and plan it out before hand - BEFORE anyone does anything. Plan it and talk it all out before you do anything. Also make sure that only the trailer moves under the boat. Make darn sure the boat doesn't move forward, or you'll lose your blocks :shock: . Get that vehicle straight and make sure that the experienced trailer driver does not force the trailer under the boat, but if needed, maybe lets off the brake or clutch to move a few inches in reverse.
Be careful please ... someone could get hurt :!: !
Now I've done boats to 18' myself, but I've been moving boats in boatyards my whole life, starting as the lumper (in the mud no less) for the boatyard pulling wood boats up that were resting on wooden cradles, where the cradle was towed down the ramp at low tide (9+' tides here) and then the boat was positioned on the cradle on a falling tide, to be towed up on the next low. Large wooden rollers (think old B&W Egyptian or Roman movies here) were under the cradle and as they'd pop out as the cradle moved. My brothers and I would each take an end of the freed rollers and run them up and position them straight under the front of the cradle ... where the towing process would repeat. It was fun!
