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A gallon of water weighs about 8.4 lbs so that means you have about 5.something gallons of water left in there. I think that's quite a lot to leave in there especially because it is concentrated in the stern where you'll add the weight of the engine. I haven't been keeping up with this thread but it is my understanding that several methods of reattaching the deck have been discussed and I would like to post a method I would use. I would glue thin wooden battens to the undersides of the remaining deck so they span the opening where you have removed the deck. After pouring the new foam simply shave it down to the surface of the battens and then place the piece of deck you removed on top of the battens and it should fit perfectly. Thinking over this I would cut 1/4 inch plywood strips about 1.5 inch wide and glue them in on edge and the foam wouldn't deform it that way.