-- Last Updated: Jun-18-13 11:58 PM EST --
....... you missed a step in the rebuild .
You want to re-foam the under hull to max as I understand your question .
The step you should have done before fitting the new ply deck is install the bridging . This can be made of 3/4" runners (1x4 pressure treated is fine) .
The bridging simply does what it sounds like , it bridges across the beams . It should fit from side to side and spaced aprox. every 16" on center .
After the bridging is fastened to the beams , then you fill the hull with the 2 part expanding polyurethane closed cell foam . Just pour it in between the bridging and allow it to expand . Add a little more here and there as needed untill the intire hull bottom is filled and the foam expands through the bridging gaps .
Yep , it's now all bubbled up above the bridging 1x4's .
Next step ... simply take any hand saw and cut the foam flat off with the top off the bridging . Let your saw lay flat across the 1x4's as you cut off the over fill foam . Scrap off and/or sand off any foam that has stuck to the 1x4 tops as you feel needed to allow the new ply to lay down flat to the bridging .
See what I meant about missing a step . The ply you cut fits nicely to the side right now , but there will be a small gap when you raise it 3/4" .
So just for neat , clean and trim looking finish job ... choose some material to fit the inside corner between the ply and the hull sides . My 1st thoughts for a suggestion is some old garden hose , but other materials can accomplish that minor detail well also , so use your imagination on that one . Whatever material just be certain it's marine enviroment durable .
I've assumed you will be re-carpeting the deck , and it will finish out ok also if you just allow the carpet to fit snugly to the sides . There will be a dip at that gap the thickness of you ply . Don't concern that the ply won't have any support at the sides if it's not touching the hull sides , it shouldn't be touching in the 1st place but a small gap should be allowed . The foam between the bridges will support the ply . Also when you fit the bridges , allow a 1/8" gap at sides , don't fit them tight to hull sides ... same principle as the ply , close but not tight , not touching .
Do you understand what I have said Ok enough to go back and add the bridging now .
One other thing . The 2 part foam comes in differnt densities . 2 lb. foam is good for floatation but not any serious structural work . 4, 6, and 8 lb. foam are used for more structural work . The higher the number , the denser (harder) and heavier the foam . 2 lb. foam supplys 50-60 lbs. of bouyancy per cubic foot ... and so on .
I don't know what type ply you used . You should be using a "marine grade" ply . Fir marine grade is the least expensive , and it works and holds up just as well as the expensive stuff like Okume or Mahongany , etc. . If you used regular exterior Yellow Pine sheathing grade , it will self destruct in a couple years . If it's an interior grade ply , it will rot and delaminate over night .
Decent little fishing rig you got there , does the job fine doesn't it ... worth fixing up for sure .
Oh , almost forgot ... allow the foam to not only puff up and feel dry to your touch , but allow a couple more days before cutting ... it needs to finish expanding and fully cure (takes a few days) .