My fresh water tank started leaking this year, so bad it would
empty the tank through the bilge and out of the pumps in a few
hours. Its a 1985 36' Gibson standard with the tank in the floor in
the back of the boat. It requires surgery on the floor after
removing the dinette and carpet. A friend of mine gave me a
suggestion that I thought I would pass on. He said the most common
fix for this at his marina is to cap the old tank and install a
polypropylene tank in the bow storage area forward of the cuddy. I
installed a 42 gallon tank that is 39"x18"x14". It fit nicely
through the front hatch and rests well on the port side. I ran new
hose through the bilge area back to the water pump in the existing
location. It required drilling a hole in the bilge area through the
bulkhead to feed the hose into the bilge under the cuddy and back
to the pump. I installed a new water deck fill in the front and it
all works fine. Took about an hour to install complete and saved a
lot of work and gave me an extra 12 gallons of fresh water. Weight
was not an issue either, no change, no listing.
Each to his own but you must never feel the need to get that Gibson up on plane because if so that water is in the worst possible place. Of course who in their right mind would want to get one up on plane at $4 plus per gallon? I would suggest you make sure that hole you drilled through that bulkhead be watertight around that hose. That way if you ever hit something that poked a hole in the bow area the boat would not sink before you could get to help.
I replaced the leaking water tank in my Carlcraft with a new 100 gl model. It sounds like it was in the same spot as your Gibson. It really wasn't to bad of a job. You only need to pull back the carpet exposing the plywood. Take a skillsaw and set the depth of the saw to cut just the plywood. Cut your access panel and then only enough joists to pull the tank out. These joists can later be sistered joist back in with stainless bolts and double 2 x 4's. I placed commercial rubber mats under and on the sides of the new tank for a tight fit and to help with rubbing on the fiberglass. Screw the access panel back down and seal around the cut. Since I didn't need to change the plumbing the job took just a couple of hours.
If it were closer to the end of the season I probably would have
cut the floor. I keep it on a buoy so jobs like that are a little
more cumbersome away from the dock. I did seal the hose to the bulk
head to prevent water transfer. It is so close to the center of the
boat that it didn’t cause any listing and there is still
plenty of room in the storage area. Regarding the weight
keeping it from planing, its a 14,000 lb boat with a
single AQ145A Volvo. Only plane around it is in the sky!