Dry Out Your Wet Core
I have an old Ballenger that I have been thinking about drying out. There are three ways to dry out a core with three degrees of difficulty, and with three levels of success.
1. Drill 1/4" holes into the core over a 2" matrix through all the foam you can access. Turn the boat upside down and let it sit outside in the sun for about two weeks. Do not use this method if you have a dark colored hull as it can damage the composite. I think using heat lamps would have similar effect. Dave Stetson used this method and said his boat lost considerable weight.
Be very careful that, when you punch through the inside skin, you do not have enough pressure behind the drill to poke outward on the outside skin. If you do this you will get little star-like cracks in the outside skin. If you get any of these, make sure you repair them!
I would recommend using a small drill bit to drill the hole initially, and then using a larger 1/4" bit to widen the hole. The reason for doing this is a smaller bit will cut through the glass and into the core very easily, with little pressure on the drill. Once you make the initial "pilot" hole, the 1/4" bit will be able to cut in easily. Contact your boat's builder and find out how thick the foam and floor skin is. If he says the foam is 8mm and the floor is about 2mm, measure about 9mm (better safe than sorry) up the drill bit from the tip and put a wrap of duck tape around there so you know exactly how far to drill.
2. Drill 1/2" holes through the top skin and vacuum bag it. You are not using the vacuum pump to suck out water as much as simply evaporate water in the core. If you do this you need to have some sort of a condenser to catch the water before it gets to the pump. Dry ice around a glass jar is good. If it works you may get a gallon or more of water. The condenser removes the water so you don't damage the pump. One boat shop in my area uses a small Shop-Vac for a vacuum pump. If you use one of these you do not need a condenser as they can handle water. I'm guessing the pump will need to run for a day or more.
Most foam is scored in blocks to allow bending. The scores are often significant so that although the foam may not actually absorb water, there is significant volume between the blocks and a lot of surface area. On older boats you can see the outline of the squares by looking across the surface at a low angle. You should drill your holes along these channels.
After you dry the core using either of the first two methods, fill the holes with epoxy and micro balloons, and then repaint the floor with a good non-skid paint.
3. The extreme approach, but the only one guaranteed to work is what Peter Alarie did with Lindsay 7318. Peter removed the outer skin and dryed out the area with heat. Then applied new skin over the now dry, damaged area, with vacuum bagging or wet lay-up. I don't think this method is justified unless you have a boat that you are willing to put time and money into, and that you know will be competitive after the drying.
Before starting, you must first identify the area of the core that is holding water. You can sometimes do this by drilling holes to see where the water is, but just because you don^Òt see water doesn't mean there isn't moisture in the core. Next, cut out and carefully remove the floor skin to expose the foam. You do not want to damage the foam while removing the floor. Use heat lamps to dry out the wet area. I think a vacuum pump may work as well. This is the only way to make sure you have a truly dry core.
Peter said that he ended up doing a huge area on 7318, about 3 1/2 feet wide, extending from the rail up over the keelson. It was scary at the time, but it was the right way to do it. I have heard drilling holes does not work well because water does not seem to travel back out of the foam easily.
Peter managed to remove enough water from the core that 7318 was underweight at the North Americans!
Before I start to sound like I know what I'm talking about, the above came from many emails with Dave Stetson, Graham Alexander, and Peter Alarie.