Last week we had a light rain and now both quarter bunks have water on them. The bunks and hull sides have a material headliner. The hull is dry so it's not coming from the stantion or hull/deck joint. I mentioned this to a friend who works on boats for a living. He suggested sealing up the boat as best I can, insert a leaf blower through a port, and spray the deck with soapy water. He claims they use this technique at work. Most of us have done this for inner tubes and inflatable boats, has anyone tried this on their boat?
I tried it on a fuel tank I built to check for leaks. BIG fuel tank and I used a vacuum set to blow. You could see the tank swell when I'd seal it up. No leaks. It should work because it doesn't take very much positive pressure. And you will definitely find the leak if you can seal it off enough. Sealed off TOO much would be a mistake I'm thinking. Tips like these from those in the business are pretty much priceless.
Seems like it would be hard to do on most boats. Sealing it off enough to create enough internal pressure. Holes from the anchor locker to the cockpit lockers and the main hatch is a problem itself. Certainly possible but a lot of work I think.
I have one small leak that manifest itself with a tiny puddle on the starboard main cabin berth, drops about an ounce of water after a good storm. The Compac beadboard ceiling, mostly a blessing for its good looks, easy cleanup and lack of mold producing material is not so helpful here. Leak could be anywhere and comes out at one of the screw projections. I did recently rebed the sliding hatch brackets and the grab rails with butyl rubber, hoping a quarter turn on all fasteners will seal it off.
Duct tape all the places that air might leak out, and should work.
Don't get too perfect a seal, as the total pressure on the hull can be quite high, 1 PSI on the deck of a 25 foot boat, averaging 3 feet wide, equals 10,000 pounds trying to lift the deck off the hull!
Note that Riley's tank was a steel tank, and visibly swelled.
The pressure to get bubbles is quite small.
For a long time I didn't think that I had a deck joint leak. Hull liner doesn't hold or show water, so in my case I would have water on the shelf behind our settees, but no sign of where it was coming from. Finally I tried taping tissue paper to the hull liner up at the likely places. That test showed that water indeed was running down behind, or through the liner. Not sure if this might apply in your situation. (I hate leaks!)
I used water soluble marker to draw a line all the way around inside the hull. A good rain left no doubt where the water was coming in.
After removing the rub strip, and tightening all the bolts for the hull to deck joint, and hitting the joint with a hose, the rub strip was replaced, and that line washed off.