Martin,

I believe that moisture can enter the aft cabin in a number of ways. Some, like leaving the aft access window open, can be quickly ruled out, as the wetness would appear on the top of the aft cabin mattress first, not beneath it as in your case. Your likely culprits (I think) are:

1. Condensation from the fuel tank;
2. Water seeping in around the binnacle and binnacle stanchions.

Condensation on the exterior of the fuel tank, if sufficient quantity, could - I suppose - run beneath the mattress and dampen that area, causing moisture and mildew problems. Condensation, though, typically would occur when the fuel in the tank were considerably colder than ambient temperature - for example, if you had recently fueled and the newly loaded fuel were very cold - and the ambient air had a high moisture content, such as in Florida in summer. You don't say where you're based, but I suspect somewhere with a high humidity this season of the year. But once the fuel temperature warmed to ambient, the condensation probably wouldn't occur again, or at least any daily events wouldn't/shouldn't cause a large "runoff" from the tank.

I have not heard of condensation on the fuel tank causing enough "runoff" to dampen the aft cabin, but this could, I suppose, happen. I will be interested to hear from other owners of their experiences.

Suspect #2 may be more likely...and I know this from personal experience. The bases of the stanchions at the binnacle, and the wire and cable passages beneath the binnacle, are extremely difficult to seal completely. Any water that enters the binnacle (such as at the joints beneath the compass) and any water that leaks through the bases of the stanchions, where wiring from the navpods exits into the aft cabin, will drip into the cover (overhead of the aft mattress) and then run down the back cabin bulkhead to thoroughly dampen beneath the mattress. I have struggled trying to seal these areas properly and finally just surrendered. I had a fiberglass guy fabricate a "dam" at the aft end of the cover, install a small tube through the dam, and then drill a hole in the aft bulkhead to allow the tube to carry any water into the lazarette. This completely solved the moisture problem in the aft cabin. See photos.

There may be a way to validate or rule out the fuel tank condensation hypothesis.

One idea would be to run a long strip of wide 3M blue masking tape underneath the fuel tank shelf (just above mattress level), from one end to the other, pressing the bottom edge of the tape for a good seal. Leave the top half of the tape "open", ie pulled away from the gelcoat, to form a trough to catch any water that might drip from the fuel tank shelf. If the aft cabin area still gets wet, but no water is captured in the taped trough, then you have ruled out tank condensation.

At the same time you could also make a similar trough on the aft bulkhead beneath the binnacle cover. Then spray the heck out of the binnacle and see if water is captured in that trough.

Hi, Martin.

Do you have a refrigerator running? Is the compressor in the aft area (as is common)? Is it venting moisture properly?

Captain Kratz: You don't offer us the year of your boat but I mine, Scot Fre, is a 1994 and I had a leak in the aft that would wet the underside of the cushions an rarely the top side. It took a while to chase it down but I found my leak in the genoa cart track on the deck where one of the screws lost its seal. Don't ask about the track of how the water ran back underneath the aft cabin cushions because I think it defied some rules of phyics. LOL

Sure.

Just FYI, I just discovered that my refrigerator line from the compressor to the box is covered with ice! ...and is dripping water into the aft cabin, or at least into its bilge area of the aft cabin. Don't know why yet. Posting a new thread on that problem, but wanted to let people know on this thread in case they have the same moisture issue, to look for that.

Oh, one other thing you can do: Run your water hose full blast onto every suspect area. The idea is to try to simulate a heavy downpour of rain. Have someone watch for drips in the various sections of the aft cabin (underneath the binnacle, tracks, cleats, winches, and so on).

That could help you localize the problem quicker, coz you don't have to wait for a rainstorm.