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I do not store my boat covered and its stored on the trailer outside.

I added a solar fan to the forward hatch and its made a major difference.
It dies not move much air, but the constant flow when it has sun has made a big difference for meI have it blowing out so it pulls air in through the gaps around the companionway cover.

I am pondering adding more but am leary of them being a trip hazard anyplace else.
Maybe into the pop top, or cabin top. Right above the potti could be nice.

I put a solar fan with battery through the window of the head. The windows are relatively inexpensive and easy to replace if needed.

I live around Galveston TX...so same high humidity and mildew growth in any closed boat. I store it covered and closed...if I left the companionway open, it would be open invitation for wasps, rats, stray cats, and god-only-knows what else!

I run a fan 24/7 and a dehumidifier (very small) which I have drilled a hole in to drain the condensate into the sink and therefore overboard. I've left my marina boat this way for 4 months once. As long as you have 120v shore power...it seems like a good solution to me.

Link to my dehumidifier: