Once known as Hartley18
Join Date: Aug 2007
Location: Melbourne, Australia
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I'm no mechanical engineer, but it sounds like your de-humidifier can't dry things out much less than 30% RH no matter how hard it tries.
As water is taken out of the air, past a certain point it takes more energy to take more water out then the thing can deliver, so at or around 30% RH, the thing will run happily but not do anything (no drips from the outlet).
Barring other electrical problems (corrosion, fire, etc.) you should be fine. I'd be more concerned with overheating if you're runing it constantly and it happens to be too big for the cabin space.
The manual "should" give you a minimum room volume and minimum RH (assuming it's not made in Asia).
A bad day on a boat beats a good day in the office