I have a small boat with a cabin sitting in a yard with no mains power
available. The inside is quite damp - if I had power I'd slap a
dehumidifier in there like a shot. The next best thing I can think of is
to spread lots of silica gel in trays or big fabric bags inside.

Is this sensible? The inside space is probably between 10 and 20 cubic
metres at a guess, and is fairly well sealed except when I'm there. I
can visit as often as necessary to swap the silica and dry it in the
oven at home. I don't really know how much I'd need.

Any pointers to a cheap source of bulk silica gel?

Cheers,

Pete