I found out by the end of the summer that there is too much humidity in the shed and not enough ventilation. After watching the boat get attacked by mildew because I was not able to open the shed for weeks while we were traveling, I decided to install a ventilation system. There are a lot of ways to do this, no doubt, but my shed is not permanent so I would prefer to keep the expenses down. After considering a lot of options, to include building the vents myself, I decided to buy two vinyl vents from Lowes and install one in each end of the shed. They are 18 X 24 and cost about $35 each. I simply took down the gable hatches and framed in the appropriate space. I cut and rolled the plastic and stapled it to the frame, installed the vent, and reattached the wooden furring strips. The gables came with bug screens too.
I'll build a shelf extending out from the top of the header, inside the shed, above the big doors but below the transom hatch. Then I can secure a box fan in place and push air out one end thus pulling cooler/dryer air in the other end. This combined with keeping a fan running in the boat should keep the mildew at bay. What I like about this option, aside from the fact is was the most inexpensive way I could come up with, is that I can still open the transom hatches when I am working on the boat. I get the best of both worlds.
I think I may also need to lay some ground cloth out on the grass/dirt area between the plywood floor work areas on each end to act somewhat like a vapor barrier. I think that the moisture in the ground contributes to the humidity in the shed when the sun starts beating down.