Spent a day working on my boat at the dock her in NW Florida and
nearly died of heat stroke while being down in the cabin even with all
hatches and vents open. I dont have too many fans because I have no
connection for shore power. There was an obvious temp differential
'tween outside and inside that got me thinking.

So..... Make a long fabric tube, say 10" diameter and 15' long with
upper end held up with the jib halyard and lower end in the forward
hatch with edges around it sealed. Upper half of the tube is black
and lower half is white. Here is what happens:

With a temp differential (hot inside, cooler outsside), you get a
chimney effect with hot air rising out of the tube and pulling cooler
air in from other ports. I can imagine getting a susbtantial flow
this way.

With little or no temp differential, sun still heats the black upper
half and heats air inside causing it to rise causing air to be pulled
through the tube giving air flow in the cabin. Steps could be taken
to maximize solar heating of teh upper part of the air column such as
better conducting baffles, passive solar fabric panels connected to
the upper tube etc. I could even imagine a fabric reflector shining
light onto the upper tube to maximize the effect.

All of this works compared to not working in a house because the
volume of the boat is fairly small compared to the volume of a house.

Let's go further. Now that we can produce air flow, can we use such
to actually produce cooling? Evaporation of water can produce a huge
amount of cooling. Close all vents to the outside except for a 10" or
so port and fill it with wetted baffles. Baffles can be wetted by
osmosis from a reservoir. Evaporation will cool the air stream.
Unfortunately, this will increase the humidity in the boat and reduce
the air flow (conservation of energy).

Any thoughts?