Couple:
Why paint only the top half black? surely you'd want as much heat in there
as possible.

Most fabric doesn't conduct heat very well.

If you have an alloy mast you have a chimny already, thermal coefficient is
good (probably don't have to paint it), large surface area , height is also
a positive factor (velocity). Only mod's req'd are to connect the bottom to
the cabin airspace and the top to "outside" without letting rain in.

Could go further if you don't mind a little extra windage up the fit a
weathercocking venturi at the masthead, any lurking breeze will add to the
effect, although it's probably something you'd only consider if performance
at anchor is more of a concern than racing.....

Mark.

"Parallax" wrote in message
om...

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.

Any thoughts?