Sunbrella, and preferably Sunbrella Plus. Any acrylic canvas will
work, just folds up a lot nicer than a vinyl or other plastics do.
Whoever makes the cover should put nice big wear patches anywhere the
cover might rub.
I like to make big covers like this in three parts...one piece from
mast forward, one piece from mast to topping lift on boom, and one piece
from topping lift to transom. It makes storage and handling of the
covers a LOT easier. I sew the zips in so I have a full 2" flap of
canvas over the zip, and the flap faces aft, so if the boat sits on a
mooring the wind will not blow it up. Also put press studs at the ends
of teh 2" flaps over the zips to take the load off the sip ends. Zips
will last for many years if sewn in this way.
I also sew a 1" webbing in a line direct from the mast to the bow
roller or similar. This hooks around the mast, and is buckled to the bow
roller or fitting. It takes the load off the canvas, and presents a nice
strong ridge to the wind, which helps cover life by reducing chafe. I
also slit up greenhide leather into 1" strips, and use it to bind
cutouts around mast, stats etc. It takes extra time to do things like
this, but it makes all the difference.
I made a cover for a 40' timber IOR yacht a couple of years ago,
covered the entire boat to the waterline. Had to be easy to put on and
off on a mooring. That cover has taken several 40+kt storms and has no
damage at all apart from some slight chafe, which is to be expected with
large covers.
I don't have any photos of it, but if you are in no rush, I could take
some within the next month or so..if the boat is still there.

Andrew.

Courtney Thomas wrote:

What is the best material for fabricating a 'year round' boat cover for
an unsheltered 40'fiberglass sailboat that is hauled or moored in the
South ?

Durability, maximal UV resistance and breathability are among qualities
sought.

Suggestions/observations/experience regarding it's design and
implementation are also appreciated.

--
Courtney Thomas
S/V Mutiny
lying Oriental, NC