Originally Posted by Petros
Racing sailboats have very costly shaped and pivoting masts so the mast does not blank off the sail as much (and with a fully contoured mast that is foil shaped, perhaps no lost sail area at all). It is less costly for the home builder to make an A-frame than contoured carbon graphite mast....
Rotating masts need not be made of carbon, nor more expensive than a homebuilt fixed mast. Gold Coast catamarans have done quite well with wooden rotating masts in charter service.
The sail area behind a mast is not as useless as it's being made out to be.
But even if it is, it performs a function - along with the jib - that is very important: reducing the drag of the mast. At the very least, the mainsail acts as a splitter plate that can significantly reduce the drag by preventing alternating vortices in the wake. The favorable pressure gradient leading to the slot results in reattachment of the boundary layer to the lee side of the mainsail, so there's not a big separated wake from the mast of a sloop rig.
The fact that the mast of a sloop is on the windward side of the jib means it sees an apparent wind speed that is significantly less than the freestream. Just stand next to the windward side of the jib when sailing to windward, and then go around to the leeward side and feel the difference in the local wind speeds. Since parasite drag scales with the square of the wind speed, it only takes a 30% reduction in the speed of the wind hitting the mast to cut the parasite drag of the mast in half.
You get none of these benefits with an A-frame mast. The mast is completely exposed. There's no splitter to prevent a Karman vortex wake. The influence of the circulation around the sails will be much weaker on the windward side, and increase the drag of the leg on the leeward side. You'd have to run the numbers to see which mast has less frontal area, but since most pictures of A frame rigs I've seen have additional struts, I'm betting the A-frame has more frontal area presented to the apparent wind.
I think there are a lot of virtues to the A frame mast, such as the ability to raise and lower it stably (still need a gin pole, though), and the use of roller furling sails for easy sail handling. But I've not yet seen a convincing case made for improved efficiency over a conventional sloop rig.