Revisiting aerodynamics...
I ran across the pro/con discussion by Brian and Tom quite some time ago, and recently the mast aft rig has popped back into my mind. I have some ideas that I have not seen mentioned before, and I thought I'd bring them up and let Brian and Tom have at them.
To summarize, mast aft advantages:
Staysails have less turbulence than a mast, and so generate more lift.
Staysails on parallel stays will provide good slotted wing performance.
Staysails are easy to roller furl, allowing continuously variable reefing.
Mast aft disadvantages:
Bare mast has lots of drag, not practical to streamline for all points of sail.
Forward inclined mast generates huge backstay tension.
So first let me address the drag issue. Why not wrap the mast in a rigid wingsail with a short chord, maybe a NACA 0012. Allow the sail to rotate about the mast 360 degrees, and balance it so that it points into the wind. This will significantly reduce the drag of the mast on any point of sail, and the rigid wingsail won't flutter and thus shouldn't need to be reefed, as feathered it will produce less resistance than a typical mast with a reefed or stowed mainsail.
Next, on stay tension, what if you were to use an aft rake rather than a forward rake? This would put the base of the mast near amidships rather than near the stern, but it would equalize the forestay and backstay tension. It moves the COE significantly forward, but with Brian's "mizzen staysail" you could correct for that. The resulting rig is a hybrid of the cutter (which puts the mast pretty far aft to begin with) and Brian's mast aft rig, with it's mizzen staysail. Unlike a traditional cutter, the mainsail is a wingsail, and thus doesn't have the "shadow" of the mast to contend with. Since rigid windsails need to be super-high aspect ratio to be light, the mizzen staysail takes up the slack behind the mast, providing sail area lost by the high aspect wingsail.
The disadvantage of this setup is that the mast, rather than being out of the way at the aft of the cockpit, is back near the center, but by basing it on a structural arch that formed part of the cabin roof, then it could be worked around. The advantages seem to be pretty good:
1. The three primary sails (jib, fore staysail, and mizzen staysail) are all fairly high aspect ratio sails that can be quickly roller furled
2. The foresails can interact with each other, and when close hauled with the rigid wingsail as well
3. The rig tensions shouldn't be significantly greater than a standard sloop or cutter rig
4. The rig can be sailed with one sail (fore staysail, fethered wingsail), two (jib and mizzen staysail), three (all staysails), or all four sails, if the wingsail is sheeted in.
Despite the apparently complexity of the 4-sail rig, it should be fairly easy to handle. The wingsail can always be left unsheeted during tacking and jibing, and the mizzen and fore staysails can be provided with booms to make them self taking; the jib can overlap, and be furled partially or fully during the tack to keep it out of the way. For jibing, all sails can be quickly furled, the wingsail can rotate forward of the mast, and the sails can be quickly redeployed after the jibe.
Any comments? It seems like this hybrid addresses Tom's issues about aerodynamics and preserves most of the advantages of Brian's mast aft rig. If anyone sees any holes in the idea, please point them out, and I'll see if I can address them. I'll also work on a sketch of the concept; I think the wingsail might have to be split at the level of the spreaders/inner forestay, and it might be the case that the upper portion is independent of the lower, but so far that's the only part of the rig that's not fairly straightforward.
--scot
