sails vs wingsails

[rlawler]

I am wondering about the difference in lift between normal sails vs wing sails. If we make some simplifying assumptions like:
both types of sail have the same shape, lets say a high aspect ratio of about 4.
The traditional soft sail can have as many of the modern "high tech" improvements as you want (battens, "wing" mast, etc).
The wing sail can have any airfoil shape you want, but to keep things equal we'll say it must be made of the same fabric as the soft sail, yet it just somehow is "magically" rigid. Oh, and it can also magically change its airfoil shape to the mirror image when you tack or jibe.
My question is:
Roughly how lift will the wing sail generate compared to the traditional soft sail?
And would the difference in lift be more drastic for certain points of sail?
Thanks,
Rick

[tspeer]

The differences between rigid wingsails and wingmast/sail combinations are mostly in the things you have ruled out.

The slotted flap of a rigid wingsail allows it to achieve a higher sectional lift coefficient than most soft sails. However, soft sails could do better if they could achieve a particular shape, especially if that shape wasn't constrained by having to be a membrane tension structure. The maximum lift has been important in the C-class catamarans, because their area limit makes it hard for them to be fully powered up on the runs.

The control system of a rigid wing allows the wing to be taller without experiencing excessive loads due to leech tension. I believe this was the main advantage of the wing on USA 17 over its soft rig. The control system also makes it possible to control the shape, such as twist and camber, in a more extensive and precise way than for a soft sail. There's no reason why a similar control system couldn't be developed for a soft sail.

There's no reason for the wing to use the same fabric as the soft sail, because the loads are taken by the ribs and spar instead of the fabric itself. Plus, there are two sides to a wing, so that would double the amount of fabric used. If the wing has the same fabric as the sail, it will be much heavier - and far heavier than it needs to be. The wing on USA 17 weighed the same as the mast, boom, mainsail and jib of the soft sail rig. So it was heavier when a headsail was added to the wingsail, but comparable in weight when sailing wing alone.

If the boat is limited by its heeling moment, then the soft sail and rigid wingsail will be trimmed to produce the same lift. USA 17 went to the wing in order to produce that heeling moment in lighter wind.

There's nothing magical about a rigid wingsail. The soft sail rig on USA 17 was faster than the wingsail in some conditions. Whether a wing or soft rig will be faster depends on the class constraints and the sailing conditions.

[redreuben]

A guy I used to work with (early 90's) built an 18ft skiff rig which used a very large diameter mast (tapered) about 150mm diameter in roughly a D shape with the flat facing aft, it had a sail track on either side and 2 lightweight sails which where controlled by mast rotation, outhaul and mainsheet. I believe the outhaul allowed the two sails to move independently to the ideal camber. I also recall that the sails were cut dead flat. It killed a comparable softsail rig around a windward course. He patented the idea and even got it on a tv show called "Beyond 2000" and tried to sell it to everything from container ships down but such is the conservatism of the yachting community he had no luck, the patent has probably expired by now.
I may not be 100% with my description as at the time I was new to sailing but you get the idea.
Food for thought.
RR