“End-Plating” the Main and Mizzen

Steve Dashew says: In the 1960s and 70s we addressed this issue on our racing catamarans with deck-sweeping mainsails. We found a good seal was worth at least five degrees in tacking angle with no loss in boat speed. If you lifted the seal just six inches, this advantage would be lost.

http://setsail.com/end-plating-main-and-mizzen/

Any thoughts for a monohull?

 

Here's some thoughts on "endplate effect" from the sail aerodynamics thread:
http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-4.html#post23118 posts 54,55,56.

 

Decksweepers were in vogue for a time as you mentioned. Decksweeping rigs turned out to be a bit much for crew movement and safety. There were visibility constraints too. Shelves sewn to the foot of sails were also popular for a while. Shelves lost their popularity when some of the class boats began to include the shelf in sail measurement which killed the whole deal. More than one Americas Cup boats tried wide flat booms with sail slides attached to the foot of the sail and tracks on the boom. These were called Park Avenue booms.

 

See also here, Tspeer's homepage. Select optimum planform.

tspeer.com

- an excerpt -

Minimum Drag Sail Rigs. For a given span, the minimum drag sail rig has the spanloading which results from a uniform downwash distribution. Figure 4 shows these minimum drag span loadings for gaps between foot and surface that range up to 20% of the span. For zero gap the result is the classical semi-ellipse. Even for very small gaps, however, the optimal loading is quite different. The spanload distribution with gap starts out somewhat egg-shaped, with the point of maximum loading near 40% of the length of the surface. The loading rapidly approaches a full ellipse as the gap is increased.

 

The problem is in comparing sail efficiency as measured by a rule and measuring actual sail area. They are usually different.

 

Of course, you are not only plating the sail, you are also plating the hull- and the hull can provide substantial lift going to windward. I have read windtunnel reports where 14% of the driving force was attributed to the hull when a deck sweeping jib was used. It wasn't clear, however, weather they figured the improved jib performance, or just assigned all added performance (with hull vs no hull) to the hull.