Looking at the mast and boom on Peter's boat yesterday the topic of span efficiency came up.
Also when I was working on the parameters for Gardners schooner rig to plug into the VPP I had to take at stab at the span efficiiency of his proposed sail plan.
This is something that I have discussed with respect to rudders and daggers but I don't think it has been discussed with sails. Although I have referred to aspect ratio rather than span efficiency. In effect the induced drag is halved if the blade is effectively end plated against the hull or if a surface piecing blade is at high speed and non-ventilating. End plating is equivalent to doubling the aspect ratio or span efficiency is 2.
Anyone who has sailed on a sloop rigged keel boat knows how the pointing ability improves when the genoa is sheeted down against the deck. This increases the span efficiency of the rig. There is little or no air spilling under the sail.
Another good example is Macquarie innovation. They went for a low aspect wing but were able to improve the span efficiency by setting it close to the water:
Wing end plating is something I proposed for my beach proa. It can also be achieved with the schooner rig Gardner is proposing.
The sail efficiency is most important when sailing close to the wind and in these angles just the lw hull provides reasonable end plating.
Gardner - For the VPP I produced I selected a span efficiency of 1.3 based on what I have read about bi-planes. If you "end plate" at the bottom the span efficiency lifts to 2.
There is some work being done with winggrid winglets that get span efficiencies above 2:
Rick Willoughby
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