I’ve spent most of my limited wakeful time in the past few days reading books by Czesław Antony Marchaj:
- Seaworthiness: the forgotten factor
- Sail performance: techniques to maximize sail power
- Aero-hydrodynamics of sailing.
These are all fantastic books. Not only is Marchaj one of the few writers to present actual hard evidence for his assertions, he writes with a dry wit that implies a great deal of criticism for the rumours and fashions that dominate sailing design. (The junk rig community seems somewhat exceptional in this regard.)
He presents evidence that elliptical foils produce more lift than rectangular or triangular, especially at low aspect ratios. (I’m summarizing here.) This is rather interesting given my earlier drawing of an enlarged sail for Tammy Norie. I think this evidence gives some clues to why the top triangle on junk rig sails is important: it approximates a curved top.
With this in mind I drew this sail plan based on the elliptical Spitfire wing.
This plan is based on 4.5m battens, but has a 3.2m second batten and a yard of only 1.9m at a steep angle of 65°. The yard forms the leading edge of the approximately elliptical wingtip.
Given Marchaj’s other results showing the critical nature of the leading edge of foils, the yard shape could be quite critical, but fortunately this suggests shaping it in a way similar to that suggested by Hasler and McLeod to provide strength. In this case it would be arched to fill in the ellipse and have a thin top.
If this plan works, the centre of lift will be shifted forward considerably, hopefully correcting the Coromandel’s balance problems.
I also can’t believe it’s a coincidence that Paul McKay’s Aerojunk looks like a Spitfire wing.