The Spitfire sail

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.

The “Spitfire Junk” sail plan, based on research by C A Marchaj, which showed that elliptical sails generate more lift, especially at low aspect ratios.

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.


Fantail even resembles it.

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15 Comments

Filed under A New Rig, 2017-2018, sail

15 responses to “The Spitfire sail

  1. stuart

    As an avid follower of all things ‘junk’ and find your adventures and experiments most interesting, did you see Chris Galliene’s proposed split junk rig with a similar elliptical planform

    • Thank you, I had not seen that. I was also wondering about having all the battens parallel like that, but I thought it might be hard to arrange the yard high enough on Tammy. I’ll write to Chris.

  2. It turns out I’m not the first to look at the Spitfire wing. Chris Gallienne pointed me at this thread: http://www.junkrigassociation.org/technical_forum/4293731

  3. Antoine Maartens

    Additional requirement: capability to heave to in any weather below force 8. What do you think?

    • I don’t really believe in the junk sloop “heave to”. Unlike the Bermudan, there no dynamic balance between the sails and rudder that keeps the boat in position. At least, I’ve not found one! What we lose there we gain in reefing ability.

      When I was stuck off Wells-next-the-Sea in the remnants of Hurricane Bertha, desperate for sleep, with no windvane, I pulled the sail in tight and used a bungee on the tiller to keep Tammy just off the wind. She made about 2kt at 90° to the wind and that allowed me to rest. Almost as good as a bermudan heave-to.

      • Antoine Maartens

        Sounds familiar!!

      • Antoine Maartens

        I used a sheet to tiller arrangement yesterday. Worked pretty good hard on the wind. I’ll try oversheeting with that line and securing the helm in the lee. See what happens.

  4. Antoine Maartens

    Regarding the balance problems: I’m thinking about moving the engine to the stern and closing the gap in the bottom. That should balance things out and then the new sail resolve the remainder of our problem.

    • I haven’t done proper experiments with the engine lifted, but I was sailing around without the engine for a while earlier this year and didn’t notice any difference. I’m not convinced it is significant. Sail shape is the key I think.

      I’ll be very interested if you go with the aero junk design. Perhaps I can visit and help construct it.

  5. Yes, that might be so. It’s on my (long) list to make a cover for the hole and see what difference it makes. Even a board held with duct tape for a day would be a good experiment.

  6. Antoine Maartens

    Currently I’m sailing to the kinselmeer on the starboard tag with 45 degrees of rudder in put. With the breaks on so to speak. This has to end.

    • At least you are sailing! I am stuck at home. Have fun!

      • Antoine Maartens

        Maybe start same drawings for the aerorig. Widest part of the wishbote has to be at least 15 cm to clear the top of the most drop installation / sleeve.

        Would help me a lot.

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