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Thread: Roach Curves on Sails

  1. #1

    Default Roach Curves on Sails

    What is the basic design attitude toward drawing or cutting the curve at the top of the roach? Is it just simply circular? Maybe hyperbolic?

    thx
    Sail honest.
    Rock music.

  2. #2
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    Which class?
    Muzza

  3. #3
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    footy i would guess...

    as for the roach, it depends, do you want a roach like on a AC boat? do you want one like a 12 metres? or do you want one like a planing dinghy? i am going with a hybrid between the planing dinghy and the AC boat... for me, the roach goes almost straight up until about 60% up the main, and then it makes almost like the curve of an inside out exponential growth graph... but, my best advice would be to go to sailinganarchy.com or xsracing.net [careful, you may get hooked and have to start checking those sites everyday...] and look at some roach/leech curves... when you find one you like, mercilessly copy it!
    the pessimist complains about the wind, the optimist expects it to change, the realist trims the sails.

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    builders of Footy hulls

  4. #4
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    So what’s your objective? Good looks? Good performance?

    In theory and in practice, the use of the large roach and square head mainsails we see on some boats these days are more efficient that their triangular predecessors. The potential is there for a higher aspect ratio for a given mast height, more power and less induced drag.

    In the absence of rule restrictions, we were in the past limited in our ability to achieve such benefits because of the sail (and batten) materials available, and their ability or otherwise to hold shape without excessive weight penalties. This has changed now.

    Whether or not benefits would accrue in practice in our RC boats - I don’t know. Because of the low wind speeds we sail in (on average) and comparatively wide difference in apparent wind angles/speeds we experience between the deck and the masthead (compared to full sized boats), we need more twist in our mainsails than the big boats. The very limited ability to adjust mainsail twist on the water (as compared to a crew-on-board boat) will, I suspect, mean that a square headed mainsail would often be operating at less than optimal trim on an RC boat. Unfortunately, in the classes I sail at present, I can’t try it – as the class rules in each case impose restrictions on roach size/shape.

    In those classes where it is possible, wouldn’t it be interesting to take two identical boats and run a series of experiments with different sailplans?

    Once again, I need a big R&D budget and a sponsor with deep pockets.
    Muzza

  5. #5
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    I think the reason for the high curved roach is "wind gradient". This shape puts more sail area up where the wind is stronger.

    I think that there is a greater percentage change in wind strength per unit of distance, closer to the surface.

    If this is correct then a high roach would be even more beneficial on a model than on a real boat, as there is a steeper gradient, lower down.
    John Ball
    CRYA #895
    IOM CAN 307 V8

  6. #6
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    Hi John,

    In theory yes - but for exactly the same reasons as you point out, the variation in apparent wind angles down low (where our boats sail) both on average up the height of the rig and in terms of extreme differences between foot and head, are all the greater. We want to have all of the sail (or wing - because a high aspect square head mainsail is much closer to a wing than the other extreme - a roachless mainsail) set at the correct angle of attack - and that angle of attack changes as we move up the mast far more radically on a short low rig (such as ours) than on a tall fast rig (such as a 60 foot tri, or a VOR boat etc). Where the sail area is spread fairly evenly up the sail, this becomes more crucial.

    On a big boat we manage this by a combination of traveller, mainsheet, boom vang, and the ability to vary mast bend as we go. On most (but not all) RC boats we are restricted to mainsheet only once we leave the beach.

    Hence my thinking is that the potential gains of such a sail on our boats may be lost by the inability to continually trim it for best effect. It's just a hunch - I'd certainly like to experiment with it.

    One type of sailing craft that has used such sail shapeas for years is windsurfers. Of course, like us, their rigs are almost set-and-forget. They approach the issue by matching the flexibility of their masts to the sails, and seek to get the correct bend in the mast (with corresponding effect on the leech) as forces increase. Perhaps this offers a clue as to how we might approach the problem in RC sailboats.
    Muzza

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