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Proposal for Variable Camber

  • 22 Nov 2013 23:30
    Quote
    Reply # 1444388 on 1441932
    Anonymous member
    Hi Kurt,

    I just reread your article on the merits of flat-cut sails (awesome!)...

    Did you use acrylic, or other stretchy fabric for your sails? If so, do you feel that its inherent camber contributed to your performance? If you were building again, would you consider stretchy fabric?

    Dave Z
  • 22 Nov 2013 23:15
    Quote
    Reply # 1444381 on 1443172
    Anonymous member
    Mark Thomasson wrote:

    Dear Dave

    if I can draw on a miss-spent youth of windsurfing - where the power of the sail is immediately felt:

    - Blocking the gap between the base of the sail and the board was like turning on a turbo.  I see you have a big gap between boom and deck, so suggest try fitting a 'water sail'  for light winds. 


    Thanks for the suggestion, Mark, that would be a sight easier than flying another shaped sail (such as a genoa)! Be an easy way to experiment with a cambered panel, too!

    A question for more experienced junkies -  is your crab-claw top panel a benefit.  Experiment with a 'normal'  triangle, it would give more area.  The outcome of interest to us all. 


    SLACKTIDE's CC top panel, its shape explicitly chosen to approximate certain traditional CC sails, has worked very well for us (can see it on our icon... all our sails have been flat-cut polyester). 

    LUNA, under  two, H&M near-standard upper panels (topmost batten unsheeted, slightly lower than standard yard), had a tendency to overpower the rudder and round us up. Her underbody was about .3M draft with full rocker and leeboardy off-centerboards... tracks very well until overpowered. We felt there was too much area left (anyway) and the CE of each sail was too far aft/outboard when deep-reefed.

    SLACKTIDE has a better tracking hull, and the deep hollowed leech in the single unsheeted upper panel moves the CE forward/inboard. Hard to separate out which contributes how much, but the round-ups are gone. Off the wind, the small panels move us right along, while feeling comfortably in hand.

    Both sails were able to drive us to windward in sustained 45kts, albeit very slowly (made good) and at wide angles. This was especially surprising under the CC, since the box barge hull isn't known for its windward efficiency. The combination of flat-cut JR and box barge hull get us around surprisingly well, though we dream of faster on the wind.

    Dave Z
    Last modified: 22 Nov 2013 23:18 | Anonymous member
  • 22 Nov 2013 22:06
    Quote
    Reply # 1444361 on 1441932
    Anonymous member
    Thanks for all the useful feedback, folks.

    So... it sounds as though too much camber is not a problem, sailing-wise, even deep-reefed in heavy going?

    It's possible I started this under a misapprehension:

    In shaped, western sails (shaped for average conditions), it's generally accepted that, as wind strength increases, the point of maximum camber moves aft, rotating the sail's drive vector toward the lee. Sails are flattened via sail controls (principally the clew-outhaul to reduce that vector, whose new orientation is dumping more power into heeling moment.

    Without shaped camber, JR has the reputation of poor-to-middling performance in lighter winds, but fair-to-middling performance when it picks up. Presumably since it's airfoil is already flattened. Our own experience (flat-cut JR with low stretch fabric) confirms this.

    What got me thinking on this is the assumption that cambered panels would develop the same effect as shaped western sails, but without a means to flatten. Thus it seemed as though changing camber to match different average conditions might be beneficial, especially for those for whom seasonal averages differ widely.

    From what I'm hearing, though (even from those with high cambered all the way up), that vector rounding effect (if present at all) is negligible in JR?

    Dave Z
  • 22 Nov 2013 06:00
    Quote
    Reply # 1443898 on 1441932
    Anonymous member
    I would tend to look to variable battens, in flat sails, that bend-and-stop to a smooth curve but can be locked straight. I've sketched, but done no more. And sailed flat.
    Cheers,
    Kurt
  • 22 Nov 2013 02:18
    Quote
    Reply # 1443797 on 1441932
    Anonymous
    Just arrived in Opua, and finally got internet. Zebedee has now done almost 10,000 Miles with 10% cambered Arne type sails in the parallel panels down to 3% in the top panel. Been in 50 kts (other boats recorded the wind) .Initially I broke battens, in the Carribean, but none since. Arne's type of sails work! I'll write it up soon for the JRA.
  • 21 Nov 2013 20:43
    Quote
    Reply # 1443555 on 1441932
    Anonymous member (Administrator)

                                                           Stavanger, Thursday

    A while ago I also did some scribbling of sketches to find a way to flatten the sail panels under way. I soon gave it up, partly because all my solutions were so complicated (example) and partly because reefing is a much quicker and simpler way of taming the forces in rising winds.

    On sails that are hard to reef (Bm mainsails for instance), it makes sense (to me) to flatten the sail by bending the mast and hardening the clew outhaul.

     

    Arne

    Last modified: 13 Jan 2015 09:25 | Anonymous member (Administrator)
  • 21 Nov 2013 13:32
    Quote
    Reply # 1443172 on 1441932
    Anonymous member

    Dear Dave

    if I can draw on a miss-spent youth of windsurfing - where the power of the sail is immediately felt:

    - Blocking the gap between the base of the sail and the board was like turning on a turbo.  I see you have a big gap between boom and deck, so suggest try fitting a 'water sail'  for light winds. 

    - In strong winds, some shape is still beneficial upwind. (which triangular fanned panels give in any case)

    A question for more experienced junkies -  is your crab-claw top panel a benefit.  Experiment with a 'normal'  triangle, it would give more area.  The outcome of interest to us all. 

    Last modified: 21 Nov 2013 13:35 | Anonymous member
  • 21 Nov 2013 05:31
    Quote
    Reply # 1442908 on 1441932
    Anonymous member
    This is an interesting idea, Dave.
    • Fanned and triangular panels are better with little or no camber, so I don't think it applies there.
    • The  10% camber, reducing to maybe 4 - 6% as you go up the sail, that we're putting into parallelogram panels is a very modest amount compared with bermudan practice, and I don't see a need to reduce it for heavy weather.
    • Whenever I've run a small line through eyelets, I've had chafe problems. This would be a problem if you half-tighten a line for medium camber. I think it's either a tightly tied line for no camber, or no line for maximum camber.
    • These eyelets would be useful for something else - reefing out a damaged panel or a broken batten. They may pay back the labour of putting them in that way.
  • 21 Nov 2013 03:39
    Quote
    Reply # 1442848 on 1442260
    Anonymous member
    Slieve McGalliard wrote:Just another thought on the same idea. If you build a cambered panel sail using Arne’s methods you could possibly adjust the achieved camber by adjusting the tension along the battens if the material would allow some stretch.
    Hi Slieve,

    I'm sure you're right. Basically, the challenges don't seem much different than the sail controls on a western rig, abetted by actual elasticity.

    Seems similar methods could be useful along the leech of your forward, split-rig panels (BRILLIANT, BTW!) for fine-tuning their shape?

    Dave Z
  • 21 Nov 2013 03:32
    Quote
    Reply # 1442846 on 1441932
    Anonymous member
    Hi Folks,

    Thanks for all the feed-back!

    In terms of reefing, yes, it's already the tool we use, often and early (getting earlier). As we currently have flat-cut sails, we're 'storm ready'. Problem comes at the other end, obtaining more power on masts of given height. We've already maxed the area, given other considerations and are now mulling over more efficient cut and/or (fanned) geometry.

    For lighter winds and/or inshore use, I've read that several owners wish for more camber... not only deeper cut, but more panels involved. We certainly often wish for more drive, especially to windward in light winds.

    Reducing shaped camber from bottom to top, as Paul suggests, is the solution we've been considering for a general sail. But if there were a way to temporarily increase the power of the sail's upper panels...

    I see that I emphasized the proposal backwards... benefit (for us) lies in relaxing the lacing, introducing more camber.


    Questions:

    Of the cambered JR folks responding here, does anyone feel there are times they'd benefit from more camber? Less?

    Presumably one reefs a cambered sail earlier than a flat one (due to increased heeling moment)... is this true? If so, do you feel you're sailing better, even though reefed earlier? That is, does the remaining camber make up for loss of sail area?

    Dave Z


    PS. As far as the work involved goes, we've dismounted and reset our sails often enough, and that doesn't take long.

    If this worked, I'd be surprised if even slow lacing took much more than 15 min per sail. In theory, we'd make that back with interest in faster transits!  8)


    Last modified: 21 Nov 2013 03:43 | Anonymous member
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