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New (cambered) sail for Footprints

  • 13 Apr 2012 15:03
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    Reply # 889029 on 829776
    Deleted user
    I think in Footprint's case, the batten camber was greater than the sail camber. Thus panel shape was kaput and max camber moved to 50%, on a rig with barely enough lead. So  by that theory, when all the battens are replaced, there should be a big improvement.
    Last modified: 13 Apr 2012 15:10 | Deleted user
  • 12 Apr 2012 22:55
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    Reply # 888329 on 829776
    Anonymous member (Administrator)

    Stavanger, Thursday

    Don’t ask me where the correct position of the max camber point is. My real world problem is that there is a mast somewhere near the luff. To avoid distorting the camber near its max point, I either have to move the mast forward (around 10% from the luff) or I must move the max camber point aft. My tactics have mainly been to use minimum balance (10% position) and the max camber point somewhere between 35-38% from the luff. What is the "correct" position from a scientist’s point of view is of little interest to me, as long as they haven’t reckoned with the mast (on either tack).

    However, what I know from experience is that the actual centre of pressure (CP) moves fore or aft with the position of the max camber point: A sail with max camber point 50% from the luff gives more weather helm than a sail with the max camber point 35% from the luff.

    Btw. the yard angle is very much a function of the sail’s balance - little balance leads to a high-peaking yard and vice versa.

    Arne

    PS: Somehow this reminds me of what my teacher of radio antenna theory once said: " Let’s assume that the earth is flat and copper-plated".

    Last modified: 13 Apr 2012 17:59 | Anonymous member (Administrator)
  • 12 Apr 2012 21:10
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    Reply # 888217 on 829776
    Anonymous member
    You can also effectively move the balance further forward by moving the position of maxim camber further forward. Most cambered sails have (I believe) been built with max camber at 40%, a conservative position (but safe). I think one could quite safely move it to 35% but more than that may make it difficult to find the "grove" when going to the windward. Sleeve, what's your experience with this?

    For LC where it's been a struggle to get the balance as far forward as I'd like, I'm hedging my bets and using 37.5% as my maxim camber position (I think Arne has done that as well).
  • 12 Apr 2012 05:34
    Quote
    Reply # 887500 on 829776
    Anonymous member
    I think our experiences are demonstrating that the more camber there is, the more lead there should be. For an amount of camber similar to that in a bermudan mainsail, the lead should probably be about the same as for a bermudan sloop. That doesn't help much when putting a new sail onto an existing mast, though.
    Today, I was looking at the plans for Shoehorn, the 26ft kid sister to Footprints. The mast is at about 23% of the waterline aft, and the sail that Gary Underwood has drawn is quite wide and low. I would say this mast is too far aft, and it's probable that Footprints' is too. For comparison, Tystie's mast is at 12.5% of the waterline aft, and rakes forward 2 degrees.
    However, we put 9% lead on Fantail ( the bermudan sailplan had 20%) and she is very well balanced. Just shows that there are no firm answers, and calculation only takes you part of the way.
  • 12 Apr 2012 02:26
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    Reply # 887355 on 829776
    Deleted user

    I thought it time to provide an update on the new sail for Footprints for those of you who have been following this project. It has been several weeks of sail trials and not a few tribulations but happy to report now that we are finally getting somewhere. I am not sure whether it is fortunate or unfortunate that our first experiences with the new sail have been  in very blustery conditions. This highlighted very quickly that my cedar and glass battens which were ideal for the old sail were hopelessly inadequate for the higher loads of the new camber panel sail. In fact I did not know that wood could bend so far without breaking. Of course the bend in the battens meant that the sail could not develop and effective shape. So began the hunt for batten options. After considering reinforcement of the existing battens with either more unidirectional fiberglass, or carbon unidirectionals I quickly dismissed this option, partly because it was unknown just how much fiberglass, and the resulting weight would need to be added. On David Tylers suggestion I have made up 3 new trial battens from 65mm x 1.6mm wall T5 alloy tube. To make up the extra length each batten has a 1.5 metre cedar extension of the same diameter as the alloy tube. Happily these battens have proven to be a success still being fairly light but very stiff. 

    So I now had good sail shape so every thing should work fine. Not so - for some strange reason Footprints was actually slower to windward than with the previous flat sail. My feeling was that there was way too much sail aft of the mast with everything loading up but no real drive. David volunteered to come out for a sail with me yesterday to try and sort things out. It was once again blustery conditions and with the same result, no power to windward. On Davids suggestion we tilted the sail well forward on the mast to what was almost the designed reaching/downwind position of the sail. This resulted in a dramatic increase in performance. I had known from the old sail that Footprints sailed best with at least a metre of sail ahead of the mast. So based on yesterdays success I think that we are finally getting there.  I need to do a lot more sailing yet in a variety of conditions and I still need to sort out the tuning of the sail, and replace the remaining timber battens with the new alloy battens. But so far I now seem to have the increase in performance that I was looking for; the sail is a lot lighter and easier to pull up; and it looks good judging by the number of cameras that have been pointed at Footprints while she is out sailing.

    This project is ongoing so stay tuned.

     
  • 07 Apr 2012 09:28
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    Reply # 884144 on 883900
    Anonymous member
    Gary King wrote:
    David Tyler wrote:,
    There's no bolt rope anywhere in the sail. 
    Must be fine with Dacron, but would it work with Oddyssey III sails? Which must be stretchier stuff than any real sailcloth. 
    I'm thinking seatbelt webbing around the sail will be the way to go in my case.
    Yes, it would work. Sailmaking practice is just as valid for Odyssey III as it is for Dacron, in fact more so. Think back to the days of cotton sails - very stretchy stuff. 
    The important point is to match the characteristics of two things that are being sewn together. If the leech edging material is less stretchy than the sail, a "bag" will develop just forward of the leech, a great way to increase drag. If the edging material is more stretchy than the sail, the leech will go slack and flutter ( a not-very-satisfactory fix for this is a leechline built into the edge). I'm not saying that webbing can't be used, just that it's more difficult to get the tension right and get a good result than it is just to cut a strip of cloth, fold it in two, tape it in place and sew it on - the way sailmakers do it. 
    Where it is a good plan to use a short piece of webbing is in way of the sheet spans, to take wear and tear.
  • 07 Apr 2012 04:37
    Quote
    Reply # 884022 on 883900
    Anonymous member
    Gary King wrote:
    David Tyler wrote:,
    There's no bolt rope anywhere in the sail. 
    Must be fine with Dacron, but would it work with Oddyssey III sails? Which must be stretchier stuff than any real sailcloth. 
    I'm thinking seatbelt webbing around the sail will be the way to go in my case.

    I did two layers, one each side on mine.
  • 07 Apr 2012 01:02
    Quote
    Reply # 883900 on 883695
    Deleted user
    David Tyler wrote:,
    There's no bolt rope anywhere in the sail. 
    Must be fine with Dacron, but would it work with Oddyssey III sails? Which must be stretchier stuff than any real sailcloth. 
    I'm thinking seatbelt webbing around the sail will be the way to go in my case.
    Last modified: 07 Apr 2012 01:18 | Deleted user
  • 06 Apr 2012 19:27
    Quote
    Reply # 883695 on 883659
    Anonymous member
    Gary King wrote:I didn't see this anywhere David, but what did you use for bolt rope in the 2 sails you made? Webbing or rope? I'm wondering if you sewed the yard sleeve over the rope or on the webbing or left them out altogether on the head.
    Gary,
    There's no bolt rope anywhere in the sail. We used standard sailcloth, and so standard sailmaking methods are OK. The leech and luff have a tabling, which is a piece ofsailcloth doubled over the edge of the panel. In this case, for a large sail, we cut a width of cloth into four strips, 230mm wide, and folded them with 150mm on one side, 80mm on the other, This is far more than is usually done on a bermudan sail, but I like to have some weight near the edges of the sail.
    The yard sleeve is essentially just a part of the top panel, and is sewn with a normal lapped seam to its top edge. We put webbing reinforcements around the window for the halyard, added some patches at the peak to spread the load, and added webbing inside the sleeve at luff and leech where it will be chafing against the yard under pressure.
  • 06 Apr 2012 18:46
    Quote
    Reply # 883659 on 829776
    Deleted user
    I didn't see this anywhere David, but what did you use for bolt rope in the 2 sails you made? Webbing or rope? I'm wondering if you sewed the yard sleeve over the rope or on the webbing or left them out altogether on the head.
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