Creating camber in the panels, with or without broadseams.

  • 23 Oct 2016 09:02
    Reply # 4328143 on 4327812
    Arne Kverneland wrote:

    David,

    it seems that my head needs to be rewired to fully grasp the 3D idea. What I am rather looking for is a practical rule of thumb, either how much they should slacken the sail along the battens or how much length I should remove by using tucks/broadseams (in percent of B)

    Any suggestions?

    Sorry, I can't help with this. I have no experience with the first, and with the second, I'm reminded of the old joke about the Irishman who was asked for road directions: "I wouldn't start from here, if I were you".
  • 23 Oct 2016 06:36
    Reply # 4328067 on 4322040

    It seems to me that using tucks, rather than broadseams, will create more of a 'box section.  With tucks, the full amount of sail material is available virtually right up to the head and foot of the panel, creating more fullness in those areas. With broadseams, the sail material contained within the sewn seam is lost for ever.

  • 22 Oct 2016 23:19
    Reply # 4327812 on 4322040
    Anonymous member (Administrator)

    David,

    it seems that my head needs to be rewired to fully grasp the 3D idea. What I am rather looking for is a practical rule of thumb, either how much they should slacken the sail along the battens or how much length I should remove by using tucks/broadseams (in percent of B)

    Any suggestions?

    It appears to me that I somehow got the tension very close to right on Frøken Sørensen. Here, the sail inflates nicely in light winds and the vertical curve is still very close the trapeze, or even rectangle shape. See below.

    Arne

    PS: Thanks for the 80/20 rule. I am there for sure, in the good-enough league.


  • 22 Oct 2016 22:19
    Reply # 4327791 on 4322040

    Then it would appear that a completely different mindset is needed for 3D panels, from that needed for 2D panels.

    With a 3D panel, whether it has tucks, shelves or broadseam, one designs one's panel with a certain amount of camber, then builds it and gets the amount of camber one has designed, within close enough limits for a junk sail. Moreover, that camber is present, not only at the mid-height of the panel, but also, to a reasonably even degree, over more of the height of the panel. Of course, the mid height of the panel is blown out into something of a curve; but overall, one gets the camber one is hoping for, which is something close to a trapezium in cross section. There is no question of "pulling harder" or "less hard", to adjust the camber, because this will have no effect. Light tension, to remove the slack, is enough, and more tension will have no further effect. 

    I would be alarmed if my sail started pulling into a concave shape at luff and leech, putting more depth into the middle of the sail. This is something over which I would have no control, and would rely totally on the stretch characteristics of the cloth. It is the kind of thing that happens if one pulls a handkerchief out by its four corners, and then drops an orange into it. One gets some camber, but the amount and position of that camber seem to be in the lap of the gods. 

    Again, I need to emphasise that I have nothing against the 2D, barrel cut panel. It has its place in the scheme of things, which is to help first time sailmakers get onto the bottom rung, and to enable others to play the 80/20 rule, getting the most return for the least investment of their time and skill. For others still, well, the 3D panel seems to me to be worth that extra bit of investment.

  • 22 Oct 2016 18:41
    Reply # 4327665 on 4322040
    Anonymous member (Administrator)

    David,

    I looked long and hard on your diagram, but I am afraid that instead of clearing things up for me, it left me rather cross-eyed and confused.

    To clear my mind, I went out in my Ingeborg to take some measurements. First of all, I found the length from luff to leech along the battens to be about 4.86m (instead of 4.90m). I guess you cannot get it that much longer without really hauling on it.. 

    Then I wanted to measure the actual camber. Since there was a tail wind in the berth of some 5-6 kts, I went out in the bay for a little test sail. The rather hastily performed measurements showed that the camber has ended on about 33-34cm. Divided on the slightly shortened  chord (4.86m x cos 10°) the camber should be around 7.0%.

    That coincides well with experience on Edmond Dantes’ and Frøken Sørensen’s sails: Unless one slackens the sail a little along the battens, the camber will end up about 1% (of the chord) lower than my chain calculator predicts.

    There is one observation more, worth mentioning: The luff and leech of each panel set with a small but noticeable hollow. It must be from here the sail gets most of the extra horizontal length to achieve the camber

    In the diagram I made of Ingeborg’s sail in the posting below, the extra length needed to get around 8% camber is 87mm or 1.78% of the B.  My guess is that if I increase the slackness along the battens from the present 4cm to 7cm (1.43% of B), the camber of the sail will approach the planned 8% camber, instead of the present 7%. This way of slackening the sail will inevitably lead to those wrinkles along the battens, as shown on the photo of Johanna.  

    If it hurts too much to watch all those wrinkles, one may instead make 2-3 broadseams which eats up the same length, (say 1.5 – 1.7% of B). This too will be an approximation.

    Something like that.

    Arne

    Last modified: 30 Jun 2019 20:18 | Anonymous member (Administrator)
  • 22 Oct 2016 09:59
    Reply # 4327146 on 4322040

    Arne,

    I’m trying to develop a graphical way of designing the edge of a cambered panel with tucks. The general principle, if I'm allowed to plagiarise and adapt the Republican slogan, is to “Make the Edge Straight Again!”  (as the batten is straight, so ought the edge of the panel to be). Here’s a particular case, based on your batten length of 4900. Please could you look at this, and see whether you understand and agree with what’s going on here. 


    Last modified: 22 Oct 2016 17:12 | Anonymous member
  • 22 Oct 2016 09:18
    Reply # 4327118 on 4322040
    Anonymous

    Go to an image stored somewhere on this site (in your photo album?). Right click and select 'copy'. Start a reply and right click in text area .. select 'paste'. Select pasted image and drag corner to resize to no more than 420 pixels wide (if the image is any bigger it causes the text to overflow the right hand edge of the box).

    It's easier if you have two browser windows open, one with your post and one with the original Image.

    Chris

    Last modified: 22 Oct 2016 09:23 | Anonymous
  • 22 Oct 2016 08:52
    Reply # 4327101 on 4322040
  • 22 Oct 2016 07:39
    Reply # 4327032 on 4324570
    ueli lüthi wrote:

    hi arne

    i intend to sew a sail based on your ar1.8 johanna style template, but with a couple of alterations, including some broadseams (not enough to remove all wrinkles – just a compromise…)

    to see if my rough geometric guesswork will do, i made a paper model.

    http://junkrigassociation.org/Sys/PublicProfile/26734434/
    Photo/46637091/57896720/0

    http://junkrigassociation.org/Sys/PublicProfile/
    26734434/Photo/46637091/57896723/0

    this doesn't look that bad, the next step will be to sew a 1:5 scale model of my sail, before i go to the real thing…

    ueli

    p.s. i don't know why this post runs over the column width – and how can i add an image preview?

    The trick is to put a shift/return into the link, as I've done above. I can't add images, though!


  • 22 Oct 2016 00:48
    Reply # 4324570 on 4322040

    hi arne

    i intend to sew a sail based on your ar1.8 johanna style template, but with a couple of alterations, including some broadseams (not enough to remove all wrinkles – just a compromise…)

    to see if my rough geometric guesswork will do, i made a paper model.

    http://junkrigassociation.org/Sys/PublicProfile/26734434
    /Photo/46637091/57896720/0

    http://junkrigassociation.org/Sys/PublicProfile/26734434
    /Photo/46637091/57896723/0

    this doesn't look that bad, the next step will be to sew a 1:5 scale model of my sail, before i go to the real thing…

    ueli

    p.s. i don't know why this post runs over the column width – and how can i add an image preview?

    Last modified: 22 Oct 2016 08:51 | Anonymous member
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