SJR and too much main panel camber

<< First  < Prev   1   2   Next >  Last >> 
  • 18 Mar 2021 10:48
    Reply # 10209217 on 10179164

    Thanks Graeme,

    I have to check the lens dimensions on my sail once again, just to be sure.

    And yes, I have thought of lengthening the hinges - not only on the jibs, but on the top mains as well. 

    Paul's jib is truly an interesting idea. I might even have enough yellow Outguard left to make a set of new jibs...

    (Like this was of any priority compared to e.g. the new rudder I was supposed to make before the splash time...)

    Last modified: 18 Mar 2021 10:50 | Anonymous member
  • 18 Mar 2021 02:41
    Reply # 10208293 on 10179164
    Anonymous member (Administrator)

    It might be worth to refer to the article by Paul McKay in Magazine No 84 in which he describes a much simpler type of jib.

    I have no idea how to relate our figures of camber and sheeting angle to this quite different approach, and need some help here. But I note two points of interest: the provision for adjustment of camber and what Paul refers to as "offset" - and the remarkable similarity in final shape when inflated, of a shelf-cut jib and the simple jibs of Paul's design.

    I would very much like to read some other people's comments

    Last modified: 18 Mar 2021 03:33 | Anonymous member (Administrator)
  • 17 Mar 2021 22:11
    Reply # 10207935 on 10179164
    Anonymous member (Administrator)

    I don't know Jami, that sail of yours looks pretty good.

    Nobody knows yet what is the "correct" sheeting angle for SJR, maybe it is not all that critical.

    Anyway, if you think increasing the sheeting angle might improve the performance of your sail, you have an easy way of trying it with your set up - I guess you have already thought of it - take a couple of jib panels off and re-attach them, temporarily lengthening the "hinges" with simple lashings, increasing in length as you move towards the clews. 

    (On Wayward Dave Zeiger went a season with shelf-cut mains with no shelfs - just the centre sections, with lashings of appropriately varying lengths. Despite the gaps, the sails seemed to work OK)


    Something like that on the jibs would be an easy way of trying out an increase in sheeting angle.

    Last modified: 18 Mar 2021 22:26 | Anonymous member (Administrator)
  • 17 Mar 2021 01:44
    Reply # 10204847 on 10179164
    Anonymous member (Administrator)

    Hi Jami

    I have been away for a few days

    Here are the measurements of the jib lens on my sail, as measured from the pattern I used. It looks as though I made a mistake in my calculations too.

    The leech of the shelf is 340mm.

    Sheeting angle

    Allowing for the 45 degree shelf angle, that gives a sheeting distance of 340sin45 which is 340 x .7071 = 242mm.  The chord is 1000mm.

    Sheeting angle is thus inverse-sin(242/1000) = about 14 degrees

    More than I meant - I don't remember now, but I think I was aiming for 12 degrees.

    Well, Sieve wanted to try a higher sheeting angle than 12, and it looks as though I have (without meaning to) been sailing around with a sheeting angle of about 14 degrees.

    This perhaps explains why I don't point quite as high as I had expected - but I am happy with the power I am getting when on the wind, and I don't think I want to change anything. Despite the larger sheeting angle, I believe the main (with tin plate camber of 8%) is collapsing at the luff just before the jib stalls. I am sure now this is a slight back-winding effect from the jib.

    As I guess you know, you can back-calculate your sheeting angle (for 45 degree shelf) by using

    inverse-sin(.7071 x shelf leech / chord)


    The above figures give a (tin plate) camber of 9.55mm for my jibs, which is about 9.55% of the chord. I had intended it to be 10%.

    In real life, of course, the soft cloth camber is a bit more than that.

    These parameters seem to work pretty well for the sail on Serendipity.

    Last modified: 18 Mar 2021 01:48 | Anonymous member (Administrator)
  • 12 Mar 2021 09:10
    Reply # 10188824 on 10179164


    could you please measure (unless you have the data already) the length of the jib lens leech? And what is your jib cord length?

    Looking at your video I'm again wondering if I have calculated my sheeting angle the right way or not. 

  • 11 Mar 2021 18:37
    Reply # 10185850 on 10180624
    Graeme wrote:

    I took this photo a couple of weeks ago, close hauled and as near as I could tell in the “sweet spot” – which is fairly narrow on my boat but it makes quite a difference if I can stay within this narrow zone.

    The lee tell-tales on the jibs are horizontal and showing correct air flow.

    That looks to be a very nice camber on the main body of the sail, quite full forward, but the sail should provide a lot of lift.

    Based on all of my years of bermudan sailing the jib interfered with flow over the main more when it was an overlapping jib, or genoa, especially those big overlaps. With no overlapping jibs this does not seem to be such a concern and it is possible to have quite tight sheeting angles for the headsail.

    Last modified: 11 Mar 2021 18:43 | Anonymous member
  • 11 Mar 2021 13:49
    Reply # 10185053 on 10179164

    As for the videos, here's mine from last summer, steered by the David Tyler -designed wind vane.

  • 10 Mar 2021 20:31
    Reply # 10183238 on 10179164
    Anonymous member (Administrator)

    Thanks Paul.  I have collected the information and will update the spreadsheet.

    Panel cambers will vary too, in some cases. If I can get enough info I will find a better way of displaying it, and post it back.

    Last modified: 17 Mar 2021 20:53 | Anonymous member (Administrator)
  • 10 Mar 2021 09:46
    Reply # 10181600 on 10179164

    Hi Guys,

    Oh to be sailing again..

    I did try to measure the actual depth of camber of my Main panels last season, can’t remember specifically but sure it came to over 10%  as opposed to the calculated/designed camber of 7%.

    BTW it’s only my Jiblets that are shelf foot, my mains are round and broad seam.

    My Slot width was designed as 160mm.

    However due to my fat tapered Lamp Post mast (199mm at deck),Slieve recommended that I decrease the chord of my lower two jiblets by 20mm for the lowest and 10mm for the second up, in order to restore the slot width accordingly at these panels.

    So slot widths are (in ascending order)-

    Panel 1, 180mm

    Panel 2, 170mm

    Panels 3&4, 160mm.

    Sorry Graeme, I can’t seem to be able to edit this on your spreadsheet.


    Last modified: 10 Mar 2021 12:49 | Anonymous member
  • 09 Mar 2021 22:58
    Reply # 10180624 on 10179164
    Anonymous member (Administrator)

    I have been wondering about these things too. I don’t have any aerodynamic theory about the SJR and have yet to discover whether or not it has any aerodynamic advantage, all I know is: it works well.

    I took this photo a couple of weeks ago, close hauled and as near as I could tell in the “sweet spot” – which is fairly narrow on my boat but it makes quite a difference if I can stay within this narrow zone.

    The lee tell-tales on the jibs are horizontal and showing correct air flow.

    The main luffs are still nicely inflated (just).

    At the “sweet spot” the main luffs are close to collapsing, but still just inflated. If I point any higher, the main luffs will begin to tremble and show signs of collapse – if I bear away any, the lee tell-tails on the jibs begin to revolve (showing airflow is losing adhesion?)

    I have to concentrate hard to keep in this zone, but beginning to find it makes a difference if I can manage to do it.

    (This is actually a screen shot from a video clip – you can watch the clip by clicking on it – then you will see the bottom panel which is setting badly due to a lack of downhaul. The luff of the slack bottom main panel is actually just past the point of collapsing, acting as a convenient indicator).

    Since the mains seem closer to the point of stall than the jibs, at this “sweet spot”, I too was asking myself if there would be an advantage if the slot were to be opened up a little. I am not sure that increasing the jib camber would achieve this, but I did wonder if the sheeting angle of the jibs could be increased a little – or would it help if the “shelfs” of the jibs were cut with a greater angle?

    Somewhere there is a limit to how much sheeting angle is possible– we don’t know what it is. We also have to consider how this might affect pointing ability.

    As for shelf angle, increasing it would open up the slot (vertically) to freer air flow – but only if there is enough wind to support the shelfs. Even with 45 degree shelf, the jibs sag a little and will not fill out nicely in very light air. 

    Rudolph (Oceaan 22) has reported something quite different -  with 10-% camber on both mains and jibs – he reports that his boat will sail to windward, when pinching, with the jibs collapsed and the mains still pulling. (see thread "new slit junk Oceaan 22). Though he also reports that jibs and mains both collapse at the same time.

    Slieve has mused in the past about possibly increasing camber and sheeting angle, but recently informed me that he strongly believes 45 degrees is about right for shelf angle.

    Without a comparable boat to trial against, I think it will be difficult to resolve the questions of ideal camber/sheeting angle/shelf angle/slot width – each parameter can be considered separately, but it is likely they have an effect on each other as well – which suggests to me that to improve this quite effective sail will involve a lot of trial and error and it may be that the current thinking is about right. There is also an aerodynamic theory approach but I leave that to others, except to observe that the apparently restrictive slot may have the effect of actually accelerating the air flow over the lee side of the main panels, in which case it would be an advantage?

    As far as I can tell, those of us who have followed the Amiina-style plan form have all settled on pretty much the same parameters, and I would like very much if the various SJR-owners would complete and correct this spreadsheet. We would then have some actual figures for comparison, between this fleet of small boats.

    Some numbers are from memory and some I don’t know, so additions and corrections would be appreciated.

    “Tin Plate” camber refers to shelf cut sails and represents the theoretical camber which would exist of the panels were cut from tin plate. Of course, for comparison with other sails, the true soft cloth camber needs to be measured when the sail is inflated – we probably all need to do this.

    It would be good if we could correct, add to and build up this database.

    (PS to change the subject:

    1. the sail catcher, supported by just fore and aft lift pairs, removes the need for lazy jacks - and the lifts interfere very little with the sail.

    2. Advantage of wooden battens - hit a marker pole the other day and broke one. Was able to add splints using screws and a drill-driver, and keep sailing. )

    Last modified: 10 Mar 2021 02:12 | Anonymous member (Administrator)
<< First  < Prev   1   2   Next >  Last >> 
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software