Conversion bm —> SJR

<< First  < Prev   1   2   3   4   5   ...   Next >  Last >> 
  • 15 Feb 2022 22:24
    Reply # 12602650 on 12185004
    Anonymous member (Administrator)

    I think you need to make a scale drawing Marcus.

    The dimensions of your sail plan can be transferred to the scale drawing, and then you can mark the position of the centre of area on the drawing, and you can place the proposed mast on the drawing , to scale, and get the numbers.

    On the photograph all you are able to show is an indication of where the mast and the centre of area would be, with no measurments.

    The drawing does not have to be of a professional drafting standard, a simple sketch would do. But the vital parts need to be to scale. The position of the proposed mast and the geometric centre of the current rig can then be shown, to scale.

    Then it will be possible to superimpose potential sail plans on the drawing, to scale, and see what best fits your fixed mast height, your fixed mast placement and your pre-determined geometric centre of area.

    Last modified: 15 Feb 2022 22:25 | Anonymous member (Administrator)
  • 15 Feb 2022 20:28
    Reply # 12602278 on 12185004

    Hi guys!

    I went to the boat and did some measuring. I also very much came to a final decision on mast position, and also placed the order for the  mast :). LAP will be 11,3 meters and total length 13 meters. 

    If someone feels like checking my calculations with numbers I have, I would really appreciate it. I really think I’ve got the CoA correct - but I am here to learn ;)

    By the way - is it only me, or is it especially windy (stormy) in Northern Europe this winter? It seems like we’re being kicked by the weathers a few times every week...


    1 file
  • 09 Feb 2022 20:18
    Reply # 12587655 on 12185004

    Thank you all for your responses.

    I will take David's advice and take a tour with the old measuring tape - and once and for all get the numbers right.

    I'll be back :)

  • 07 Feb 2022 21:02
    Reply # 12582310 on 12185004

    Hi Marcus,

    before proceeding any further I would suggest accurately measuring your existing rig and creating an accurate to scale drawing of it. The dimensions you will need are height and foot length of the foretriangle, boom length and hoist length of mainsail, height of boom above deck, both at the mast and the outer end, and rake of the mast. From these dimensions an accurate drawing can be created and the geometric center of the sailplan can be calculated. This will give you an accurate basis for your design and much more certainty going forward.

    All the best with the project.


  • 07 Feb 2022 08:33
    Reply # 12580496 on 12185004
    Anonymous member (Administrator)

    Centre of pressure, CP

    Graeme, your question is relevant but not easy to answer.
    Back in the early nineties, I sailed my Malena  (Albin Viggen) with three versions of Hasler-McLeod style junkrigs, all sloops.

    • ·         The first, flat sail generally gave easy steering. However, when heading up to tack, a strong lee helm developed, as I understand is normal. This made tacking less easy, but also ensured good self-steering when close-hauled. In other words, the CP moved a lot with the angle of attack.
    • ·         The very same sail was then given hinged battens with the max camber at the middle of the chord. There was a noticeable increase in weather helm (but not adverse). Great improvement in performance.
    • ·         The new  (1994) blue cambered panel sail, with mostly the same profile, but with the max camber point about 35% from the luff, initially gave a little lee helm when close-hauled in light winds. I put it right by adjusting the mast just a little (maybe moving the mast top 10-15cm aft), and then reducing the sail’s mast balance a bit.

    I have concluded that the CP moves a bit forward in a cambered panel junk sail with the CE put in the same position as in the Bermudan rig, but I don’t dare to quantify this. Remember, the state of the original Bermudan sails also plays a role here. Maybe the CP moves forward 1-2% of the waterline on a broad, cambered sloop JR.


    PS: With CE I mean the geometrical centre of area, which is fixed. The CP, centre of pressure is the real thing, and that moves around a good deal, depending on the shape of the sail and its angle of attack.

    Last modified: 07 Feb 2022 10:52 | Anonymous member (Administrator)
  • 06 Feb 2022 22:26
    Reply # 12579588 on 12185004
    Anonymous member (Administrator)

    Arne’s drawing of the two superimposed rigs is worth a thousand words.

    With the mast position you would have preferred, Arne’s rig  may be the more appropriate choice.

    I would keep an open mind. These rigs have been carefully worked out and each should be regarded as a package. Some running parrels are needed to manage Arne’s rig which should not be needed on the SJR, but the other side of the story is that the SJR package includes running parrel-downhauls (which need to be tweaked each time the sail is reefed) which Arne’s rig does not need – and if you are going to have 5 lower panels in your SJR, then you will probably need at least three of these.

    With the conventional junk rig you have a little more “wriggle room” too, ie the balance can be varied a little.

    I have a question for Arne.

    A number of SJR conversions (including my own) have demonstrated that if the geometric centre of the SJR rig is about the same as that of the original Bermudan rig, then the helm balance can be predicted to be about the same. This rule of thumb was pointed out a long time ago by Slieve, and saves the need for a lot of theorising if you have a drawing of the original Bermudan rig.

    However I have not seen this approach applied to other rigs, such as Arne’s, so I don’t know if that rule of thumb would apply. Maybe Arne’s rig would need a little more (or a little less) lead compared with the original  Bermudan rig?

    (This post crossed with Slieve's.

    I feel a little uncomfortable with the photograph, and if you don't have an accurate scale drawing to work from, then you do need to be confident you have the correct geometric centre (CoA) for your original bermudan rig, and use this as the basis for future calculations.)

    Last modified: 06 Feb 2022 22:45 | Anonymous member (Administrator)
  • 06 Feb 2022 22:02
    Reply # 12579553 on 12185004

    Hi Guys,

    I've just found these recent posts in this thread and am trying to get up to speed.

    Marcus, I must admit that your latest posting, Reply # 12576711, has left me a little confused as I thought you had settled on a setup for your rig. Still I think Graeme has answered most of the points that have to be considered. If you are convinced that the balance is better with one or two reefs in the Bermudan mainsail then it would appear to make sense to recalculate the CoA of the Bermudan rig with, say, one reef down and place the 50% overall chord of the SJR on that same longitudinal point. I suspect that doing so would only move the CoA a very few cm forward.

    When you say that you think the present CoE is actually a bit further forward than calculated you are getting into a very dangerous mindset. It's called guessing. Once you start that then you are on a dangerous slippery slope. Forget CoE. The only thing you can rely on is CoA, centre of area, and thankfully that is easily calculated when you have the Bermudan rig information. Then by placing the SJR 50% chord on the Bermudan CoA you are following those who have gone before which suggests you should have a successful rig.

    As I've said off line, I'm not into specifying masts, but common sense suggests that it's easier to fix a 'too long' mast than a 'too short' one.

    Cheers, Slieve.

  • 06 Feb 2022 20:07
    Reply # 12579379 on 12185004

    Thanks guys, I really appreciate your input. I do realise that helm balance is indeed important and that it’s probably quite nice to have it from the start. So, the present CoE will have to be met. And the mast will also have to be as I have drawn it in my last drawing. 

    Arne: Thanks for drawing your sail on my boat. I recall that you reasoned on the H/M SJR, would that be an option for me? What I really like about the SJR is the balance, which allows for less parrels. Scale is approximately 1:70, the boat is 32 feet and I am aiming for 50-55 sqm of sail. 

    Graeme: I did decide on the SJR, however the mast position has higher priority - if I can’t have both, I am open to other sail plans, preferably one of Arne’s. And I agree on the mast length, I most probably will go for the longer one (even if my earlier attitude was to have as short mast as possible - maybe old dogs CAN be taught to sit ;) or just give up facing the obvious)

    Paul: It sure would be nice to not have to increase the batten length, since it feels like “more to handle”.

    Thanks for helping me with the perspectives on things :)

    Good night guys


  • 06 Feb 2022 14:16
    Reply # 12578829 on 12185004
    Anonymous member (Administrator)

    Hi Marcus

    Now I took the liberty and plonked one of my sails into your boat. I happened to have a master sail with the yard angle lowered to 65 degrees. This lets me increase the sail's mast balance to about 20% and bring the CE just a little forward of that of the Bermudan rig.

    However, I am a little unsure of the scale of your drawing, so my sketch may miss a little.

    Anyway, good luck.

    Last modified: 06 Feb 2022 14:22 | Anonymous member (Administrator)
  • 06 Feb 2022 10:16
    Reply # 12578508 on 12185004

    Good morning Marcus, I agree with all Graeme’s comments. If you want to put the mast where you have shown it then the sail would have to have a much longer boom so a lower-aspect sailplan. (Something along the lines of Arne’s conversions). That way you also increase the necessary ‘lead’ which also justifies a further-forward mast position. I can send you the calculation formula I or you can look up the AeroJunk article which also gives it I think. 

    Paul McKay


<< First  < Prev   1   2   3   4   5   ...   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