Conversion bm —> SJR

<< First  < Prev   ...   2   3   4   5   6   Next >  Last >> 
  • 11 Dec 2021 09:26
    Reply # 12185210 on 12185004

    Thanks for your input, Graeme!

    Yes, it does really look rather hefty that sail.

    About the top panels, I actually just tried to copy Poppy’s sail plan. I really have no deeper thoughts about that. The sail on Poppy looks really nice and when I drew it on my own I liked it. 

    I will definitely give a shot at calculating the CE of the bermudan sail. At the present I do not have the scale drawings (she’s a on-off). As I am very pleased with the present helm balance, it should work out fine if I can find the right spot. 
    Your proposed mast length is way over what I am opting for, maybe the two masted alternative is better, in regards to keeping down the height of it all. 

    Well, back to the drawing board ;)

    /Marcus 

  • 11 Dec 2021 08:55
    Reply # 12185152 on 12185004
    Anonymous member (Administrator)

    I’ll just comment regarding the SJR proposal.

    Your updated diagram looks more realistic to me – with SJR it would be normal for the mast to be just a little forward of where a Bermudan cutter mast would go. However, when it comes to the actual numbers, a scale drawing will be better – it is a bit dodgy trying to guess the CLR position off a photograph – even an exactly side-on photograph is not exactly a profile drawing.

    If you don’t have any scale drawings, there is another approach you might take, regarding a single-mast SJR. It has been found that the centre of effort of a SJR will be pretty close if it is placed to match the centre of effort of the original Bermudan rig. (This may not necessarily work for other junk plan forms, but it does seem to work for SJR).

    Since you have the old rig, and you know the old mast position, it should be possible for you to calculate exactly the position of the centre of effort of the old Bermudan rig.

    Now the centre of effort of a Amiina-type SJR is near enough on a vertical line through the midpoints of the lower battens. You can now move your sail plan until the SJR centre of effort coincides with the original Bermudan centre of effort. That puts the SJR sail about where it should be.

    The mast centre-line of a Amiina-type SJR runs through the points on the lower battens which are 1/3 of the batten length. That defines where the mast position will be. Usually that works out just a little forward of the original Bermudan mast position and I think it will in your case too. (Though in the case of a Bermudan sloop with a relatively small fore-triangle, which is unusual these days, the mast positions might even coincide).

    So that’s a method you could use to get a SJR to have about the same helm balance as the original Bermudan rig, without the need to try and guess the CLR position.

    Just for an example, I took the original drawing of Amiina’s sail (as designed by Slieve) and scaled it to 55 square metres. I think most people will say this is getting pretty big for a single sail. The scale factor for area, to achieve that, is 3.41. The lineal scale factor is the square root of that (1.85) and if we apply that to Amiina’s mast, it comes to 13.6m above the partners. Amiina’s mast is actually a little higher than it needs to be (though that is an advantage) – which means you might be able to reduce the mast height a little – maybe just over 13m – above partners – but it must be at least as high as the peak of the yard, or otherwise you will have other problems.

    I think these calculations will fairly closely match the single-mast SJR that you have proposed – it will be a starting point, anyway. It does look to me like a rather large sail on a rather tall mast, but those are matters you will have to evaluate for yourself. By increasing the number of panels, you have made jibs which have an aspect ratio of less than 1 (ie they are wider than they are tall) so you are in new territory with your sail design. Also I am wondering why you would divide the top panel into two parts. Sail design is evolving, of course, and there may be some ideas that you are keen to try. I think you might enjoy a discussion with Slieve, who would be the best person to advise you on designing this type of sail.

    I took the profile photograph of your boat, and from its length (32’ = 9.6m) I established a scale, and just took a guess at where the Amiina sail plan looked about right (just "eyeballed"). By following the procedure outlined above and establishing the original centre of effort, you will be able to get it closer to correct. It does look like a pretty lofty rig to me.

    I wonder if 55 sqm really is a suitable sail area for this boat, it does seem rather a lot.


    sketch modifird 12/12/2021

    correction 14/12/2021 should be 6.01m battens

    Last modified: 13 Dec 2021 23:11 | Anonymous member (Administrator)
  • 11 Dec 2021 07:52
    Reply # 12185030 on 12185004

    Thanks David.

    I did the adjustment of removing the rudder. Does the photo seem right?


    /Marcus

    1 file
  • 11 Dec 2021 07:27
    Reply # 12185023 on 12185004

    Hi Marcus,

    the CLR is usually calculated without the rudder area. This will make quite a difference to your CLR location and mast position.

    All the best with the project.

    David.

  • 11 Dec 2021 07:06
    Message # 12185004

    Hi there. 
    So, I bought a 32 foot cutter rigged bermuda, long keeled heavy displacement boat. Since she needs a new set of sails and new standing rigging, I am thinking this is an excellent chance to convert her to SJR. Reading some of Slieves articles has convinced me of the ease of handling and perhaps even finding a better mast position than with a ”traditional” junk. 

    I’ve done the ”balancing on a ruler ” thing to get my CLR, but it seems to be very far aft. However, if it is correct it is better for my mast position. I’ve also considered the two masted option. This would probably be better in forms of getting hold of the masts (this seems to be the biggest challenge).

    The sloop version would be the simplest, but the mast has to be 13m. I contacted Hawk Marine asking for a price for a needlespar mast, but got the reply that they don’t make them anymore. 

    I’m thinking of copying Poppy’s sail with an alteration of balance, more sail fore, but the same sail area would be suitable. The present sail area is a total of 54sqm-ish (main, staysail and high cut headsail)

    Well, I’ll post a few photos here, please let me know what you think about my ideas. 

    /Marcus

    4 files
<< First  < Prev   ...   2   3   4   5   6   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