Conversion project - Trismus 37

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  • 13 Dec 2020 17:58
    Reply # 9426555 on 9390257

    The reason SJR allows the mast to be a little further aft than a contiguous JR is that SJR allows a “balance” of sail area of up to 33%.

    If you put the centre of area of the SJR to coincide with the centre of area of the Bermudan rig, you will have placed the sail in about the right position. A vertical line dividing the battens in the ratio 1:3 (33% balance) indicates where the mast would go.

    (Someone else can advise you how to place a contiguous JR sail on your drawing, then a vertical line dividing the battens into a ratio of, say, 15% to get the approximate mast position. You will find that puts the mast a little further forward.)

    As far as I am aware all SJR sails have been cambered. If you read Steve’s notes in the Technical Section you will see how it can be done, using the “shelf foot” method. 

    I have only ever made one sail and it was a small small shelf foot SJR. I would expect most people would agree that lofting and sewing a cambered sail entails more work than a flat cut sail. However I would not use the word "difficulty" - just a bit more work is all. 

    Last modified: 13 Dec 2020 18:34 | Anonymous member
  • 13 Dec 2020 12:22
    Reply # 9426050 on 9390257


    Thanks David for the suggestion.

    No I had not thought about it because I was initiall more tempted by the 2 masts option. However, due to the complexity and cost of installing 2 masts, I may revise this option.

    Could someone tell me why the SR allows to put the mast further aft ? Beause of the larger sail area fore of the mast ?

    Longer battens ? Isn't it a bit difficut to have more than 6m long which is the industrial standard for alu tubes.

    I had a look at the article by Slieve McGalliard on Poppy. Very nice rig but she does very well also because the sail area is very large (main + 135% genoa of Bermudian rig). For a Trismus that would mean >70sqm sail. Can I reasonably plan such a large sail ?

    Finaly, what is the difficulty of designing a cambered sails SJR ? In comparison with the more basic flat sail ?


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  • 07 Dec 2020 18:13
    Reply # 9410802 on 9390257


    have you considered a split junk rig for your conversion? It would provide less heeling moment for the same sail area and would move the mast aft to about the forward end of the raised cabin section. The boom would be longer and the hoist and mast length less allowing you to set more sail area for the stability that your boat has.

    Just a thought, David.

  • 07 Dec 2020 16:28
    Reply # 9410524 on 9390257


    I have a Freedom 30 Cat ketch that I am going to convert to a junk rig soon.... One comment I have about the 2 sail rig on a long keeled hull is that it is very easy to adjust the sails to balance the boat on most points of sail.  Adjustments are usually in the order of:

    1. Trim the foresail and then trim the mizzen using the sheets

    2.  Reef either the mizzen or foresail as required.  I usually find that I reef most of the mizzen first before needing to reef the foresail. 

    It may be ok to accept being slightly out of balance under full sail in light winds knowing that it can be corrected as the wind increases and you reef.


  • 07 Dec 2020 10:02
    Reply # 9409810 on 9390257

    My experience with ketch JR was positive, on balance.

    Going to windward, and reaching, I liked the way that the larger foresail did most of the driving, using the mizzen to adjust the trim, with reefs taken in it first so that the CE moved forwards. 

    Going on a dead run was OK, but there was a slight downside to not-quite-a-dead-run, in that the foresail has to go out to the windward side, so that it's by the lee. In strong winds that makes it a little harder to handle when reefing and gybing than a smaller sail would be; but it's not a deal breaker. 

    Perhaps you could talk directly to the two subsequent owners of Tystie, Martin and Gordon, to see how they find the ketch rig.

  • 06 Dec 2020 14:20
    Reply # 9408347 on 9390257


    2 more attempts added.

    One is a ketch with reduced aft sail. My concerns here are 1. for the space to control the sheets (between both sails and aft) and 2. for the lead. I am absolutely not sure of the CLR position and if right, whether 10 % lead is enough. I don't have much option to increase it.

    Other follows my reading of the posts about the conversion of the Cheoy Lee 38, particularly Arne's message. I tried the sloop. A big sail ... A Trismus has a shallow draft, 80 cm boards up, and less ballast than the CL38. Closer to 30 % ballast ratio.

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  • 29 Nov 2020 20:27
    Reply # 9393400 on 9390257

    Hi Andre,

    the CLR is normally calculated without the rudder, this means it will be a little further forward than shown on your sail plan #2. I find that a lead of about 12% is generally about right for a split rig, although on Arcadian it was not enough and I increased it on her to about 17% in order to balance the helm.

    All the best with the project, David.

  • 29 Nov 2020 16:40
    Reply # 9393101 on 9390257


    Thanks for the suggestions.

    I had a second try with masts further aft and equal sails. 12 m tall masts, and approx 2 x 36 sqm sails.

    Badger seems to have equals sails.

    I tried to locate the CLR (if I understood well from the forum) with Arne's rough method (see pics in my album, link in first post). I am not sure it is valid with a centerboarder though ??

    I have difficulties to find a way to increase the sail area by 150 % ! There is not much space between the sails and I need some space aft. Maybe increase the sail surface forward of the fore mast ?


    Last modified: 29 Nov 2020 16:41 | Anonymous member
  • 28 Nov 2020 08:32
    Reply # 9391024 on 9390257

    Hi Andre

    What a great design and how innovative!....I remember it well from when I was a much younger young designer recently graduated from the Westlawn School of Yacht Design in the US. I was very prompted to build it but never did. It became the precursor of the big trend in France of centerboarders. Patrick though was from Belgium and he died, fairly young few years latter.

    The design is somewhat dated, with the long overhangs etc, and today a would reconsider the stern daggerboard if I were to rig it as a Schooner, and aluminium wold be the way to go .....for me!

    I would see no reasons why you could not rig it as a junk rig but I would base my sail area calculations to start with on at least 135 and better, 150% of the original surface meaning main plus 150% genoa surface.

  • 27 Nov 2020 20:23
    Reply # 9390347 on 9390257
    André wrote:

    Hello all,

    The current circumstances are not favorable to boat projects, so I try to take advantage to build up the plans and improve the preparation for a conversion that will take place, hopefully, not too long down the road.

    I like this boat, a Trismus 37, centerboard with some keel, designed a while ago for ocean voyages. My project is for life onboard, and long distance on ... not that high budget. So I need simplicity for autonomy.

    I have been looking around on the site and am more and more convinced that my next boat (third after a 33ft sloop and a 43ft ketch) will be a junk rig. I am hence thinking of converting a Trismus 37 to junk.

    I would be happy to have some opinions given the characteristics of the boat ?

    After reading a recent thread about the conversion of a 36ft boat,  I am afraid 1 mast would have a sail a bit too large. Original has 65 to 75 sqm for 11.8m or 13.5m tall mast.

    I have tried to play with images pasting from images of Ron Glas (sorry for the blasphemy). Nothing serious in terms of calculations though.

    Positions of the masts I tried on the drawing are more related to the interior layout than on real data. I don't have much choice due to the centerboard position.

    So 2 masts, equal size or forward sail smaller ? I would prefer 2 equal masts to have identical sails.

    I try to put a link to pics of the original plans of a Trismus 37. First time I do that so I'm not sure.



    Hello André,

    It helps to put a shift/return in the middle of a long link like this, as I have done above.

    I think I would try to put both masts further aft. Perhaps the foremast goes through the exisiting forehatch, and the after mast goes aft of the double cabin doorway. This could mean that the sails are of equal size, or even that the after sail is smaller, making the rig a ketch, not a schooner. Maybe the sails could be of equal width, to share battens, but one of them could have more height than the other, to get the helm balance right (like Badger).

    Last modified: 27 Nov 2020 20:26 | Anonymous member
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