Conversion project - Trismus 37

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  • 18 Oct 2022 22:38
    Reply # 12958993 on 9390257

    Hi Ulysse,

    have you tried a schooner rig? It looks to me as if this may work better than a ketch. The main mast would fall at the aft end of the saloon table and the foremast would not require as much bury, so could be located further forward.


  • 17 Oct 2022 19:27
    Reply # 12957112 on 9390257

    Hi Graeme,

    Many thanks for taking the time to give your views to my feable tentative in drawing a junk rig for this Trismus.

    Your comments are very much appreciated, and sorry it took me so long to reply.

    Indeed I was not happy with the main mast so far forward, with very little bury. I tried putting it there in order to not interfere too much with the accomodation, but it obviously des not work. 

    If I don't mind the accomodation, having the mast further aft could work with the structure: the deck is already quite strong there, and there are bulwarks that reinforce the hull. But then I am afraid the main would interfere with the mizzen. Sheering might become complicated.

    I hve done some more research on the design and the use of centerboards. The forward centerboard is intending for close hauled sailing only. One a beam reach or down wind only the aft centerboard is to be used.

    Not sure if I'll go with this particular Trismus though. Also the boat is great it is not in such good condition and the asking price is quite high. But there are other Trismus! So I'll keep trying with the sail plan.

    Again, thanks a lot.

  • 12 Oct 2022 21:54
    Reply # 12952074 on 9390257
    Anonymous member (Administrator)

    Since no-one else has yet replied, I offer the following comments  and ask some questions, in the hopes that further discussion will follow.

    If all the engineering is sound, this hull, with its rather shallow draft and tandem centreboards looks like an exciting and attractive proposition, especially for junk rig. A lot of shallow draft hulls tend to develop weather helm when sailing off the wind or when hard pressed, and junk rig can exacerbate that - but with that aft centreboard down it should track down-wind like a train on rails.

    Normally, where there is an existing sail plan from the original design, which has been proven satisfactory from a “helm balance” point of view, it should be straightforward to build a junk rig around the centre of area of the original sail plan. In the case of a split junk rig, the new centre of area of the sail plan can coincide with or be very close to the original. In the case of a contiguous (unsplit) ie conventional junk rig, there may be a slight adjustment on that, as you will see from Arne’s notes, but the point being: a successful original sail plan gives a confidence-building start-point for the design of a suitable junk rig. Evidently you have followed this principle, and placed a junk rig which has taken into account the drawing of the bermudan rig which the previous owner has given you.

    To my eye (and I don’t claim to be an expert) – the result does not look correct.

    The centre of immersed lateral area of the hull (with both boards down) looks to be about where it is shown on the drawing – well aft of what is normal (but with the ability to adjust centreboards this can have a positive advantage especially if this is the type of hull which easily develops a lot of weather helm). However, the centre of area of the sail plan seems set so far forward in relation to it, that I would be concerned about possible lee helm, or even difficulty tacking – I imagine sailing to windward with the aft board fully up and the forward board fully down and still wonder if the rig is a little too far forward. The large mizzen you have drawn might compensate for that, but it still does not look right to me. (On a full ketch the mizzen is meant to be a working sail, not an air rudder).

    The other thing which would worry me a bit about your proposed junk rig is having the mainmast itself placed so far forward – ahead of and over-hanging the waterline in fact, with barely enough bury for the integrity of the mast, in addition raising the question of whether the hull itself is structurally capable of supporting a free-standing mainmast at that point – together with the effect this new weight distribution may have on the vessel’s tendency to pitch in a head sea.

    In summary, from an unqualified lay-person’s perspective, the combined centre of area of the proposed junk ketch rig "just looks to be" too far forward, and in addition that mainmast also looks to be placed too far forward.

    So, I ask the following questions: (1) What is the intended purpose of the vessel? (Live aboard? Coastal sailing? Canal cruising? Long distance ocean cruising? For how many people?) 

    Not wanting to be negative. This shallow draft, capable-looking long keel vessel with its tandem centreboards strikes me as a very exciting proposition with a lot of possibilities. In my younger days I would have dreamed of owning a vessel such as this. If the hull engineering is sound, I’d say go for it. Perhaps some discussion with the original designer regarding the original rig would be helpful, especially regarding its handling characteristics - and get some good advice from someone qualified before settling on your design for the new junk rig.

    PS on second looks, that bermudan sail plan as a sloop, with just the 3/4 headsail, looks as if it would go quite nicely hard on the wind with the aft board up and the fore board down. Perhaps that was meant to be the working configuration in heavy weather. Down wind, you could cram on as big a genoa as she would carry, and with the aft board down (and maybe some of the fore board up) I can imagine a good down-hill ride. Maybe this was the rationale for that rig.

    I just found on the internet the sail plan for the original Trismus 37 with a bit more detail. It seems to be a cutter rig and and it looks to be the same sail plan as you were given, but with a  bit more detail.

    Anyway, for a junk rig, I still think the mainmast wants shifting aft from where you have placed it, and perhaps a junk sloop (or a two-masted junk rig) with centre of sail area moved aft (from where you have it) might be better. With tandem centreboards to play with, you have a certain amount of choice. You really need to find out more about the boat's characteristics from someone who sails one of these vessels. There seems to have been a few different hull and rudder configurations.

    To my amateur eye, its an unusual and interesting proposition.

    Last modified: 13 Oct 2022 04:36 | Anonymous member (Administrator)
  • 10 Oct 2022 18:12
    Reply # 12948976 on 9390257

    Hello all,

    I am bringing up this topic because I just visited a Trismus 37 offered for sale without it's rig and I am toying with the idea to convert it to JR.

    It's the first time I'm having a go at it so this attempt is likely to be all wrong, all comments and suggestions much welcome.

    While trying to fit the masts in likely places and also respecting a somewhat working CE, it seemed to me that a ketch rig may work. I have been thinking a bit like Tystie's current rig... The main is derived from Arne's master sail AR 2.20 with one more pannel to get about 52sqm. The mizzen is another of Arne's sail that should be around 20sqm. Not sure if the scale is perfect but it gives an idea.

    The owner has drawn the plans as DXF so I attach here the rendering along with my efforts.

    Thanks for reading!



    3 files
  • 13 Dec 2020 17:58
    Reply # 9426555 on 9390257
    Anonymous member (Administrator)

    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 (Administrator)
  • 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 ?


  • 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
    Deleted user


    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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