Dual pair shrouds on trimaran

Discussion in 'Multihulls' started by revintage, Jul 7, 2023.

  1. revintage
    Joined: Nov 2016
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    revintage Senior Member

    I have seen some images of Peter Quorning´s Dragonfire 28 and the shroud system looks interesting. Both the forward and the rear shrouds are attached to the forward and rear beams respectively, at the same point as the waterstays.

    One question is how the shrouds are arranged at the hounds, as it will be very crowded if we have 5 lines to the schackle? Probably with the shrouds as very long bridles with a very short single line just below the hounds would solve this.

    My guess is that the highest tension should be at the forward shrouds, with the rear acting as "backstays".

    Is this a system that could be used to stiffen up the hole boat and are there any serious drawbacks, balancing the tension is my biggest concern? Another one is if the rear shrouds are in the way of the main when going downwind, but that must be tried out IRL.

    Thinking of using it on my 19" tri, thread found elsewhere here.

    Sketch added below the Dragonfire images.

    Attached Files:

    Last edited: Jul 8, 2023
  2. SolGato
    Joined: May 2019
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    SolGato Senior Member

    I like the concept of running back stays, especially on a smaller Tri that will have a powerful headsail flown from a pole like a Schreecher, and ended up adding them to my standard Farrier Tramp Tri using Farrier’s Super Tramp drawings for reference as it had a taller more powerful rig with running backstays from the factory.

    With the lighter duty beach cat mast extrusions those boats had, the mast was sensitive to loading (bendy), and big headsails would induce back bow in the mast affecting the main sail shape. The addition of running backstays allowed me to maintain proper forward bow and kept the rig well aligned to better maintain sail shape, especially in rolling swell and downwind, which kept speed more consistent and the boat sailing flat.

    Thanks to the wide beam and its outer anchoring tangs which were welded to all Tramp models, I only had to modify the mast to incorporate the back stays, and they weren’t much of a nuisance when the main was eased out.

    On the Super Tramp setup, the forestay and upper shrouds all had their own connection points, and per the drawings I added the running backstays between the two using the thru bolt with and inner bushing to which outer tangs were attached each side to connect the back stays.

    I like the idea of separate connection points for independent adjustability, but also as a backup in case a main shroud lets go.

    Isn’t your setup going to be a rotating rig?

    I would think with a rotating setup, you would almost have to go the bridle route at the mast with the shrouds and backstays connected and splitting off to their respective points on deck.

    And Dragonfire is an interesting design in that the floats (judging from the photos) don’t seem to adopt the current high volume philosophy as they appear very streamlined (when for example compared to the boat behind it in the one photo whose floats bulge out above the water line)m and they appear to be more of a semi submersible design when under power, and yet the boat seems to perform very well with the main hull lifted out of the water, which I assume is in part due to the foils of the dagger boards.
    Last edited: Jul 10, 2023
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  3. revintage
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    revintage Senior Member

    Yes, rotating rig, using a NOS Marstrom alu Tornado mast tube. Note that the forward shrouds only take up side forces. they do not help against forces from the rear, as they are on the same plane as the mast base. So as long as the main sail is sheeted, everything is fine. The rear shrouds/back stays must be there to take the rest.
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  4. Skyak
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    Skyak Senior Member

    My view is that this stay setup is a winner by allowing a lighter stiffer construction when designing to the same safety margin. It is not a trick that can be applied to a trimaran that was designed for a bridal at the float to get any gain.

    The rules of compression members are always the same -don't apply loads that result in bending, and avoid shallow stay angles. The Dragonfire moves the stays off the ama to the water stay position to reduce bending stress on the aka ends in one plane (this is the limiting factor in constant section aka). The bending introduced in an orthogonal plan is likely not a limiting factor that would result in needing to increase strength and stiffness (weight) of the akas. My view is that this is the "best design" only because the Akas are limited to being constant section, strength, and stiffness and it is not beneficial on a tri with less aggressive narrower beam.
    Last edited: Jul 11, 2023
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  5. revintage
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    revintage Senior Member

    Took the 1st image from a Capricorn F18 manual, but it is the same hounds fitting I will add to my Tornado tube. As it will be enough with forestay and two shrouds in the the lower shackle, my idea is to add the rear stays(red) in the upper shackle instead of trapeze lines. I guess the difference in depth wrt mast rotation center will not be a problem.

    Hounds.png IMG_2787.jpeg IMG_3086.jpeg
  6. patzefran
    Joined: Feb 2011
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    patzefran patzefran

    I first used a Tornado mast on my Strike 20, without backstays, and shortly I bent the mast sailing downwind with genaker !
    Regular stays induce very high compression downwind on the mast, as the fore and aft distance of the attachment point to the mast step is way too litle .
    Since I use backstays from aft beam at waterstay intersection and never had any problem. Backstays are mandatory ! I use a big shackle at the hound and there is no problem to
    host forestay, stays and backstays ! as my Strike 20 beam is high (5.10 m !) the backstays doesn't interfere with mainsail downwind.
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  7. revintage
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    revintage Senior Member

    Patzefran, suppose you began with the shrouds to the original 5.8 chainplates that are to far forward. The 5.5 akas I have, has the chainplates ca 65cm behind the main beam, while 135cm is what at least should be needed. I will splice four stays and try it out. Will go up from 1/4" to a 8mm shackle. My beam is close to the same at 4.8m.
    Last edited: Jul 11, 2023
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