Twiggy MKIII

Discussion in 'Multihulls' started by rapscallion, Feb 5, 2015.

  1. ALL AT SEA
    Joined: Nov 2013
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    ALL AT SEA Junior Member

    Congratulations, from this photo she looks tidy and light.

    From my experience sailing a twiggy, i often wonered what adding some volume to the main hull bow would do- making it vertical.

    My only worry would be tacking, as the pivot point is already a long way back. I think, though, that if the forefoot knuckle was at the waterline, not below, tacking wouldn't be impaired too much.


    I think any modification to the amas would have to be fairly dramatic to have much effect... I dont have enough understanding of foils and structural loads to comment.

    Enjoy her.

    Thomas.
     
  2. triple jim
    Joined: Aug 2015
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    triple jim Junior Member

    All at sea,

    Thanks for your message.

    Vertical bow / tacking, your comment really makes sense . i sailed recently a Twiggy with a vertical bow extension. The owner has also lengthened the aft which allowed him to moved the rudder backwards.

    My Twiggy has just been weighted : she is 1600 kg with all sails, anchor/chain on board., included a "not so heavy" diesel (80 kg). No food, nor cooking ustensils, 80 l to add in the tanks. It seems quite good even if it's way more than Lock's numbers.
     
  3. rogerf
    Joined: Apr 2012
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    rogerf Junior Member

    Took me a while but here it is :)

    image.jpeg
     
  4. Twiggyman
    Joined: Jan 2013
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    Location: Vancouver Island BC Canada

    Twiggyman Junior Member

    Twiggy Mk 2 vs Mk 1

    You may find the modifications Lock made to generate a Mk 2 Twiggy informative.
    He changed to plumb bows, topside flare, cold molded round bilge which gives more volume down low, and he added more volume forward in both the floats and the mainhull.
    I did takeoffs of float frame areas for both versions and calculated volumes for comparison.
    Float displacement: Mk1= 4010 lb, Mk2 = 4950 lb. , a 125% increase.

    also
    float displacement forward of the front beam: Mk1 = 1800 lb, Mk2 = 2500 lb.
    So quite a bit more stability up front.

    If you compare the plan view shape of the hulls you can see the forward shift in volume, and the additional forward deck width in the float and mainhull.
    Using the design racing displacement of 3200 lb, the % displacement of the floats is: Mk1 = 125% , Mk2 = 155% .

    The Mk1 version was intended to fit a Jester class racing rule for max waterline length, and the overhanging bow would have added forward stability to that length. The Mk2 was not intended to fit this rule - so the longer waterline and greater volume forward were included for more stability.

    If you want to modify your Mk1 Twiggy the changes Lock made in the Mk2 indicate how he thought the performance and stability could be improved.
     

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  5. oldsailor7
    Joined: May 2008
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    oldsailor7 Senior Member

    has anyone still got Twiggy plans available.
    We should all be trying to save Locks plans before they all disappear for ever.:(
     
  6. tane
    Joined: Apr 2015
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    tane Junior Member

    ...I wonder about the rearward diagonal stability what with the float being fine at the rear & ending well forward of the main hull's transom...
     
  7. Corley
    Joined: Oct 2009
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    Location: Melbourne, Australia

    Corley epoxy coated

    I've never heard of a Twiggy capsizing diagonally rearwards over a float transom it has happened with some F-boats though.
     
  8. tane
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    tane Junior Member

    ...my multihulls-expertise is limited to cats (& there wharrams ...), but in the thread abt historical multis the much improved diagonal stability of contemporary multis was emphasized: longer floats, real "transoms" on them & of course in general more volume
     
  9. triple jim
    Joined: Aug 2015
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    triple jim Junior Member

    There is one Twiggy in Formentera with longer floats. The floats have more volume forward but have also been made longer of 1,80 /6' aft. As said above, the main hull of this Twiggy has got a vertical bow and the tiller has been moved backwards along with a main hull aft extension.

    I found this Twiggy more difficult to tack than mine. All is a question of balance.

    As for now, knowing my Twiggy lacks a bit a volume forward, I'm quite glad to see my floats short as their are at the aft.
     
  10. Corley
    Joined: Oct 2009
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    Corley epoxy coated

    Trimaran hullshapes are important you have to be a bit careful with big float transoms they can act to force the opposite float bow down in a following sea. Most modern trimarans exhibit moderate transom volumes in the floats and it's often rockered up a bit to stop immediate immersion. You do want some volume back there to prevent squatting upwind though.
     
  11. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    The Twiggy was designed in 1977. In this time reducing drag was a very important factor. Also Twiggy was designed for the OSTAR - an upwind race. Designers of this time were producing boats that were not fast enough to blast through waves like tris of today. They had to conform to the wave surface. A short float stern allows the tri to sail to windward without the float stern pitching up the whole boat. (This is because the boat is going over the wave top at an angle of 45 degrees.)

    This article from 1982 shows how Shuttleworth used the idea of reducing float stern volume to reduce pitching.

    http://www.shuttleworthdesign.com/BritFertalk.html

    IIRC Lock alos liked the idea of keeping the rudder well immersed by ensuring the float stern would not lift the main hull stern.

    cheers

    Phil
     

  12. oldsailor7
    Joined: May 2008
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    oldsailor7 Senior Member

    Excessive forward extension of floats is not always a good idea, especially if the floats are fine forward, or have a broad flat fore deck.
    Extending the stem downward to get extra buoyancy may, if overdone, slow the boat in turns.
    Bulbs at the bow are better, as, if they are correctly designed, they provide extra buoyancy without the sideways drag.
    Lock first employed this on the ocean racing catamaran "Bagatelle" with great success.
     
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