Twiggy MKIII

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

  1. rapscallion
    Joined: Oct 2006
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    rapscallion Senior Member

    Hasta la Vista is a very modern looking design - very appealing to the eye.
     
  2. Twiggyman
    Joined: Jan 2013
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    Location: Vancouver Island BC Canada

    Twiggyman Junior Member

    Best curved lifting foil location

    The Twiggy Mk 2 also has 150% floats at the vessel racing displacement of 3200 lbs. Each float displaces about 5000 lbs when fully immersed. In cruising mode when the vessel is loaded to max 4300 lbs they provide about 120%.

    Any thoughts:
    To make a Twiggy Mk 3 where should curved lifting foils be located longitudinally along the floats?
    Just forward of the forward beam, or aft?
    Forward or aft of the center of float displacement?
    Catri has them forward, Farrier has them aft......
    The forward beam on Twiggy is already located at the center of float displacement.
    Pro's and con's?
    Assume a T foil on the main rudder, too.
     
  3. rogerf
    Joined: Apr 2012
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    rogerf Junior Member

    A vessel racing displacement of 3200 lbs, or 1.5 tonnes, should be revisited. I would say that once loaded for racing the vessel will be well over that weight. In fact I doubt that few if any Twiggys ever achieved that weight dry. Just look at pictures of Ian Johnstons boat prior to racing - bow down with both floats in the water.
     
  4. Gary Baigent
    Joined: Jul 2005
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    Gary Baigent Senior Member

    I always put the float foils just forward of the main beam - and my single beam designs rake forward too, so say close to 55% position on main hull WL ... but it was very educational to see foil positioning on the AC72s, at 50% or maybe even a little less. But then my boats have never tipped/crashed their bows in like the 72s have done. But the 72s are, of course, very much faster.
     

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  5. Marmoset
    Joined: Aug 2014
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    Marmoset Senior Member

    if the mast moves back would that de-sensatize foils?

    Barry
     
  6. Gary Baigent
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    Gary Baigent Senior Member

    Take a look at foil and rig positions on the new, soon to be launched maxi trimaran VPLP Macif.
     

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  7. Tom.151
    Joined: Jul 2009
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    Tom.151 Senior Member

    I'd love to see which picture you refer to rogerf and dated if you have that info

    Ian and Cathy Johnstone sailed long distances to get to race venues, so a heavily laden boat under some conditions would hardly be a surprise -- and is hardly a basis for evaluating the as built weight of the boats.

    My Mk1 (NOT built by me and clearly built from best materials and very carefully as could be seen from her unpainted interior) sat in flat water with one ama more than 18" clear of the surface. That was with Gougeon B-section, 6 bags sails, galley & nav gear, outboard, ground tackle - but no food stores. waters, or fuel. To me that implied the boat was near the 2200# bare weight expected.
     
  8. catsketcher
    Joined: Mar 2006
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    Location: Australia

    catsketcher Senior Member

    Roger owns the lightest Twiggy Mk1 ever built in Australia. My Twiggy was built well and light out of rainforest timber and had its float almost 400mm out of the water when superlight. I remember the outboard didn't sit properly in the water when I took the mast off and its weight was gone.

    Roger's Kiska is the pre-eminent example of a timber Mk 1. Jeff Owen, the builder, reputatedly allowed to weigh each individual sheet of ply that went into the boat. Kiska still floats high last time I saw her. I am sure Roger can see the effect of adding small weights to Twiggys's. When we were cruising we sent home 32kg and I could see its effect. They are very weight sensitive.

    Some back of the envelope calcs

    LWL - 8.5m
    Bwl main hull - 0.8m
    prismatic - about .62
    Waterplane area = 8.5 x 0.7 x 0.62 = 4.21 m^2

    Waterplane area is what gives you the immersion rate, To sink the boat one cm you have to push down with a force equal to the bouyancy force of the immersed volume = 4.21 x 0.01m = 0.0421m^3 - 42 litres or roughly 42 kilos of water mass.

    So if you put 400kg of crew, food, fuel safety gear, and water on a Twiggy it will sink about 10cm. As the float lever arm is double that of the main hull the float will sink double so it will go down 20cm.

    It is always easy to underestimate the weight on boats. There are very few that float high. On top of immersion rates Twiggy Mk 1s also can bend their crossbeams differently. I kept mine too loose until I sailed on another offshore. I then tightened mine up and the floats were appreciably closer to the water on the mooring.

    cheers
     
  9. rogerf
    Joined: Apr 2012
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    rogerf Junior Member

    There is a picture in Cathy Hawkins book "Return in the wake: The true story of a woman's adventures at sea" of their Twiggy prior to a race. The book is well worth a read.
     
  10. rogerf
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    rogerf Junior Member

    The story goes that Jeff Owen drew up a schedule of timber and ply and calculated that the build weight would exceed the design weight. Apparently after contacting Lock he replaced all the stringers with WRC leaving Oregon (spruce) for mast compression and other vital pieces. Regarding the ply he had the specs for the bruynzeel gaboon marine ply and rejected any sheet that was overweight.
     
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  11. Corley
    Joined: Oct 2009
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    Corley epoxy coated

    Just on ply weight it's amazing how much sheets can vary I've been slowly buying 3mm sheet for my CM project and have been finding 200-300gm variation not uncommon. Luckily my supplier is a patient sort and doesn't mind me weighing each sheet. I'm sure he is rolling his eyes behind my back though, lol.
     
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  12. Marmoset
    Joined: Aug 2014
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    Marmoset Senior Member

    yeah when your feeding a 40 i'm sure they'll tolerate it, but bust there chops over a dinghy, It'll be a different tune.


    Barry
     
  13. triple jim
    Joined: Aug 2015
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    triple jim Junior Member

    I just bought a Twiggy Mk 1. I was aware of the aft volume issue but I have a few questions that may be some of you could answer.

    Is an extension of the volume of the amas at the bow making sense ? It will stay marginal en terms of %. The owner of the "Multisailing" Twiggy told me that he once had to throw his Twiggy sideways down some steep 4 m deep, short and violent waves, as he trusted the lateral stability better than the fore/aft stability. His Twiggy has been modified with longer floats, with vertical bow offering more volume but it doesn't seem to really solve the problem.
    May be some of you know of some successfull amas extensions ?

    Keeping the amas as they are, could you advice on sail reduction related to wind speed in a downwind situation ?

    Thanks in advance

    Jean Marc
    Twiggy Cajka
     

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  14. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Hello Jean welcome to the forum,

    I have not owned a Twiggy for 17 years but I will give you a few tips from what I learnt sailing a great boat.

    The Mk1 has a low prismatic hull. It was designed in 1977 when designers were trying to reduce drag rather than cope with power as they did later. As such Twiggy has what today's designers would call fine bow shapes. Lock put on a large rig and tried to get around the problems of having a large rig on a fine boat by moving the rig aft. This gave more forward stability.

    Lock also designed the Twiggy after playing around with some simple computer models. He worked out that if you tacked the boat downwind then the thrust from the rig would always be over the ama - around where the front beam joins the ama. So he put a lot of volume there. You will notice the ama is deep under the front beam.

    Two Australian Twiggys capzized in the 80s. Twiggy went over twice in Europe and Sailmaker went over on the Australian East coast. All times when racing. What is more important is that in all times they were square running under kite and then ran down a large wave.

    Mono sailors will learn quickly to bear away when hit by a squall. When hit by a large squall you may bear away onto a square run - this is when all the Twiggies pitchpoled. I found out something similar when hit by a large front and ran downwind. I was scooping up the waves with the foredeck and filling up the cockpit IN THE HARBOUR. The bows were driven down by the pressure of the mainsail - the jib was blanketed.

    When I came up in the wind to pull down the main as soon as I turned onto a broad reach the boat was well balanced, even and easy to handle. Lesson 1 - never square run a Twiggy when heavily pressed.

    Adding volume to the ama bows is not as important as adding it to the main hull bow if you want to change something. The Twiggy's Achilles Heel is the volume up front and not diagonally. So volume in the main hull bow will assist more than the ama bows.

    That being said - Twiggys being cruised around suffer few effects and I have not heard of any flipping in the last 30 years. Square running is fine if done without an enormous press of sail and the boats love a beam reach in any wind. They are enormously stable sideways. Once we rounded up, rare in a Twiggy as we were under headsail only, and a very large swell tipped us over so that my friend slipped from the windward seat and fell onto my head as I ducked down to help the autopilot from the leeward seat. Then we straightened up and headed off again. I don't know how far we went over but I was glad of the wide beam and easily permeable nets.

    cheers

    Phil
     

  15. triple jim
    Joined: Aug 2015
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    triple jim Junior Member

    Phil,

    Thanks a lot for all the infos. It's really helpful.

    Cheers

    Jean Marc
     
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