Small trimarans under 20'

Discussion in 'Multihulls' started by Doug Lord, Jun 24, 2012.

  1. Dolfiman
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    Dolfiman Senior Member

    Doug, Wayne, my tentative specification for a daysailer 20' trimaran is :
    - amas with 50% to 75% of buoyancy (as designed) + potentially 100 % to 125% of dynamic lift (as expected) when boat speed is around 10-15 Knots (% means % of the total weight with crew).
    - with light winds 6 Knots, to rely on the buoyancy of the leeward ama while minimising the wetted surface : 50% buoyancy amas should be sufficient for that, and board amas inclination should help the wetted surface reduction.
    - with wind 10-12 Knots, to rely on 50 % buoyancy + 50% dynamic lift
    - with wind 16 Knots, to rely on 25% buoyancy + 75% dynamic lift
    - in case of gust at low speed or speed recovery after a tack (your comment Doug), well it is a bit the unknown part, in the other hand the crew good reaction is well known : to ease the sails and to progressively recover speed, we are in a daysailer context. It is there the question of the right trade off for the ama buoyancy %.
    - my reference boat for the study : the last VPLP design Astus 20,5 with high volume floats
    L'Astus 20.5, un petit trimaran pour aller partout - Mers & Bateaux https://mersetbateaux.com/lastus-20-5-petit-trimaran-aller-partout/
    >> I have not of course the pretention to design a faster tri, it is just the exercise of a "think different" approach aiming a lighter and easy speedy tri. Windsurfers can go very fast with a lot of dynamic lift, can boards shape amas cope with that kind of spec ?
     
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  2. Tink
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    Tink Junior Member

    I used to sail a mono-maran, or International Canoe. The sliding seat not on provides righting moment but also so degree of buoyancy and lift in the event of a leeward splash down.

    A trimaran with a wide buoyant Aka where the crew can sit would be appealing to me.

    This goes in the direction I admire Secret Harbor Boatworks http://canoetri.com/canoetri.html#p1

    PS - should I be using Thumbnails or Full image and is there an easy way to get the picture in the bottom. Normally the picture inserts at the top
    C49EDC67-3A78-4A91-AAD8-809F8C569FDA.jpeg
     
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  3. Corley
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    Corley epoxy coated

  4. Doug Lord
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    Doug Lord Flight Ready

    Corley, very good looking little tri-like all of Kurt Hughes stuff-thanks.....
     
  5. Doug Lord
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    Doug Lord Flight Ready

    ==========================
    Dolfiman, would you want your boat to fly the main hull?
     
  6. Cholsson
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    Cholsson Junior Member

    Today I was pushing my poor little trimaran a bit further. Almost manage to fly the centre hull, but not 100%
    But... its wet :)


    (No 360 video today, just plain Garmin Virb)
     
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  7. Dolfiman
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    Dolfiman Senior Member

    Not the priority, the first objective would be to demonstrate that one can go easily fast with an ama surfing thanks to a strong dynamic lift (and within an excellent lift/drag ratio, the key point) while the main hull still flush to the water surface.
     
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  8. Doug Lord
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    Doug Lord Flight Ready

    Thanks!
     
  9. waynemarlow
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    waynemarlow Senior Member

    Dolfiman
    That's the conundrum, at 16ft its not unreasonable to expect to be able to self recover from the inevitable, at 20ft its got a whole lot more messy and more cumbersome and probably out of the ability of most day sailors. My guess is that at 20ft a wide trimaran would be pretty impossible to recover once knocked over and I think Farrier talked about and proved that was so. He came up with the some solutions such as sinking one of the amas and letting go partially of one of the side stays to cant the mast, but as soon as the mast totally inverts its all over. I tipped my 16footer over and had to be righted as I couldn't self right and eventually the mast filled with water. I'm going to have to look further into this as the Weta seems to be able to self right.
     
  10. Corley
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    Corley epoxy coated

    The weta system allows you to flood a float in order to be able to remove its buoyancy. I've seen some photos of reasonably big trimarans like a 23' Tremolino be righted with a water bag fore and aft. Looks like a time consuming process and you would need plenty of sea room to do it. Our club is getting reasonably experienced with righting small trimarans the F22's they tow them back over bow over stern using a bridle a Drogue doesn't seem to be necessary but could make the process easier in certain circumstances. The rescue boat needs some power I've heard a figure of around 300hp bandied around for larger multihulls but not sure on that front.

     
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  11. Dolfiman
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    Dolfiman Senior Member

    Yes, it is the tricky point and I have changed my spec on that point : I have sketched attached an alternative "triangular shape" ama with a buoyancy of ~ 105 % to stay in the current standard / capsize risk. Of course the weight reduction is then small (~ 25 kg / 430 kg of the reference boat) but, as long as the expected dynamic lift is there with speed, the RM power can be equivalent and can support the same sail surface.
     

    Attached Files:

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  12. Doug Lord
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    Doug Lord Flight Ready

    Dolfiman, in comparison with the Astus it appears that you keep the beam(cl ama to cl ama) nearly the same-is that right? Could you make some gains with a bit more beam?
     
  13. Dolfiman
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    Dolfiman Senior Member

    Many thanks, very interesting. Clever but yet not easy to do, of course the flooding of one float is a prior but does not help a lot the recovery itself which rely on the righting moment that can create the crew. And as you noticed, that needs sufficient sea room. This tri seems to have small amas, of volume < 100 %, if can judge from the capsize sequence 00:20 to 00:24.
     
  14. Dolfiman
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    Dolfiman Senior Member

    I keep all the same, my study is focused only on the substitution of a classic ama by a board shape one. May be in a second step I will consider a central hull with a dose of scow bow, so to stretch at the max the water lines and favour the surfing mode here too. I am searching reliable formulations for board lift and drag to introduce in my VPP. I have found this interesting article about the maximum speed reachable by a windsurf, by Dietrich Hanke, fluid mechanism engineer and founder of Maui ultra fins, the text is in English after the short introduction in French. Includes a comparison without and with foil, fig. 13 and the text around :
    Quelle vitesse est-il possible d'atteindre en windsurf? https://www.mauiultrafins.fr/en/module/smartblog/details?id_post=12
     
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  15. Tiny Turnip
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    Tiny Turnip Senior Member

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