Small trimarans under 20'

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

  1. Lurch723
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    Lurch723 Junior Member

    its A very interesting discussion “hull volume”, I guess it comes down to the simple question: do you want to be a part of the righting moment or do you want the ama’s to do the work for you?

    For me I knew I wanted them to give static roll stability and in addition to trapeze off, as it turns out they work exceptionally well for launching and recovery as you simply sit on a hull and it takes most of your weight while you make your way to the central hull. Now too much volume will create drag as the profile hits the water and cause a helm load to leeward or windward (but only a little)depending on which hull hits. If you design the right profile so the wetted area and bouyancy increases slowly at first then at more extreme moments of submersion the full volume kicks in you get the best of both worlds.

    Of course you must sail the boat flat to make the most of this approach and trapezing really helps here, you can also cut down on outright beam. From experience I can honestly say that directional stability on a tri is excellent and this cuts down on roll and turning moments that you would normally get on a skiff or trapeze monohull.
     

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  2. revintage
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    revintage Senior Member

    Hi Lurch723,

    Admit, interesting subject, some kind of balancing act to get it all together without adding to much wet surface, the tri will anyway have much less than any skiff. I am going for planing amas and later on V foils.

    Noted you used a 49er rig on your tri. I also have two 49er rigs, old style, one with a 1m cut down main and the other original. Thought of using the original but with with the foot cout down to 2,2m on the tri. But will use the already cut down when testing the tri build. Note the old style is much smaller than the present. A little worried though, as the rig needs so high tension to work properly.

    Do you have more images and data of your boat and how you modified the rig? At the moment I am using the cut down on an old dinghy.

    cutfoot.JPG cutluff.JPG View attachment 142316
     
    Last edited: Aug 12, 2018
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  3. Lurch723
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    Lurch723 Junior Member

    Yes they need a lot of tension, I made a boat breaker like I had on my B14 and you just pile on the pounds! What I would say is the standard rig really powers up and you need a lot of righting moment to handle the power. My design was specifically penned around a 49er rig as standard, it made life easier sourcing and maintaining the design. I also learnt that you simply don’t need enormous rigs for offwind speed as a tri generates so much apparent wind

    Looking at your proposed options I would probably go for the second idea where you are losing a panel from the bottom, if you choose the first idea you will need to bring the rig further aft to compensate for the Center of effort moving forward. The one rule you must be aware of is it’s a dynamic rig, the mast and leech work together. As an example sailing upwind while twin wiring I can add and remove power simply by pulling literally 6 inches of main and that tightens the leech and literally switches on the power.

    I have drawings but they are on good old fashioned drafting film, concentrate on focusing the rigs centre of effort over the boats centre of lateral resistance and so long as you are within a couple of percent I doubt you will feel any imbalance.

    Planing AMA’s sound interesting, I guess you are after their righting moment?
    I decided to sail the boat flat and ignore the AMA contribution. Foils are what these things are all about so get creative and don’t be afraid to try stuff.
     
    Last edited: Aug 12, 2018
  4. Doug Lord
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    Doug Lord Flight Ready

    One of the great lessons of Hydroptere's development is the change in her amas from typical low wetted surface displacement hulls to the stepped, planing amas she finished up with. They weren't designed to be sailed "on" , but designed to have minimum drag during contact with the water at speed.....

    original displacement amas:

    Hydroptere displacement amas.jpg

    stepped planing amas:
    Hydroptere planing ama.JPG
     
  5. revintage
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    revintage Senior Member

    Hi Lurch723, think you misunderstood it all, or is my English so bad :) ? My center hull is a 30cm wide and from an A-cat. You might be interested to rephrase, as I get the impression you are refering everything to the fat and heavy dinghy?

    The old dinghy is what I am using the rig on at the moment! It was to show the existing cut down main with 1 meter removed at the bottom panel.

    The problem might be CoE gets to far back, but that can be compensated with a jib.

    I am not worried about the tension on that old one, it is tensioned up like it should. I use a 10:1 boat breaker on the dinghy. I know the rig quite well, as I also having a complete 49er.

    I was worried about how to make the new design with tubing and center hull cope. But 100kg isn´t that much ;-) .
     
    Last edited: Aug 12, 2018
  6. waynemarlow
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    waynemarlow Senior Member

    love the picture with the guy standing beneath the foil, look at the head :D
     
  7. Gary Baigent
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    Gary Baigent Senior Member

    Incorrect, Douglas, your minimum contact is not the point of the stepped floats, it is to quickly promote planing and minimum drag (like hydroplaning powered race boat) - so Hydroptere quickly gets the float clear of water surface and onto the even more efficient foils.
     
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  8. Doug Lord
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    Doug Lord Flight Ready

    The fact is that the planing amas would be an extremely small fraction of the lift required for takeoff*-their primary purpose is to reduce drag when the ama contacts the surface at high speed. The displacement amas worked for the relatively slow speeds for takeoff which required a boat speed of 12kts but when speed doubled or tripled and much more --and they contacted the surface they were a big drag on the boat.

    *in fact at slow takeoff speeds the stepped hulls probably were more drag than were the displacement amas but at high speed-3-5 times takeoff speed- the stepped amas were much, much better.



    ama planing surface touching the water:

    Hydroptere-1 ama touching the water.jpg
     
    Last edited: Aug 12, 2018
  9. OzFred
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    OzFred Senior Member

    Don't bother, you know you will never win. It is a bit weird to use as evidence an image where the drag from the foil itself resulting from a severe ventilation event would be hugely greater than drag from the float, which is barely touching the water.
     
  10. Lurch723
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    Lurch723 Junior Member

    Sorry revintage it’s my fault for not going back through the discussion far enough, please accept my apologies. I would say this about the 49er rig, it’s really difficult to get the mainsail up the luff track when rigging and I hate it. If I had more money I would have bought a carbon Nacra 17 rig and used that.

    Please keep posting because I will be watching your progress
     
  11. Gary Baigent
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    Gary Baigent Senior Member

    As usual, Douglas, you see what your mind wants to envisage ... and not reality (like your fantasies of your momentarily hopping toy of years ago) - this regarding your posted image of Hydroptere where the planing float sections are not working because the float is airborne from foil lift.
    Actually, and forgetting the usual LoRd nonsense, it appears that Hydroptere in that image is skidding savagely sideways, hence the huge area of spray and turbulence being thrown to windward. Here is an old 1970s photograph of a real foiling pioneer, Noel Fuller's Sabrina with also 45 degree foils somewhat similar to the much later Hydroptere - check the different foil spray thrown to leeward.
     

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  12. Doug Lord
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    Doug Lord Flight Ready

    Hydroptere-1 ama touching the water.jpg
    =============
    That picture shows a perfect example of the planing ama working just exactly like it was designed to do!!


     
  13. Doug Lord
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    Doug Lord Flight Ready

    Interesting and remarkable comparison of weight and SA for Hydroptere--and for a Moth for reference:
    Hydroptere SA=5375 sq.ft
    Hydroptere weight: (not incl extra ballast)
    1-7.5 ton-2204.62lb/ton=16534.65lb
    2-crew weight est. 6@ 175lb each=1050lb
    Total= 17584.65lb
    ---------------------
    Hydroptere Sail Loading=17584.65/5375= 3.27lb/sq.ft.
    ------------------
    Moth 175lb crew =66lb boat=246lb/86sq.ft=2.8lb/sq.ft. sail loading
     
  14. revintage
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    revintage Senior Member

    Hi Lurch723, no worries, no apologies needed.

    About the the 49er rig I hate that little problem to. Partly cured it by making it 2:1, with a small block at the mainsail head and the fixed end of the halyard above the mast top pulley. Works like a charm, but plenty of rope ;-) .

    On the dinghy I showed you I am using synthetic standing rigging to keep weight down. Have also calibrated my Rigsense for this.

    I have an old Tornado rig to cut down for the new project, but I am reluctant due to the weight.
     

  15. Dolfiman
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    Dolfiman Senior Member

    This recent video of the new Astus 20,5 , where one can see the central hull spray rails work and some ama wavepiercing sequences :
     
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