Ama % references

Discussion in 'Multihulls' started by AnthonyW, Feb 13, 2015.

  1. AnthonyW
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    AnthonyW Senior Member

    Hoping someone can assist - many articles and books reference ama %s as a function of displacement. Is this % volume over total bodyweight, and per ama, or collectively for both? Battling to find the definition of his calculation.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    The percentages normally given(100% thru 200+%) are the percentage of total displacement of each individual ama's total buoyancy.
    Some ama designs are significantly below 100%. For instance the hull portion of the ama on my Test Model is 28% and the whole 2 stage ama is 56%. But those low figures are suitable only for small tris, preferably square or over square. Most offshore tris tend to be over 200% these days. But the "flotation" of many offshore tri's is augmented by lifting foils on the ama allowing the boat to push harder in racing conditions. At least 200% and/or lifting foils is needed on most designs that want to fly the main hull.
    Hope that helps.....
    UPDATE- This is better ,more precise answer than is the first sentence above:
    The design figure for the weight of the amount of water the total immersed ama displaces is a percentage of the TOTAL designed weight of the boat including crew.
     
    Last edited: Feb 15, 2015
  3. AnthonyW
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    AnthonyW Senior Member

    "The percentages normally given(100% thru 200+%) are the percentage of total displacement of each individual ama's total buoyancy...." presumably as a % of the total weight of the boat?
     
  4. AnthonyW
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    AnthonyW Senior Member

    Ps

    I saw your model foiling - nicely build model. Lot of work into that - well done. I know not every one is that into foils - which must seem to you like people resisting the combustion engine in the early 20th century - so well done for perservering. I must confess it is not something I understand, but I imagine once you experience it it must be quite addictive.
     
  5. Corley
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    Corley epoxy coated

    A good way of considering the % of ama immersion on is to look at the displacement of the boat at its drawn waterline and make it say 200% of that figure. The 200% also caters for dynamic events for example only half the float is immersed coming off a wave etc. It's good to have some spare volume in that situation. A problem for many trimarans is they end up being heavier than drawn or are overloaded when built so tend to see more float immersion than might be desirable for best performance.
     
  6. Doug Lord
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    Doug Lord Flight Ready

    =========================
    Anthony, 100% ama buoyancy*= % of total displacement in my response-just what Corley said above.
    * that is with the whole ama submerged not on its design waterline. The percentage can give you a rough idea of how much the ama will submerge if the boat were to fly the main hull.
    ==
    Thanks for your kind words about my test model-hopefully I can get a full size version built to prove what the model has already shown is possible.
    The design of small tri's has been stuck in a place where most everybody accepts that a beach cat is always faster than a beach trimaran. Taint so and more and more I think we'll see examples by innovative designers who recognize the full potential of the trimaran platform-not just for speed but for excitement , ease of sailing, and room especially in boats from 12' to 20'. Exciting new designs are about to burst onto the scene like the 19' Exocet trimaran with UptiP ama foils and T-foils on each ama. Ted Warrens Ultralight 20 is an example of a light fast 20 footer where ama foils are an option. The next few years will be most interesting for small cats and especially for small trimarans.
     
  7. AnthonyW
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    AnthonyW Senior Member

    Thanks

    Thanks for all the replies - very helpful and much appreciated.
     
  8. ThomD
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    ThomD Senior Member

    No, boat weight and displacement are different things. On a tri, design displacement is roughly 50% more than all up weight of the boat, rig, etc... The 50% being the load carrying ability of the boat. On cats the load can be 100% of the boat.

    ---------------------------------------------------------------------------------

    So if you had a trimaran that was say 10K pounds, load potential of 5K, but empty, you take it out for a day sail, you add 200 your 100% amas, could be functionally on that day 150% amas, and you are short the extra 50% of weight that constitutes a big part of your righting moment. Zero boat weight, zero stability.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    The design figure for the weight of the amount of water the total immersed ama displaces is a percentage of the TOTAL designed weight of the boat including crew.
     
  10. ThomD
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    ThomD Senior Member

    I don't think that is right, or clear:

    Are you saying "TOTAL designed weight of the boat including crew"; or total designed weight plus crew. I don't think either is right by the way.

    What does including crew mean, in the sense of what is the weight for that? Never seen a boat plan that specified the weight of the crew as a design factor, other than perhaps solo, though the weight was not specified there either. Have seen canoe plans that specified weight.

    My kurtboat is supposed to weight 800 pounds, it displaces 1820, and the amas at 150% would be 2730 fully immersed. That is how I see it. If thee is a fully displaced number for the amas on the sheet, I will give it.
     
  11. catsketcher
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    catsketcher Senior Member

    Thom I think different designers have different methods. Another term is cruising payload (on top of boat weight) or cruising displacement - both added together.

    The physics of the situation is clear however. As an ama is needed to float whatever weight the boat is then this should be the figure that is used when talking about ama percentages. If the boat is designed to be 1000kg with 200% floats then each float should have a volume of 2000 litres (approx). When this racing boat gets older it will (as we seem to do as well) grow heavier. The ama percentage then decreases correspondingly.

    Interestingly I saw a little piece in the Shuttleworth design article I first read 25 years ago that is somewhat incorrect. When I was designing a tri I worked out the angle of heel when the main hull lifted and worked out the force the rig would produce in a downward vector. I then added this to the ama volume to get 150% floats. That is what Shuttleworth said to do in the article.

    The after reading "The symmetry of sailing" I realised Shuttleworth was wrong, at least when sailing in a static sense. When a boat is sailing at a constant velocity it is in balance - all the forces are equal and opposite. So the downward force from the rig is exactly opposed by a corresponding upwards force from the angled centreboard and rudder. So you don't need bigger floats for heeling -

    Until you jump off a wave and hit the trough. This is where you need to extra volume. If the boat falls off a 2m wave it will accelerate at 9.8m/s/s. A normal sized 30ft tri will probably immerse its float at least 0.5m so it has to decelerate 4 times faster - about 4 G. The figure gets worse if the waves are bigger and the fatter the floats the higher the G and the heavier the boat gets momentarily. As the boat is accelerating the board will not oppose these forces and so you will need much larger floats to cope with landing from a wave. This seems to be the case with tris being easier to drive hard inshore rather than in big waves.

    Easiest thing to do is to get an accelerometer app on your phone and go sailing with it. This is something I will be doing with my little folder in a few months.

    So your 200% floats are 200% of whatever weight the boat happens to be in a purely flat water - until you start going up and down and then they are anyone's guess.

    cheers

    Phil
     
  12. Doug Lord
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    Doug Lord Flight Ready

    =============
    Thom the total load of the boat including crew and "normal" supplies has got to be figure used to give accurate(and meaningful) information when compared to the weight of water displaced by the totally immersed ama.
     

  13. hump101
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    hump101 Senior Member

    Even in a static sense this is not correct, since the foils are one of the components resisting sail side force, but not the only one, particularly at heel angles where the displacement of the lee hull is becoming critical. The lee hull itself will provide an increasing amount of the side force as the boat heels, and typically will not develop a significant vertical component of dynamic lift even when heeled for a long slender hull.

    Consequently, you do need to consider a proportion of sail downforce to determine appropriate lee hull buoyancy. This force can exceed vessel displacement in racing boats at very high heel angles (>45 degrees), hence 250% displacement lee hulls.

    As an aside, the mass quoted as the displacement of the boat is literally that - the mass of water displaced by the hull in whatever condition is under consideration - so typically reflects the sail-away mass with all crew and stores on board. Problems arise when the definition of "all crew and stores" used by the designer is different to the owner, so it is imperative that the operater understands what were the design limits, and what they actually have on board.
     
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