Advantage and merits of Ama daggerboards

Discussion in 'Multihulls' started by waynemarlow, Nov 30, 2008.

  1. aussiebushman
    Joined: Oct 2009
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    Location: Taralga NSW

    aussiebushman Innovator

    Actually, the depth of the hull has increased by only the thickness of one plank, namely 10mm, because all of the additional volume was achieved by reshaping the sides into "fatter" curves. You will note that each hull is no longer symetrical. This means the current daggerboards extend 10 mm less into the water than originally, but due to the extra volume, maybe it would be prudent to make them longer without changing the shape and therefore the case. This would be a very simple job.

    The revised ama shape was achieved by epoxy-bonding foam formers to the original hull, then strip planking with Paulownia (not much denser than Balsa and with all the best properties of WRC). This created the additional volume with remarkably little weight penalty (up from 35 Kg to around 50 Kg - a small price to pay for the benefits.

    Before posting a picture I was going to wait until after this hull was glassed and faired, whereas it is currently only bogged ready for glassing. For interest, the beams have been reinforced with keeper plates under the bolts, also 90 degree tangs glassed to the original hull.

    The existing beams will be telescopic and If you can see the upright "boxes" above the beams in the picture, these are where the new upper beams lock into the lower beams when extended - more on this later

    Regards

    Alan
     

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  2. Tom.151
    Joined: Jul 2009
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    Location: New England, USA

    Tom.151 Senior Member

    For shorthanded cruising there are some rules ;) ...
    * you will foul the boards on pots or trash (maybe even the bottom)
    * this will happen when you're not paying attention
    * this will happen at the worst navigational moments
    * this will happen in the worst weather conditions

    I love the idea of boards in the floats for smaller (20 to 30 feet) trimarans to open up the interior for accommodations, but I remember the rules....

    Vastly simpler, IMNSHO, are boats that have the mast located at the front of the cockpit so the daggerboard can be fitted immediately behind the mast in the sole of the cockpit. Handy, easier to adjust, less intrusion into the cabin proper. Nice boats like the Newick Val and Crowther Twiggy are some v.good examples of shorthanded 30' cruisers.

    Sure there are compromises you have to make - just make good ones ;)
     
  3. Fanie
    Joined: Oct 2007
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    Location: Colonial "Sick Africa"

    Fanie Fanie

    I have a Q for you trimaranners. Why are you always trimming the ama's to as small as possible ?
     
  4. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    --------------
    I can only answer for myself: my primary interest is in small tri's less than 20'.
    It has been my contention that designers, for too long, have designed these boats like larger cruising trimarans rather than raise hell sport boats they could be. Now, don't get me wrong there is a place for every kind of sailing from low key gunkholing to blast out speed sailing. But what has sorta startled me is that, as best I can tell, there is not one-not a single design or production small trimaran anywhere designed to fly the main hull!
    So in my research on this subject I've found a couple of things that may be worth considering:
    1) the ama must be small-less than half the size of the main hull to save weight and allow high speed,
    2) a designer that is trying to do a blast out, fly the main hull small tri must understand hydrofoils because to harness all the power of a square, small tri foils are a necessity not a nicety.
    So Fanie, for the tri designs I'm interested in very small ama's are critical in keeping weight low while at the same time being designed to support the weight of the whole boat at speed.
    Why do you ask?
     
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  5. Fanie
    Joined: Oct 2007
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    Location: Colonial "Sick Africa"

    Fanie Fanie

    Personally I favour larger ama's, for various reasons, one is capsizing is easier with smaller ama's.

    There would probably be a balance where the larger surface area vs the buroughing of hulls equals out for max speed - in which case you should have the ama at about 30 deg mounted to the vertical to obtain max performance with the rest flying. Then of course you are back to square one - it's a small mono carrying a lot of weight :D

    My own experience re tri's is also small ones, I somehow manages to sail the ama right under the water. I'm sure if they were larger, and can carry the sail force the speed would go up. The advantage is a little in weight and it's fine to about 18km/hr where the ama is already low on booyancy, more wind and the ama bury and I'm stuck at 22km/hr, even with the turbo on :D

    A planing ama should have less drag than a sunken one, even if you add a bit of weight, right ?
     
  6. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ===========
    I think a planing ama and/or lifting foils on the ama should do the trick. The idea I'm exploring is using lifting foils on the daggerboard and rudder with a wand(altitude control system) that would enhance the pitch stability of the boat while controlling the pitch attitude of the ama(and main hull).
    In exploring this I found that a square 18' tri with two guys on trapezes could have TWICE the righting moment of an F18 cat with two guys on trapezes!
    I also discovered that you don't need all that power to have an 18' tri beat an 18' cat.....
    But for an all out speed boat a tri should be the fastest multihull-bar none-under 20', in my opinion.
     

  7. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    Here is a 22' tri designed and built by Henrik and a friend. Just launched. boards and rudders in each ama. Designed to fly the main hull......

    See the SA thread here: http://forums.sailinganarchy.com/index.php?showtopic=82523

    There is a thread here but not as advanced as the SA one....
     

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