Hollow Sections aft on ULTIME Trimaran Amas

Discussion in 'Multihulls' started by Doug Lord, Mar 28, 2018.

  1. philSweet
    Joined: May 2008
    Posts: 2,166
    Likes: 122, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Those hollow sections aft are what happens when you need to develop something like 20 ton - meters of bow up moment so you don't stuff the bows when you have 150 ton - meters of heeling moment on. When the proportion of lift coming from the foils set, hull, and ama is varying constantly due to waves and speed changes, it is important that all have a fairly close pitch moment center. Those amas push the center of buoyancy and the center of dynamic lift further forward. I suspect they are designed mostly with an eye towards stable take-offs and landings.

    In airfoils, this is called reflex. It refers to the presence of negative camber in aft portion of the camber line. It causes a strong change in pitch torque with changes in angle of attack. Somewhat similarly, these ama shapes result in higher pitch stiffness, which helps to manage the foil angle of attack.
     
  2. Doug Lord
    Joined: May 2009
    Posts: 16,372
    Likes: 279, Points: 93, Legacy Rep: 1362
    Location: Melbourne, Florida

    Doug Lord Flight Ready

    Interesting shot of Gitana 17 planing on the aft portion of the stb ama:

    Gitana 17 at the Route du Rhum start.jpg
     
  3. bjn
    Joined: Jul 2014
    Posts: 130
    Likes: 7, Points: 18, Legacy Rep: 16
    Location: Stockholm

    bjn Senior Member

    I don't think it's planing. I think it's crashing down from having lifted out of the water. Either from pitch instability or from exiting the crest of a wave.
     
  4. Doug Lord
    Joined: May 2009
    Posts: 16,372
    Likes: 279, Points: 93, Legacy Rep: 1362
    Location: Melbourne, Florida

    Doug Lord Flight Ready

    Maybe, but in all the video I've watched of the boat flying I've never seen a "crash" but in one video she did a pitchpole wannabe with the port hull submerging up to the forward cross arm and then doing what she was designed to-pop right back up.
    I think the planing shot occurred when she was just starting up with possible max downforce from the main foil and got hit by a gust-hence the pitched up attitude. All three hulls are designed to plane....
    When Gitana starts up ,say on port tack, she frequently is seen planing on the stb ama until she reaches take off speed.
    This video shows her planing on the stb ama 0-16sec in then at about 1:35 in:


    Start at 3:47 in on this video to see the whole start up sequence: 1st planing on stb ama then liftoff of stb ama:


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

    Scroll down to first video to see the "pitchpole wannabe" incident at 1:38 in. Scroll down to "video 3d animation" to see start-up sequence:
    All systems go! - Gitana : Offshore racing stable created by Baron Benjamin de Rothschild http://www.gitana-team.com/en/a-1284/all-systems-go
     
    Last edited: May 2, 2019

  5. Doug Lord
    Joined: May 2009
    Posts: 16,372
    Likes: 279, Points: 93, Legacy Rep: 1362
    Location: Melbourne, Florida

    Doug Lord Flight Ready

Loading...
Similar Threads
  1. nzdavo
    Replies:
    29
    Views:
    3,230
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.