Bridgedeck centreboard why don't they work???

Discussion in 'Multihulls' started by valery gaulin, Jan 10, 2017.

  1. brian eiland
    Joined: Jun 2002
    Posts: 4,788
    Likes: 157, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    You would surely think they would say something about it on their website,...or there would be other chatter somewhere?...or maybe at least one photo (not just a dwg).
    I can find NOTHING yet.

    In fact I am still so confused about their website and their subsequent builds, that I am unsure as to whether they are still utilizing any type of skeg, and/or how many vessels they built skegs on.
     
  2. brian eiland
    Joined: Jun 2002
    Posts: 4,788
    Likes: 157, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

  3. pogo
    Joined: Mar 2010
    Posts: 342
    Likes: 9, Points: 18, Legacy Rep: 73
    Location: Germany Northsea

    pogo ingenious dilletante

    "Hissy Fit" , the first Dazcat 1495 , dart shaped keels ( lifting bodies) , conventional positioned daggerboards.

    Here is a test :
    http://www.yachtingworld.com/reviews/boat-tests/dazcat-1495-boat-test-a-cutting-edge-catamaran

    With hoisted boards the center of lift is pushed back --this results in a shorter lever arm for the center of lift for initializing the above explained curve ( post 502).

    “The dart-shaped keels move the lateral resistance further aft when you hoist the boards,” explains designer and builder Newton. “This knocks out a lot of yaw and makes it easier to steer. Daggerboards can be tricky to handle in waves, which is also why we put big rudders on – it’s got to be able to take the rough stuff.

    The primary " leeward preventers" on this boat are the normal big boards.
    With other words , the designer regards the performance of dart keels as insufficient for a Dazcat, but takes all the other advantages of dart keels.
    --less draught
    --sufficent pointing ability ( also in shallow waters)
    --well protected rudder
    --good protection thru delta shape when running over obstacles
    --enough volume deep in the belly for an engine or tanks

    --the advantage of dartkeels with conventional positioned has been described above

    Concerning the tiny canard board, the " zero degree stabilizer", that one that seems to exist only on the drawing :
    perhaps it's advantage , the " new feature" ( avoiding broaching ) , is still too unproofed, too new, too hot to handle ?
    Or, simply unknown.


    pogo
     
  4. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    Attached Files:

  5. pogo
    Joined: Mar 2010
    Posts: 342
    Likes: 9, Points: 18, Legacy Rep: 73
    Location: Germany Northsea

    pogo ingenious dilletante

    Note the shadow, i guess that this keel is not a dart keel, it's a delta keel- a skeg.
    Also interesting , board's slot is not in the delta.
    Newton made a lot of compromises.

    pogo
     
  6. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    The skeg is an interesting design point. You could say its the fixed equivalent of the bow centerboards (mirrored asymmetrical? probably) of the design drawing. The skeg could have positive values but do they outweigh the extra drag? Probably this is for another thread.
     
  7. pogo
    Joined: Mar 2010
    Posts: 342
    Likes: 9, Points: 18, Legacy Rep: 73
    Location: Germany Northsea

    pogo ingenious dilletante

    The mirrored equivalent of the fixed canard board ?
    Mmh.....
    No.
    The farer the delta keel , respective it's center of lift , is positioned towards the stern, the less effective it's function as stabilizer---lever arm.

    The rudderblade , directly under the stern , thus farer aft , can't act as a stabilizer ( a course holding zero degree stabilizer) at all, ' cause of the position of the pivoting point -- at cl of delta keel ( or conventional daggerboard)
    The rudderblade is set to leeward, lifting the stern back to windward , avoiding the curve to windward.
    Note ; we are talking about surfing , broaching , yaw effect.

    pogo


    P.S.
    Few hours ago i explained the above ( everything) my brother. I needed all my hands.
     
  8. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    should have been more descriptive. The port bow canard mirrors the stbd bow canard is what I meant.
     
  9. pogo
    Joined: Mar 2010
    Posts: 342
    Likes: 9, Points: 18, Legacy Rep: 73
    Location: Germany Northsea

    pogo ingenious dilletante

    Oh ****.
    I
    t's too late now to figure out this new constellation ....
    Sorry, but it's 1.41 am.
    I gonna have a drink now , thinking about it while watching a well known movie...
    " Glengarry Glen Ross" , or " A streetcar named desire" ?

    Cheers, c u

    pogo
     
  10. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    and dream of lifting bodies.:eek:
     
  11. redreuben
    Joined: Jan 2009
    Posts: 1,670
    Likes: 69, Points: 48, Legacy Rep: 349
    Location: Beaconsfield Western Australia

    redreuben redreuben

    So.....How does the Dazcat keel prevent leeway ? Is it purely a function of surface area resistance or does it create lift ?
    If it does create lift, how does it do that ? IE; does it have a profile in that it is thicker at the front than the trailing edge ?
     
  12. UpOnStands
    Joined: Nov 2015
    Posts: 681
    Likes: 14, Points: 18, Legacy Rep: 16
    Location: Sydney

    UpOnStands Senior Member

    Edit: Joking aside the Dazcat D1495 & 1395 use regular daggerboards as leeway preventers; the fin-like skeg on the dart-like keel provides yaw resistance and a solid exit point for the props and ??.
    Dazcats would have tell us how much lee resistance the keels provide.
     
  13. brian eiland
    Joined: Jun 2002
    Posts: 4,788
    Likes: 157, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    I think you got it all correct there Pogo, .....including that unexplained canard fin/board/whatever. I do know for a fact that I would NEVER consider such an item on an ocean cruising boat.

    And this correct as well
     
  14. redreuben
    Joined: Jan 2009
    Posts: 1,670
    Likes: 69, Points: 48, Legacy Rep: 349
    Location: Beaconsfield Western Australia

    redreuben redreuben

    Yaw

    Getting back on topic, the dart keels beneficial affect on yaw downwind can be covered by our centre mounted board if it is allowed to swing back a bit to move the pivot point aft.
     

  15. pogo
    Joined: Mar 2010
    Posts: 342
    Likes: 9, Points: 18, Legacy Rep: 73
    Location: Germany Northsea

    pogo ingenious dilletante

    Yepp, but on deep downwind courses only -- apparent wind deep.
    One has to take the sails also into account.
    The cl of the sails under this condition is also further aft, no , it's vector is "deeper" ( related to the sails)
    (And remember --on deep downwind courses normally the board(s) are hoisted.)
    When the boat broaches to windward , cl of lift of the sails goes effectvely forward --to the new apparant wind coming from forward ---the boat suffers on leeward helm now. It dorsn't matter if the centerboard is half down or hoisted, even with or without dart keel. CL of appendages is too far back.
    ( Here we also see again the reason for board(s) hoisted under such condition. Piviting point is further forward which results in a better leverarm for " turning" the boat back. The farer the pivoting point is set forward, the less leeward helm, the easier the recovery down to the former course to the wind)
    You have to get the boat back on the former downwind course now---by ( ventilating? or stalled ?) rudder (in the wavecrest?).

    You see, setting back the appendages ( e.g. via dart keel w. hoisted boars , or back swept centerboards) for avoiding or minimizing the yaw effect is a compromize only.

    ( I guess that the Dazcat 1495 got delta keels for troublefree drying out; rudderprotection. One such a big boat a retractable rudder is complicated and expensive. Speculation: knowing about the avove discribed disadvantage of such a delta keel, set back skeg, the first drawing showed the cure -- the little canard board.
    A better , less complicated and cheaper solution would have been the lifting body, the dart keel)


    Back to bridgedeck centerboards now.
    Yaw.
    Are we searching for a self correcting device ?
    Setting back the appendages is not self-correcting.


    pogo
     
Loading...
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.