Dagger Board with Kick-up Bottom Section

Discussion in 'Boat Design' started by bill price, Aug 22, 2017.

  1. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    I assume your boat is fairly small???
    We had a 21' sloop with a swing keel that was essentially shaped like a daggerboard. Worked great in deep or shallow water and onto the boat trailer.
    Use a daggerboard (one piece), make the slot (casing) long enough for the 'swing', you should be fine.
     
    bill price likes this.
  2. bill price
    Joined: Aug 2017
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    Location: Atlantic. NC

    bill price Junior Member

    Well , I certainly respect knowledge of a 35 year aircraft structural designer, and wouldn't want to waste my time.
    How about tell me why it won't rotate before I build a model?
    Also can you give me a reference to info on gybing CB.
    BP
     
  3. bill price
    Joined: Aug 2017
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    Location: Atlantic. NC

    bill price Junior Member

    JSL, I've had 6 or 8 small sail boats ( up to 21') w/ DB and CB. DB is just simpler than CB.
    I'm designing a rowable 14' x 5. 25' x 4" <> 4' Flat-head sloop, and DB would be very convenient.
    I also know that the DB striking bottom disturbs folks, so the idea of a DB with kick up lower section came to mind.
    Not sure if it's worth the trouble, but maybe worth trying.
    BP
     
  4. Angélique
    Joined: Feb 2009
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    Location: Belgium ⇄ The Netherlands

    Angélique aka Angel (only by name)

    Here's a Dutch link.
     
  5. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    A good rule on boats is KIS ..Keep it Simple.
    Single piece, hinged = simple.
    As to the pivoting (gybing?) for different tacks - don't bother... just another thing to go wrong.
     
  6. bill price
    Joined: Aug 2017
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    Location: Atlantic. NC

    bill price Junior Member

    Very good idea.
    Thanks ,,, BP
    Works really really well.
    Very simple.
    Leading and Aft edges can be shaped.
    I'm designing a Rowable 14' x 5.25' x 4" <> 4' Flathead Sloop.
    DB would be desirable.
    Some folks are uncomfortable with DB strikes.
    This idea should greatly alleviate that concern.
    THANKS FOR ALL the COMMENTS // Bill Price Atlantic NC.
    ( Hope you can see the PDF file showing the Model. )
    BP
     

    Attached Files:

  7. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    not sure how big this is or how much area* the 'red' (swing up) [*if adequate and effective]. (ie: why bother)
    I hope the strike is 'straight' & 'clean': one little twist and the whole thing could be jammed in the case.
     
  8. bill price
    Joined: Aug 2017
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    Location: Atlantic. NC

    bill price Junior Member

    Over 60 years of sailing small boats in North Carolina Sounds,, most strikes are at the fore foot of the DB or CB, so a foot or so of depth of swing section should work well in these waters.
    Many leagues with board up , and strike on kick up rudder, I've never had a jamb of the ( long) rudder blade in the cheeks. ( Of course there is some sideways movement of a rudder, , but in a small boat there is also movement in the hull. ) As for jamb in the case,,, I don't see how there is any more risk of that,,, In fact, the kick up of the lower section will Greatly reduce torque within the DB box, in ny directipn .
    Also ,, for this proposed hull, DB will be 1.25' to 1.5' wide, resulting in maybe 9" to 12" wide cheeks even with a significant airfoil . That should be stout enough for most lateral stresses.
    We'll see..
    BP
     
  9. Angélique
    Joined: Feb 2009
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    Location: Belgium ⇄ The Netherlands

    Angélique aka Angel (only by name)

    The bottom line of the linked page says the guy changed the Gybing Centerboard on his Dufour T7 for an only pivoting and ballasted centerboard, which also reached a bit deeper as the original gybing centerboard, since the pivot could be more forward and also lower then on the original gybing board.

    End conclusion:

    The a bit deeper reaching only pivoting centerboard turned out to be more effective then the original gybing one, and it doesn't rattle anymore, which was a pain of the gybing board.

    English: ‘‘Gybing Centerboard’’ = in Dutch: ‘‘Midzwaard met zelfrichtend toespoor’’ = in French: ‘‘Derivé Auto-oriëntable’’

    The above is my own summary of the below quoted bottom line in Dutch...

    ‘‘ Het voordeel van een ‘Gybing Centerboard’ is nooit echt bewezen. Mijn eigen T7 is - in samenwerking met scheepsarchitect Peter Bosgraaf - voorzien van een een ‘fixed’ geballast midzwaard. Doordat het draaipunt meer voorlijk en onder de waterlijn zit kon de lengte van het geprofileerde zwaardblad langer worden. Meer lengte is effectiever dan het toespoor. Bijkomende voordelen zijn dat het zwaard niet los in de kast rammelt op voordewindse koersen en zelf-vallend en oplopend is. ’’
     
    Last edited: Aug 25, 2017

  10. bill price
    Joined: Aug 2017
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    Location: Atlantic. NC

    bill price Junior Member

    I think an issue with many Board and Bottom ideas , is that what looks good on even keel, may be adverse when heeling. i.e. I wonder if a gybing CB or DB is effective when beating to w'ard.. In fact, would the greater angle of attack of the Board on high heel act to pull the hull lower = longer WL = more resistance= slower speed?? More heel would = slower speed?? Just a thought. BP
     
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