Deep Keel Position

Discussion in 'Sailboats' started by ClaudioD, May 18, 2016.

  1. ClaudioD
    Joined: Oct 2011
    Posts: 13
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: France

    ClaudioD Junior Member

    This is a typical sailing model application where in principle, for a non written Rule, the Fin CG, the Bulb CG and the Hull LCB are all aligned under the same vertical.

    A couple of days ago I decided to trace the Curve of Areas for the same hull tilted by 30°.

    As expected the LCB moved backward. See drawing:

    My question:

    "If the Hull LCB move backward, should the Fin and Bulb be also repositioned backward of the same amount ?"

    I note that the Fin and Rudder are acting against the Sail's forces.
    Most of the time the sailing model as the real boats are travelling under tilted conditions where the anti drift surface is most wanted in particular when in Close Hauled .
    Off course, moving "by design" the Fin-Bulb backward to meet the LCB at 30°, the boat will raise de bow and sink the transom when floating upright in the harbour.
    Thank you

    Attached Files:

  2. messabout
    Joined: Jan 2006
    Posts: 3,380
    Likes: 526, Points: 113, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    When the boat is sailing the CE is never where it is in elevation (side) views. The CE always moves forward, the extremes at full boom out as on a run. That is why it is conventional to put some "lead" in the layout of the CLR and the CE.
  3. ClaudioD
    Joined: Oct 2011
    Posts: 13
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: France

    ClaudioD Junior Member

    Thanks for your answer, but it is not what I'm searching for.
    As explained, watching the COA at 30° of heel, it can be seen that the LCB is sliding backward proportionally by about 1.2% of LWL.
    Before I was use to fix the Bulb CG just under the LCB at 0° tilt.

    Of course the CLR is dependant for the majority of the Fin position and surface.
    Lead and CE are just further consequences of what happen above the water and are not part of the question that stick to the under water conditions:

    Shall I move the Fin to match the LCB Shift when boat is heeling to get the best efficiency ?

    Let say that when the sailboat is moving and heeling the CLR is also moving backward as the LCB, then:

    Which Lead shall the considered the one starting from 0° or something between 0° and 30° of heel ?

    Thank you
  4. jehardiman
    Joined: Aug 2004
    Posts: 3,811
    Likes: 1,197, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    But the boat just doesn't heel over to 30 deg, it is pushed there by the sail. So you also have to consider the bow down moment generated by the sail. With that I'll bet the LCB at equilibrium does not move as far aft as you think. Old square riggers would often reduce sail forward to prevent the head from being forced under (i.e. "driving her under") for the same reason.
  5. gggGuest
    Joined: Feb 2005
    Posts: 867
    Likes: 38, Points: 28, Legacy Rep: 76
    Location: UK

    gggGuest ...

    I don't believe you should be ignoring the rudder either. In practice, on dinghies anyway, rudder and centreboard are both taking sideload, and if the rig is moved forward or back all that happens is a tiny redistribution of the lateral resistance between the two.

  6. ClaudioD
    Joined: Oct 2011
    Posts: 13
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: France

    ClaudioD Junior Member

    In my opinion one should analyse the two sub systems separately before joing them at system level.
    I named the two subsystems "below water" and "above water".
    Starting with the "under water" the first thing is to find the CLR.
    Now, if for any reasons, I move the Fin forward or backaward, standing the rudder where is, the CLR position will change.
    The CLR is the Starting Point for the Lead.
    Thus before fixing the CLR position is necessary to consider the relationship between Fin and a 'variable' LCB.
    I do not know if I have explained my point of view and therefore refreshing my question :
    The Fin shall follow the LCB changes from 0° and 30°/35° of boat Heel , bearing in mynd that a sailing boat is spending most of the sailing time in the heeled position ?
    Once satified this question will be possible to jump to the second subsystem "above water".
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.