Logic in different length hulls

Discussion in 'Boat Design' started by Thule, Feb 19, 2024.

  1. CarlosK2
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    CarlosK2 Senior Member

    Now

    (A) A bad Longitudinal Balance, plus (B) a not very high but normal Longitudinal Metacentric Height (= 1 x LWL) plus (C) a sharp bow

    A+B+C =

    Bow pitches 2-3 degrees Bow down

    And

    Mr. Munk goes all crazy

    (and to top it all off a big daggerboard/Keel strangles the yacht)

    The yacht capsizes or again and again seems to want to capsizes/roll/somersault:

    the yacht does not sink, in fact it crossed the Atlantic Ocean alone, but the crew's spirits sink.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

  3. CarlosK2
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    CarlosK2 Senior Member

    IMG20240410111338.jpg

    (Water plane and LCF)

    We do not need a gigantically wide bow as is now fashionable in France.

    What we need is:

    (A) Longitudinal Balance: LCB = LCF, and the center of gravity (LCG) a little more aft, for example -3% LWL

    (B) a hull that gives us a quick attitude of the hull bow Up when accelerating facilitating a quick transition from displacement mode to Surf mode.

    and

    (C) if possible also a Longitudinal Metacentric Height between astronomical and sidereal (= 3 x LWL)
     
  4. CarlosK2
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    CarlosK2 Senior Member

    Starring

    LCF: Longitudinal position of Center of Flotation

    LCB: Longitudinal position of Center of Buoyancy

    LCG: Longitudinal position of Center of Gravity

    MMM: Max Michael Munk Moment

    Moment de Munk — Wikipédia https://fr.m.wikipedia.org/wiki/Moment_de_Munk

    Max Munk - Wikipedia https://en.m.wikipedia.org/wiki/Max_Munk

    IMG_20240411_101118.jpg

    I propose this simple expression, and the Factor "F" is 0.8 upwind, 1 downwind and 2 if the bow sinks.

    Obviously it is very crude and it is not very precise because it depends on the geometry of the bow but it is useful to think with Numbers.
     
  5. CarlosK2
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    CarlosK2 Senior Member

    Surfing with Mr. Munk

    We bring (Froude > 0.6) the bow out of the water with a nice and decisive Hull Attitude bow up (+4 degrees):

    So at the same time we minimize the High (H) side pressure on the hull and the hull is willing to overcome the bow wave from above instead of going through it.

    And a very wide stern well seated in the wave destroys the low (L) lateral side pressure.

    And on top of that two big rudders. Wooow.

    IMG20240411105415.jpg
     
  6. CarlosK2
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    CarlosK2 Senior Member

    Screenshot_2024-04-11-11-17-52-89.jpg

    In other words: it is not worth moving the keel back and forth with the lead torpedo, because even upwind it is worth adjusting LCG to LCF.

    And on the other hand, it is no use cheating yourself and calculating LCF in port with the yacht stationary and drawing a stern out of the water, because the big wave will wet the entire stern.
     
  7. CarlosK2
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    CarlosK2 Senior Member

    IMG_20240215_114951.jpg

    (0.60 Froude and Big Wave)

    1) Prandtl-Munk, 1924
    2) Savitsky, 1964
    5) Prandtl-Munk, 1924

    4) HydroStatic Force
    3) Earth Force
     
  8. CarlosK2
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    CarlosK2 Senior Member

    IMG_20240409_125131.jpg

    And the daggerboard fits a superb, comfortable, versatile and functional Fractional rig: Mainsail = 2 x Jib
     
  9. CarlosK2
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    CarlosK2 Senior Member

    Screenshot_2024-04-11-12-51-31-59_f90b96e7af3c5a594eb0c92de7fc5fe1.jpg

    After enjoying like a child Surfing and Surfing and Surfing big Waves ...

    we loaded 200 liters of sea water in windward (C19) ...

    Upwind

    as the sailboat is small the effect of 200 liters of seawater is tremendous because the center of gravity moves and also increases the Displacement.
     
  10. CarlosK2
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    CarlosK2 Senior Member

    And now comes the terrible question: what is the cost of this 1 Ton Surfboard

    And here comes Mrs. Inflation: I estimate 60-64 K (!)

    OM f G
     
  11. Paul Scott
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    Paul Scott Senior Member

    Not worth? Unless I’m missing something, pull up the canard, move the weight back, and things change. Get on plane and wetted surface increases, but surfboard shapers like to point out that for fast planing, you need to have more wetted surface, which can go along with increased displacement, or at least Disp/Length ratio. Bow up planing helps deal with waves. A lot. On a Serenity type windsurf board you can get by with keeping your weight in the middle down wind with the daggerboard down, but on a shortboard, Formula, or even longboard, the foot straps are where they are aft for a bunch of reasons. A fault I find with Minis is the immense amount of sail forward downwind, which does what for balance at that point? What would change with a bigger main, forward pulled up canard daggerboard, aft water ballast, and longitudinally moveable lead keel, fewer or no foresails in the big stuff at least, that might allow CE to migrate aft with LCF, LCR, and LCG without the need for large foresails as the mast and CE in effect rotate aft as the bow rises to plane, leaving the fore sails for light airs and Archimedean sailing?
     
    Last edited: Apr 11, 2024
  12. Ad Hoc
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    Ad Hoc Naval Architect

  13. Paul Scott
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    Paul Scott Senior Member

    Cool design :) Possible to steer with the keel?
     
  14. CarlosK2
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    CarlosK2 Senior Member

    Rogers

    Rogers Yacht Design Rogers Yacht Design : info@rogersyachtdesign.com Rogers Yacht Design : info@rogersyachtdesign.com Rogers Yacht Design : info@rogersyachtdesign.com http://www.rogersyachtdesign.com/sections/transat.html

    The ballast moves backward (downwind) and forward (upwind) and also moves to windward.

    When Upwind the Ballast is to windward it produces very little lateral force and then the daggerboard function is performed by the daggerboard further forward.

    There are a handful of Mini Transats with this idea. One of them had the mechanism break down and the daggerboard got stuck in the aft-most position and then they discovered that it sailed superbly without the need to move it.

    On the other hand, the designer of the Aqua Quorum*, who was used to designing fast motor boats, came up with the same idea (LCG = LCF) thinking about sailing Upwind against the waves.

    Document sans titre https://www.histoiredeshalfs.com/50%27%20monos/C10%20Aqua.htm

    That is to say, by both ways (Upwind and Downwind) some of us have reached the same conclusion which is completely logical. The strange thing is that it is necessary to argue something so obvious: it is absurd to sail with tons of lead ahead of the Pitch Axis.

    *: Let's remember that in that race a good number of sailboats capsized or disappeared.

    ---

    In the case of my 1 Ton WindSurf Board:

    LCF: 58% LWL
    LCB: 59% LWL
    LCG: 62% LWL
     
    Last edited: Apr 12, 2024

  15. CarlosK2
    Joined: Jun 2023
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    Location: Vigo, Spain

    CarlosK2 Senior Member

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