Gitana 17-100' Trimaran Foiler-Launch 7/17/17

Discussion in 'Multihulls' started by Doug Lord, Jul 13, 2017.

  1. Doug Lord
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    Doug Lord Flight Ready

    Macif / Gabart on DOWNFORCE: (found by Dolfiman in Macif thread)

    In François Gabart 10 nov. message :
    ""You have to find the balance to be able to stay in support on the central hull, but it's not so easy when you go fast. I cannot stay on the central hull all the time and I'm still not comfortable sleeping like that. It's starting to come, it will take training again. But my sleep is damaged, despite fatigue. I still managed to take a few naps."

    Is it not in this case that a foil on central daggerboard, when adjusted to provide a small downforce, which can add passive safety when the singlehander is sleeping ? In case of gust of wind with a move to more heel, the inflow incidence on this foil increase instantly and can provide an extra RM which adds to the one due to the weight. And moreover in steady state condition, this foil RM component is in squared speed boat, can increase in parallel with heeling moment in squared apparent wind speed. I am right ?"

    Yes, you are right,DL!
     
    Last edited: Nov 11, 2017
  2. Doug Lord
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    A sad day for Team Gitana with her loss to Sodebo in the Transat Jacques Vabre! Seems to me that the race was decided by a colossal course mistake at about the halfway point . The foils worked well through the roughest part of the race.
     
  3. TANSL
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    TANSL Senior Member

    But the weight can only produce a heeling moment, never righting. Am I wrong?.
     
  4. Doug Lord
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    The weight of the boat, when the main hull is flying, acts just like the downforce on the mainfoil and certainly does produce major righting moment-not heeling moment.
     
  5. TANSL
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    TANSL Senior Member

    Sorry, I'm trying to understand how the foilers work, asking questions, which may seem silly to an expert, because I read things that go against, apparently, what I know about boats. If the ship's CoG is on the center line, above the surface of the water, it seems logical to think that the moment it produces is heeling. Thank you, once again, for answering my questions.
     
  6. Doug Lord
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    Doug Lord Flight Ready

    Gitana NOT foiling! I missed this earlier today:


    On this her first trial run in race configuration and up against particularly stiff competition, the Transat Jacques Vabre has certainly lived up to expectations. The high standard of play pushed Sébastien Josse and Thomas Rouxel to their limits and has given them a better understanding of the fabulous machine that is the Maxi Edmond de Rothschild. Some technical incidents on the second day of racing, did however prevent the duo from expressing the true potential of Gitana 17 by forcing them to do battle without all their weapons. Indeed, in addition to the AIS black-out from the start, which could have had disastrous consequences in terms of the sailors' safety, the duo had to deal with repeated engine failure; damage which cost the skipper of Edmond de Rothschild many long hours in the bilges, whilst his co-skipper busied himself with maintaining the rhythm at over 30 knots. Last but not least, it's an injured bird', deprived of her wings', which has just pulled alongside in the Marina in Salvador de Bahia. Indeed, on Tuesday, after the front rolled through, the sailor from Nice informed his shore crew of serious damage to the boat's port foil. Unusable, the precious appendage was raised to avoid any further damage. Two days later, it was the turn of the starboard foil to show the same signs of damage as its twin. To put it plainly, our duo was deprived of her major assets for ¾ of the race, which explains the speed deficit everyone observed.


     
  7. Doug Lord
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    Doug Lord Flight Ready

    ===================
    Assume the boat is level and flying on its lee ama foil(disregard any input from the center foil for the purposes of this question). In that configuration, the CG is in line with the daggerboard and the weight produces righting moment. I don't mind trying to answer any questions.
     
  8. TANSL
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    TANSL Senior Member

    I'm sorry but you do not see it. I'm going to use a picture previously used by you where I added the CoG (which may be there, a little lower or a little higher). It is very clear that the moment generated by the weight of the vessel is in the opposite direction to that of the force at point "B". If one of those moments is righting the other should be heeling. Unless I'm doing something very wrong or Ihave to take into account something that I am not able to guess.
    Downforce-02.jpg
     
    Last edited: Nov 13, 2017
  9. Doug Lord
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    The modification you made to the drawing is accurate for the CG of the heeled boat. I suggested ignoring the heel for the sake of simplicity.
    Point B is supporting all* of the weight of the boat under sail at its center of lift. The CG is where you show it on my drawing for the heeled boat. The weight of the boat at the CG is multiplied by the distance from the CG to point B giving the Righting Moment of the boat*.
    Point B is lift -it is supporting the weight of the boat. The CG is acting in almost the same direction as downforce -therefore the moment is calculated as above.
    "B" is not a "moment", neither is the weight at the CG. The weight at the CG multiplied by the distance between it and Point B is the moment which happens to be the righting moment for the boat.(*disregarding the lift or downforce from the mainfoil)
     
  10. TANSL
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    I am sorry but I do not understand anything. I already know that "B" is not a "moment", "B" is the point of application of the downforce and "G" is the point of application of the weight. If the point of rotation of the ship is at the intersection of the water line with the central foil, the moment of the weight can not be more than heeling. But forget it and answer me only if what I have drawn is wrong and why. Many thanks.
     
  11. Doug Lord
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    Doug Lord Flight Ready

    The point of rotation of the boat is point "B". Forget about the main foil for now. "B" is taking the whole weight of the boat. Once you understand that we can add in or subtract the mainfoil loads........
     
  12. Dolfiman
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    Dolfiman Senior Member

    Dear Tansl,

    This video shows you how a big trimaran can capsize with rotation around "point B" as explained by Doug :


    , and demonstrates that if a foil is mounted on the central daggerboard, adjusted to give a downforce, it can help counteract this fatal heel rotation : as long as this heel rotation begins due to the gust of wind, the inflow incidence on the foil instantly increases (in the coordinate system linked to the foil), and so do the CL of the foil, the downforce and eventually the contibution to the RM = downforce x distance central hull to ama.

    PS : skipper of the trimaran was J- Dick, presently skipper of the leading Imoca St Michel in the Jacques Vabre race. He had a double chance in this accident : first he felt in the water from 15m height without hurting, then when the mast broke and the hull finished upside down, he does not received any hull or akas on his head.
     
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  13. Doug Lord
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    Doug Lord Flight Ready

    Dolfiman, what a terrific video-thanks!
     
  14. David Cooper
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    David Cooper Senior Member

    If B is rising and A is falling, both are acting to reduce heeling. It's only if you have the central foil generating lift upwards as well that it's adding to heel. If you have that foil applying downforce, it reduces heel. Using a centre foil for downforce is still a risky thing to do though unless you have it automatically adjust in the way Doug's design does because it will inevitably come clear of the water on occasions and tip the boat over sideways at much greater speed than normal, giving less time to catch it. I'm sure these big tris will only be using lift upwards to reduce drag by keeping the hull clear as much as possible, and that means it's adding to the heeling.
     

  15. David Cooper
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    Remember that the French word "chance" often needs to be translated to "luck" in English (to prevent it from sounding like a great opportunity is up for grabs).
     
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