11th Hour, the new Imoca for solo, short-handed or fully crewed races

Discussion in 'Sailboats' started by Dolfiman, Aug 10, 2021.

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  1. Dolfiman
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    Dolfiman Senior Member

  2. Skyak
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    Skyak Senior Member

    Looks quite radical. I understand the value in shedding water faster and minimizing wetted surface, but I don't understand how they can keep the weight down with that much surface.

    I can't wait to hear sustainability developments.
     
  3. pironiero
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    pironiero Senior Member

    Old one was more stylish, this one looks like a clown...
     
  4. pironiero
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    pironiero Senior Member

    Also these chines, they probably will improve low wind performance, but what about med to high speeds? are they really rely on foils that bad?
     
  5. pironiero
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    pironiero Senior Member

    also sucks that there is no content about building process
     
  6. container
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    container Junior Member

    The chines are there for dynamic lift at speed, they will do nothing for low speed performance. The IMOCA class has a rule limiting bow volume which is why they are not using scow bows like the mini 650s and class 40s
     
  7. pironiero
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    pironiero Senior Member

    how are they supposed to work, can you please explain?
     
  8. container
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    container Junior Member

    When a boat starts planing the water tends to stick to the hull, wrapping around the turn of the bilge and up the topsides (see attached image of ABN Amro VO70) for this to happen there is a suction force created which, besides increasing wetted surface area by pushing water up around the hull, also tends to "suck" the boat down into the water, compounding the issue (ever seen a float plane with round bilge floats? This is why)

    A large chine flat eliminates this problem, but you also create lift by deflecting the bow wave down and away from the hull. Older generation IMOCAs all had small chines , spray rails, whatever you want to call them, but they went away from these when the foils were introduced. I guess they were trying to reduce drag at non foiling speeds and rely on the foils to keep the bow out at high speeds, This is the reason the foiling boats all had narrower hulls with a softer chine, a couple of them even have fully round bilge hulls (Charal, Hugo Boss)

    However this new generation of 60s which are sailed fully crewed can be pushed alot harder in all conditions so the designers need to bring out all the guns to keep the boat from nose diving, especially when launching off a southern ocean wave at 35kts and crashing off the foils which is what they are primarily designed to do.
     

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  9. container
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    container Junior Member

    You may also notice a couple of the boats have removed the foam fairing around the bow knuckle, which was there as a bit of a safety measure in case of hitting something at sea, essentially creating a flat surface to increase dynamic lift when dipping the bow at 30+kts. Looks ugly, but is really the only option they have to increase performance short of designing a new boat.
     

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  10. pironiero
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    pironiero Senior Member

    Ha ... didn't know this
    upload_2021-8-13_8-21-2.png
     
  11. wet feet
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    wet feet Senior Member

    That was one of the most notable features of L'Occitane to my eyes.

    [​IMG]
     
  12. ChrisVJ
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    ChrisVJ Junior Member

    At some stage the designers decided that long narrow bows that drove easily were less drag than traditional broader bows. (And for those pesky rules too!) Even so in some videos you could see the boat slow as they plowed the water for what appeared to be several waves at a time and the narrow topsides did nothing to lift the bow. One had to wonder (I certainly did,) when they would find a way to lift the bow out of the water to sustain the best planing speed.

    Probably a rules thing but I am surprised no one has built lifting foils at the bow, maybe quicker lift and less resistance than broadening the cheeks.
     

  13. Skyak
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    Skyak Senior Member

    The number of foils (appendages) is limited by rule. I think I heard that this year's boats have some limited ability to adjust foil angle -to tune the trim after they hit the water. The point is that IMOCA bows are changing shape because they are not in the water anymore. The rare case of the bow hitting the water, is driving into a wave at ~30Kn and the best move in that condition is up and over.

    The other note is that the scow-bow is not always faster. Old narrow bows are still superior upwind and in light wind. More important to designs for mortals, the scow bows need large sail area to displacement to be superior. It's not a stand-alone feature.
     
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