Nacra 17 / Curved Foils for Cats-Pete Melvin

Discussion in 'Multihulls' started by Doug Lord, May 4, 2012.

  1. Doug Lord
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    Doug Lord Flight Ready

    This is an excerpt from an article on the SA front page: Full article- http://www.sailinganarchy.com/index_page1.php
    According to Melvin, the Nacra 17 was chosen by the majority of sailors at the evaluation trials for a new Olympic catamaran for 2016.

    In the last few years curved foils have appeared on A Class catamarans, the new NACRA 20 and SL33 catamarans, and some custom and limited production catamarans. All of these designs successfully incorporate constant-curvature daggerboards, commonly referred to as “c foils”. The innovation with the curved foils on catamarans is that they are used to enhance performance for both upwind and downwind sailing.

    Performance improvements with the c-foils have been well documented in the A class catamarans and c-foil equipped boats have won the World Championships for the last several years. Other foil geometries have been tested on the A Class cats but so far the c-foils have proven to provide the best all-around performance and are by far the most user-friendly and practical solution.

    Lifting foils on catamarans are an area of intense development at the moment, especially with the next AC being contested in catamarans that allow lifting foils. I do not think anyone knows exactly what developments will prove to be best in the future. Each class of boat will evolve toward a foil geometry that is best suited to their specific rules and requirements.
    For the NACRA 17 we chose c-foils for the following reasons:

    A) C foils have proven to make catamarans easier to sail, especially downwind.
    They provide lift which helps keep the bows up and reduces the tendency to pitchpole.
    The extra lift generated from the foil helps reduce hull wetted area and markedly reduces hull drag.

    B) There are other foil geometries that could provide efficient lift but all the ones we evaluated for the NACRA 17 had issues that we determined were not suitable for this type of boat. For instance, L foils are not able to be retracted from the top and would make launching and beaching problematic. We are also not convinced that L foils would be faster on this type of boat. They would certainly create more drag in light air, due to having more wetted area. The windward L board would also need to be lifted to reduce drag and unwanted vertical lift on the windward side, thereby making the boat much harder to tack. C foils can be left down on the windward side since they automatically reduce their vertical lift when on the windward side.

    C) C foils make the boat more exciting to sail. The ISAF Olympic Multihull Evaluation Committee report and individual sailor reports confirms this. There is an added dimension of control and speed that can be attained after gaining some experience sailing with them. We believe that this is an area that should be exploited with a modern racing catamaran and is why you see them on the NACRA 17, NACRA 20, and SL33 catamarans.


    click on image:
     

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  2. Doug Lord
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    Doug Lord Flight Ready

  3. bjn
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    bjn Senior Member

    I've watched some videos of the NACRA 17, and to me this boat looks hard to sail.

    Why are the foils so far aft?
    It looks like that creates pitch stability problems.

    "C foils have proven to make catamarans easier to sail, especially downwind."
    They seem to be pretty hard to sail downwind to me. At a certain heel the boat will lift out of the water and create all kinds of problems.

    "They provide lift which helps keep the bows up and reduces the tendency to pitchpole. "
    To keep the bows up, the sailors has to be in the short space aft of the foils, and use gravity to balance the boat on the foils, more or less. And that seems to be a pretty hard task. I've seen a lot of pitchpoles in the videos. It also looks like if the pitch angle gets too high, the trampoline will act like a wing in ground effect, and lift the boat up in the air!

    I think that maybe the designers real intentions is to create the kind of boat we see in the videos. Because it's pretty entertaining to watch! =)

    https://www.youtube.com/watch?v=pehYM0RfyyU
     
  4. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Nacra 17

    Nice video, thanks. I've never had the pleasure of sailing a 17 and probably would have enjoyed it 20 years ago. There's a bit on www.catsailingnews.com about the 2015 Worlds and apparently, the winners did extraordinarily well by actually foiling as in the picture below:

    [​IMG]
     
  5. bjn
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    bjn Senior Member

    I compared the NACRA 17 to the M32 and AC45, see attached pics.

    Both of the compared boats have the daggerboards/foils closer to the bow. Just by looking at the pics, you get the feeling of more stability. And I'm sure the math says the same thing.

    Also, I've watched footage of the M32, and they look far more stable in pitch.

    Does your pic show a "jump", or were they actually able to sustaing foiling for long runs?
    Is there a video of them foiling?
     

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  6. Doug Lord
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    Doug Lord Flight Ready

    N17

    The only comment is at the link I gave you last. I think you probably make a good point if starting a foiler design from scratch, but the 17 wasn't(theoretically) designed as a full flying foiler.
     
  7. bjn
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    bjn Senior Member

    The M32 wasn't designed as a foiler either.

    I guess I'm just trying to understand what the designers are thinking when they put the daggerboards so close to the transom. The centre of effort of the sails looks far ahead of the centre of lateral resistance, so to me the foils looks to be in the wrong position also in that regard.

    Just some thoughts from looking at the pics. I'm not a boat designer.

    I clicked your link before, but I couldn't find the pic in any of the three blogs about the event.
     

  8. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    N17

    It's there-right under the Day 4 headline for the Worlds in Denmark. You have to scroll not quite halfway down.
     
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