AC 36 Foiling Monohulls

Discussion in 'Sailboats' started by OzFred, Sep 13, 2017.

  1. Doug Lord
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    Doug Lord Flight Ready

    NZAC* is going to ,apparently, have a lot of one design elements not including the foils but maybe including the struts that support them, not including the hull but parts of the rig not including sails. So it seems to me any real innovation will be left ,primarily to the foils ,sails and hull design-which only has value when not foiling. With foiling beginning somewhere between a windspeed of 6 and 9 knots and a "legal" top end windspeed around 25knots the hull may not be in the water much. The main "edge" possible from a great hull design will be a shape that allows quick acceleration I would guess.

    * a possible New Zealand Americas Cup boat

    Seems like a very skinny hull(at the waterline) like NZAC One(except maybe with chines and a rectangular /square section) may be the way to go depending on how quickly the foil struts can move?
    A lesson from the Moth is that a high L/B skinny rectangular hull section beat a semi-circular low wetted surface hull. In other words the reduction in wavemaking drag with the skinny rectangular section was less drag than a semi-circular section even with a bit more wetted surface.
    NZAC ONE     9-25-17  dl 002.JPG
     
    Last edited: Nov 30, 2017
  2. Joakim
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    Joakim Senior Member

    NZAC One has a deep keel with a bulb providing stability before foiling. AC75 would have very poor stability with a very skinny hull. I don't think it is possible to reach foiling speed without some form stability.
     
  3. Doug Lord
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    Doug Lord Flight Ready

    NZAC One was there just to illustrate the narrow waterline-nothing else. The degree of form stability will have to be determined as time goes by but I would bet that a very narrow hull would be considered. The boat doesn't need form stability from the hull once its moving so I wouldn't be surprised if they push it toward the minimum. The lee foil will add stability when the boat starts moving -even relatively slowly. The ability to get on foils first may depend on the choice made for waterline beam.
    Also, I may have missed whether or not racks on the hull for the crew will be legal or not?
     
  4. OzFred
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    OzFred Senior Member

    Which Moths are you comparing?

    Even the more square–sectioned Moths like Prowler, Bladerider and Mach 2 have round sections from the bow back to about the mast. Later Exocet, Ninja and Rocket designs (which are at least the equal of Mach 2 going by results) have round sections for their entire length.

    The Moth squarish section (e.g. Axeman, Prowler) was developed before foiling was introduced (around 2004) to provide a planing surface. Once foiling was ubiquitous, planing became irrelevant. I don't think replacing square sections with round has any significant practical benefit for Moths, it just gives the sailor a warm feeling knowing they've optimised the shape for certain factors. Considerations of wave–making and wetted surface between round or square sections would be insignificant in the overall mix of variables, particularly in such a small boat where the hull is rarely in the water.

    Newer boats also have more rocker, which may also be part of the equation as it allows (fractionally) easier control of pitch by moving crew weight. But again, it's a tiny, tiny difference even at the extreme (e.g. the almost board-like Prowler vs the much more "rocked" Rocket).

    A more valid comparison is with non–foiling off–the–beach catamarans, which seem to have oscillated between square and round. But I don't know enough about them to comment on which has been "better" in what conditions, probably someone familiar with developments in the A or C Classes prior to foiling would know more. I do know that the debate over square/planing or round/fast displacement is argued with religious zeal, with no clear winner as far as I can determine.

    But having said that, I expect that in a foiling class like the AC75, minimising hull drag at low speed will be the primary criterion. So round sections to minimise wetted surface, not square since the boat will foil before it planes. Even in planing skiffs (e.g. everything from 12 to 18 footers), the rear planing sections are rounded between chines, not flat, and the chine is usually fared–in about where the centreboard is, then fully rounded to the bow.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    My comments were based on an article that I think was written by Ian Ward and a comarison of midship hull sections of the same area one semi-circular* and one square or rectangular. The L/B ratio of the semi-circular version was something like 9.7/1 and the square/ rectangular version was something like 11/1. I also have a drawing that illustrates this but can't find it or the article. There are infinite variations but the point was that a square or rectangular section was narrower for the same displacement as was a semicircular shape and that narrower was faster despite around 11.5% more wetted surface.
    At any rate, I think at least some of the hulls will be skinnier than any renders so far except NZAC One.
    * not rounded chines

    Andrew McDougall photo of Mach II:
    Moth Mach II Andrew McDougall-photo.jpg
     
    Last edited: Dec 1, 2017
  6. OzFred
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    OzFred Senior Member

    So all you can say is that in a comparison between two unknown hulls using some unknown methodology, a skinnier square hull was faster in unknown conditions than a wider, round section hull. Without knowing exactly what was being compared and how, it's a very long bow to draw to extrapolate from a 3.35 m boat weighing 35 kg to a 23 m boat weighing around 7,000 kg.

    For Moths, if you go by results in regattas (which is what actually matters), there is no perceptible difference between square and round hulls. A far greater influence is the rig, foils and sailor.

    The Mach 2 image shows that the forward third (at least) is round, similar to the forward sections of all modern skiffs, so the distinction between "round" and "square" sections is pretty meaningless. I expect the AC75s will have round forward sections, flattening toward the rear with possibly a hard chine above the waterline (because it looks modern, but isn't) so they might look a lot more like Wild Oat XI than Hugo Boss.

    Beam will also be influenced by the electrical, hydraulic and mechanical equipment that has to fit into the hull and control the foils, but it may not be a limiting factor.
     
  7. Doug Lord
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    Doug Lord Flight Ready

    I'd be interested to see a Moth hull with a semicircular midship section starting at the static waterline......... The article I read by a well known Moth designer was adamant that a semi circular shape takes longer to get on foils than does a rectangular/square shape.
    I imagine some NZAC versions will be as narrow as possible....
     
    Last edited: Dec 3, 2017
  8. Doug Lord
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  9. CT249
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    CT249 Senior Member

    That only applies at speeds that are considerably lower than a full-blown Skiff can attain, so it may not translate to larger boats.
     
  10. OzFred
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    OzFred Senior Member

    You only need look as far as the Maguire boats' Exocet, which has an excellent record in Moth Worlds and UK championships in the last couple of years. Clearly unhindered by it's round hull.

    [​IMG]

    Maybe. What criteria define "possible"? Comanche is not skinny, but has a remarkable record and great speed. It may be that a wider hull with form stability is a bonus, with the canting, ballasted foils being used to induce heal to reduce wetted surface as required.
     
  11. Dolfiman
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    Dolfiman Senior Member

  12. OzFred
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    OzFred Senior Member

    I've used Google Translate, which translates what I guess is the same phrase as "But according to our information, the hull width should be 5.3 metres for a displacement of 7 tons".

    The interesting part for me is that the overall length is 68', so the 75' must include the bowsprit. That makes the boat about the same length as a 12 Metre, and not that much bigger than an IMOCA 60 (there's a section that translates as "We were a little afraid they go to something traditional or more basstard with a keel and foils as seen in Imoca.")

    Also interesting that "Guillaume Verdier had already worked on a [similar] project of 39 feet", so the Kiwi's have a bit of head start, though the boat in question wasn't actually built.
     
  13. CT249
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    CT249 Senior Member

    Just to clarify; I was looking at pre-foiling Moths, where the hull shape was arguably of much more importance and, in the later boats, getting fairly uniformly designed around square sections through the middle.

    Emmett Lazich (two time world Moth champion, 12 Foot Skiff champion and the 49er gold medallist's coach) noted that before foiling, the narrow Moth hull was capable of a much lower top speed than the fat, round hull of a 12 Foot Skiff. Obviously the two are very different craft, but the point was that the Moth had a top speed limit (albeit a very high one considering its other dimensions) that the 12 Foot Skiff hull didn't. Similar issues were commented on when comparing seahugging Moths to longboard windsurfers.

    The seahugging Moth was remarkable at what it did, but the hull shapeconcept doesn't necessarily translate to craft that can have an inherently higher top speed. It just shows that you can't take the optimum hull from one style of craft and assume that it will be the optimum hull for one that operates in different speed and power regimes, as you and I know.
     
  14. Doug Lord
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    Doug Lord Flight Ready

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  15. Dolfiman
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    Dolfiman Senior Member

    Porto Cervo is a famous spot in Mediterranean sea for superyacht regattas in sunny / windy conditions, e.g. the Loro Piana superyacht regatta organised by the Yacht Club Costa Smeralda, could be a serious candidate for the location of the AC36 pre regattas :
    Yacht Club Costa Smeralda - Home http://www.yccs.it/en/#1
    Loro Piana Superyacht Regatta - 2018 - Porto Cervo http://www.yccs.it/regate-2018/info/loro_piana_superyacht_regatta-254.html#1

     
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