AC 36 Foiling Monohulls

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

  1. CT249
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    CT249 Senior Member

    One thing.... I wonder if this AC will attract even fewer entrants than the last couple. Apparently the US team has gone quiet. One can assume that the extra expense will probably kill the Aussie and French challenges. So BAR, Italy and NZ? BAR are sounding upbeat, but will beating the Kiwis be enough to get BAR's sponsor's going? With due respect to NZ, beating them may not mean as much to the person on the street as beating the US.

    So could it be a two-team AC? Probably not, but three or four is still far too few.
     
  2. OzFred
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    OzFred Senior Member

    No. From the article:

    Which can only be interpreted as 9kn of true wind, not apparent. It will be a remarkable achievement if they can actually do it.
     
  3. OzFred
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    OzFred Senior Member

    It's difficult to see how any of the new technology for AC75s will be relevant to normal boats. Certainly cruisers and weekend racers aren't going to start fitting large midship articulated foils any more than multihulls have adopted J, Z, L, T or whatever foils. I've yet to see a non–racing production multihull with C foils (though likely there are a couple, they're certainly not de rigueur on new designs).

    Neither the (yet to be confirmed) batteries nor hoistable, reefable wing sail need the AC 75 to promote development in sailing applications, it's happening anyway.
     
  4. TANSL
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    TANSL Senior Member

    Naturally, when the boat takes off, the speed of the boat and that of the apparent wind increase a lot and they do it progressively until something limits them. Btw, what defines the speed limit?
    I, who know very little about this, I guess a wind, sideways (excuse the awful terminology), 8 knots does not make the boat take off but that an apparent wind of 8 knots makes the boat take off easily (the figures are an assumption only). Probably I'm wrong, I do not know, that's why I ask the question of what wind you mean when you talk about the minimum wind necessary for the ship to take off.
    So we do not have the certainty of what they are referring to, but each one "interprets" what they think is right.
     
  5. CT249
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    CT249 Senior Member

    I've never heard of anyone talking of a boat taking off in terms of apparent wind - only true wind. Otherwise you are talking about two factors (true+ apparent) of which the latter depends on boatspeed, which is completely unknown and will also be non-linear with a big foiler and therefore make prediction even harder.

    So I think we can assume that Dan B. meant 9 knots true. As noted, Frank Bethwaite said that was the peak wind at famous sailing venues, which (from vague memory) included Kiel, the Solent, Long Island Sound, Sydney etc. Apart from LIS, they are windier than many other places people sail. So what we could have here is a very radical machine that may not go all that fast at all in the conditions a huge number of people sail in, therefore it will lead to less development that can be used elsewhere.

    My radical thought are not about boat design, but about the fact that we almost need an entire new sport, governing body, and major events for "mainstream seahugging" sailing that 99.X% of sailors actually do, in the same way that road cyclists have an entire sport that ignores the minute percentage of those who use the much faster recumbent streamliner bicycles. If sports as similar as rugby league and rugby union, softball and baseball and (to a large extent) longboard, SUP and shortboard surfing are all counted as separate sports, then why not seahugging and foiling? It could allow the vast majority of sailors to once more have major events to aspire to, and a controlling body that is concerned about their issues and not those of a minute minority.
     
  6. TANSL
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    TANSL Senior Member

    @CT249, OK, thanks for your opinions and ... I very much agree with your last paragraph.
     
  7. David Cooper
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    David Cooper Senior Member

    The boat will keep accelerating until the acceleration force is equalled by a force acting in the opposite direction from drag. With ice yachts, the friction between runners and ice is a lot lower than foils in water, so they can hit speeds over 100mph (and there are unconfirmed reports of some being timed at 150mph+). Most of the drag with ice yachts comes from the rig and whatever parts are needed to connect that to the runners and hold the pilot. With boats there will be extra drag from the hulls, though you could jettison the hulls once up, leaving you with something as minimal as an ice yacht but with foils in place of runners. Put the pilot on one side 0nly and in an aerodynamic capsule and you can maximise RM for a single run on a specific tack. Although, foilers are already hitting speeds where cavitation becomes a major issue in strong winds, you should redefine speed limit to mean speed in proportion to the true wind speed rather than absolute speed. What would be most useful would be figures on the proportion of hydro-drag caused by foils vs. the aero-drag caused by everything else. The latter should be similar to ice yachts while the former should be a sizeable addition to it (where ice yachts add almost nothing from runner drag). If you could eliminate all drag, the speed would increase infinitely regardless of windspeed, but there will always be some drag to prevent this, so if you're looking for a realistic speed limit, it will depend on having the most efficient foils and rig possible, and while we still don't know exactly what those would be like, if you draw graphs showing the rate of improvement of the best designs over time, you'll likely find they have already become nearly flat, leaving little room for substantial gains, except that new materials with the same strength for less weight will have a greater impact on foiler performance than ice yachts as any saving in weight will reduce the amount of foil needed in the water.
     
  8. TANSL
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    TANSL Senior Member

    Great explanation, thank you for the first time I start to understand some issues that these machines raised me.
    I am going to prepare some thoughts in the light of all this, which I will publish later. I am worried now about the cavitation of the foils.
     
  9. TANSL
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    TANSL Senior Member

    I think that finally I managed to dispel the fog that clouded my mind. It is not about the speed of the true wind, or that of the apparent wind, but of the speed of the boat in relation to the water.
     
  10. Joakim
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    Joakim Senior Member

    This should really be discussed. Foiling, canting keels and kites are just too far from "normal" sailing. AC and VOR boats are technically interesting, but I don't get excited about the races. AC was very interesting with 12 MR and IACC boats, but since then and in the middle it has been another case. This year I was voting at the Rolex Awards. All the four male nominees were outside my idea sailing. I didn't really care who of them would win. Luckily the female nominees were all within it (well one from foiling Nacra).
     
  11. OzFred
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    OzFred Senior Member

    Seriously, it's about the true wind, nothing else.
     
  12. TANSL
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    TANSL Senior Member

    I am not an expert and, therefore, I should not contradict you. I just think that the lift is generated by the foil, when it is submerged in the water, and moves at a certain speed relative to it. The foil out of the water, no matter how much true wind there is, does not generate any lift (almost no lift). It is probable that I am wrong but I do not know at what point.
     
  13. OzFred
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    OzFred Senior Member

    Yes, you're correct, lift is from the foil.

    Different boats will start foiling at different speeds through the water (boat speed), and different boats will go at different speeds in the same wind. So even though boat A starts foiling at 5kn boat speed and boat B at 6kn boat speed, doesn't necessarily mean that A will foil in less breeze than B. It might be that in the wind required for A to foil, B can also achieve it's foiling speed through a more powerful rig giving it greater boat speed in the same breeze. *

    So it all depends on whether they can reach their foiling speed in the available wind. For simplicity of comparison, performance is typically reduced to the true wind speed required to get foiling. Hence "foiling in 9kts of breeze", as it removes all the other variables and reduces the comparison to an easily measured and understood value.

    * Breeze is also quite variable. A "steady 10 kn" may dip as low as 7 or 8 kn and go as high as 12 or 13 kn over of a few minutes. And two anemometers 10 m apart might have a variance of ±20% at any particular instant (and much greater on lakes and rivers). In marginal conditions with boats of very similar performance, one or two might get just enough of a puff to get foiling, leaving everyone else behind. Those are the conditions that everyone dislikes, because it can be a huge advantage if you're lucky enough to be in the right place at the right time.
     
  14. TANSL
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    TANSL Senior Member

    I see that we now agree on many things.
    Whatever the speed of the breeze, the ship's skipper must maneuver to get the speed of the boat over the water to rise to a sufficient lift. If the skipper does not maneuver correctly, no matter how high the speed of the breeze, the boat will not take off. It seems to me, therefore, inappropriate to relate the ease of take-off with the speed of the wind.
    Thank you, sincerely, for your explanations.
     

  15. OzFred
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    OzFred Senior Member

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