Drivers for the "sqare top" width?

Discussion in 'Hydrodynamics and Aerodynamics' started by jmf11, Dec 23, 2021.

  1. Mikko Brummer
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    Mikko Brummer Senior Member

    I´m afraid I have to disagree with Steve, even if I have huge respect for his experience & knowledge. Rather than cantilevers, battens act as compression beams. Properly designed, a sail needs hardly at all battens to support its roach. I recall from my youth a rather windy Kiel Week race, where we lost the top batten of our 470 just before the start - we finished 2nd without the top batten, seeing hardly any difference at all. Only if the sail was totally flat would the battens act as cantilevers,

    Also, I don’t think a square head is needed for depowering - probably the “most depowering”of all, the Finn with its 11 sqm sail has no roach at all, but manages to sail upwind in 35 kn with its bendy mast. Likewise for the Star.
     
  2. CT249
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    CT249 Senior Member

    Isn't the "disagreement" down to;

    1- that Steve is talking gust response and you are talking depowering?

    2- that a batten in a pinhead is a compression beam whereas the batten in a squaretop is under compression AND also acting as a cantilever? If the squaretop's batten isn't a cantilever then what is stopping the squaretop from just folding straight away to leeward in the portion aft of the area of tension between the clew and head?

    I've been in a couple of classes that have adopted bigger heads and the gust response certainly seems to be better; however whether they are better sails overall is another issue. Arguably the concentration on improving gust response has created some sails (especially windsurfer sails) that are heavy, costly and not particularly fast.
     
  3. CT249
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    CT249 Senior Member

    I may have been getting confused between apparent sweep and apparent aspect ratio. When I've seen it sketched it's shown that when a rig is raked to windward the effective aspect ratio drops, while leeward heel increases the apparent aspect ratio.
     
  4. Mikko Brummer
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    Mikko Brummer Senior Member

  5. jmf11
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    jmf11 Junior Member

    Thanks Mikko for the link to the above thread. I went through it and learned several interesting things: the important relation about this head design choice and the mast behaviour, how fractional rig work.

    If I want to wrap up my ideas:
    • most of the rigs are optimized according to racing rules, which are an important driver. So it is important to understand that context to see if the design optimization in a class is applicable or not to another,
    • Boats are root moment constrained, the downwash distribution (not the span loading) should be linearly decreasing, which leads to a bell-like span loading. This heads to more tippy heads than wings optimized for eliptical span loading,
    • The above point about bell-like span loading is especially interesting if there are no rules limiting the mast length, as it implies a longer mast span for the same lift,
    • The optimun with a rig, can be in some conditions with negative lift at the top (consistent with some statements from Tspeer in Minimum Induced Drag of Sail Rigs and Hydrofoils http://www.tspeer.com/Planforms/Planar.htm)
    • Pinhead sails can perform very well in gusts (as well as fat heads) if finely tunes to the mast and correctly tuned (Finn, Star...)
    • Maybe there are not so big differences between pin head / fat head /square head (possible fashion effect). A-Cat and Moth are reducing head sizes (even with stable set of rules).
    I will still keep in mind a statement from Ben G " It can be difficult to get the correct twist in the head of a pin head sail across the range, ie. not to overload the upper leech down range, and be able to flatten the head sufficiently up range - especially boats that cannot access sideways tip bend due to mast head rigs etc. If setup properly the square top achieves both of these - controlling the camber and twist of the sail more readily to the conditions at hand. Up range, the head should be completely flat and twisted off, while leaving the lower third with camber. The downside will be the extra skin friction of the square top, less so the incorrect downwash." Could the fat head have more drag, but easier to design good enough?

    I have received tmy copy of CA. Marchaj "Sail performance". This book is fine for my knowledge level. Introducing the theory and its application to sailing, in an effort to understand the principles without going to the equations. This will bring me one setp further...

    The thread Wake Wash https://www.boatdesign.net/threads/wake-wash.48661/ is definitivelt too advanced for me at the moment.

    Thanks a lot all for the sharing of all those information !

    Best regards,
    JMF
     
  6. Skyak
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    Skyak Senior Member

    I think you missed the correct answer a page ago. Wind speed is greater the higher off the water you go. Sail "optimization" for racing must consider two different states -righting limited, and wind limited. Fat-head puts more sail area in more wind so it wins the wind limited case and we move on to the next question "Do the rules penalize the fat head design?" and the answer is no generally (I don't know of any). Finally we need to figure how frequently race boats are in the wind limited state 1) when wind speed is low but not so low the race is canceled, 2)almost all downwind legs (and some upwind) of windward/leeward races, 3)almost all of the big offshore races are down prevailing winds and weather routing tech makes sniffing out favorable winds ever more precise. In the rare righting limited conditions the fat head is not a big negative. With a tapered tip you can design a nice gust response, and with simple extrusions the flat sail lowers the drag of the chunky extrusion tip.
    For me, this completely explains why racing sailboat mains are rectangles despite the fact the "optimal" lift profile is elliptical.
     
  7. Skyak
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    Skyak Senior Member

    I think I understand (and highly respect) both of you. Battens are in compression holding the leach out from the mast and that makes the full battened sail a bi-stable structure. The fat head of that structure gets it's angle of attack from fibers running from the head to the clue like Steve said and stiff high tech fibers (and powerful vang setups) have enabled high aspect square top sails that were impossible with the stretch of polyester.
     
  8. CT249
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    CT249 Senior Member

    Mikko, who has access to better info than we do, reckons the ORC penalises squaretops.

    It's very arguable whether most offshore races are down prevailing winds. Most US ones may be, but not the Fastnet, Middle Sea, Hobart, etc.

    The squaretop isn't the only way to increase sail area up high - a pinhead on a longer mast can do it too.
     
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  9. CT249
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    CT249 Senior Member

  10. Mikko Brummer
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    Mikko Brummer Senior Member

    Can you elaborate on this, CT?
     
  11. Skyak
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    Skyak Senior Member

    I said most, not all, and your list of race examples is a good place to look for 'revenge of the pinheads'. More 'coastal' than 'offshore' that brings up another point -near land the windspeed gradient with altitude is greater. Shorter rigs sail further away from land (longer path) to avoid wind shadows.

    Of course any complete handicap will account for the height of sail area, but mast height is absolutely in all handicaps, and when big buck designers put out their latest they always have significant square top mains.
    Pinheads seem relegated to one design, and budget conscious "sailing as a service" programs.
    I agree with Mikko that pinheads can be designed for gust response, but what is the selling point?
     
  12. Skyak
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    Skyak Senior Member

    This brings up an important point (observation) -reverse heel favors pin-head planform. Or better stated, pin-head planform benefits greatly from reverse heel.

    If a W/L development class limited sail area and not mast height, and had reverse heel, pinheads would rule.
     
  13. philSweet
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    philSweet Senior Member

    No Skyak, Reverse heel is the #1 driver for Fat heads. It has to do with the change in effective sweep. Reverse heel is like a forward-swept wing, You need less taper or negative taper to achieve low induced drag when you have less sweep or forward sweep. When you stand a boat up or run reverse heel, you are moving the masthead up wind and reducing the sweep of the rig.

    The other driver is a change in the lift slope with change in tip ratio for a given aspect ratio. This changes the sail twist and tip washout that has to be designed into the sail, and hence gust response. But this stuff still needs to be sorted out on the water. The effects are small and depend on sail control effectiveness.


    I should also comment that the optimum maximum draft location of the sail's camber at the head changes quite a bit with changes in sweep, so a sail wants to be held at a particular sweep for best performance. (What really happens is that the draft is aeroelasticly adjusted forward in higher winds to account for additional heel. But there is only so much of that available).
     
  14. AJB
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    AJB Junior Member

     

  15. AJB
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    AJB Junior Member

    Phil n all
    Correct above, clear distnction between optimum with windward heel (moth, 49er, 18 etc) and 'statutory' 20 degree keelboat leeward heel.

    The aeroelastic gust response though is appearing to converge, with (uprange for keelboats and skiffs) the top ~ 20% of the span operating at minimum drag (say 4% camber) and not intended to generate lift...'lying in the onset flow'

    The actual width of the head might (absent rules etc) be driven by two optimisations:

    1. The L/D improves as the trailing edge ( i.e. main leech) nears vertical;
    2. Minimum head chord around 700 mm for Mr Reynolds

    Check in on the small rigs of the top two 18s from the recent JJ Giltinans. The Andoo rig profile in particular tells us a few things...
     
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