Drivers for the "sqare top" width?

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

  1. Erwan
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    Erwan Senior Member

    In addition to what is above mentionned, I would add:
    According to an Australian A-Cat sailmaker , when questionning him in 1993 or 1994 about why he gave up the idea of a wishbone boom decksweeper /stepped leach he started to develop in 1988, just like the Moth'sails today.
    His answer was that the area was more usefull "upstairs" in order to sail the "Wildthing" earlier for downwind performance.
     
  2. jmf11
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    jmf11 Junior Member

    Thanks Steve, and also others, for the responses to my question. I'm comforted by the above statement that a key driver is the de-powering of the rig. However I have not found parametric studies or academic papers about the optimisation of that criteria.

    The AC75 have a quite big roach. Depending on the ship caracteristics, what would be the optimum roach for de-powering feature? I imagine that there can be too much, or not enough roach.

    This is a key feature for sailability of spart catamarans, windsuf, landyacht...

    Best regards,
    JMF
     
  3. Remmlinger
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    Remmlinger engineer

    I am wondering, that nobody mentioned the negative influence of the mast on the sail-section behind it. In the top part of the sail, the mast diameter is relatively large compared to the chord. At a relation of d/c = 20% the performance of the sail is already significantly deteriorated: http://www.remmlinger.com/2D aerodynamics.pdf
    The top part of a Bermuda sail is therefore almost useless, it creates drag with only little lift. A square top main ads useful chord length in an area that is far enough away from the mast, the top part of the sail will now contribute to the driving force.
     
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  4. jmf11
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    jmf11 Junior Member

    Thanks for the paper Remmlinger,

    This is a good point to take into account. For the sails I'm interested it, there is a fabric luff pocket in which the mast slides in. I believe that this can make the things a bit better, action as a soft wing mast. However the shape may not be perfect at the mast tip. So your comment is also applicable.

    JMF
     
  5. Erwan
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    Erwan Senior Member

    You must let something to chew for the sailmakers, their experience would probably answer your question regarding the optimum chord ratio between tip and sailfoot, and this optimum probably changes with the boat, the crew's weight and the different luff & leach tensions.

    Not sure there is a lot of performance to be gained optimizing this parameter alone.

    Thkx for this kind of topic, all these "design" issues have a strong appeal to me.

    Fair winds
     
  6. Remmlinger
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    Remmlinger engineer

    If the sail can slide around a round mast, the situation is generally better for the whole sail. The effect is similar to a rotating mast. In my paper I investigated also rotating masts, using XFOIL. The improvement in performance is quite impressive on reaching courses. The situation at the top of the sail with large d/c - ratios is not significantly improved by a rotating mast.
     
  7. CT249
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    CT249 Senior Member

    How much of the top part on a conventional rig is almost useless? In practise, the flow over the area around the top batten often seems to be very significant, judging from observed performance and telltales.

    I've been shocked by how hard fairly high-aspect wingmast squaretop cat sails (ie F18) are trimmed in light winds, with very little twist and the best performance apparently often obtained with the top leach telltales stalled.

    At the other extreme, a truly world-class high-performance una-rigged squaretop wingmast cat going downwind in about 5 knots of breeze has optimum trim of 19% camber at mid height and about 20+ degrees twist, which seems to be a huge amount of twist and depth. I still can't get my head around the amount of twist that is fast downwind in cats, even old ones like Hobie 14 types; as with windsurfers, getting the tufts to flow often seems to be of little help in finding optimum trim.
     
  8. Doug Halsey
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    Doug Halsey Senior Member

    Getting the tufts to flow means there is no flow separation, which usually means that the foil is not operating at CLmax.

    Since CLmax is the condition which maximizes the sail's thrust when the apparent wind angle is 90°, the best trim often results in significant amounts of separation.

    For upwind sailing or high-performance "apparent-wind sailing", the word "often" in the previous sentence should be replaced with "sometimes".
     
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  9. Remmlinger
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    Remmlinger engineer

    It depends on the ratio d/c. I can only report about my calculations with XFOIL. To obtain numerical values of the 2D-lift-coefficient, you can use my computer program: 2D Sail aerodynamics http://www.remmlinger.com/Sail2D.html
     
  10. CT249
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    CT249 Senior Member

    The exact definitions of "sometimes" and "often" are not available. :)

    Yep, I'm aware of the need in many classes to sail at, or very close, to separation in the upper leach. I'm going from classes in which I have been national champion against Olympic team members and world champs (J/24s, offshore yachts, Windsurfers) or state champion (Lasers) so the observed performance is from craft that were not exactly sailing slowly.

    On many of those craft the upper leach telltales are stalled a lot of the time when at optimum trim (to use an old example, when for example winning nationals with an America's Cup winning trimmer on the sheet) but at other times, getting the same telltales to stream 50% or more of the time is critical. Obviously to some extent the head could be of little relevance aerodynamically and it may only be important because it impacts the lower regions of the sail in terms of leach tension, draft, a of a etc, but my gut feeling is that the achieving flow over the head IS actually producing significant drive and therefore not almost useless.

    This could be because here in Australia we tend to sail in choppy conditions where the masthead is moving back and forward a lot and therefore the angle of attack moves around a lot, perhaps. I live inland where we also seem to get very severe wind shear, and perhaps giving attention to twist at the head is merely a good way of giving attention to the lower sections that actually produce significant lift. On the other hand, on several occasions I have seen what seems to be a dramatic sudden change in performance between my Laser and my club arch rival, one a boat of slightly greater speed that is also cat-rigged but with a rotating mast and roachy head. The other boat slows dramatically at about 4 knots of wind. Because it's a step change it doesn't seem to be related to wsa, foils, SA/WS ratios etc and it seems to be related to the bigger head section stalling. The tight-leach Laser sail would have already stalled. I wonder whether the issue is that the pinhead of the Laser sail, when stalled, creates a smaller trailing vortex or tip vortex to the roachy main when stalled. The other boat is sailed by a sailmaker who is an Olympic medallist and multiple world champ in roachy-sailed boats so I think we can infer that he's not sailing badly.

    I wonder whether the difference is the unsteady flow experienced in real life and the effects on a of a, etc.
     
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  11. Mikko Brummer
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    Mikko Brummer Senior Member

    I’m not a big fan of fatheads - to me, they are like the plumb bow, a product of rating rules & fashion. Certainly, if mast height is limited, it is a way to add area (where the wind is stronger).

    A triangular sail goes to depower just as well, especially on a bendy mast. Sailboats are righting moment limited, see a discussion about bell shaped lift distribution. I must admit I don’t have much experience on sailing with fathead sail, but aren’t the heads in Acats or Moths getting narrower and narrower, with the deck-sweeper starting to be a norm?
     
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  12. philSweet
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    philSweet Senior Member

    One very important factor is the effective sweep of the rig, a figure that accounts for heel, AWA, mast rake and boat trim. The wild thing reduces effective sweep and hence it favors a much larger tip cord. An old leadmine sailing at 20 degrees heel in a blow is optimized with a modest roach pinhead. A cat doing the wild thing wants as much as 40% head cord percent for optimum load. Windsurfers have the best solution - independent rake and mast heel. They can manage the lift distribution for best total thrust and sideforce over a wide range of conditions with a single planform.
     
  13. jmf11
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    jmf11 Junior Member

    Dear Mikko,

    Your post looks thought provoking to me, as it goes against the general idea that fatheads sails are the norm now. Your experience with academic FSI studies for several olympic supports gives credit to your ideas. Thanks for having shared this knowledge through different articles and papers.

    AC75 have a quite large fathead, maybe deriving from the class rules... And yes I checked the evolution of A-Cat sails, and the balance seems to have evolved. Seems that they even use shorter masts in some cases now. A-Class https://www.landenberger-onedesign.com/en/productsshop/multihull-sails/a-class => long mast and tippy top or short mast with a bit larger head.

    So at this point of discussion, the only explicit driver for the head size is to reduce the drag by optimizing the lift distribution. And I have not identified yet a demonstrated contribution from "quantitative gust management / dynamic behaviour" (which I still feel could be an important driver).

    By the way, the trend for windfoil performance sails also goes to smaller heads.

    Bottom line: I'm puzzled in front of those different designs/planforms and not identifying the clear design drivers that could be subject to an explicit optimization process...

    Best regards,
    JMF
     
  14. CT249
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    CT249 Senior Member

    Phil's point about sweep etc is very interesting; Lock Crowther was interested in the issue and felt that it was one reason that conventional monos were (in the '80s) often surprisingly quick upwind against multis. In windsurfers there's a lot of attention paid to keeping the rig upright for better pointing, and the issue of effective sweep has seemed to me to be very relevant. Almost everyone believes that the upright rig is quicker because of its (allegedly) greater apparent projected area but Lock's remarks always made me think that effective sweep angle was also very significant, as Phil notes. In many windsurfers, effective fore-and-aft rake and windward heel aren't very free but are restricted by CLR factors and it's interesting that better upwind performance is often found when increasing apparent sweep. Conventional theory says that it's due to "closing the gap" of the foot but as I understand it, CFD studies show that's not really critical. In the Windsurfer LT (modern version of the original Windsurfer and just about the world's fastest selling one design apart from the Opti) it seems that increasing apparent sweep causes a significant advantage in pointing even though the old-style rig doesn't allow for effective closing of the gap.

    Now that A Class cats are often foiling a lot, they aren't wild thinging which must significantly change the apparent sweep angle. Moving to the smaller top makes a lot of sense, along with the other issue of increased apparent windspeed which encourages a smaller head. I haven't seen Landy for a couple of months (because his cows are keeping him too busy to come sailing!) but all my recollections are that the increase in apparent is seen as the driver towards smaller heads in Moths and As.

    A couple of years ago I was talking to a top pro windsurfer who said that foiling windsurfer sails were moving towards fuller heads, but if I recall correctly that was more to increase the leverage the sailor had over fore-and-aft trim when on foils.
     

  15. jmf11
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    jmf11 Junior Member

    I don't really understand the benefits of that "sweep".

    For fore-aft = rake, I had looked at the wings design and the "sweep" was interesting at trans sonic speeds. I'm not there with my practice yet :). The interesting thing I had found about the rake is Span Squared | North Sails https://www.northsails.com/sailing/de/2020/01/span-squared. And there is also the "closing the gap", now implemented in other supports by deck sweeping configurations.

    About heeling and windward heeling, I only see benefits to the windward one as for Moths. But this is more about canting the foil and improving righting moment.

    JMF
     
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