Aspect ratio on headsail vs shroud angle?

Discussion in 'Hydrodynamics and Aerodynamics' started by sailingdaniel, Aug 18, 2011.

  1. Eric Sponberg
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    Eric Sponberg Senior Member

    For this example, one would expect small differences in upwind performance of the square top main over the conventional main. This is, in fact, what we see here, if I am reading your results correctly. I am presuming the ORC chart on the right is for the square top main, the chart on the left is for the conventional main, corresponding to the order of your sail diagrams. Also, this is the VPP program results, not your CFD results, so we are not looking at apples-to-apples here regarding your initial statement.

    At the lower wind speeds, the beat angle of the square top main is measurably lower--fractions of a degree, certainly, but measurable here. Remember, this is an extremely small change in planform shape that appears to have a measureable difference in performance. At the higher wind speeds, the beat angles are the same. VMG is every so slightly better for the square top at low wind speeds, the same at higher wind speeds. Boat speed holds better for the square top main up to beam reach. After that, both rigs appear to be pretty much the same.

    For such a small change in sail shape, I'd say that these are expected results. It is small, but measurable and favorable to the square top main being the better performer. And this runs counter to your statement earlier in post #48: "We found virtually no difference in performance upwind, between a regular and asquare top main, in a CFD study." In a case such as this, the differences are going to be small, and if what you say is true about the CFD study, then the CFD has not picked up what the VPP has.

    Also, in post #59, you said, "We did an upwind simulation, where a small fat head mainsail betters a normal triangular sail by some 3% (3% more drive), and a nearly square top main again betters the small fat head by 0,5% only (all sailplans have the same area)." OK, there you are, the square top main was the best of the three, and measureably so, the square top main over the triangular main by 3.5%.

    I welcome your continuing thoughts.

    Eric
     
  2. brian eiland
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    brian eiland Senior Member

    Upwash to Masthead Genoa

    I could fully imagine that the mast tube itself would produce a healthy amount of upwash for a masthead genoa...but of course the sail area of that genoa in that region is so small. More importantly would be to look at any negative effects of extra vortices generated?
     
  3. brian eiland
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    brian eiland Senior Member

    That sort of gain would certainly be enough to command their use on the new America's Cup cats as well as many other RACERS.
     
  4. viking north
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    viking north VINLAND

    That 3.5% combined with the wing keel, the fattened cheeks on the hull (forget their name) computer assist tacticion, and a few more speed enhansing tricks squeeze out fractional winning knots over the competition. If it out performs it's an improvement regardless of how little that might be. Great thread, we all gain from this gentlemens approach to the give and take of info. The other great feature here is it's marketable and thus a driving factor to keep the ball rolling---

    P.S. The use of multi hulls in traditional monohull racing leaves a bad taste in my mouth-- I think it was sour grapes for Dennis Conners to do so-- the very thing that promotes that non gentleman, win at all cost, bad name, that we Americans should strive to overcome.


    A yacht is not determined by the vessel but by the care and love of her owner--
     
  5. Mikko Brummer
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    Mikko Brummer Senior Member

    To me a 3,5% difference in mainsail drive was "virtually no difference", or 0.02 kn in speed...

    This is for a sloop rig. For a cat rig, the square top would probably be more beneficial, even if the heeling moment costraint still applies.
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    In sailboat design, this is a huge difference.

    Eric
     
  7. Mikko Brummer
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    Mikko Brummer Senior Member

    Hmm... 0,02 kn is about 37 m, or maybe 3 boatlengths after one hour of sailing - not huge, but something a racing sailor would still appreciate.

    The 3,5% difference in drive is considerable, but in case of a sloop rig much of the drive comes from he jib so its effect is not so big.
     
  8. Mikko Brummer
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    Mikko Brummer Senior Member

    I don't think masthead rig is the best, even if you consider purely aerodynamics. It could have to do with the tip vortices interfering with each other in a negative manner.

    The "best" sailplan if aerodynamics only is considered would be maybe 85-90% fractional, although the optimal fractional ratio decreases as soon as heeling moment comes into play. The real reason why the fractional rig is so good lies in adaptability. With a fractional rig and a bendy mast you can control power to the extent that you can have much more sail area in light air and still cope with strong winds than with a masthead rig. Bruce Farr was probably the first to realize this with his 1/4-tonner 45.South, and the french half tonners later.
     
  9. outside the box

    outside the box Previous Member

    Eric is this what you mean? JFWIW movable headsail and a freestanding mast on a Catamaran.
    Plus movable prodder. Photo's.
     

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  10. Eric Sponberg
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    Eric Sponberg Senior Member

    Yes, those are some ways to accomplish moving the jib tack.

    Eric
     
  11. outside the box

    outside the box Previous Member

    Thank you Eric
     
  12. brian eiland
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    brian eiland Senior Member

    I continue to be amazed by this almighty worship of high aspect ratio and ever smaller headstay angles...particularly where cruising vessels are concerned. In general cruising sailors are not sweeking that ULTIMATE pointing capability. Yes they would like some of that capability, but not at the expense of other good attributes of lower AR.

    Can anyone on this subject thread give me a good argument that in terms of aerodynamic wind flow over a headsail mounted on a 19 degree headstay will be any better that one mounted on a 22-25 degree one.

    ......Sorry I can not continue this posting of mine at this time as I'm being called to a great dinner here in St. Thomas where I am visting some old friends. Brian








     
  13. Mikko Brummer
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    Mikko Brummer Senior Member

    The angle the headstay makes to the mast creates a "kink" in the leading edge of the sailplan. That kink causes a disturbance on the mainsail, so probably the best angle would be zero, or the forestay parallel to the mast ;-), but I doubt a few degrees makes a huge difference. This could be one reason, though, why it is so beneficial to rake the mast aft on fractional rigs - the forestay gets more parallel with the topmast.
     
  14. Mikko Brummer
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    Mikko Brummer Senior Member

    Star is a good example about how a fractional, bendy rig can provide a wider wind range. Totally over canvassed, the Star is fully powered in 9 kn of wind, yet it can sail in 30 knots (without reefing). If you look at the enclosed photo, it's like above the top of the jib (aka. above a masthead or a gaff rig, if you like), there is still like a Finn-sail to extend the sailplan. Sure helps for power in light winds, and on the other hand the topmast acts as a strong lever to bend the whole mast when pressure increases on the leech. All happens automatically, with no need of user input, making it possible to sail the boat in a breeze.

    With ref. to my previous post, you can also see that the Star achieves a topmast parallel to the forestay, by virtue of huge rake and the bendy mast.
     

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  15. Mikko Brummer
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    Mikko Brummer Senior Member

    The modern 91% fractional rig of the X-35, on the other hand, is an example of how the rig automatics don't work. With the forestay going high up, the topmast lacks the leverage to bend the mast at all in gusts. This is worsened by the super tight backstay (750 kgf), necessary due to the lack of running backstays, effectively locking the rig in place (as is the case with the masthead rig). This leads to the whole mainsail flogging like a flag behind the tall and narrow jib that extends close to the top. If you try to twist the jib top open to give the mainsail some more living space, the luff of the high-aspect jib will backwind halfway down, which is even slower.

    I think the old IOR, in-line fractional rig with running backs was so much better than these modern rigs, inspired by the IMS-rule.

    If you think of the sails as the engine of the sailboat, then the rig is the suspension: The modern rig is like an old fashioned car with a stiff rear axle and no shock absorbers, while that of the Star is like a Citroen's pneumatic, perfectly comfortable suspension.
     
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