Squaretop mains

Discussion in 'Sailboats' started by Mikko Brummer, Apr 10, 2013.

  1. Doug Lord
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    Doug Lord Flight Ready

    Rectangular planforms vs Triangular planforms

    ====================
    I'm not sure theres an argument. I was a bit ticked about being quoted w/o attribution but Mikko explained that satisfactorily. There may be a difference of opinion about the effectiveness of squaretops and Mikko has quoted a single article printed in French that I can't afford to have properly translated. I hadn't heard the argument(for or against squaretops??) that they are only used on boats with a mast height limit before. Of course, thats not true and I'm sure there are many people like myself that have lots of squaretop and pin head experience that prefer the adjustability of the square top. With any new popular technology there will be these kinds of "arguments"
    but experience usually settles the matter.
     
  2. motorbike
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    motorbike Senior Member

    Evidence would suggest the ST mains are generally better performing, but perhaps the difference may be offset by hull form, ballast distribution etc. More sail area does not necessarily mean more efficiency. Plenty of other factors come into play such as weight, spar dimensions and drag inducing components. Clearly squaretops get more area into clear stronger wind, they can also depower the upper part of the sail which may be an advantage (or not) and they also mimic to a degree an eliptical planform which has lower drag. But its not that simple nor is comparing modern squaretops to trad gaff rigs as clearly they are not as fast or weatherly. As usual one size does not fit all. This has been discussed at length by very well educated and experienced NA's
    http://www.boatdesign.net/forums/sailboats/square-top-mains-9668.html
     
  3. Richard Woods
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    Richard Woods Woods Designs

    As always a simple sounding debate gets much more complicated when you look at the whole picture.

    I first sailed a Merlin Rocket in the 1960's and, given the technology available at the time, I think it was a major first step towards todays squareheaded sails. In the 1970's I raced a number of International Moths, all rigged with big roaches and almost square tops. I was one of the first to try a sleeved luff sail. As my final year design project in 1977 I designed a 10m cruising catamaran which featured a short "Dutch style" gaff rig. That rig was developed in part from several discussions I had with Tony Marchaj (a friend of my fathers) at his house.

    However, later I realised that a gaff rig was much heavier and ultimately more complicated and expensive than a "Merlin Rocket" style rig. So that is what I drew for the first catamaran I built and also drew on even my early catamaran designs, like the Strider for example.

    So I don't have anything against squaretop rigs per se. What complicates the issue is, as I said at the start, when you take everything, not just performance, into account. As CT249 points out, adding a square top rig, twin trapezes and a spinnaker to a Tornado only added 4% to speed. And obviously much of that speed increase came from the extra stability and offwind speed, not from the mainsail which only added 2%, say, to speed.

    A production yard has to weigh up the options "if the boat goes 2% faster would any of my customers actually notice?" "if the boat cost 20% more would they notice?"

    I always tell people "don't bother with a square top mainsail if you have it made in dacron" , "don't bother with high tech sails if you are happy with LAR keels on a multihull". Studies have shown that a laminate sail offers 5% more drive than a dacron sail, or roughly 2% more speed.

    Performance is the sum of all parts. A successful boatbuilder will also factor in the purchase price, the running costs and the ease of sailing. Never mind customer resistance to something new.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Square top mains

    In several areas of the discussion I've seen references to the extra complication of square top sails due to the full length battens.
    Why does a squaretop have to have full length battens? On numerous models I used a "gaff"(very light weight) instead of a diagonal batten and no full length battens at all. Of course square top jibs(so far as I've seen) don't have full length battens(except the one on my foiler!)
    Just a note: the "gaff" allows for an upper outhaul that could be adjusted while sailing or set beforehand for the conditions-this has a profound effect on the top third of the sail and was used on numerous models as well as on the square top jib on my foiler.
     
  5. Richard Woods
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    Richard Woods Woods Designs

    As you know, the Merlin only uses one full batten as a "gaff". One reason for reducing the number of battens is to save weight. Another is because a "soft" sail is easier to read. Never mind the extra cost and chafe of full length battens.

    As I said in my earlier post, a detailed weight analysis on a 30ft cruising catamaran showed that a full length top batten and a square or squareish top mainsail was lighter than a gaff sail. Remember that it isn't really practical to have any rigging except from the masthead on a gaff rig. And that clearly means a bigger heavier mast section is required.

    The top batten does need to bisect the leech angle, so typically is at 45deg. It's unlikely you'll get a well setting sail with horizontal battens (Mikko?). That means the top batten will normally have to be removed before stowing the mainsail, something many people don't want to do.

    One advantage of the square top sail over a gaff mainsail is that by and large the sail maker determines the shape. Whereas a gaff sail can be adjusted in four corners (plus maybe gaff end vangs) which means much of the shape is instead determined by the sailor, who may not always get it right

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  6. powerabout
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    powerabout Senior Member

    you also forgot to mention the massive sheet loads that you need to use on a square head on a multi
    which then causes you to use more battens at the top
     
  7. Petros
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    Petros Senior Member

    Usually when referring to aerodynamic efficiency you are considering the lift to drag ratio [L/D]. On a sailboat better L/d will result in more net drive for the same size sail in the same wind in almost all points of sail. The other benefit to a higher L/d on a sail is there will be less heeling moment for the same amount of drive on a reach or pointing high into the wind. Likely you can also point a bit higher, or make better speed when close hauled.
     
  8. DCockey
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    DCockey Senior Member

    But you have to be careful what is kept constant and what varies when making comparisons. As Petros said, L/D is a meaningful measure of aerodynamic efficiency. But often CL/CD is the actual comparison made and the ranking can be different in that case if the area and corresponding CL changes as the configuration or planview shape changes.
     
  9. Mikko Brummer
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    Mikko Brummer Senior Member

    When it comes to windsurfers, I believe they have square tops for more efficient pumping or "paddling" with the sail. Windsurfers use heavy pumping in light airs all the time, and pump themselves into plane as soon as it is possible when the wind comes up. In the Olympic RS-X, they have a centerboard for the light wind regime, which they take out when they have a chance of planing, and sail with the fin only.

    It's obvious that a wide, elastic top works for pumping. In sailboats you don't have the possibility of swinging the rig around (in some way you do with the freestanding rigs in dinghies like the Finn).

    HTML:
    <iframe width="420" height="315" src="http://www.youtube.com/embed/VVBZzfVE0Eo?rel=0" frameborder="0" allowfullscreen></iframe>
    How do I embed a Youtube video? Anywy, google "windsurfer pumping light air" or see for instance http://www.youtube.com/embed/VVBZzfVE0Eo?rel=0
     
  10. Doug Lord
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    Doug Lord Flight Ready

    ===============
    Mikko, to embed your you tube video:

    First you type: [you tube]
    Then the code and then [/you tube] with no spaces between you and tube
    ---the code for your video
    is VVBZzfVE0Eo?rel=0
    ===================================
    SO:

     
  11. Doug Lord
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    Doug Lord Flight Ready

    Pumping

    It may be legal but I just hate seeing people do that-it goes against the grain......
     
  12. Richard Woods
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    Richard Woods Woods Designs

    I have always assumed that windsurfer sails work in a slightly different manner to regular rigs.

    The simplest possible rig is the triangular single mainsail set on an unstayed mast - like a Finn, Laser/Torch etc

    But they don't all work the same

    For example, to sail a Laser fast off wind you run by the lee with lots of leech twist and a fairly flat sail. Try that on the very similar Byte and you'll go to the back of the fleet very fast. Instead you need a very full sail and tight kicking strap. Still sailing by the lee of course.

    Essentially the same rig, but two totally different ways of generating maximum drive.

    It's all too complicated to theorize, you have to get out there and sail to get the real answers.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  13. gggGuest
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    gggGuest ...

    The trouble with this sort of debate is that firstly one has to establish what a fair comparison point is - you could make cases for equal area, equal span, equal heeling moment, equal cost and probably about half a dozen others, and secondly how to answer a question I have never really managed to figure out - did Bill win because of his boatspeed or in spite of his lack of it?
    I suppose I've been sailing with sails of something over a true elliptical distribution for something like 25 years, but on the other hand I've never had one which actually has the tip parallel to the waterline: they've always been somewhere in between.
    My interest is only in racing dinghy rigs of quickish boats with moderate to large rigs - things like multihulls and leadmines march to different drummers.
    I suppose the first thing we can be certain of is that, should all else be equal, sail planform can only be a tiny fraction of the potential difference between boat a and boat b round the track, and will always tend to be submerged by other variables, most notably the ones pushing the stick and pulling the strings.
    A second thing that we can be certain of is that, no matter what the planform, a sail that does not match the mast well will be very much inferior to one that does. And that means if you change the sail planform on a mast that matches the current sail well you won't get an improvement.
    But subject to those monster caveats, yes, I think over elliptical sails are worth having on racing dinghies. I see two advantages.

    The first is all about twist, pressure distribution and induced drag. We know that theoretically the lowest drag comes from an elliptically loaded pressure distribution. You can achieve that with an under elliptical sail which is more heavily loaded at the top, an elliptical sail which is evenly loaded, or an over elliptical sails somewhat twisted off at the top. But which one is more readily achieved? Especially when we also consider apparent wind changes from wind shear? Don't need to answer that do I?

    BUt the big one is all about dynamic behaviour of the rig and gust response. This only really applies to fully battened sails. The ideal dynamic behaviour, to my mind, which of course isn't really achievable, is for the top of the sail to twist off at a given point prgressively down the sail, but without affecting the mid leech. You need the leech up to at least half way up to stand firmly and not twist off to maintain pointing ability. And this only really happens on full battened sails with a soft leach at the top. The extent to which the sail twists off is a a factor of both the area at the top and the flexibility of the mast, so the most important thing is that they match perfectly. But once you've got to the stage where you are getting a mast made to suit your intended sail, well then I think its possible that with a larger head you get less movement on the mid leech for the same depowering effect, which I suspect is probably advantageous.
     
  14. Richard Woods
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    Richard Woods Woods Designs

    So what boats do you sail?

    I was discussing B14 rigs with Chris Davison at Ullman Sails UK a couple of weeks ago. Their rigs are set up very different to a multihull. The headsail is much fuller, the mainsail much flatter. Yet in theory, as the sail plan drawings look similar, one might expect both a B14 and a beach cat rig to be designed in the same way

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

  15. gggGuest
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    gggGuest ...

    Canoes these days, used to be Cherubs.
    I'm not sure one should expect a Cat rig and a B14 rig to be very similar: think about dynamic behaviour and the amount of inertia to overcome when a gust attempts to heel first a B14 and secondly a cat flying a hull. I've extensively sailed a monohull with a beach cat style over rotating mast and it could be a brute to handle. Quick if things were just right, but if not it would bite hard and fast.
     
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