Square top mains?

Discussion in 'Sailboats' started by kenwstr, Dec 6, 2005.

  1. edvb
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    edvb Junior Member

    I am not an "in the box" kind of guy.:cool:
     
  2. JosephT
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    JosephT Senior Member

    Aerodynamically, it's a matter of surface area on the sheet. A square top offers more surface area at the top for a mast of equal height. Thus, you're getting more surface area out of your sheet with a square top. The end result will be improved performance.

    Compare an airplane wing. How many wing tips to you see that come to a perfect point (Not too many)? Most aircraft wings are squared or rounded off on the tips. The same general concept applies here. More surface area = improved lift = better performance.

    It's really as simple as that.
     
  3. Mikko Brummer
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    Mikko Brummer Senior Member

    Airplanes don't heel...
     
  4. Silver Raven
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    Silver Raven Senior Member

    Mikko - think you should do much more home work. Try to study - sailplanes - high performance gliders. Look at some very high end wing-masts.

    Look into the new 4 mtr plus extension on the AC 45's - then make your comments with great care. Regards, james
     
  5. Mikko Brummer
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    Mikko Brummer Senior Member

    Thanks, but I don't think I can do any more homework. I was trying to refer to the heeling moment, look at for instance post #47 in this thread, by Tom Speer.
     
  6. CT 249
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    CT 249 Senior Member

    Errrr, have you seen the stuff that Mikko has done and published in respected international arenas? I think that his knowledge is pretty well grounded.

    It's pretty well accepted in aerodynamics that if you are limited by heeling moment (or by wing root strength in an aircraft) rather than span, then a square tip is less efficient than a taller foil with a smaller span at the tip. This has been said several times here by Tom Speer and by other sailing aerodynamicists such as Neil Pollock.

    Sure, many classes have square tops these days but these are classes that have lots of righting moment and/or other restrictions such as a mast height restriction. In conventional dinghies and monos, righting moment is limited and therefore the extra heeling moment of a square top can make them actually slower once the wind picks up.

    One thing I sometimes wonder is whether we correctly calculate the cost and handling issues of square tops when we look at them as being the bee's knees. They don't come without cost and complications such as highly-loaded battens that have to be bought, installed by the sailmaker, and then usually taken out every time the sail is folded. Even after growing up with full head battens I'm struck by how much extra hassle having to remove them can be, when you are stuffed after finishing a shorthanded overnight multi race or de-rigging in the rain in the dinghy park. And the really deep camber ones that are sometimes seen on cruising multis seem to spend a lot of their time in light winds choked up and offering nothing but drag, whereas a pinhead can be more "set and forget" in lighter airs.

    Squaretops may be great in their place, but not always better.
     
  7. gggGuest
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    gggGuest ...

    For whatever my opinion is worth Mikko needs to do stuff all homework in this sort of area, because he certainly knows a great deal more about it than I and most other people do.

    Glider rigs are very admirable, but the detail problems are entirely different because of the heel problem. If you put a glider wing on a boat it will be unsailable.
     
  8. Perm Stress
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    Perm Stress Senior Member

    Originally Posted by Silver Raven:
    Mikko - think you should do much more home work.

    Completely agree.

    As noticed by F. Bethwaite in "High performance sailing", glider wings work at ~3 degrees angle of attack, while sails work in all the range15 to 90 degreees angle of attack. And this is only the beginning of differences.
    So any implications that sails work similar or should copy the gilder wings simply has 0 physical meaning.
     
  9. themanshed
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    themanshed Senior Member

    About close to 15 – 20 years ago I converted my G-Cat 5.7 M catamaran from a pointed main to a square top at the recommendation of my sail maker. I noticed increased performance around the marks with my boat. I had a chance to speak with Bill Roberts about the concept back then. As you may or may not know Bill is the designer behind the SuperCats and the RC catamarans. Bill is also an aeronautical type of engineer that worked for a world known Aircraft / Rocket Engine manufacturing company.
    So we are sitting in a mutual friends shop and I ask Bill to describe the difference to me in layman’s terms – no graphs or scientific terms. He looked at me and smiled and said have you ever seen a modern airplane with pointed wings? He explained that flat heads as he called them was nothing new the old sail ships in the orient had them. The concept is the wind as it leaves the trailing edge of the sail or wing, leaves it clean. In the case of a pointed sail or wing the wind does not leave the trailing edge clean and swirls around down the face of the sail or wing. He looked at me and smiled and that is the difference now I can make charts and graphs and give you all kinds of data but you are smart enough to figure out which is best. I looked at him and said nope that is enough information for me as I took another gulp of my Budweiser.
     
  10. Mikko Brummer
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    Mikko Brummer Senior Member

    I may have posted this earlier in another thread... the JPK boatyard built 2 identical boats, one with a square top main and another with a "normal" main. Both were optimised for the carbon mast tube etc., to best serve its purpose, and sailed extensively against each other by professional crews. The boatyard ended up choosing the normal mainsail, which the sailors felt had a slight edge upwind, pointing higher, with no difference between the 2 in off the wind. The normal main with one backstay only (square top had twin) was easier to trim & simpler to handle, and boat a little cheaper.
     

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  11. michael pierzga
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    michael pierzga Senior Member

    Sounds correct to me. Square tops are also high maintenance. All that mass aloft chafing and compressing the batten. For normal folks I would regard the square top as a fashion statement and a poor choice.
     
  12. DCockey
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    DCockey Senior Member

    A fundamental question in any comparison of different sail shapes is what is held constant, if anything:
    • Sail area, actual - Perhaps the least meaningful
    • Sail area, rated - Critical if a rating/adherence to class rules is important. Otherwise not meaningful.
    • Mast height - Important if an existing mast is to be reused or there is an "air draft" restriction due to a bridge, etc, or from class rules.
    • Heeling moment - Arguably the most important for pure performance
    • ????
    Unfortunately what's held constant is rarely stated an may be implicit. Usually performance comparisons are made in the context of racing which doesn't alway coincide with what is important when not racing.
     
  13. DCockey
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    DCockey Senior Member

    An over-simplified and mis-leading at best explaination of the aerodynamics.

    Have a look at recent jet aircraft where drag reduction is important. They have effectively "pointed" wings which are turned upwards. One of the primary reasons many aircraft have square wing tips is cost and weight of wing construction.
     
  14. themanshed
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    themanshed Senior Member

    My example was basic and perhaps lives in the light multihull racing world–but I’d like to point out a few things:

    Aircraft speeds compared to sailboat speeds. This is a sailing forum and when your sailboat can break the water surface from speed ---- COOL!
    Racing compared to "Yachting"
    Mulltihulls compared to Monohulls (weight / performance).

    A test done with two like boats and no difference was noticed were the boats of a design that would benefit from a sail design change like the shape of the main sail head?. For example two heavy monohulls the change could be insignificant that the crew would notice - slow and steady is slow and steady. Where light multihulls are fast and nimble and a change like this can be noticed. Then you have to asked is it worth the extra work if the sails are reefed and are back stays used?


    My thought in an analogy is why they don't make just one pair of shoes for everybody; life would be simpler that way.
    Do you want sport shoes for example: track shoes, golf shoes, or mountain climbing shoes? Do you want wing tip shoes for the formal? Or do you just like to wear your Top Siders all the time! Some like slip-on, and others like to tie their shoes. So bottom line it is a matter of choice for performance, will your design get the performance, or just style?
     

  15. CT 249
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    CT 249 Senior Member

    Good two-boat testing is surely good two-boat testing, irrespective of the boat's speed potential. Even if it's done in slow and steady boats, a performance improvement should be noted.

    But yes, as you say, things like squaretops can effect different boats differently. An old "Sailing World" article said (IIRC) that the modern square top was first seen on the A Class cat sails that Greg Goodall cut for his world-champ brother Alan. Of course, square tops of other types were seen in many other craft and a development like that can't really be pinpointed, but in a boat like an A Class a squaretop seems perfectly at home; it's got a thick wingmast so gust response may be better obtained by twisting off a squaretop rather than active mast bend, it's got flat sails so the squaretop doesn't choke in a lull, and if it's like some other Goodall designs it has very high mainsheet tensions so working the main in gusts is difficult and you need to design in fairly good gust response on a rigid rig.

    But the fact that a squaretop works on some cats etc so well doesn't mean that it works on everything, as you noted.

    In windsurfers you can change in seconds from a flat fully-battened squaretop to an old-style deep "pinhead", just by swapping rigs. The thing that always fascinates me is how rarely the cool modern squaretops actually work outside of the hull/board drag and windspeeds they are designed to work in. On the wrong "hull" in the wrong conditions they are slower than a '70s dacron pinhead.

    Below is a classic illustration, from the biggest British windsurfing event a few years back. Out of around 200 entrants in the "sport" class, all of them on modern sails and on a mixture of funboards and raceboards, second overall in light wind (over about 4 races) was an original Windsurfer, the brightly coloured sail right of middle. In the conditions of the day, it just worked better!

    [​IMG]

    PS DCockey, cost and ease of use (including rigging) are of course other vital considerations, even in racing boats! A classic case is (as I am told from a very good source) one of the last of the unrestricted performance dinghy/skiff types. Some years back, they moved into more pre-bend and fatter heads, meaning that the boats had to be rigged on their side. The problem was that about half of the clubs that sailed the class had very limited rigging space. The "advance" in sail shape meant that you could no longer race from those three clubs, and that was a factor in the class losing numbers.

    So sure, they've got massive heads on their main (at least some mains have had wider chord in the head than at the foot) and go faster, but few of them race......
     
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