Square top Mainsails

Discussion in 'Sailboats' started by ChrisVJ, Jul 7, 2022.

  1. ChrisVJ
    Joined: Nov 2021
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    ChrisVJ Junior Member

    I have been looking but I can't make out the rigging on the boats with the new square top mainsails. I understand that the purpose is to broaden the leech for more sail area and perhaps better set but do these boats have running back stays. They can't have standing back stays or the sail couldn't tack.

    One had to wonder, seriously, if the extra sail area is worth the hassle. It maybe OK on the old gaffers with their big crews but for the rest of us, why not just add a few feet to the mast and add the little triangle? You could have a full batten to stiffen the sail at the original square point if you wanted.
     
  2. Rumars
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    Rumars Senior Member

    A lot of the boats with square top mains do have running backstays, but this is not primarily because of the sail shape. You can just as well rig them on a mast with angled sailings or on freestanding masts. How much "trouble" they are depends on what the designer wanted.

    The purpose of the square top is a more efficient planform, and it's not about what we see but what the wind sees. In simple terms, with a classic triangular sail approx. the top third of the sail contributes nothing to drive, while a square top uses the entire area for drive.
    If a square top is worth it is a "horses for courses" thing. For example some argue that cruising boats don't need them, just like they don't need full battens, big roach, flexible masts, and other refinements found on racers. Others say it's stupid not to use the entire panoply of modern refinements. Where one positions itself on this scala is personal preference.

    Simply lengthening the mast is not always acceptable, weight up high affects stability. Gaff rigs solved this with sliding topmasts, but the modern square top is more closely related to the sprit rig then it is to the gaff. The complete planform of a gaff rig is with topsails hoisted, and that is a triangular planform. The gaff rig is just a bermudian sail with a single big sailbatten and where the first reef takes area from the top and not the bottom of the sail.
     
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  3. CT249
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    CT249 Senior Member

    Rumars, surely the top third of a triangular main is doing a LOT of driving. If not, why else would ensuring the correct sail trim in that part of the sail be so vital to being competitive on the racecourse? A well set triangular main will normally have tufts showing attached flow, so surely it's doing a fair bit.

    The racing trim of a squaretop or other roachy main is often quite similar to that of a pinhead, which seems to indicate that they are not dramatically different in terms of aerodynamics.

    If the top third of a triangular mainsail is doing nothing and a squaretop is providing valuable drive, where is the changeover and why does it not show up on the racecourse? For example, comparing performance between similar boats with and without squaretops shows that there is actually not a dramatic performance difference. Many classes here in Australia are rated as only 1% faster, or less, when they move to squaretop, and some of that performance improvement is created by simply adding extra area.

    If the top third of a roughly triangular sail added nothing to performance then moving to a squaretop would cause far more than 1% extra speed, and boats with squaretops would leave those with "pinheads" about 6% astern at least.
     
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  4. ChrisVJ
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    ChrisVJ Junior Member

    The top of a main is less efficient in terms of useful energy usually for two reasons. 1. It generally has less control than the lower part of the sail, though recent pictures of racing boats show that, compared to the 'old days' the sails are remarkably congruent for their whole length. 2. Very simply and logically, for any particular square foot, even giving the same drive as lower down the heeling moment is greater by the distance from the point of lateral resistance of the hull and when well heeled the added moment of the mast and sail being outboard of that lower down.

    My original point though was about the difference between the use of back stays and of running back stays made necessary by the use of square topped sails.

    In the very highest degree of racing the last 1/4% may be the difference between winning and not. I get that in such circumstances the inconvenience of running stays might be warranted but in semi cruising boats of longer distance racing, or maybe in particular in short around the buoys is it worth it or just another fad to follow?
     
  5. gggGuest
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    gggGuest ...

    I suppose one difference is that with a big head on the sail there is really no choice but to have an efficient kicker/vang and keep the leech properly under control, whereas historically cruising 'pinheads' reasonably often had inadequate gear and poor sail setting. There is no reason why leech control should not be as good or better on a triangular sail as a 'square top', but that's not always what's seen on the water.
    A possible advantage, looking at pros and cons, is that a square top sail permits a shorter mast and more drive for the same heeling moment, or even the same drive and less heeling.
     
  6. AJB
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    AJB Junior Member

    Guys,
    The tip of a triangular main is quite inefficient, adding tip vortex and requiring a heavy bit of mast ... with minimal attached flow.
    Roughly, Reynolds number wise, minimum squaretop width around 700 mm gets you to a better place.
    So on a typical 3:1 AR main you can achieve a better result with a ~ 12% reduction in mast height.
     
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  7. luckystrike
    Joined: Feb 2010
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    luckystrike Power Kraut

    A friendly Hello to all,

    I have practical experiences with a "go to sqauretop" conversion. . To have more power for our weekly wednesday races and our local regattas in bought a slightly used H26 mainsail for my Waarschip 730, which was a conventional light displacement performance cruiser from the late 1970's. The sailmaker cut off 5' from the luff and added two full legth sailbattens to make it a squaresail. The W730 is top rigged with angled shrouds, so I prepared two running backstays from dyneema and got rid of the normal backstay. The runners are not critical in useage because they must not be handled immidiatly during (normal wind) tacks or jibs. Except jibs in strong winds or when sailing with gennaker.

    The performance differece was remarkable. We were around two minutes per hour faster, which gave us the potential to win the races. The bigges gains were downwind under gennaker.

    We treated the squaretop as the racing sail and changed to the old pinhead main for cruising (except light winds).

    Have Fun, Michel
     
    Last edited: Jul 11, 2022
  8. ChrisVJ
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    ChrisVJ Junior Member

    From what little I know about aircraft wings, tip vortices also occur on broader wing tips. How about semi stiff tips placed perpendicular to the sail (as on a lot of commercial aircraft wings these days) to prevent them?
    A following post advises that a square top offered about a minute per hour advantage (in that particular case.) As I said, in racing it counts but it's a minimal percentage for the cruiser.
     
  9. AJB
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    AJB Junior Member

    Chris
    The top part of a fractional main is at least mildly analagous to the upturned jet tip. The squaretop helps to efficiently taper the aero load as one additional mechanism.

    Horizontal add ons not really practical....
     
  10. CT249
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    CT249 Senior Member

    So why is that not seen in action?

    A Laser has a low aspect main. If you reduce the mast height by putting on a Radial rig, the boat goes slower.

    Where is the evidence that the attached flow is minimal?
     
  11. CT249
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    CT249 Senior Member

    How did the sail area compare? It sounds as if the new mainsail had considerably more area.

    Classes I sail have a lot of practical experiences, and normally even on a cat (which work well with squaretops) there's a fairly reliable improvement of 1%, but some of that is extra area.
     
  12. CT249
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    CT249 Senior Member

    The squaretop's running back is basically the same as a standard backstay, but you have to swap to the windward one each tack.

    The issue I was raising is that the theoretical advantage of squaretops, such as it is, does not seem out to be borne out in practise as much as is often claimed. For example, the speed difference in the 14' development class sloop I mentioned, now that they have often gone for very big squaretops, seem impossible to see from my vantage point racing against them on a different class developed from the same roots, but with a smaller head. There must be some advantage, of course, but the point is that if the top 1/3 of a "pinhead" was doing almost no work as claimed then "pinhead" boats and those with smaller roaches would be left far, far behind.

    We can already generate the desired elliptical span loading much of the time with pinheads, using depth and leach tightness to compensate for the lack of area. In boats that are fast because of their other characteristics, squaretops work well, but don't seem to make as big a difference as some would claim - and that advantage may not translate to boats that are inherently slower because of their other characteristics. For example, the fact that higher CofE increases heeling is a problem for the squaretop, all else being equal, rather than an example of the "pinhead's" supposed almost complete lack of drive. In an intrinsically fast boat, which normally has high crew stability, the higher C of E is not normally such a problem. In an intrinsically fast boat, you normally have flat sails and sophisticated rigs that allow squaretops to respond well to twist. But in an intrinsically slower boat, normally of lower righting moment, with fuller sails and often a less adjustable rig, the squaretop can twist too much in a breeze and not enough in light winds.

    I'd be shocked if there was actually less control per se over the head of a mainsail in general. I can tune the head of some of my rigs very accurately over a very wide range, and having done the occasional race or blast with guys like Grant Simmer, Julian Bethwaite, John Bertrand and talked main trim with them etc I can't recall any of them saying that the head was particularly hard to trim. In some boats it may be, but in others we have (for example) jumpers or caps that allow very precise mast bend adjustment.

    It's hard to see squaretops as a "fad", though, given that the first really "square" topped modern sails (to the extent that one can define them) were probably the Marmion/Goodall A Class Cat sails of about 1988. But non-triangular sails for racing and non-racing boats have been around for eons.

    Arguably it's not that they are a "fad" at their age, but something that (as time has shown) has benefits and drawbacks and therefore are good for some people and boats and not for others.
     
  13. Rumars
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    Rumars Senior Member

    Ok, I might have exaggerated, I admit. Of course there is some drive from the top third of a pinhead, just much less then possible with other planforms. This of course has to do with the additional area as well as its form. Pinhead I define as having a straight leech, wich many sails don't have anymore.
    The triangular tip of the sail is not optimal, this has been established for a long time, and is to be avoided when possible. It doesn't have to be a fat square head, sometimes it's added roach and trapezoidal headplate. Bending the mast also ads to changing what the wind sees.
    When you do the sums pinheads loose, even if it's just by 1%.

    The decision to convert or not is not really complicated, if you don't want runners and the mast won't stand without them, it's a question of how much you want to spend to change the standing rigging.
     
  14. AJB
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    AJB Junior Member

    A designed from scatch keelboat or skiff squaretop rig (main and jib) should:
    1. Be lighter for a given sail area
    2. Be more adaptable across a wide range of AWS, because a greater range of twist profiles are enabled
    3. Practical C of E height may be lower than an equivalent area pinhead, with the head, when optimized able to be depowered and just lie stably in the onset flow
    4. Well tuned skiff and sportsboat rigs do not use running backs....
     

  15. luckystrike
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    luckystrike Power Kraut

    Hello CT 249,

    of course my squarettop had a bigger sail area, I guess 1 to 1,5 squaremetre more area. For my feeling the main advantage was in the better overall shape and attached windflow at the top. With the pinhead we always had problems to let the upper telltale fly free and that was 0.5m below the top. We located a telltale at the upper sailbatten and no problems to let em fly. Trimming twist at the top was easy with mainsheet and traveller from full power to zero power as you need it upwinds in a blow.

    Heeling forces and weaterhelm were no problem.

    The runners are not critical in use if you have a cruisers rig with angled shrouuds. you can easy the "old" runner before the tack and tighten the "new" runner at leisure time after the tack. The angled shrouds will take care of the mast during the manouvre. This is the way the Minitransat boats (producrion class) deal with the runners.

    Have Fun, Michel
     
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