Squaretop mains

I know many canoe and Cherub sailors but not sailed either.

Agreed. I tried a beach cat rig on my Stealth dinghy but it wasn't that successful so I changed it to the more normal fixed-gooseneck-boom-rotates-with-mast rig.

What I really meant by my comment was that, just looking at the sail plan drawing without knowing the hull, you might expect the rigs to be equally effective. But they are not, so one cannot make blanket statements without considering the boat as a whole

Richard Woods

 

A very lightweight gaff may work on models, but to make one that will take the loads of a full-size boat is very different. On the 30'/9m tri I did offshore shorthanded races in a couple of winters ago, we had a two-speed winch the same size as a 34 foot keelboat's primaries on the mainsheet. There is no way that any cheap and light gaff is going to handle someone whaling on that like you do. At the smaller sizes, something like the F16 we had used (IIRC) a top-line Harken 8:1 mainsheet system and at top end of the range it could only be pulled on by bending and then extending your knees from trap. Guys like Glenn Ashby said that mainsheet loads were higher than on the Tornado because of the sail and mast design.

If someone can create a cheap and simple structure that will withstand such loads then it would be surprising - the industry can't, and it's tried. Instead they provide large and expensive systems to handle the loads of squaretops on big boats.

 

This doesn't look too expensive nor too complicated. Tested for generations
http://www.selvenforsale.de/spars-1/boom-gaff/

 

Went through the IOR days when running backstays held the frac rigs up and all the work that was. Then it was nice and easy when we just had masthead rigs with a standing backstay. Now with the big top mains it's back to managing 2 backstays, at least boats seem to still have swept spreaders, instead of the inline spreaders of IOR days, so the rig doesn't fall over the bow if you miss the backstay. Though I'm thinking one of these days sailmakers will remember how easy mast tune was with inline spreaders and we'll go back there.

 

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I used a small gaff with an adjustable upper outhaul supported by the jib halyard, which was the forestay. Lots of load. I didn't use the gaff on the main because I got in a rush but I see no reason it wouldn't have worked as well as the one on the Tantra did with very high mainsheet loads.
The "modern gaff" or Wing Tip Rig offers some potential advantages worth exploring further including having an adjustable upper outhaul, the elimination of full battens and ,of course, no diagonal batten. The "peak" of the sail can be adjustable as well.

Pictures: L to R, 1) Tantra(EXP-1) with 1975 square head supported by a gaff encased by foam for masthead flotation. Adjustable upper outhaul.,2&3) Aeroskiff foiler with square top main and jib-jib supported by gaff with adjustable upper outhaul.

 

Sure, but the conventional gaff option creates significant problems with extra weight and windage, as well as (from everything I can read about it) difficulties in controlling twist. So it's not as effective as highly-loaded battens on a modern sail.

Some people claim that gaff rigs are effective but if so, why were they given 5% rating advantage (to use one example) over bermudan rigs way back in the '30s? The contemporary information is pretty clear when one reads old books and mags - the reason that bermudan rigs became popular was because of their simplicity and efficiency in many situations.

I'm not saying that all gaffers are slow and in fact I'm a bit of a fan of gunter rigs/high peaked gaffs, but they don't match the efficiency of a good bermudan much of the time.

 

Off course the big advantage of a gaff is when you crack the sheets you can use all that area and really fly without resorting to hanging all the laundry up. On the wind all things being even a Bermudan will walk all over a similar gaffer, but it all depends on how much you rate that ability. Sail handling gets harder too if the gaff is a big lump, dont forget topmasts and running backs and tree stump masts. A lot of people romanticise about gaffs and lament the lack of development, but in a way the modern ST sail is one modern interpretation of the gaff.

 

The lowest drag does not come from an elliptical distribution, for a sail or sails often heeled, in a sheared wind, with the gap under the boom and the deck etc.

What makes you think dynamic behaviour of the rig & gust response only applies to fully battened sails? Dinghies with long topmasts above the hounds, like the 470, 505 and the likes have a very positive gust response, as do keelboats such as for instance the Star. The best gust response probably comes from cat rigged boats, like the Finn, but it's of course a special case and not readily scalable to bigger boats (although some designers do even that).

 

Nothing, but I do claim that with fully battened sails the mid leach stands up much better which I think makes for a more effective upwind rig.

 

I don't understand why full battens would help the mid leach stand up. The battens are supporting positive roach. There isn't that much positive roach in the middle of the sail. Seems to me whether the mid leach will stand up or not has to do with sail design not battens. After saying that I do realize that changing batten stiffness effects the leach, but more as fine tuning.

 

I would agree with this based on my own experience and observations.

Although I have also learned that the leech control now enjoyed by the unrestricted skiff classes partly comes from having the leach virtually straight below the gaff batten. With a very stiff mast and square top main, pulling in luff tension using the cunningham lets the top twist off progressively, whereas the earlier mains with a rounded roach below the top batten would open up at mid leach and pointing would suffer. In either case a powerful vang and downhaul relative to the rig size is requried to make this work.

It seems intuitive to me that the full length battens contribute to this, but I dont propose a scientific reasoning. I just know for myself that this planform allows for progressive shape changes through the wind range without some of the compromises I have experienced with short battens and traditional roach profiles in the past.

 

Interesting and very good comments, gggGuest, sean9C and JRD. I haven't sailed much with fullbatten sails, belonging to the earlier 470-505-FD generation, but I was involved a little bit in 49er optimization work prior to London 2012.

It appears that the full length battens have a role in bending the mast and flattening the sail, through the use of cunningham (luff tension). Tensioning the cunningham pushes the batten front ends against mast, bending it and flattening the sail.

Full lenght battens may well support even a straight roach from clew to the top corner, as long as there is compression in the batten. If there is no roach at the leech end, the batten can get compressed from the other end, at the luff, through the luff curve cut into the sail. As long as the sail luff curve is not absorbed by the mast bend, you will have compression in the batten and support for the leech.

Leech battens cannot get compression from the mast or luff curve, but leech support in a sail with shorter battens is provided with good design, properly shaping the leech area so that you get a suitable compression into the short batten. Very tricky, and at the core of sail design at highest level. Leech support is crucial to performance in most boat types, for instance the 470 will not perform if the mid-leech opens too early. Yet it is depowered very nicely as the wind picks up. Leech pressure increases with the wind, the topmast is bent back and the mid-mast forward around the pivot axis formed by the forestay and the hounds.

While JRDs experience about the square top working better than the pinhead, in respect of lower leech support, is tempting, it could also be due to design. Since the square top is inherently earlier powered up than the pinhead, and since it loads up more the topmast, it could be that it runs earlier out of luff curve in the upper part, which leads to dumping the head corner.

Attached two shots of a 49er main. The only difference between the two is that in the first the cunningham is slack and in the second it is taught. You can see the dramatic increase in the mast bend, induced by the battens pushing the mast forward, and in result the flattening and twisting off of the sail. The red, thick stripes are used by the automatic tracking system to record the sail shape, the battens are the green lines above these. You can see the wave forming on the lowest draft stripe, witnessing that it is indeed to push from the batten above it that induces the mast bend. The photos are best flipped back and forth to see the effect.

Note that the 49er (and sport cats) "gust response" is rather enforced than automatic, as it requires adjusting the cunningham and/or vang, while in the "oldfashioned" style rigs such as 470 or the Star (or the Finn) the response is truly automatic, you don't need to do anything.

 

Mikko, the response of square tops I've sailed with and without full battens and with and without a diagonal upper batten seems to be automatic within certain ranges-ranges set with downhaul and vang. After the range is set the sail does the depowering automatically in response to gusts.

 

You are certainly right, the last paragraph of my posting is not well formulated. For fully battened sails, the luff tension provides an additional way of adjustment, on top of the usual gust response of rigs with long enough top masts (as opposed to masthead rigs). That said, cunningham tension does work for leech batten sails as well, even if the mechanism there is a bit different.

 

I have some observations based only on physics and not on any significant experience compared to the posters above. Perhaps you can enlighten me or find a more fundamental explanation in my opinion.

My understanding is that horizontal battens are needed to achieve a flat leach under load. Achieving the same flat leach under load without battens would require outrageous tension and stiffness -along the lines of the catenary equation.

The condition that we have been talking about for the past page is righting force limited, highly variable. I think the distinction is important because a flatter sail (with good active angle of attack) will always outperform in this condition.

In the fat-head main, when the sail is flattened sufficiently, the battens are so close to straight that the highest loaded ones (the top) can buckle trough. To snap through the diagonal batten is bending the tip of the tapered fractional mast. If the battens are curved, as in a full, deep sail, the battens are committed to the curvature and more stress will cause them to deepen this curvature -gaining power in gusts which would require feathering the entire main to get back to upright.

So what is the source of the performance?
1-flat sail carefully feathered in a righting limited condition ...flatter=better
2-'de-powering' the top without having to give up drive lower in the sail ..autoreefing
3-some improved aerodynamics of a twisted off tip -brings to mind the wing shape of soaring raptors...improvement in induced drag