Sharp edge foils, a taut luff, and the entry and exit shape of a panel foil
These are some thoughts, as I have tried to unlearn what I thought I knew in the past, and attempted to learn anew what makes a good junk sail.
The first difference between a junk rig and a bermudan rig is that the bermudan rig is taut, everything under stress – headsail luffs as tight as a bar. To maximise efficiency, the modern bermudan rig needs high-tech, non-stretch canvas and twang-tight rigging. And, unlike the Chinese lug, a western lug sail too needs a taut luff, under quite a lot of tension.
The junk rig is not stretched out taut – just a little tension perhaps, but essentially it should drape, or stand, without the stresses and forces used in western rigs to achieve a nicely shaped foil. And, of course, there is no twang-tight forestay either. Choice of sail cloth is less critical. Stresses on the hull (and the mind) are greatly reduced. True, a somewhat more complicated arrangement of running rigging is often necessary, together with judicious sail design, to get the sail to drape and set nicely. But no need for the high forces applied by a tight fore stay, cunningham, downhauls, kicking straps, central sheeting systems, sheet winches etc as seen on western rigs.
Mauro wrote: “…my luff looks also quite taut…” Actually, looking at the photo, the luff is not “taut” at all. I know what he means. Mauro just used the wrong word to express what he means. Mauro’s luff is as it should be on that sail – nice and straight, and aligned with the rest of the cloth, standing nicely. It looks very good.
Paul Th wrote: “…the layering enables the luff to stand without distortion or collapsing…” and I think that’s the right word to use: “stand”.
With some sail designs, for example those with parallelogram lower panels, it looks nice if the luff is straight. Whether it adds to windward performance I do not know, but if the sail design drawing shows a straight luff, then it will look better if the sail itself has a nice straight luff. But not taut.
However, Some of the traditional Chinese sails had a curved luff. Slieve’s Poppy sail has a curved luff in the upper panels which is aesthetically very beautiful – and testament to the statement that a junk sail should not have a taut luff but should “stand” without undue tension.
Here is the very latest from Paul Th.
(You saw it first here!) just off the press, a cute little dinghy sail, designed and built by Paul Th and being trialled by Marcus on a Welsford “Tender Behind” dinghy.
It’s a different sort of sail from the Van Loan and Hasler/McLeod derivatives we have been discussing in recent times – it is a low A.R., cambered, fanned sail which owes more to the Hong Kong junk, and Vincent Reddish, with Paul’s emerging flair combined. It has a pronounced curve in the luff. Not taut. Not straight. Standing nicely.
Entry angle. Tabling, webbing or bolt rope?
A bermudan sail will usually have a bolt rope on the luff, for a number of reasons. One of them is that the camber in a bermudan sail is derived partly from the luff being cut with a curve, but pulled tight and straight with a strong downhaul at the tack, or modestly tensioned and attached to a bar-tight forestay. This induces some of the camber in the sail. The luff needs to be well-reinforced against stretch, with a bolt rope, if for no other reason than that.
We don’t do that on a junk sail. Camber is built into the sail in other ways, and the luff is not swigged down tight like a lot of western sail types need to be, including the western lug. The individual panels of a junk sail might each be straight cut at the luff, whatever, the luff detail needs to withstand stretching from the weight of the rig, and wind forces, but will not be subjected to the extra load of a tack downhaul.
I like Slieve’s bolt rope detail at the luff of his SJR jibs for different reasons – a more exaggerated entry angle can be built into the jibs, the camber running smoothly all the way to the bolt rope. A tabling or a wide webbing on SJR jibs might be a little stiff? Those little jib panels run out with a tight curve right from the very bolt rope, with no flat spot at the luff. It’s a very small detail which I doubt makes much difference, but as perfection can be achieved here in a very easy way, why not?
Also, the parrel downhauls on a SJR can be set firmly enough so that the modest downhaul forces on the battens can extend and come into play at the jib luffs, to induce just a enough tension to keep the luff nice and tidy and straight – but not with anything resembling the tension applied to the luff of a western sail. Still, it makes sense to me to make the bolt rope from a non-stretch material such as dyneema. (I doubt if this luff dwnhaul notion can be applied to a contiguous sail in any simple way, although Paul Sch has been investigating it. I wouldn't have thought that it matters).
However, for the somewhat less exaggerated entry angle of the mains aerofoil - or for the aerofoil of a normal, contiguous sail, the entry angle might be a little less as the curve is a little less pronounced at the luff, and I can’t see that it would make much difference to the entry shape if a bolt rope or a webbing or a tabling were used at the luff. There are reasons why some people prefer one detail over the other, but I doubt if entry shape comes into it.
Leech The leech is a slightly different story. I never had the pleasure of owning a bermudan sail with a nice leech – I never had the money for a new sail, and all I ever had were always a bit stretched and fluttered a bit, sometimes hooked at the leech and the battens necessary for a roached leech were always a pain in the neck. It seems bermudan sails need an adjustable leech line but I never figured out how to adjust one without inducing more hook. Quite frankly, I never owned a bermudan sail that had a decent-looking leech, roached or otherwise, and glad to turn my back on that rig and not think about it again.
I would not like to have a bolt rope or a leech line on the leech of a junk sail. It seems to be sufficient to lay the cloth parallel to the leech if possible, and to use traditional tabling - or a strong webbing - (or a light webbing in combination with a little bit of tabling which I tried on the dinghy sail I made, and I thought it looked quite good). The aerofoil shape is more or less flat at the leech, so the shape will be, if anything, improved by a tabling or a wide webbing which is stiff and flat. There are other considerations, such as structure, but I am discussing shape only. Paul Th’s tabled leeches are beautiful and an inspiration to me. I can’t quite get my head around the single webbing that Arne has used so successfully, and I compromised last time with a light webbing and some extra sail cloth overlapping, just to try and make it look a little bit like Paul’s trademark detail !
Having said that, as Arne has reminded us many times – the proof of the pudding is in the eating, and either tabling or webbing must be OK at the leech, because they have both been proven to perform perfectly well, and to last. Either way, the shape of the sail at the leech seems to remain good.