In this article from the PBO archives, Vincent Reddish unravels some of the secrets of the junk boat’s enduring popularity in his quest for the optimum sail plan...

Facing retirement with angina and a persistent back injury, I was persuaded by John Campbell’s book Easier Rigs for Safer Cruising that a junk rig would be the thing for my intended new boat, a Vertue; but to gain experience I first decided to convert my existing Leisure 17.

I bought Jock McLeod’s design folios, and in due course Hasler & McLeod’s Tactical Junk Rig Design. This proved an invaluable book; without it I doubt if I could have rigged the sail successfully: it’s a gold-mine of practical information.

Because I wanted to experiment and learn as much as possible about the Chinese sail I decided on an historical approach, and began with the simple rectangular fully battened lugsail, twice as high as it was wide, of the kind used on river junks and sampans and described such detail in Worcester’s The Junks and Sampans of the Yangtze.

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I used a very cheap material for the sail, a woven and laminated polyethylene sheet sold as tarpaulins by Bradshaws of York and found it so good that I still use it.

This simple sail, set vertically, is used on river boats to catch light airs coming over the river banks; I found it alright in such conditions but not in strong winds or choppy seas. My Mk 2 was similar but angled forward at the top, giving some improvement.

The Mk 3, a standard Hasler-McLeod design, was much better but suffered from the well known faults described by Hasler: lack of drive to windward especially in choppy seas and light airs, and a reluctance to tack in some conditions making it frequently necessary to wear ship.

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Somewhat disappointed, I was fortunate enough to come across two books which led me to a detailed study of the original rig of the Chinese ocean trading junks: The Last Sailors by Hollander & Mertes, and Setting Sails by Maitland & Wheeler, both of which contain many photographs including some taken in the last century, of such junk boats under sail.

With a long background in scientific research, I found the investigation increasingly absorbing. All the photographs show sails more or less side-on, eleven of which were measured in detail, scaled to similar overall sizes, and averaged.

There were usually five battens, less often six in which case the bottom two were averaged. There was a surprising degree of uniformity about the sails.

The boom and battens were all the same length to within 5% of the average, although the lengths of the battens seemed to decrease slightly upwards, but that may have been partly a perspective effect.

The length of the luff was equal to that of the yard, to within the surprisingly small error of only 8% on average, the ’dispersion’ of the individual sails about the average being less than 25% (that is to say the dimensions of more than two thirds of the individual sails lay between 0.75 and 1.25 of the average dimensions of them all.)

The luff was divided equally among the six panels formed by the five battens, boom and yard, to within the same sort of accuracy. The length of the leech of individual panels, however, increased progressively up the sail, which was fan-shaped.

Again, the dispersion about the average for the panels of the individual sails was only some 25%, the error (or uniformity) in the dimensions of the average sail being less than 10%.

It was apparent that there was some systematic pattern in the planform of the sail but it was some time before the obvious occurred to me; the Chinese who developed this sail would not even have had tape measures let alone digital calculators; what they would have had were lengths of rope which could be doubled or trebled to give simple fractions — ½, ⅓, ¼, ⅙, ⅛, and so on — of some standard length such as the boom.

I converted all the measurements into such fractions of the length of the boom, choosing the simplest fraction that was within the 10% uncertainty; that is to say, all the fractions chosen represent the average sail measurements to within 10%.


Fig. 1: The dimensions we finally adopted.

The result is shown in Fig. 1. and is self-explanatory, if rather remarkable, and merits close study. The luff and the yard are both two-thirds the length of the boom and battens; the length of the leech of the topmost (sixth) panel is half the boom, the next one down, a third, and the remaining four panels decrease by an eighth of that until the leech of the bottom panel is a half that of the fifth.

The total length of the leech is 1¾ times the length of the boom. Other useful features to note are the horizontal distance of the top of the yard from the vertical line of the luff which is a third of the length of the yard; the area of the sail is 10% greater than the square on the boom; the aspect is then 1.1; and on average only 8% of the width of the sail rig forward of the mast, Fig. 1 aiso shows the location of the centre of effort.

Having determined the shape of the Chinese ocean-going junk sail, the question then arose: “how did they make it?* The answer is found briefly but clearly in Worcester’s The Junks and Sampans of the Yangtze, p72.

I can do no better than use Worcester’s own words: “The rough dimensions of the sail having been determined, the yard, boom and battens are laid out on the ground out connected up to the bolt ropes so as to form a frame…

“The sailcloths are then laid over the framework, and the family set to work to join them together in inch-wide seams. Strengthening pieces are affixed where necessary.

“The sail is next sewn to the bolt ropes. Usually the sails are double roped, the sail being sewn to the inner rope, to which the outer one is secured by twine stops at irregular intervals of about 6 inches. The sail is next laced to a bamboo at head and foot, which in turn is lashed to the yard or boom at intervals of about 46 inches.”

My wife and I followed this procedure. The yard and boom (boom length 11½ft) were made from pine to the shapes and sections given by Hasler & McLeod.

The battens were made from 10ft lengths of garden bamboo, strapped together butt ends outward using self-adhesive strapping tape from B&Q (which show no signs of coming apart after a year and a half at sea).

Bolt ropes were 5mm matt polyester. The sail, cut to shape and hemmed round a thinner rope, was made from a Bradshaw’s tarpaulin.

The yard, battens and boom were roped together on the lawn, with dimensions along luff and leech as in Fig. 1. A ‘halyard’ was fastened from the centre of the yard to a stake; a downhaul, fastened one foot from the forward end of the boom, was pulled tight to another stake, so the frame was tightly stretched. The sail was then laced to the frame all round, and along the battens.

This procedure is fundamentally different from that of attaching the battens to the sail as in the Hasler-McLeod westernised junk boat. The framework of the Chinese sail has a certain cohesion — the luff and leech ropes providing some stiffness — so it moves and changes shape as the whole.

The battens don’t shake about loosely, spilling the wind out of the sail in a choppy sea. Look at the photographs in The Last Sailors and in Setting Sail of Chinese junk boats sailing close-hauled.

Clearly, the sails are not flat. The leech is curved giving twist in the sail and curvature to the upper part; the lower battens are flexible, giving curvature there; the individual panels all have some fullness in them.

I sought to achieve the same features. Each batten was made just sufficiently flexible to have a 10% depth of curvature at the maximum load its associated panels would transmit to it at a windspeed which made reefing desirable.

Some fullness was put in each panel by pressing it down into the grass while it was laced to the luff and leech ropes {stretched tight) and to the battens.

This creased the corners of the panels, with no detrimental effect. All the battens were sheeted, with spans across battens 5 and 4, 3 and 2, 1 and the boom. A luff hauling parral was put on the upper four battens, using spans across pairs in the usual Chinese way.

The tapered aluminium mast from Needlespar had been put through a foredeck strengthened with extra GRP, and stepped on the keel. The sail wag positioned so its Centre of Effort would be exactly on the same vertical line as that of the original Bermudian rig.

The result? A transformation in performance. She goes about surely in all winds, and is close winded, making 80 degrees between tacks, although in choppy seas it makes good faster to windward if sailed a bit freer, 50 degrees from the wind; but then so did the Bermudian rig.

The boat is a twin keeler with a 7ft beam and measures only 14ft on the waterline. I sailed several hundred miles with this rig in the late summer of 1989 (the whole project only began in May) in winds varying from the lightest airs to the top of Force 6 gusting 7, by which time I was down to two panels and still thrashing fast to windward in great clouds of spray. In all respects it outperforms the original Bermudian rig.

In this happy condition I ordered my Vertue from Bossom’s Boatyard at Oxford. It was delivered on 5th September, with a mast from Needlespar. The foredeck had been strengthened by the builders and a pilothouse built over the forward end of the cockpit. My wife again helped me to make the sail and rigging on the lawn.

In this case the length of the boom was 17ft (5.2m), the sail area 320ft2 (30m2). Installation of the deck partners and mast step, mast, sail and rigging, anchor, chain and windlass, seacocks and Lavac WC, and Seagull 170 outboard with remote control (no inboard engine), together with various odds and ends, were completed ten days later and the boat launched on the 19th September at Renfrew.

She was motored down the Clyde and sail was hoisted off Dumbarton with feelings of hope and trepidation; as usual I was singlehanded. This sail had also been positioned so the Centre of Effort would be on the same vertical line as the sailplan of the Bermudian rig originally designed for the Vertue by Laurent Giles and Partners.

In head minds rising from Force 4 to Force 6 (and continuous torrents of rain which made me thankful for the pilothouse) she tacked surely along the narrow buoyed channel, sailing fast close hauled in breaking seas, making 80° between tacks and was so well balanced on the helm that the boat would sail to windward without a hand on the tiller.


Fig. 2: The finished result

Later experience, in a variety of wind strengths and on all points of sailing, has confirmed the rig’a excellent behaviour. Fig. 2 is a photograph taken abeam, showing the planform of the sail.

In his book Sailing Theory and Practice, C. A. Marchaj writes, on p100: ”The rigging… in the original Chinese version could be used to adjust the camber of the sail within wide limits”, and on pp.107 to 116 draws attention to the need for sail twist.

In his Aero-Hydrodynamics of Sailing, p429, writing again about the Chinese rig, he notes that “in one respect the junk type rig is remarkable. This is the sheeting arrangement… an independent sail shape and sail incidence control system… Such an arrangement permits accurate changes in camber and sail twist independently of changes in incidence angles.”

The photographs of the old Chinese junk boats in the books referred to above, and of my own Vertue, show these possibilities clearly. Separate mainsheets for the upper and lower part of my sail may be worth trying too.

Some of my results complement those in the fascinating article by Group Captain Smith in Newsletter 20 of the Junk Rig Association. One last bit of useful information: bales of bamboo in long lengths can be obtained from Jacobs, Young & Westbury Ltd.

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This feature appeared in the January 1992 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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