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Amateur Yacht Research Society, February 1997 Newsletter

AYRS Lecture on junk rig given by the Junk Rig Society

Lecture held at London Corinthian Yacht Club, December 1996

John Perry, 90 Tavistock Road, Fleet Hants GU13 8EZ, UK

Having had a traditional engineering education I attended this lecture on junk rigs with a slight anxiety. My anxiety was that I might be about to learn that the aeronautical engineers ideal of a smoothly curved aerofoil section is misguided and that we would actually be better off with a flat plate sail. In the event I went away happy, it turns out that the junk rig enthusiasts have now discovered that junk rigs do work best if they have a curved aerofoil section just like Bermudan rig has.

The latest junk rigs have abandoned the use of rigid wooden battens which hold the sail almost dead flat and instead they have adopted flexible battens made from pultruded fibreglass tubing.

Flexible battens allow the sail to take up a camber and apparently this gives significantly better windward performance than flat plate junk rigs. Downwind the performance is not noticeably different to the older style of junk rig. Unfortunately, flexible battens do not allow enough sail camber in light winds and they may give too much camber in strong winds. Some junk rig enthusiasts are therefore experimenting with battens made up from rigid sections which are joined by hinges which allow only a certain amount of angular movement. One approach is to make battens from tubular sections which fit together with deliberately loose fitting spigot and socket joints - something like a worn out tent pole. In principle this gives a fixed amount of camber regardless of wind strength. This is much better than having no camber in light winds and lots of camber in strong winds but it could be even better to have a large camber in light winds, this camber decreasing as wind strength increases.

Achieving the ideal of reducing camber as wind strength increases seems to be one of the fundamental problems with cloth sails. Any piece of cloth held up to the wind will tend to bulge more as the wind increases. This is particularly the case if the edges of the cloth are stiffened with reinforcement or attached to spars. The most successful approach to controlling camber in cloth sails has been the use of flexible spars, particularly flexible masts, which are rigged in such a way that they bend as the wind strengthens and so pull camber out of the sail section. This is not likely to work with junk rig since the mast is only loosely attached to the sail by loops of rope. Suppose however, we take a junk rig as figure. 1 below and then we move the mast a bit so that we can fix the ends of the battens to sliders running in a track on the mast as figure 2. With only minor modifications we end up with a fully battened square headed Bermudan rig showing that the latest ideas for Bermudan multihull rigs are in some ways rather similar to junk rig.

The mainsheet arrangement is one noticeable difference between the junk rigs and Bermudan rigs. Bermudan rigs almost invariably have a single main sheet attachment point, whereas junk rigs have multiple part sheet(s) taken to each batten. Various pulley arrangements are used to control the distribution of sheeting load between the battens. One effect of this complicated sheeting arrangement must be a reduction in sail twist, another effect must be increased windage.

Various AYRS members, Ian Hannay for example, have written in support of allowing twist along the height of a sail. Because of the wind shear with height, a tiny amount of twist is needed to keep the angle of attack constant along the height of the sail and a bit of extra twist on top of this serves to lower the centre of effort of the sail at the expense of some loss of lift coeficient. This is obviously an advantage in strong winds and sailboarders utilise sail twist to help maintain control in difficult conditions. Sailboard sails have developed to optimise this effect.

Although some sail twist is beneficial, it seems likely that most rigs most of the time have too much sail twist and reducing sail twist certainly seems to be more difficult than allowing it to increase. This leads to the thought that multiple part sheets as seen on junk rigs might be worth a try on fully battened Bermudan rigs. They would produce some extra windage but perhaps not much more than permanently rigged slab reefing lines.

Apart from junk rigged craft, Thames sailing barges are another type of craft which have lines to the mainsail leach to control sail twist. Two steel wires are lead from the upper end of the Thames barge sprit to a pair of winches on opposite sides of the deck. On starboard tack the starboard winch is loaded and vice versa. I have not sailed a craft of this type but it would appear that these lines control twist by controlling the angle of attack at the top of the sail independently of the angle of attack at the foot. No doubt they are also useful to keep the sprit from waving around when at anchor and to allow it to be used for cargo handling.

The message which came across from this fascinating talk was that junk rig gives ease of handling, long sail life and adaptability to cope with strong winds. Two model rigs were displayed, these being almost large enough for a small dinghy. For demonstration purposes these model rigs were fitted with a full complement of lines and blocks and it was shown how easy it is to hoist and lower sail and to reef using a minimum of adjustments.

When the sails are lowered, they flop down into a cats cradle of lines rather than falling all over the deck or blowing out in the wind. Junk rigs are nearly always used with non stayed masts which avoids heavily loaded rigging components. The non stayed mast may require some deck reinforcement but this is not usually difficult and a number of fibreglass boats such as the Corribee and the other Newbridge boats have been successfully adapted for junk rig without major structural modification. Another advantage of junk rig is that the sails tend to last a long time. The sail cloth is in small panels which are well supported and there is no need to use huge tension from sheets and kicking straps to keep sail twist under control. Also, having no shrouds reduces sail chafe off the wind. One junk sailor has made his sails from cheap plastic tarpaulins and they have lasted reasonably well. Another has used cloth made from recycled plastic.

It was admitted that in general the windward performance and pointing of a junk rig is inferior to Bermudan but off wind performance can be comparable to Bermudan rig without the need to set special downwind sails.

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Subject: Junksails, Polynesian sails

From: Peter Wynn

Given that one of the main problems with junk, lateen and similar rigs is moving the sail round to the leeward side of the mast isn't it possible using carbon spars to have a light weight bipod mast athwart ships and simply(?) hang the sail from the centre.

Assuming I can find the enthusiasm I want to try the idea with a model of a Polynesian rig but somebody must have tried it somewhere before now surely to goodness.

From: FISHWICK Simon

Yes it's been tried before, though I suspect with limited success. One of the advantages of junk & lateen rigs is that set on unstayed (or limited stayed) masts, they are free to weather cock if the sheets are let go. You lose this with a bipod rig. You also have problems of yards and sails getting out of control as they are lowered eg to reef. This of course is true of any sail set flying. The third disadvantage is windage. Nevertheless, they do have some advantages. eg as a means of taking compression stresses directly to the hulls of a catamaran instead of putting large bending moments on a bridge deck.

I'm not sure though what you mean by a Polynesian rig. If you mean something like the micronesian lateens, then on a cat you should not need to shift it. You can bring the "tack" in to the mast foot (or even dispense with the mast and stay the yard) as they did in (I think) Tahiti.

On a proa though you do need the facility to adjust the rig both end-to-end and athwartships. My proa used a single spar, stepped on the windward gunwale stayed to the end of the outrigger beams and to the (current) stern. I also had a preventer stay to the lee gunwale, but it was a nuisance, and I never really needed it. Whilst essentially I had a bipod mast+stay arrangement, I found needed to adjust the length of the stay (and the rake of the mast) to change the balance of the rig with wind strength - the less wind, the more the rig had to lean to leeward, else it collapsed.

From: Jim Champ

I would have thought the extra windage of two spars would be a problem... Also IIRC one of the traditional virtues of junk rig has I believe been the simplicity of hanging the whole thing off one relatively unstayed spar...

However I wonder about a "gallows pole" affair mounted well forward in the boat with the junk sail hanging from that... Still more windage though I guess... If one wanted to go for gratuitous technology that could maybe be combined with some sort of "padded" leading edge to the sail to give it a nice rounded edge to discourage separation of flow... The sail would be clear of the turbulence of the mast in most situations like that but OTOH the mast would be a separate source of drag, not mixed up with the sail flow... It would still be pretty mixed up with the leeward side of the sail flow upwind though:-)

From: Craig O'Donnell

Mmmm, so use a bipod mast and a staysail and just flip the staysail around. Polynesian-Indonesian vessels have used bipods and tripods. Windage did not worry them.

Dixon Kemp's book has a diagram of a bipod mast referred to as the "Algoa Bay Lateen" (S. Africa).

Or how about a wingsail amidships and two small wingsails at each end. Drop the 'endsail', rotate the wing, raise the new 'endsail'. The endsails could even live on the ama, I suppose, thus being out of the bad air from the main sail.

From: Ian Clarke

Assuming brainstorming is now in order, how about a double-thickness sail with each skin separately battened, set on a pole mast up the middle? The trailing ends of the battens would be rigidly connected, and the leading edges joined by hinged quadrant-sections, in such a way that the camber of the aerofoil "popped over" when you go about. Sorry I can't draw it from the keyboard, but sketch it yourself.

If extra push was needed, put scoops along the luff like a parafoil kite to shape the aerofoil dynamically.

This should keep all the advantages of junk rig (except cheapness!) I think. Somebody must have done it already, tho'.

From: wynn

I was rather assuming that the foot of the sail would be secured to the deck by a line directly below the suspension point

From: FISHWICK Simon

Yes, OK, but what about the head? As you lower the sail it will blow all over the place, rather like an out-of-control spinnaker, but with battens?


For lateen rigs I think I understand what you are saying, and I like this too.

Offset the mast to one side and don't let the mast rotate. The sail is then stretched between the crane at the mast head and the deck. The only down side is that, unless the distance from the bottom of the sail to the deck is short, or the downhaul tension is fairly high, the foot of the sail will sag to leeward.

From: FISHWICK Simon

Sounds like half a bipod. A Unipod?

Trouble is I'm not quite sure what you will have gained over either the traditional form of the rig, or a Chesapeake-style raked leg-of-mutton. The more you offset the sail from the "Unipod", the greater the bending stresses on the mast. The less you offset it, the more likely you are to get interference between the mast and the flow across the lee side of the sail (at least on one tack). Putting the mast up the luff avoids this, which I think is probably where we came in!

From: Craig O'Donnell

Suddenly we're back to the idea of a dual staysail boat (mast, two staysails, fly the appropriate one on the appropriate "tack").... proa schooners, anyone?

From: wynn

I don't see that at all. Trying to imagine the situation it seems to me the two situations have very little in common and as someone who sailed a cruising cat with full battens some thirty years ago when everyone said it was mad and stupid, I suspect the battens would be a plus factor.

But anyway I'm interested primarily in the Polynesian rig (I think people are calling it 'crab claw' sail but as I haven't been in touch for a very long time I can't be sure but that would be a pretty good desciption of the sail I have in mind) and that sail has a spar top and bottom which I would think makes it somewhat easier to handle with a bipod mast than a junk sail.

From: FISHWICK Simon

Hmm, yes, (stops short). I forgot we were talking about proas! Sorry!

Now the big problem with a proa is getting the static balance right on BOTH shunts, ie getting the rig Centre of Area ahead of the hull Centre of Lateral Area. Then when you are lying stopped in the water after a shunt, the rig has enough bearing away moment to counteract the (LARGE) luffing force due to outrigger drag. Otherwise you just sail in circles round the 'rigger. I know, I've done it! That means either moving the rig centre of area (shift rig, or furl/unfurl sails) or the hull ditto (raise/ lower leeboards/ rudders). Just sticking a mast in the centre of the boat, and pivoting the rig around it is no good on its own. It works once you are moving, because you can take out the excess turning moment using the rudder, but once you stop you have to get out the paddle.

The Micronesians knew what they were doing! But their way of using the steering paddle to give lateral resistance, and outrigger drag to steer, is not compatible with western ideas of windward efficiency and rudder effectiveness! Bolger may well be right when he says the best way of combining the two is to use a bow rudder. I used to get rudder stall problems though, but that may have had something to do with lack of rudder "feel".

From: Craig O'Donnell

Right; how is this a problem with a staysail? I don't have problems with either a bow-rudder or dual (dueling) boards.

Besides, in _my_ proa design, there will be an outboard motor in the ama ....

<just kidding>.

From: FISHWICK Simon

It's not really a problem, you just need two - and furl the "aft" one on each shunt.

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