What is wrong with a Junk rig

I have been designing a catamaran using the formulas in how to dimention a catamaran PDF. It is easier to use the spreadsheet. I was intending to have a single lugsail. Richard wood has posted that a junk sail sails badly on a cat and Wharram gave up on it. Could someone please tell me why a junk sail isn't good I think it is an oriental lugsail.
mik

 

Hi Mik.

There are two basic reasons:

1.) the rig is quite heavy, due to all its Boomlets (sometimes called battens). These and that the shrouds and stays which can only attach to the top of the rig, make it up to four times as heavy as a more conventional rig. Being so heavy it can make the catamaran more top heavy under full sail than it would otherwise be.

2.) It is generally a flat cut sail, or at least much flatter cut than a more conventional sail, including a Balanced Lug. This makes it less effective when sailing upwind. On a light catamaran, this is a double disadvantage. It can not only not sail as well to windward as it otherwise would have, but, because catamarans generally sail faster than mono's of the same Length, the Apparent Wind moves further forward when a catamaran is on a reach than it does when the same size mono is. This requires better windward sails to get the best performance, even on a reach.

Assuming you chose the catamaran hull form for performance (speed) reasons, the Chinese Lug, is probably not your best option.

 

Thank you

 

I would not say a junk rig is inferior to a lug on a cat. Neither is common on cats because they tend to be low aspect which is less desirable on a cat with low drag and lots of righting. Between the two I would say the junk has more potential.

The weight of a junk does not need to be more than a lug and the junk does not need to be flat cut. It is a mater of work and care to make a light, strong, well cut sail and battens. Look up "soft wing sail" and you will see how high performance a 'junk' rig can be.

I think you need to look at context or ask Richard what he meant. I am pretty sure he was saying that a junk rig does not work as well as a mainsail or sloop -due to rigging, aspect, and aerodynamics.

 

Hi Skyak

I agree with you in principle.

But the truth be told, the Chinese Lug rig is a heavier rig per material used to construct it. If it is made of carbon fiber, this is true, if the competing rig is also made of carbon fiber.

A Balanced Lug rig does not necessarily have a lower Aspect Ratio (AR). Even if it does, it is still likely to outperform the Chinese Lug to windward, due to being lighter and having a better sectional shape, which, BTW, can be flattened when need be, something one generally cannot do with a Chinese Lug.

They can be improved, but they still cannot compete well with just about any fore and aft rig, when sailed to windward.

But I am not at all a detractor of the Chinese Lug (CL). I have defended it ardently in other threads.

Like just about any other rig, there are things its very good at and things it is not so good at.

For an ocean voyaging mono hull, which is expected to have adequate windward performance, but otherwise, to be very easy to work with a small crew, in all conditions, the CL is hard to improve on. It has been used on one very early western built catamaran with considerable success. But this catamaran was very heavy and rarely if ever exceeded speeds of a mono of the same LWL

But modern multis are much lighter built and have much higher performance expectations. With these boats, Bermudan Rigs (BRs) are almost certain to offer superior performance, even if they are low AR.

This is because they are not only lighter, but they are less top heavy too (per material used). Not only that, but they have a low Center of Area (CA) per given AR (using the formula: AR = (span^2)/Area ).

For instance, a BR with a foot equal to its hoist, a very low AR version, still has an AR of 2.0, if it has no roach built in.

Granted, on a mono, with a narrower Beam and a rolling deck, this sail would be an utter menace. But on a catamaran, not nearly as much so. The cat has a much wider deck, per given displacement, which, along with a much shorter roll cycle and much smaller angle of heel, keeps the end of the long Boom out of the water.

The biggest advantage of a CL is the ease in reefing. This is almost exclusively why the rig is used today. If this is the top requirement for MIk, then perhaps the CL should be considered for his design. But I thought it was only fair to warn him that such a choice could carry heavy costs in other areas, such as performance.

As for the "Soft Wing Sail", is it really a version of the CL (which it certainly shares some characteristics with), or is it merely a collapsible Wing Sail?

The real test would be ease in reefing.

Can this sail be reefed by merely easing the halyard?

 

I thought a few minutes before posting. What you said was quite reasonable, but it relies on assumptions about the construction that might not be true. My response is entirely about what CAN be done. If there had been something more interesting to talk about I would have skipped answering.

That said, you have a good attitude toward learning and seem likely to use it in a simple effective boat, so let's have at it!

I think I can prove that a junk rig can be lighter than the same area and general plan in a lug. Same materials (polyester and wood), same safety factor, and same conditions.

Think how big the yard is on a lug -longer than all battens in the junk and 5 to 8 times as thick -and it's position is higher than All the battens so the junk has a weight advantage at the top. The lug boom is much thicker and heavier than all the junk battens combined but of course it is at the low point of the sail. The key consideration is that the lug needs big thick spars because they are in bending trying to flatten the leach and control twist of the sail. The junk rig battens are lightly loaded because they are supported on both ends and only have a small amount of sail to control. The key is to consider the catenary curve -as the span increases, the tension to hold a given deflection increases exponentially. This is why the halyard tension of a junk can be significantly less than a lug -all those battens are looped to the mast and have control lines at the leach -so the mast has less compression and less bending force and can be smaller and lighter. The taller the rig, the greater the disadvantage of the lug so your statement about high aspect ratio -not practical with a lug. Total standing rigging would be about the same. The sail material could be significantly lighter for the junk due to the lower stress. Conclusion -the junk is lighter.

Next consider aerodynamics and control -the advantage of the junk is that it has a better plan form (close to elliptical) and has direct perfect control of twist. The lug is a big bag by comparison -very hard to control, always too much twist, leach always too curved.

The main reason I would go with a junk rig is that the sail does not flog in big gusts -just ease the sheets and the forces go to near zero. The lug flogs so much a gust often capsizes the boat. The value of precise twist and feathering to zero force can not be overstated. The junk can safely carry more sail in variable conditions and get more thrust out of the limited righting moment. The cost of this performance is complication -more parts, more sewing, more lines to tend... but for me it's worth it for the control, no flogging, no capsize, precise twist and never being hit in the head by a boom.

About 'soft wing' -There is a thread called 'junk rigs for cruising' I think, and there is a guy making his own tadpole shaped battens for a junk rig. It is a great improvement in aerodynamics and wonderful control. If the OP wants to pursue a junk rigged catamaran that would be worth consideration -it's the only junk or lug that could out perform a Bermudan rig. There certainly are soft wing sails that DON'T have junk rig attributes.

 

Square Rig Variation

You might have a look thru this older subject posting:

http://www.boatdesign.net/forums/sailboats/square-rig-variation-30031-2.html

 

Most modern junk rigs use a free-standing mast, therefore have neither shrouds nor stays. However, to have a free-standing mast in a catamaran generally requires a bi-mast setup; one mast in each hull.

Many modern junk sails are being made with camber, which increases their windward performance. The Junk Rig Association forums (free to read, small fee to join), or the JunkRig Yahoo group (free), can answer your questions.

A few catamarans have been made with junk rigs. Here's a post I made a while back mentioning some of them.

http://www.boatdesign.net/forums/multihulls/biplane-rigs-41811-2.html#post530077


Dragon Wings had flat-cut sails.


This second photo is of PHA, with junk-like wing sails. Her replacement, Grand PHA, is currently doing a west-about circumnavigation.

 

Square Rig pointing

http://www.boatdesign.net/forums/sailboats/square-rig-pointing-questions-5991.html#post35214

You might find this subject thread interesting.

 

That link's dead, Brian.

Googling Mir did bring me to one link that claimed it can get to within 37 degrees. If they're talking VMG and TWA (and nothing else makes much sense) then the square rigger points higher than a Moth, an A Class, a 12 Metre and an AC72. Hmmmm.

Interestingly, the same link says that although Mir can get to 37 degrees, fore-and-aft rigs do better. So what boat points higher than 37 degrees?

 

There are a number of links to his research that are dead,...don't know exactly why.

The point I was trying to make is, that it is NOT necessarily a given that all square rigs will not point. I think Maltese Falcon proved that.

 

It's more believable to me that the 37 Deg. is toward the wind. That is 53 deg. off the wind. I just don't see how it can do better than that.

There are two reasons I say this:

1.) Square riggers usually have long shallow keels, if even that. Some have just a skeg and a cut-water, so there is not a lot of underwater "lift" to windward, and

2.) Their Yards are usually controlled by "Braces", which sheet to aft the mast, on the boat's center-line. As the windward end of the Yard moves more forward, the sheeting angle of its Brace becomes more and more acute, making it less and less effective, as it does more to pull the Yard across the mast than it does to hold its end to windward.

This is probably how various forms of the Western Lug rig evolved. Though it looks a lot like the Mid-East Settee rig, it is vastly different, in that the pitch of the Yard is usually much less, and it has a much longer free Luff. It is easy for me to imagine that The square rig was first sheeted as a fore and aft rig (see my comments below, as to how this was done), then the Luff was made shorter than the Leach, by pitching the yard. This way, even if the Yard crossed the mast at or near its half length, it would have a bias towards pointing to windward and therefore be easier to sheet for upwind sailing.

The "dyna-ship" concept adds another wrinkle by elliminating Sheets and Braces all together, and replacing them with a rotating mast, with fixed Yards. This causes two problems:

1.) The mast must be free standing, so has to be a rotating, cantilever spar, and,
2.) it must be able with stand some terrific torsion loads.

This is probably why we don't see many of them around.

Now it is conceivable to have a pair of "Windward Braces", which are before the mast, and sheet to extreme sides of the ship, one to port and one to starboard, which would only be used when sailing to windward. Kind of like "bow lines", but attached to Yard instead of the mid-luff of the sail.

ADDED 4 DAYS LATER [These "Windward Braces" would not work. The reason is that they would all have to be sheeted to the deck. For the higher sails, this would create an even more acute angle than the original aft, center line arrangement.]

But frankly, Scarlet, I think the chap is pulling your leg.

If it sailed 60 deg. off the wind, I would think of that as excellent performance. He may have had the tide helping him.

 

Chinese Lug Weight.

Hi Skyak.

Before I get into my relatively lengthy investigation, I’d like to nail down some nomenclature, as to what is the difference between a “Batten” and a “Boomlet”.

Both Battens and Boomlets stiffen a sail, so that definition is out.

Battens and Boomlets both can be used to increase the Sail Area (SA) set within Mast and Boom, or within Mast, Boom and Yard (or Gaff), so that definition is out.

So this is what I came up with:

A Boomlet, which is long enough to span the greatest width of a sail (from Luff to Leech), must be stiff enough to be set on two saw horses, which are at least 90% of that Boomlet’s length apart, without bending enough to fall through.

This test might be too easy, but I believe it creates a sharp distinction.

Now on to my investigation..

It is based on simple Beam theory.

To explain it in simple enough terms, I’ll create hypothetical sail, with two versions.
The first will be a Balanced Lug (BL). The second will be A Chinese Lug (CL).

Since this hypothetical sail is to determine the relative weight difference between the two, I will follow the same material and section shape rules for both versions. Here are the rules:

1.) Spars are to be made of Douglas Fir,
2.) Spars are to be solid,
3.) Spars are not to be tapered at the ends, and
4.) Spars are top be square in section.

Both sails will be rather crude in construction to keep the construction labor to a minimum.

Now for the dimensions:

1.) Yard, Boom and Boomlets will be 5.0 ft long.
2.) Yard will be at a 1:1 pitch.
3.) The distance between the Boom and the front end of the yard (the Hoist) will be 10 ft.

The BL version of this sail will be approximately 51.5 sf. The CL will be the same. It’s hoist will be adjusted to produce the same Sail Area, so its mast will be a bit shorter, because the equal length Boomlets will create a slight roach.

The needed stiffness of the spars is based on the maximum righting moment of the boat.

Since we have no boat here, let’s assume that it was discovered that 1.0 inch square was sufficient section for both the Boom and the yard, for the BL version. These 5 ft spars would weigh approximately 1.20 lbs each.

The mast section will likely be 2.0 inches square or more, due to its great length (approximately 15 ft). For the sake of this argument, we will put it at 2.0 inches square, so it will weigh about 15 lbs.

So the total weight of all three spars will be about 17.4 lbs. The sail cloth will weigh about 0.75 lbs, including re-enforcement patches. Add another lb for the halyard, down haul, and a sheet line, and the all up weight comes to a little over 19 lbs. No one would ever accuse this rig of being weight efficient. With tapering (not allowed in this case) the rig might weigh a little under 16 lbs, still no feather weight.

Now let’s convert this to a CL.

To do this, lets divide the sail into eight panels, which will require one Yard and Eight Boomlets, for a total of ten spars, including the mast.

These Boomlets will be splayed, so that the 45 deg. angle, between the lowest Boomlet and the Yard, will be divided by eight, giving 5 to 6 degrees for each of the seven middle Boomlets. Fortunately, this is a whole lot easier to do than it sounds.

Now, we will assume that each Boomlet will have its own Sheetlet.

This is where the math comes in.

Since each Boomlet effectively has its own sheet, the bending load on each can be divided by eight, so it would seem that each Boomlet can have one eighth the sectional area as the old Boom, on the BL sail.

But this is clearly not the case. This because the stiffness of a beam is governed by its elasticity multiplied by its inertia, and limited by its material breaking strength.

The formula for inertia, for a rectangular section is:

I = w(d^3)/12

“w” = width and “d” = depth, and “d” always faces the load.
What this boils down to is that, with one eight the distributed load, we need to find the cube root to 0.125, or one eighth.

This works out to be 0.50, so our Boomlets will be 0.50 inch x 1.0 inch and will therefore weigh in at about 0.60 lbs each. But we can do better by complying with my rules and insisting on a square section. Doing this, I come up with a Boomlet which is 0.595 inch square. This now comes to 0.425 lbs per Boomlet, which is far better than 0.60 lbs, with the rectangular section.

So, with eight of them we will end up with 3.4 lbs - 1.2 lbs (for the old Boom), or 2.0 lbs more weight for the Boomlets.

The Yard will have the same duties it had before, so will probably remain unchanged.

But now, because the roach created from the Boomlets adds some SA, we can shorten the hoist of the mast to make up for this. The roach will add about 13% more SA, so the 10 ft Hoist can be shortened to about 8.8 ft.

Hmm. At about 1.0 lb a foot, this subtracts 1.2 lbs from the rig, which leaves only 0.80 lbs of additional weight. So this turns a 19 lb rig into an 19.8 lb one, an increase of less than 5%.

This kind of makes me wonder what Tom Colvin was talking about when he said a CL rig weighed four times as much as a Gaff one.

Now that I look at this, I see most of the weight coming from the mast. With a Gaff rig, you can have a system of stays and shrouds to help support the mast, so maybe the mast could be lighter. But there is no reason you couldn’t do that with this rig.

Another error I may have made here is that this rig is quite tall, with a 15 ft mast only netting 51.5 sf of SA. With a shorter Mast, the Yard and Boomlets would have to be longer, to get the same SA, so would have to be bigger in section. Still, it doesn’t seem to add up to four times the weight. I wish I still had the book I read this from.

I do remember him saying that the Yard on a CL had to be relatively massive, in order to carry the weight of the Boomlets without drooping. Perhaps he was just talking about the weight of the sail itself and counting the Boomlets as part of it.

 

Sharpi,

I would quibble with your definition of a boomlet, but it isn't germain so let's just go with it.

One of the issues I see is that stiffness increases much faster than thickness of the span of a spar. So your boomlets at 1/8 the thckness of the boom don't have 1/8 the stiffness, but something on the order of 1/74 the stiffness. At best each boomlet needs to be about 1/2 the thickness of the original boom to retain 1/8 stiffness, and since stiffness not strength is what's important here that's our constraining factor.

If we have 8 boomlets at 1/2 the thickness then in total we have used four times the amount of material. Worse much of that weight is high up the rig, subtracting from RM, and adding substantial loads to the mast. The extra compression loads from them need to be taken into account, requiring a heavier mast section to be use further reducing RM, and the mast weight itself adds to compression loads.

 

Turning into an interesting conversation, ....I just have to find more time to read thru it.

I did find this other subject thread,...with this particular quoted reference:
http://www.boatdesign.net/forums/sailboats/junk-rig-refinements-48551.html#post656551