double sided sails

I am attracted to the properties of a double sided wrap-around mainsail on a cantilever mast.

Very clean entry, simplified rigging, much lighter and very much simpler.
From what I can understand it's efficiency is comparable to a wingmasted sail. You do not see any of these mainsails around though. I wonder why.

Does anybody have any thoughts about this style of sail?

cheers all

 

I suppose it would need to be limited to day sailers because of the difficulty of lowering such a sail on the water. Although some saving in weight would be gained from reduction in hardware, I suppose there would be a significant increase in sail cloth area (being double skinned) and an increase in sail weight.

 

For an interesting wing-sail look here: http://www.qei.co.uk/wingrig.html

If you are talking about a mast-sleve then isnt the problem reefing and stowing the sail in a manageble mass - and removing the sail from the mast (in large sizes) would be interesting.

 

Yes Kevin, but I would have thought that since the sail is doubled around, the breaking strength of the sail cloth would be halved and hence you would be carrying the same weight aloft as an ordinary mainsail, given that it is twice the area.

As far as reefing goes. I would think it would come down a lot easier than a sail on slides, although there would be a lot of chaff where the sail wraps arount the mast.

The SkyeSail looks good doesn't it. I don't think the gaff at the top is necessary though. Still it looks good. Thanks for the link MarkC.

When you start to add battens, and internal camber lines etc, you may as well go for a cantilever wingmast.

 

The attempts to do this (wrap around sails) have had mixed successes. I remember reading about trailing edge flutter and air between the layers, plus a few other things to cause the discontinuation of the idea. Check with Freedom Yachts, they may have some of the early work still on hand.

 

thanks

Thanks Par. I will check them out.

 

There are all kinds of wrap-around sails. The Ljungstrom rig meets your description. "This system consists of a doubled, boomless, battenless, legomutton mainsail that furls (and reefs after a fashion) around an unstayed rotating mast. For beating and reaching, both layers of the sail lie together and are sheeted to the same point, that is they act as a conventional single sail. Off the wind, the sail can open like a butterfly's wings to double the effective area." (http://www.outyourbackdoor.com/OYB8/boats/boatspark.html)

Here's an example from Dick Newick's pen, built by Southport Island Marine:

The mainsail is a Ljungstrom, the mizzen is a conventional full-batten sail, but with a wishbone boom.


I think a trimaran is ideally suited to this rig. A monohull doesn't have the sheeting base to get a good shape in the sail under most points of sail. Note the wide traveler that could be effectively extended even further with a Barber hauler.

 

beautiful

Sure is nice; and so simple too. Too bad the Lungstrom rig is no good for a mono. Boomless sails have to be a blessing. I think that the wrap around sail, as distinct from the pure Lungstrom would be a lot more effcient, especially if the mast is parabolic forward and can be turned into the wind. Thanks for the pic Tom.

 

Actually, the Ljungstrom rig was designed for monohulls. There's a picture of one in Marchaj's "Sailing Theory and Practice" on a classical 40 ft keelboat. But when you see it running wing and wing with both sheets led to the same point on the stern, you can see it's just crying out for Barber haulers to spread the clews apart. Apparently Vosper-Hasler developed a boomed version that doesn't suffer from this problem

Still, it has one advantage few other rigs share - it can be easily reefed downwind.

I think any discussion of double-sided sails has to ask, "Exactly what deficiency in single-sided sails does the double-sided sail aim to solve?" There's a generally acknowledged drag penalty for the mast, but wingmasts address this without having to resort to a double-sided sail.

Sails have to operate efficiently at high lift. Thin sections do this better than thick ones (http://www.desktopaero.com/appliedaero/airfoils2/highclsections.html). Bob Liebeck (a famous aerodynamicist at Douglas Aircraft) set out to design the ultimate high-lift section, trying to produce the maximum lift with a single section with no constraints, and this is what he came up with:

It looks remarkably like a sail! (The plot at the bottom shows the design velocities, starting at the underside trailing edge, working around the leading edge, and back to the upper side trailing edge.)

Attached below are two other examples of the effect of thickness, this time showing the effect of two extremes in wingmast size. The two sections share the same lee-side contour, and the velocities on the lee side are similar. But the added thickness of the big wingmast results in higher velocities on the windward side that reduce the lift.

I think the biggest benefit of a wrap-around sail is producing a smooth lee-side contour. However, this can also be done with a rotating wingmast. Even with a wrap-around sail, it may still be preferable to have a rotating mast to be able to optimize the leading edge shape and ensure there isn't a kink in the lee contour where the sail leaves the mast. An egg-shaped mast may be best for this.

Next, the windward side shape should be aimed at eliminating the separation bubble behind the mast. The double surface doesn't need to, and probably shouldn't go all the way to the leech. The more thickness you add to the section, the less lift it will produce. So a large sleeve may be the best way to go.

 

"Exactly what deficiency in single-sided sails does the double-sided sail aim to solve?"

I suppose Tom, I was looking for the simplest approach for the greatest efficiency. That is where you have to make the tradeoffs.

I presume that the xfoil data for the 5 and 50% wingmasts hold for the double sided sail as well. Although the wingmast's ability to opimise the leading edge, has to give it the final advantage.

 

What I meant was, you need to find out specifically what the problems are with whatever rig you choose as your starting point.

For example, if you have a round mast with the sail attached to a track or groove on the back side, then you're likely to have a separated region on the lee side that is the real peformance killer. The double-sided sail should be aimed at eliminating that separated zone.

If you start with a rotating wing mast, the problem may be the windward side separation bubble. This causes drag, but is not as serious a problem as separation on the lee side.

If you start with a sharp leading edge, like a staysail, then the problem is a separation bubble on the lee side and a lack of leading edge suction. This might benefit from some thickness at the leading edge to allow the acceleration to happen there, and produce a forward-facing low pressure region that will reduce the drag and increase the drive from the sail.

Each of these problems might be attacked a little differently with a double-sided sail.

 

I gotcha...

I originally started with the wraparound sail on a circular mast and a wishbone boom, then the mast needed a parabolic leading edge turned into the wind, to take care of leeside lift separation. Then you have the added complications of quite a large volume of sail, and it's attendant reefing processes. That is when I thought: why bother, just go straight to a conventional wingmast sail.

They seem to be the best compromise all round - that's why there are so many of them.

My primary aim was to minimise sailarea for a given thrust in an effort to minimise heeling moment for a very narrow monohull design. If weight and inertia could be controlled, a solid sail with separate flap and a dedicated downwind sail is one way to, but no good for offshore...pity.

 

Questions on Modifying Ljungstrom Rig

...from a posting on another forum...interesting addition to Ljungstrom rig subject


A while back I was looking at ways to control my boomless and battenless mainsail (free standing Ljungstrom style rig) on our Newick tri. After much fooling about, which is always fun, the conclusion was reached that a boomless and battenless sail is difficult to control in a racing type of environment. So, I picked up a used F31 mainsail which is about 100sqft larger (20%-ish) yet 8 feet shorter that my original sail. We've tried it out a couple of times now and it works very well.

The full batten sail gives a good shape even in very light winds and small adjustments in heading don't require barber haulers or hand holding sheets at just the right angle to keep everything flowing well. With the addition of a small jib, the sail area is boosted some 225sqft (40%-ish) over the original rig. This is much needed in our light wind area. Plus, with the addition of the jib the boat is much easier to tack and drive. ...telltails are a good thing.
Over all, the bigger sail set lower does absolute wonders for the boat. If cruising was the only use for our boat I'd be very content with the original rig, but as a racing rig it's lacking.

So, the proposed changes are....
The top 8 feet off the mast should be cut off. With the much shorter sail the bare mast looks a little goofy up there, like I've got a huge reef in all the time. Cutting down the mast seems a good idea as it takes away the windage and weight. The mast is cedar and carbon so alterations are easy enough. The general plan here is... cut the top off, use old mast head sheave box lower down. Lots of epoxy.

The jib halyard is external and I really don't like that. The set up, as is, is nothing more than a turning block tabbed to the mast that the halyard runs through. I'd think a good gust would rip the whole works off the mast. The halyard clangs annoyingly against the mast while sailing upwind. I'll have to put a bullet block in the mast and another down low to exit again. While I'm at it I think I'll put another in up a foot or so, so I can fly an extra of some sort off the wind. Cutting two small slots in the mast high up in the front face of the mast can't do any harm. Opinions?

The mast is currently freestanding. I'd like to add a forestay so my little jib can go on a furler. What will that do to the mast bearings? Which is where we start to get into the crazy ideas... Maybe while I'm cutting the top 8 feet off the mast I should be cutting off the bottom 5 feet as well. This would be the below deck portion of the freestanding mast. Run some rope stays back and turn my freestanding rig into a more "traditional" 3 point multihull rig. The advantages of this would be a modest weight savings and as the freestanding mast now leaks rainwater, that would be fixed too (the mast now leaks into the head area so isn't a real problem, more of an annoyance).

"Why not ask the designer?" you say.
The mast is a bit of an unknown quantity. The boat was custom built 12 years ago for an owner who is no longer with us. Was the mast built to plan? No one really knows. Other than "Spark" a smaller sistership no other boats have been designed by Newick, or anyone else as far as I can tell, with the same type of rig.

This is too bad because I think the set up has great potential as a cruising rig. But, I'm more into the racing and performance sailing and require a rig that is more tunable and easier to control in our very light and changeable winds. While I will certainly ask Dick what he thinks before I start cutting away bits of mast I'm interested in what patrons of this pub think.

 

Cross References

It might be timely to cross reference a few other discussions on this Ljungstrom subject:

All-in-one-mast:
http://www.boatdesign.net/forums/showthread.php?t=11072

Split mainsail:
http://www.boatdesign.net/forums/showthread.php?t=20229