The design of soft wing sails for cruising

Im curious at what your trying to achieve David? I can only assume 1 of 2 things... 1 - your either trying to make something better than you already have, (2 iterations of a ketch soft wing rig) or have had in the past (pure junk rig). OR 2 - you like experimenting with stuff like this as a kind of hobby... either way i can understand entirely.

Im also curious if you could quantify your improvements? when i say quantify, i mean in numbers, such as boat speed vs apparent wind angle?

Reading through what youve done in the past - there is a PDF available for download on Davids building of the wing junk rig here btw - http://www.google.com.au/url?sa=t&r...=kmDXnD1A2dahF5AJZGb0Vw&bvm=bv.60983673,d.aGc

I can see youve put in alot of effort into this over the years...

Besides the easy handling, easy reefing, safety side of things, the only benefit i can see your after with a wing junk rig, is to improve the upwind performance in light airs compared to the pure junk rig? Is this a fair assessment?

Another way of achieving a similar end, is via the rotating wing masts with roller boom furling... easy reefing, everything controlled from the cockpit, very clean and no fuss... downside is very expensive... But the point is, a rotating wing mast with conventional sail, can be just as efficient as a wing. The double surface around the entire windward side (not including the leading edge of course) is not as important as you might think - assuming we are comparing to a rotating wing mast that is. A standard fixed mast used by most is very inefficient tho.

I do alot of kitesurfing, have done for over 10 years... these kites are simply flying wing sails, pure and simple. There are full foil kites with double skins top and bottom, and also single skin kites on the lee side with a round leading edge. There is not a discernible advantage to the double skin kites, they do not win all the races or competitions - in fact the single skin kites win more races - mostly due to their increased popularity no doubt. If there was a clear performance advantage, the double skin kites would be winning everything...

The way i see it, is the inflatable leading edge single skin kite designs are effectively like a "leading edge slat" on an airliner wing - which forms part of the high lift devices used to increase the Cl for take of and landing.


You can see in the above pic, there would be a turbulent and discontinuous flow area where the number 4 is circled. This doesnt present a problem for the most advanced aircraft in the world today and they extend these leading edge slats to improve the Cl on take off and landing. Not to be confused with the trailing edge flaps, which are only fully extended on landing as they also desire increased drag for landing. On take off, the trailing edge flaps are only slightly extended to limit the drag and acheive acceptable L/D ratio.

With correct trim or AoA, there is not some huge drag or lift penealty if there is a slightly turbulent area directly behind the leading edge on the windward side of a heavily cambered sail or wing thats trimmed properly. So the idea that there is great benefit in a double surface in this area is kinda misplaced. Provided there is sufficient area and shape to get the stagnation point in the correct location with clean flow, thats the important part. A rotating wing mast with correct trim achieves this - and is why its still used on the fastest ocean racing yachts in the world today. Im only mentioning racing yachts because its proves the point of efficiency, they need to be efficient for maximum power to weight ratio. We can use the same ideology for cruising, but simply reducing sail area to reduce the total horsepower. So again, reducing rig size can be acheived by increasing efficiency. Decreasing rig size, decreases all the loads on the boat, decreases weight, and decreases costs - all of them exponentially.

 

groper,
What am I trying to achieve? You’re kind of right with both your 1) and your 2) - I worked as a designer, and I can’t stop designing and building things. I’m always trying to make something that’s better in some way.

Can I quantify in numerical terms? No. I have no anemometer, I do not sail with my eyes glued to a VMG readout. Watch my lips, while I say this very slowly: "Speed to windward is not at the top of the priority list for an offshore cruiser".

But you have to look at the whole history. Tystie started off with a single sail, which was fine for coastal sailing around the UK, but I did not consider that particular design to be suitable for ocean sailing, as deep reefing was an issue. I converted to wing sail ketch, and sailed it a lot of miles, successfully. In NZ, I improved the shape of the battens, and improved the performance, but those battens were not going to last very long, as I could only get soft alloy, and there were fatigue issues. Also, the sails were getting old. So the rig was due for replacement. I had designed the ‘Fantail’ planform to be suitable for a 26ft Raven in NZ, and it had turned out very well, and had been much admired. I wanted to see how large this design could go. So I converted back to the single sail, and proved its worth by sailing from NZ to Alaska and beyond. So far so good. But now I’m within hailing distance of three score and ten, and I have to accept that if I’m to go on sailing for very much longer, I have to split the rig again, and soon - the single sail of 635 sq ft needs a younger, stronger man than me. I have to do it now, before old age catches up and overtakes me, and I can’t work on my knees on the sail loft floor. So my motivation now is not to make a rig with earth-shattering performance, that will stun the sailing world. My motivation is to replace a single sail that’s too big for me with a split rig that sails well - which is not necessarily the same as sailing fast. And my definition of sailing well is not necessarily the same as other people’s.

You are talking as though I have a blank sheet of paper and a fat bank account, and can make whatever I can design. That is a long way from the truth. I have a boat, it is my home, and I do not want to replace it, even if I could afford to do so. It has a mainmast in good condition, and I can replace the mizzen mast at acceptable expense. I can make the sails and the battens that I have drawn at acceptable expense. I certainly cannot afford to buy a wing-mast, and I certainly cannot make one. The mast I have is stiff enough and strong enough for a variation on the junk rig, which puts much less of a demand on it than does a variation of the bermudan rig.

Whatever they may do for a racing boat’s performance, rotating wing masts redefine the term ‘PITA’ if put into a cruising monohull. As I have said, they will not stop sailing ( on a biplane cat, however, there is a proven answer - the masts are toed in when at anchor, and the boat does not constantly try to sail the anchor out).

You may well be right about the relative performance of a double-skin forward, single-skin aft sail versus that of an entirely double-skinned sail. But then we have to look at the practicalities. With the double-skinned sail as I have envisioned it, it is much easier to devise a nose section that can be conveniently assembled around the mast, and most importantly, can be removed for repair and re-installed without total dismantling of the rig. The angle of rotation of the after batten is less, and the rig will not tend to fail to articulate in light airs. It is much easier to devise a strong hinge and stop mechanism to limit rotation. And more. If someone can tell me how to design a sail with a single-skinned after section that solves the practical problems that I encountered in my first attempt at the form - well, bring it on.

 

David,

Good look with your choices.

I see your choices as being as well thought out and practical as possible given what you are trying to implement.

I have not bothered to try an figure out your mechanisms that will allow both reversible camber and reefing, but you seem to know what you are doing.

I am curious at how you plan to control twist and how effective you think your controls will be.

Making a functional soft skinned wing is an accomplishment. Bringing in features that improve the practical aspects make it even more so.

Please post back from time to time with updates.

 

The answer is very, very simple! I shall use a chinese sheet! And it will be as effective as I choose it to be!
To learn more, buy a copy of Practical Junk Rig, by Blondie Hasler and Jock McCleod, wherein all the basic principles are expounded, and you can see how to achieve whatever anti-twist effect you are trying to achieve.

 

David, would there be any advantage to add a little forward force to the riblets between the battens - perhaps get away with less riblets or better leading edge shape definition?

If so, the concept I have for achieving that involves a simple piece of shock-chord, anchored at the very leading edge of the battens above and below the rib, but passing behind the rib.

 

I had a closer look at your batten design. I see you do not have any means of sealing between the front and rear sections. Unlike your old wingsail, the articulating point and gap is situated close to where maximum lift is produced. Any air leaking at this point could lead to great losses.

Best solution would have been if you could take the fabric all the way around, leaving a little excess in the vicinity of the gap so that the leeward side is stretched tight at full camber, while the windward side just crumples up until the next tack. But I take it chafe or some sort of material fatigue could be an issue?

Alternatively, try and seal off the windward side of the gap as best as possible. Perhaps by extending the rear seam on either side of the front section so that the high pressure will push it back into the gap and so blocking it. The extended seam on the leeward side should lie fairly straight backwards, but might lift a little depending on how much air is still leaking through.

Please get second opinions from aero guru's like Tom Speer and Mark Drela before taking any of my advice!

 

David, its clear now that we have very different goals and reasons for exploring these ideas... but we both already knew that, it just helps to know the reasons behind others decisions to help other minds understand a point of view.

My point of view is much different... im a young fellow in peak physical condition... im interested in blank canvas ideas, no compromises or modifying an existing framework with all of its limitations... im experienced with fabrication in carbon fiber and resin infusion. I dont have a fat wallet either, but it doesnt need to be just because carbon is considered. A 13.2m freestanding carbon mast can be built for under $5k AUD in materials, weight approx 100kgs, if you do the work yourself... and as the rig gets smaller it gets exponentially cheaper and lighter...

IM interested in wing sails to maximize the L/D ratio, maximize the Cl, and reduce the rig size as much as possible for a minimum size, maximum efficiency rig for higher speed cruising - not racing. Higher speed cruising means to cover more miles in a day to bring further out destinations within reach of short term vacations, rather than full time cruising without time limits.

The ability to reef a wing sail or leave it on a mooring in strong winds is main problem i see and the focus of my dilemma. dropping the sailcloth is one way of doing it, but is not necessarily the only way... perhaps a rigid wing could be folded away and stowed flat on deck. Perhaps it could be telescoped inside itself like a collapsible fishing pole -there are people working on this already. There could be many ways of eliminating the windage whilst at anchor... i still find it hard to justify anything beyond a standard rotating wing mast with battened main tho, it ticks all the boxes besides the propensity to sail at anchor. Perhaps an easy method of dropping or telescoping the wing mast could solve this tho...

I also very open to the ideas of electronic and or hydraulic control. As an electrical industry worker, i feel confident in the reliability of electrical systems and the ability to repair them myself. I see electrical systems as more reliable than mechanical systems, based on my experience. I know many others would emphatically disagree on this, but each to their own based on their personal knowledge and experiences.

Im also very interested in multi element designs - or 'high lift devices' in aerospace parlance. I see this as an area of real possibility - imagine a sail which can provide the same drive but half the size of a conventional sail? Such small sails means real gains in terms of safety and handling, nevermind performance on all points of sail...

 

You're looking at the photos of the first set of riblets, made from thin ABS sheet. These soon failed, due to UV attack, and in any case the edges were cutting the sail. Another step up the learning curve. I replaced them with riblets made of alloy bar.

I'm not sure how vital they are. The upper luff stays straight enough, it's only the lower panels where the luff can tend to fall inwards too much, without enough weight of batten below them. I'm going to make the panels narrower and try without, in the new rig, and maybe retro fit just one in the centre of a panel if needed.

 

I have this in mind, and did in fact first draw the sails with a Z on the windward side, and the two parts connected. This might work, but re-introduces something that I'd hoped to get away from - having to use velcro or a zip between battens. I have also considered stiffening the aft edge of the forward section, and extending it past the nose, so that the windward side of the forward section will be pressed against the after section, and this may also work. But my best thought so far is to make the seal internally. I propose that in between the battens, the edges of both fore and after sections of the sail shall have a triangular or trapezoidal flap extension attached, and that these shall meet at the centreline and be fastened together. It's not possible to do this at the position of the batten itself, but for a large percentage of the height, a reasonable seal might be made.

My understanding is that by the time we've got this far back, we're past the high plateau of the Cp curve, and into the pressure recovery zone, but i'm open to correction.

 

Been there, done that - and never ever want to do it again! Yes, you can make such a thing, but when you could have bought a tapered alloy light pole tube for much, much less, you have to be sure that your carbon end result is going to be much, much better. I just managed to make my current battens, nearly 7m x 55mm dia, but only just, and I have had some failures due to dry spots. Now I'm further up that learning curve, and I know I can make batten tubes and other components up to 3m, with a lighter wall being needed because of the shapes I've designed. But masts? No, leave that to the guys who have invested in filament winding and ovens and temperature controlled premises. I have no doubt that Selden Mast, whom I used to work for, now having that technology and knowing that Beneteau are going to want some free-standing masts, are working behind the scenes on such things. I could never persuade them whilst I was there - I was ahead of my time.

 

True, you are in the recovery zone - at least for the high AoA you will be using.

But have a look at the pressure distribution on something like Designfoil. You will notice that there remains a significant "hump" on the low pressure side at around 30% chord. The way I understand it, it acts like a "stepping stone" for the pressure recovery. In other words, it allows the low pressure at the leading edge to recover at a less-steep gradient than it would have otherwise. A gap that allows partial pressure equalising at 30% would affect both upstream and downstream pressure, the result of which would make the Cp graph look like you grabbed the line at 30% and dragged it down - see my illustration below (in which the AoA is an arbitrary 13deg).

Besides a likely earlier stall, it is easy to see how much area under the Cp graph (i.e. "lift") is cut off, even if the steep initial recovery does not create a stall.

 

Thank you for continuing to probe and find the weak spots in the design. This is the way I like to work, even if I get tetchy at having to explain things again and again, and draw and redraw

So i've gone back a few steps, and tried to find a geometry that gives a narrower after V-batten, allows for the two parts of the sail to overlap by a good margin (250mm), keeps the angle of articulation within reason and still permits a strong, sound structure to be made. This drawing shows the reduced camber on the top two battens, and below them the form of all the battens below that:

 

Thanks for taking the time to describe how your rigged worked. But I'm having trouble visualizing why a tapered mast would cause some of the bearing plates to not ride against the mast. I can understand why a bowed mast could cause problems, but a tapered mast has a straight sidewall, so I would think that if the plates would bear against a straight tube it should also bear against a tapered tube. Obviously, I'm missing something.

 

Exactly so, Will. CAD has made it easy to dream up and draw half a dozen variations on a theme before cherry picking the best of each and combining them. So now I have before me the "single aft batten" section, based on Wortmann fx77w153; and the "narrower V batten" and the "broader V batten" sections, based on Eppler e422.

Someone earlier in this thread expressed the opinion that actually, the "single aft batten" configuration would perform better than the other two, in theory. It's very attractive to go along with that thought. In practical terms, it's obviously less expensive to use 25% less cloth, and a single tube is obviously easier to make than a built-up V batten. The sailmaking is easier, with less of a problem to overcome to permit the sail to twist away at the head.

So the conclusion is inescapable - I already had the right solution to the section shape, in my previous wing sails. All I have to do is to engineer the battens better, and CFRP will allow me to do that. Combine all this with an improved planform, higher aspect ratio and with its peak aft, instead of forward, and the way ahead is looking clear. I'm working on it.

 

Yes, the inevitable mast bend is a large part of the problem; but perhaps you're missing that with a tapered mast (only tapered in the upper half, these tapered poles are often parallel in their lower half) there is this problem: the top of the sail is pulling back, the upper battens are pushing forwards. Thus, they are working on opposite sides of the mast, and if the diameter they are working on changes, as it does when reefed, there is no correct position for the bearing surfaces.