The design of soft wing sails for cruising

If its just a matter of a concave shape to the windward side perhaps you could try the windsurfer approach and induce camber? The rendering was done to illustrate the concept and is not intended to represent a particular foil shape but it seems to me that using some variation of a camber system it might be possible to induce any shape you want, at least at the batten locations,

 

A complete non-starter, I think, Timothy.
From an instructional website:
"To get all the battens sitting in the correct position, sailors may have to give the entire sail a sudden flick to force the camber inducers to move. Or it can be common for sailors to have to kick the lower battens to get them to flick around to the other side."
On a 40 sq m sail? Yeah, right.
Please try to understand that I've spent the last 35 years thinking about this subject, and have thought of, designed, built and discarded all kinds of ideas and notions like this. What I'm seeking in this forum is input from other sailors and experimenters who are working, hands-on, in this field. The ideas part is the easy part - anyone can do it, and many do. The hard part is turning an idea into a practical ocean-going rig.

 

As I said before David, my experience in kite sailing AND the world titles of international kite sailing course racing results, proves that there is no great disadvantage to a slight separation bubble on the windward side. You've seen the kites with inflatable leading edges, and it's a much bigger bubble than you are contemplating...

I think the main disadvantage will be at low alpha, as kites don't fly in this regime. So you may consider this when deciding what's important to you.

Also, regarding ram air systems, despite your non interest, the problem would have been the exact opposite of your concerns. They work well in strong winds, better in fact, it's the light winds which cause not enough pressure inside and it doesn't fully inflate and you end up with a poorly shaped high drag lump of rubbish in the sky...

 

To take your last point first: There's no problem with strong steady winds. There's a problem with the turbulent winds found nearer to the surface of the sea than kites fly, and downwind of obstructions. There's a much bigger problem in light winds - cruising boats find no difficulty in sailing at hull speed once the windspeed is, say, 10 knots or more. It's the range of windspeeds from 5 - 10 knots where we need all the power we can get, and we don't want a poorly shaped high drag lump of rubbish hung on the mast.

I've already understood and accepted your point about there being no need to go to the trouble of making a triangular after batten, as a sail also operates at the high alpha of a kite ; Obviously, a single straight tube is more convenient in practical terms, and that's where I'm directing my thoughts.

 

You're expressing a common misconception. Aspect ratio is not about being skinny - it's about being tall for the area. Span is what matters for the lift-induced drag. Dividing the area into multiple shorter sails will increase the drag, no matter how high the aspect ratio of the individual sails is. When you add up the loading on all of the multiple sails, the induced drag will be much the same as for a single large sail that has the same total loading.

If it's important to reduce the rig height for whatever reason, then there will be a performance penalty for that.

 

I believe that my thoughts have now solidified (like very slow setting epoxy, it takes a long time!). Airing the design on this forum resulted in the aerodynamicists confirming:
1) that the foil section I used before was as good as any, with no great differences in performance between the high lift foils considered.
2) that there is no point in going for a double after part to the sail, because at a high angle of incidence, the single after part should perform at least as well, and possibly better. I failed to find satisfactory solutions to the problem of how to deal with the difference in spanwise length between the windward and leeward sides of a fully doubled sail, and that question does not arise with a single after part.
3) that the head of the sail should have its after end higher than its forward end, so a short yard should be used. Otherwise the planform is fine.

No alternative suggestions for the basic type of the rig convinced me as to their suitability for “go anywhere” cruising.

So I have given a great deal of thought to the design of the battens, and how to make them in carbon fibre. The results are attached.

The design of the hinge borrows from my original GRP battens. These had the battens going into moulded triangular boxes, lined with HDPE, and with no horizontal pivot. There was no vertical pin, the battens having a protrusion that butted against the aft face of the box, thus simplifying assembly. These worked well, except that there was too much friction in light breezes, due to the vertical forces on the batten. I have added a roller to the batten end, and added the horizontal pivot to overcome this. The amount of assembly work that has to be done on the deck, with an inconveniently strong breeze and cold fingers hampering the work, is kept to a minimum, with only the two horizontal screws to be inserted.

It is important to keep the upper and lower surfaces of the battens smooth, so that they do not snag on each other and disrupt smooth articulation when the sail is reefed down. This design achieves that goal, better than my previous attempts.

The hinge converts the couple between the batten pivot and its roller into a push/pull force on the parallel tubes on the inside of the nose. I will make these robustly, as they are the structural part of the nose. The outer component of the nose only forms the curved shape, and is lightly loaded, so can be moulded more lightly than the inner component.

I have worked out the methods of moulding the components, but won’t detail them here. When I start work in March, I must keep a photo record of all the stages.

 

Came across this today;



Might be useful to someone considering a soft wing sail...

 

Camber control look interesting. It looks like a two-hinge articulated rib that is loosely connected (i.e. lines and pulleys) to a batten that is stitched onto the sail cloth.

I wonder if the lines and pulleys are for manual adjustment or set up in a "self-tacking" nature?

 

Interesting.

A year or so ago, there was a big tri with foils on the Swiss lakes, with just this design of rig, very much bigger of course. There was a website, a video, and Big Things were being spoken of. Then everything all went very quiet, and I could never find out what had become of it. My guess is that there was a hushed-up failure of some kind.

But in fact, when I tried soft-wing.ch it's still around. From that website:
"the Pi28 "Gonet & Cie" on Lake Geneva: Pi28 is a boat foil developed by Hugues de Turckheim and Sebastian Schmidt, aiming to win various speed records on Lake Geneva. This yacht was built in 2011 and has already now possible to test two prototypes of flexible wing. A third prototype, including improvements studied in previous trials is underway navigation during the 2013 season: the results in terms of performance, maneuverability and reliability are extremely positive to date (see also next section)." (sorry, Google translation)

Now it seems that there is a much smaller cat with this rig - incredibly complex when you look up inside the sail - and they are learning the lessons about strength and shape in a less expensive way.

 

In an earlier post I think you suggested that in previous versions of your wing sails you've had some problem with keeping the surface of the sail smooth due to the mast bending? How does your current design address that concern?

I'm also wondering about the horizontal pivot you're putting between the wishbones and the tail? Since typical junk rigs have straight battens, with, at best, only modest capability of bending in the vertical plane, why do wing sails need that degree of flexibility? If you do indeed need a horizontal pivot in your battens, how will the sails bend to accommodate that movement?

 

I.N.,
I addressed the first part of this question in #5 of this thread, as follows:
"4. The LHP is actually a good way of shaping up the sail. In my old rig, I attempted to make each batten butt against the mast, with a semicircular plastic bearing. This reasoning is foiled by a) the taper in the mast and b) the inevitable bend in the mast. The plastic bearings in the lower part of the sail could be observed to float clear of the mast, contributing nothing. The leech would then go slack. In contrast, a LHP could be used to haul the luff back and straighten the leech, independent of whether the mast was bending; or whether the head of the sail was on the upper, smaller diameter of the mast, or the lower, larger diameter of the mast, when reefed."
- if the luff is trying to take a convex shape, and the leech is thus slackened, the obvious answer is to haul the luff back.

Why is the horizontal pivot needed? if it is not there, when the battens try to rise - as they do in a gybe and at other times - the demands on the vertical pivot, of strength and ease of movement, are much greater. My first battens (GRP) had the after section articulating within a trapezium-shaped box, and though they were strong, they would bind under certain conditions. My second battens (alloy) did not have any vertical movement initially, and suffered from fatigue. I had to add the second degree of freedom.

With the simple batten of a junk rig, with no vertical pivot, there is no need of a horizontal pivot. As soon as you add the one, you must add the other.

There is no problem with the sail. Under normal conditions, the battens are straight, viewed from the side. The horizontal pivot is needed to cater for abnormal conditions.

 

What is the maximum angular movement that the horizontal pivot experience?

 

In your main hinge assembly drawing --

main hinge assembly.dxf

-- I'm curious how you will cover the green hinge portion with sailcloth? Since the triangular hinge component is rather long (475mm), if you use the sailcloth which covers the wishbone to also cover the triangular hinge, then there could be a lot of sliding motion between the wishbone sailcloth and the batten sailcloth. But if you use the batten sailcloth to cover the triangular hinge then you might have a more complex batten sail configuration, plus possible problems connecting the wishbone sailcloth to the batten sailcloth.

There must be a good reason why you don't use a single universal joint right where the batten enters the end of the wishbone assembly? It appears you plan to assemble the wishbones around the mast by sliding them aft-end first over the mast. Are there other piece(s) to the wishbone assembly to be used to position the wishbone on the mast?

 

In an uncontrolled gybe, about 20 - 30 degrees.

 

Did you look at the camber inducers used by Windsurfer sails?