The design of soft wing sails for cruising

I know you've said that you're beyond the brainstorming aspect of your sail design, but here's an idea for those who might wish to emulate your airfoil shape with less batten complexity.

This is a drawing of an articulated wishbone batten you posted, which didn't have the airfoil you wanted.



This is the latest iteration of how you are thinking to approximate an Eppler 422 airfoil with an articulated double-wishbone batten.



And here is my idea of how to approximate that airfoil shape without needing an articulated double-wishbone batten. I'm not sure how to firmly attach the straight batten to the wishbone. (minor detail, I'm sure ) The more I think about it, the more I like the idea of a single narrow elongated wishbone batten instead of a wishbone/straight-batten assembly.



The idea is to have the windward side of the batten assembly approximate the E422 foil shape, realizing that the sail will sag in further between the battens. The leeward sail panel will be shaped by a web that connects the sailcloth to the battens. Think of this web as a large specially-shaped batten pocket. This web (and the sail) will collapse against the battens on the opposite tack.

The lower portion of my sketch is somewhat confusing, as I'm trying to show both a series of sectional views of the battens, as well as showing how the sail panels will bulge toward or billow away from the battens.

 

I.N.,
Tried it, a long time ago! This was exactly where I was circa 1980. You can draw the cloth billowing outwards into the shape you want, but the cloth has other ideas. It doesn't self-inflate like that, or if it does, the curve is much further aft than you want it, and the section is nothing like the "curve forward, straight aft" kind of shape we want, for high Cl/Cd.

No, you have to push the cloth into shape from the inside. It's the only way.

 

Your drawing shows the sailcloth billowing away from the battens on the leeward side, so I figured that it would billow for my sail as well. The crucial difference in my sketch is that the shape of the billow is controlled by the web tying it to the ridged batten. You should be able to create whatever leeward shape you desire. Much more accurate, guessing from the comfort of my armchair, than not attaching the leeward panel to the V-battens. The shape of the windward side, however, is less controlled.

 

Crucial difference: I have left the cloth free only in the after half of the sail, and its spanwise length is tightly controlled. The forward half of the lee side must be pushed into shape, believe me. otherwise, it simply collapses inwards. For an example in another sphere, look at model aircraft with soft wing covering - they always put a shaped spar at the LE of the wing to get the all-important first part of the upper surface shaped as they want it.
Also, have you have thought about the enormous difference between the spanwise length of cloth on the windward side and that on the leeward side?

 

I have no experience with soft wing sails or with model aircraft wings, so I must accept your assertions. However, my apparent misunderstanding of this graph posted by Will Fraser fooled me into thinking that there was strong negative pressure along the forward portion of the the leeward side of an airfoil, which I assumed would assure that the sail billowed out.

Yes, but since you had that same problem with your wishbone-vee design (or else had air leaking through between the fore and aft sail panels), I figured it wasn't such a bad problem. Guess I was wrong, again.

 

The twin skinned kites use no battens, same as the para glider wings as well. They use the ram air effect where there is an air entry port near the stagnation point which inflates the wing and keeps it firmly inflated. I see no reason why IN's idea won't work using ram air and webs to control the shape.. But I also fail to see a reason to do it as a single skin in the aft half of the sail should work just as well with less weight and complexity. Same as the omer wing has done really...

 

Kites and para gliders don't tack and gybe. There's an insoluble geometric problem with this idea. The board sailing kites that I've seen have a permanently inflated tube at the luff, to give it a non-collapsing shape. All well and good in a kite but in a sail, you want to reef and furl ...

The Omer wing is double skinned throughout; it's the Beneteau and Matin Bleu wings that have a single skin aft.

 

Ok - i thought the omer and benetau were the same thing, nevermind.

Kitesurfing kites come in 2 distinct types - the Inflatable Leading Edge, and the Ram Air.

The ILE kites are by far the most popular - these are the ones you are referring to.

The ram air kites for kitesurfing are more like a paraglider wing, with double skins over the entire wing separated by webs in the middle - most of these are manufactured by the paraglider wing companies.



You can see the Ram Air intakes near the leading edge.

 

Here's how the design stands at the moment (of course, I'll change my mind again tomorrow, but as long as I'm just chasing pixels around a screen, that's OK).

 

The problem, or rather, "challenge" with the ram air inflation idea is, like David inferred by "tacking and gybing" - the need to reverse camber at some point. Then the current ram air inlet becomes a suction outlet. So you will need a set of ram-air inlets on both sides and be able to seal the leeward set.

 

Seems like an easy problem to solve... a simple one way valve made from a flap of fabric on the inside would seal it when air tries to escape, it blows inwards and allows air in when on the pressure side?

Not required at all if the front part of the sail is supported by hoops in Davids current design and a single skin aft half of the sail... still not sure why double skin all the way to the trailing edge is desired? Can someone remind me? Surely eliminating 50% of the battens - all the V battens - is desirable?

 

Whenever off the wall ideas like this come up, I test them against my practical experience. How will this work in the thunderstorms of the ITCZ, squally weather in the SE Trades, in the fiords amongst the fall-winds and williwaws, in a full gale in high latitudes, a vicious cold front passage in the Tasman Sea ... ? The Omer wing does not yet pass these tests, let alone a ram air wing. This topic is not entitled " the design of rigs for leisure sailing on a nice afternoon".

Points for the full double skin:
1. A good match can be made to off-the-shelf high lift foil sections, designed by aerodynamicists who know a lot more than I do.
2. More room inside for the necessary pivot and limit stop mechanism.
3. The Omer wing is using it, and is sailing very close to the wind (but to do this, the camber needs to be forced in, using hydraulic rams - a freely articulating sail such as the MatinBleu, or mine, does not sail that close).

Points for the single skinned after section:
1. Less cloth needed = less weight, less cost.
2. A simple round tube for the after batten = less construction time, possibly less weight.
3. Perhaps no worse aerodynamically, at high alpha, and might even be better.

Points against the single skinnned after section:
1. Re-reading Tom Speer's paper on wingmast and sail design: discussing windward surface flow patterns, " ... a turbulent separation bubble near the mast-sail interface. This isn't good, but it isn't disastrous either. It's a price we have to pay for the symmetry of the wingmast ... The skin friction within the separation bubble is acting on the backward facing surface of the mast. The projected area of this surface is the depth of the mast/sail junction relative to the separation point, or approximately the maximum thickness of the separation bubble. This is a significant contributor to the profile drag".
But often, he's talking about conditions at low alpha, as happens when an ice boat or fast multi is sailing to weather. This situation is where a hard wing mast comes into its own, but it's not the same with a slow monohull. Here, we are using maximum alpha most of the time. Again, to employ the automotive analogy, I am not looking for a high-revving sports car engine, I am looking for a diesel with bags of torque.

 

Peter,
Yes, all these wing sails will sail at 10 degrees alpha, or more, and thus can do as most sailing boats can do, and point at about 30 degrees or more to the apparent wind and make good boat speed.
But the point is that hard wings, hard wing masts with soft sails and the Omer soft wing can sail at less than 10 degrees, and therefore point higher; if a freely articulating soft wing tries to to do that, the sail starts to articulate (which is the equivalent of the way that a bermudan sail will lift if pointed too high).

From an article about the Beneteau in the "Voiles et Voiliers" magazine, Fevrier 2014:

"Au près, abandonné le vieux réflexe de placer la bôme dans l’axe par exemple. En ouvrant un peu, nous accrochons 6,1 nœuds à 45 degrés du vent par 12 nœuds de vent réel."
"Aussi long- temps que possible, il faut conserver la flèche alignée sur le secteur, formant un angle de 15 degrés par rapport au vent apparent". (15 degrees alpha)

And they give a polar diagram showing the boat making 5 knots at 45 degrees to the true wind in 10 knots true windspeed. Good enough, but not earth-shattering, for a light 43ft boat.

The Omer wing is claimed to sail at 20 - 25 degrees to the apparent wind.

 

Here is a comparison between three configurations of batten that I consider to be practical to make and functional in use.

Broad after V batten
For:
Conforms almost exactly to the shape of e422
Least angle of articulation
Against:
More difficult to seal the gap between fore and after sections
Long length of unsupported sailcloth - more stress, less control over sail shape.
Pivot point furthest forward - worst articulation

Narrow V batten
For:
Easier to seal the gap between fore and after sections
Less unsupported sailcloth
Against: ?

Single batten
For:
Easier construction
Less weight
Less cloth
Pivot point furthest aft - better articulation
Easier to carry spares; replacements can be found anywhere (in alloy or timber)
Against:
Possibly extra drag due to separation bubble on weather side
Greatest angle of articulation

Which to choose?