Jib + Wingsail combination

I was going through some old Seahorse Magazines the other day, and came upon two pieces they did on the Catamaran Stars & Stripes (the America's Cup Cat), and in the article the designers said that even with the efficient wing they had, they found better performance with it when they added a blade jib. I also remember Steve Clark mentioning on the IC forum that a Class A cat might benefit from a jib downwind.

It got me to thinking :

If you sail with a jib and a fairly high aspect ratio symmetrical 3D wing (like a 0012), does that wing operate more effectively in light conditions than it might otherwise?

Ignoring twist for the moment.



Paul

 

I would suspect that most of the extra benefit may simply be due to the added sail area of the jib. However, the solid wing may have been subject to laminar separation at low wind speeds (often a problem for wing sails) and the jib may have helped to maintain attached flow. I would expect that in general adding a jib would exacerbate rather than reduce 3D effects, particularly if it is less than a full height jib.

Mal.

 

Thanks for the reply, Mal.

I guess I'm focused on 2 things- getting the flow smoothly around the front end of a thick foil in light airs, and keeping the flow from detaching towards the leech in light airs.

What I'm wondering is whether

1. the jib would help light air flow get around the gentle, rounder front curves of a 3D foil smoothly and viscously enough to compete with a cambered 2D sail with a mast, or even a flat plate, and

2. whether the jib induced smaller pressure gradient on the lee side of the thicker foil in light airs translates into a smooth enough flow over the rest of the chord to get a thick wing to perform enough in light airs to at least keep up some with the thinner sails while still keeping the fat stuff's advantages in heavier air.

3. is jib overlap an advantage with a wing?

4. how do you see the jib exacerbating the 3Dness of the wing? The front curves on the windward side?

If I remember right, Steve was not proposing extra sail area for the A cat. S&S? The sky was the limit!


Paul

 

Paul,

I think you need to take a step back and ask why you want to use a solid wing in the first place. There is a general perception that solid wings are always “more efficient” than soft sails. Sure aircraft generally use rigid wings, but the aircraft design problem is different to that of the sailing boat.

Aircraft wings are required to deliver high lift during take off and landing, but the rest of the time, low drag is the overriding concern. Thick wing sections deliver low drag, but the trade off is that you can’t get as much lift as you can out of an equivalent membrane wing. Hence at critical times high lift devices such as slots and flaps are deployed.

A sail on a boat is generally required to deliver maximum lift at all times until the maximum righting moment of the boat is exceeded. Beyond that point the sail is then de-powered either by reefing (in which case the reduced sail is still delivering high lift) or by flattening or feathering the sail. Most boats use a jib/main combination. The jib is basically a high lift device.

So wings and sails function almost at opposite ends of the design spectrum (low drag vs high lift). There are of course always exceptions e.g. ultralight aircraft which operate at low speed can get away with cheaper soft wing arrangements. The exception for sailing boats is in the very high performance area. At speed, apparent wind becomes a barrier to upwind speed and the only way around it is to improve the lift/drag ratio of both the hull and the sail. Some very low drag hulls, such as C-Class cats can afford to sacrifice a bit of lift from the sail in order to achieve a significant drag reduction. Not to mention the additional weight penalty.

But this is not the whole story. C-Class cat rigs are not one piece rigid wings. They are usually double slotted wings capable of large flap deflections which can generate large lift coefficients for downwind sailing. One article I read quoted Lindsay Cunningham as claiming maximum lift coefficient of over 4 for The Edges’ wing, from wind tunnel test results. These sorts of figures are the result of detailed calculation and testing, which is then realised using complex and expensive wing structures.

I don’t think that chucking a jib on a rigid wing is really going to cut it. I suspect that Stars and Stripes was basically underpowered, resulting from a fast tracked development program with a limited budget. The jib was a quick fix.

The proximity of the jib att he lower leading edge of the wing creates a large variation in downwash angle along the span of the wing (3D flow effects). Hence an untwisted rigid wing will be operating at a less than optimal angle of attack along most of its span.

Mal.

 

It seems you are correct. To quote Duncan MacLane and Dave Hubbard from their series in Seahorse Magazine (Feb - May 2005):

(here discussing the first, smaller wingsail of 1,350 sq. ft.) "On the wingsailed boat the solent was only carried in light winds, where hull flying was not possible or was inconsistent. The jib would be dropped and stowed as soon as the wind velocity allowed continuous hull flying..if the wind was puffy the solent was kept aloft to provide power i the lulls..Hull flying is critical for upwind performance..When we could fly consistently with the wing alone, we would drop the headsail to reduce aero drag. As she was designed for heavier air, the rig and platform were balanced for sailing under wing alone, so this was an ideal condition..tacking the wing was never a problem..."

The second wing, the one used in the Cup races, was significantly larger at 1,860 sq. ft. This allowed hull flying at 7 knots.

 

Actually, both of you have put your finger on it-

Limited budget
Efficiency at different wind speeds
Depowering

I'm thinking about doing an DC. A mainsail/mast/rig is going to be at least $2500, and I do need 2 new jibs, a mainsail, and a recut on my assym on my 40'er, so doing both, hmmmm...$$$$$$!

Here's what I was adding up, gedanken experiment wise;

1) Bethwaite; twist diminishes as wind speed goes up- after 6K etc. And around here it's pretty true.

2) Reynolds number (if I can believe info) says that thick foils are better after 110,000-160,000 or so. Below that, thin cambered (whether with foil LE or not) are better. Leave out flat plates for the moment.

So I was thinking, based on my quivers of sails/masts from my windsurfing days, and when IC sailors say they are starting to depower, how much sail would be controllable on an IC above Re say 200,000, and 5 Square Metres popped into my skull, mast head ~20' above the H2O, ~2.5' chord, and I figure with the seat leverage and me at 180 lbs., I can probably hold on to that from, I don't know, 12K up to maybe the low 20's, since a wing, if the right section (I dream), will feather controllably, which was my experience with SOME RAFs and the one semi wing sail I tried on a windsurfer. Don't know if it would be competitive upwind at 5 Sq M in 12K upwind, but it does have max DC rule span. I can build a simple wing myself a LOT cheaper than I can have a sail built every 2-3 years, recutting, etc, buying masts etc. It probable will be heavy, but who knows? And since I'm building the thing myself, I can modify it myself. And get some first hand experience on the nature of the absence of twist induced drag. It might even be that 50/50 isn't the best split between the two foils.

Ok, there's my stronger wind sail, how do I give myself a low wind speed single membrane sail? A jib! 5 sq metre jibs are not that expensive, so I'm ok with having a sailmaker build that. And I can mess around with AR, planform and all that fairly inexpensively, and figure out where the crossover point between having both jib & wing up, and having just the wing up upwind. It's not perfect, but the jib's the main power source in a main/jib system, and I'd be looking at the wing in this case as an imperfect enabler. Granted, twist is an issue at low wind speeds (Bethwaite), but there are some dodges available (Marchaj). Which is why I was asking about the pressures around a wing modified by a jb in light airs. And when I look at Gentry for example, the pressures on the lee side of the main with a jib up are tiny and it seems like the jib could modify things enough on the wing so it might work in lighter wind, and the jib would also, if set up right, help bend the air more downwind than an una rig. (Which is where Steve's comment comes in.) When using the wing alone, I figure I could carry a 5 Sq metre assymetrical, which might give me deeper angles, and bend the air even more than an upwind jib would. If I were really masochistic, In light airs I could swap the jib and the assym at the upwind mark.

Couple all of this with a hull in the low 20's (sq feet) for wetted surface, so it's easily driven (it's ok, I used to swim competitively, and I sailed Div. 2 boards a lot), and I might have the same sort of dynamic as S&S; an underpowered wing augmented with before the mast sails. Instead of flying the hull, I'd be flying me. And as much as I like sailmakers, I'd be paying myself Boat Units $$ and with any luck learning something. Granted, it probably won't be state of the art, but there are no other IC's around here, and maybe I could bug Mr. Speer, as he's a local. Who know, maybe I'll wind up with partial wing mast!

Paul

 

On my IC, of which I have admittedly limited experience, I seem to be fully powered up to at least 10 knots, maybe 12. I think you would be selling yourself short with only a 5m^2 wing. Why not go for broke and do a fully articulated 10m^2 wing? Now that would be something! Talk to SHC about how to build one. I think a DC might well be in the L/D range to take advantage of it. Certainly you could use the extra lift downwind where the IC is currently lacking.

Mal

 

Mal-

I'd love to go for an articulating wing right away, but I've not done it before.

1) So I'm taking Steve at his word- keep it simple, learn to exploit that simple system & then move on to the more complex.

2) I want to explore the different Reynolds number plateaus (?) that give the nod to 2D or 3D foils. Sailboats seem live in two different re regimes. It would be cool to find a way to exploit and bridge the two.

3) I'm intrigued by main/jib pressure/flow interactions, esp since I've been a single sail snob without apparently much reason.

4) There's an idea floating around out there that the main (abstractly?) can act like a flap to the jib, and if it can....?

Paul