sail aerodynamics

[CT 249]

If the Marchaj test results are right, then everyone - Moth foiler sailor, America's Cup tri racer, round the world racer, windsurfer, speed record canting-rig proa sailor, all of them - is wrong. With due respect to Marchaj, maybe we should go with everyone else.

We have seen people try flip rigs, assymetric rigs, kites, captive kites, delta rigs, jib only rigs, cat rigs, rigs with endplates, side-by-side rigs in cats, boomless rigs, bi-pole mast rigs, lattice-mast rigs, gollywobbler-carting schooners, staysail-setting cutters, wishbone-flailing schooners, lunjstrom rigs, dyna rigs, aero rigs, lateen rigs, every other sort of rig imaginable - and time and time again, the bermudan sloop proves to be superior in many ways.

Considering the ingenuity that has gone into design of sailboats, it's a big call to say that the crab claw sail is the best, but that no one has ever been able to show it to perform better before a wide audience. It's an especially big call when boats and boards with crab-claw type sails have been tried and have failed to perform. I can recall a pic of one at a speed week in the late '80s, and boards with similar rig were tried (by De Vries, IIRC) at speed weeks. Performance was definitely lacking.

Interesting to see that according to the Proafiles site, "Other experimentors have been unable to duplicate Marchaj's test results".

One reason often used to support the prevalence of bermudan sloops and bermudan cats is (as the Proafile says) "the modern sport of yacht racing has developed around the triangular race course, a course that highly favors windward sailing efficiency." However, that doesn't apply to many ocean races (which is why we've had ketches win the Round the World race) or in speed records, or in many windsurfing events - so why would they have stuck to bermudan-type rigs if they were inferior?

The thing that makes me dubious about many rigs is that the claims are so over the top. For example, there's been some talk here about the test that 'showed' that a sprit rig gave something like a 30% increase in windward performance over a sloop.

If that was true, you could re-rig a Snipe with a sprit rig and it would go upwind with a 49er (which is around 30% quicker than a Snipe). Or you could re-rig a Hobie 16 with a sprit rig and it would be about as quick as a 25' wing-masted C Class cat upwind. Or you could re-rig a Catalina 22 with a sprit rig and it would beat a J/44 upwind and leave a Farrier F 27 trimaran astern at the rate of 30 seconds per mile. Or you could sort of chuck a spritsail on an original Windsurfer, and it would beat or scare the latest Formula Windsurfers (with twice the sail area and 1/3 the weight) every beat.

Call me a conservative, but somehow I can't see a simple re-rigging job turning a Hobie 14 into something that can wipe a 6' longer Olympic Tornado off the planet upwind. So it seems more logical to think that the 'problems' with the bermudan rig are greatly exaggerated, and that other rigs tests that are not replicated on the water are also incorrect.

[Paul Scott]

One thing that might help a genoa is pointed out by Gentry- If the leach of the Genoa is located at the point of maximum flow (airspeed) of the main, the Kutta condition at the leach of the jib is satisfied at a higher windspeed than free flow, for example, which means that the flow over the suction side of the genoa doesn't have to recover to as low a speed, and as a result is not stressed as much, which would tend to keep the reattached turbulent flow closer to the leeward surface the Genoa. So stall is harder to achieve.

The genoa is bigger in chord, so the flow has more time to reattach.

At lower speeds, the re of the genoa is larger, so it's easier to get into, or retain, a more reliable flow regime than a blade.

Hope I got those right.

[yipster]

Originally Posted by sharpii2

1.) How do you reef it?
2.) Wouldn't comparing it to a well vanged, low aspect ratio Bermuda cat rig be a more fair comparison.

It seems that the big advantage the Crab Claw has is that the boom and the yard are connected ahead of the mast, allowing next to no twist.

I wonder how well it would compare to a low aspect ratio, Boomed Lateen sail, which can come close to matching both advantages?

yes there are planty questions, still that were some serious results.
was reading the new multihull magazine on jib vs genoa on cats
bought and just received Marchaj revised edition optimising sail
so can compare with some 100 fotocopy whats reised, i'll be back
was thinking lateen too but Marchaj's tunnel tests were not so good on the latin
i understood it was the vortex lift of he crab claw that made it score so high
as with many things, the rite setup makes a big difference

[RHough]

Originally Posted by Paul Scott

One thing that might help a genoa is pointed out by Gentry- If the leach of the Genoa is located at the point of maximum flow (airspeed) of the main, the Kutta condition at the leach of the jib is satisfied at a higher windspeed than free flow, for example, which means that the flow over the suction side of the genoa doesn't have to recover to as low a speed, and as a result is not stressed as much, which would tend to keep the reattached turbulent flow closer to the leeward surface the Genoa. So stall is harder to achieve.

The genoa is bigger in chord, so the flow has more time to reattach.

At lower speeds, the re of the genoa is larger, so it's easier to get into, or retain, a more reliable flow regime than a blade.

Hope I got those right.

Thanks! That makes some sense.

The longer chord of a Genoa means it takes longer to establish lift than a Jib.

In Classes where total sail area is measured the sail plans have tended to evolve into fractional rigs with non-overlapping jibs. It was the rules that did not rate overlapping sails at their full area that spawned the big Genoas.

I don't think the AR of the sail itself has much to do with it. The projected area remains the same for a any amount of overlap. So the only thing that changes is the location of the vent at the leech of the Jib or Genoa.

Paul B's comment about a Jib being about as fast as a Genoa during J24 testing agrees with my experience. At one time I tried a "rule beater" light #3 to get a rating credit and still have a very powerful light air sail. It worked well some days and not so well other days, sea state might have been the difference.

The reason I asked the question is because most sail makers just build #1's at LP = 150-155%. I suspect there is a "too much overlap" size depending not on the percentage of the Genoa that overlaps, but the percentage of the Main that is overlapped. The same "150" might be at 40% of main chord or 60% of main chord depending on the relation ship between "J" and "E" ...

I've never seen this relationship explained very well.

Thanks for the responses!

Randy

[Paul B]

Originally Posted by RHough

I don't think the AR of the sail itself has much to do with it. The projected area remains the same for a any amount of overlap. So the only thing that changes is the location of the vent at the leech of the Jib or Genoa.

Perhaps in steady-state conditions we have perfect interaction between the sails, but probably in lump the flow is interrupted and the sails have to act alone for periods until the overall flow is re-established. In this case the aspect ratio might be a factor.

Originally Posted by RHough

The reason I asked the question is because most sail makers just build #1's at LP = 150-155%. I suspect there is a "too much overlap" size depending not on the percentage of the Genoa that overlaps, but the percentage of the Main that is overlapped. The same "150" might be at 40% of main chord or 60% of main chord depending on the relation ship between "J" and "E" ...

I think you are correct about the relationship of J to E. The other thing to keep in mind is for IRC many people are sailing with smaller genoas, what we might have called #2s in the old days. If the shroud base is sufficiently narrow to allow the same sheeting angle for a 140% sail as a 150% sail, and if this puts the vent in the optimal place, it might be as fast or faster than the 150% sail.

[Paul Scott]

If a genoa alone works well in smooth water, but genoa and main better in bumps, do the two act as 2 individual sails in bumps/gusts, or does the system as a whole with a genoa and main have enough flow hysteresis to average out the bumps, and/or gusts.

I'm wondering if some of the small jibs that the 30 sq m class tried out that would have been called blades if they hadn't been overlapping to the point of the jib leach being located at the point of max camber of the main. The striking thing about these jibs was how little the forstay was separated from the mast. Wasn't there a thread here somewhere about what the minimum separation was necessary from forestay to mast for a jib to work in circulation?

Paul

[Doug Lord]

On page 638 of the Aero-Hydrodynamics of Sailing Marchaj says:" The powerful effect on the character of the flow exerted by a relatively small auxilliary foil situated in proximity to the leading edge of a much bigger foil(roughly 20times the chord of the small foil-dl) is dramaticaly exposed in Photo 3.29(I will try to reproduce below). It will be seen that at an angle of incidence of about 25 degrees, the full separation and stall is already developed on the main foil alone, while with the help of a small auxilliary foil, the rear separation just begins at an incidence angle of 31 degrees. As, a matter of fact, such a configuration reflects in princible the tall-boy type of sail, a short-footed sail set between genoa and mainsail for beating in light winds, or set across the foredeck for running.
A range of positions of the auxilliary foil with respect to the main foil was tested to find out which one of them gave the most substantial gain in terms of maximum lift. It was found that the best efficiency was achieved when the auxilliary foil was located(in relation to its trailing edge) 15% of the main foil chord ahead of the leading edge of the main foil and 12 % above the main chord line, both chords being parallel to each other. In the case of the Clark Y main foil section shape the maximum lift coefficient was about 1.8, which is about 40% greater than for the foil alone."
-----------------
Below is a picture of the above illustration taken directly from the book and a couple of poor pictures of an experimental version of this rig on a model catamaran. Compared to a "normal" rig the results with this rig in light air were astonishing-simply holding the rig and moving it with and without the ribbon jib showed a greatly increased lift with the ribbon jib. I came close to trying this on my first foiler but chickened out though the rig is based loosely on the idea. I think the concept has enormous potential at least for experimentation. The downside besides a little complication at the top and bottom of the "jib" is that ,as best I can tell, the mast must be very straight.....

[Paul Scott]

There was a piece in Seahose a n issue or so ago about some wind tunnel work on tall boys and jibs for downwind work that showed a real increase in performance, but my qusetions about the Marchaj photo that Doug uses have always been whether there's more lift developed as a system, or whether there's less drag because the flow is smoothed out (a small difference I know): is there less lift from the main, or more lift from the slat? Didn't one of the German aero firms mess with this on their wings before WW2?

[BobBill]

Mssrs. Speer and Eiland, and one or two others, taught me something. (Old school on slot, I was), rest is mostly banal chatter. I noted they likely ceased adding (reading) to this thread, by and large. Still, amazing what one simple question can whip up.

It is good, nevertheless.

One question lingers, for me and I will ask here:

America's Cup BOR American Challenger runs wing and headsail, it seems thus far; so,

when the gun booms, will BOR pull out the stops and run wing only, or run with both?

If so, why so; if not, why not? Keep it simple.

[Paul Scott]

A wing can generate lift at small flow angle, which is advantageous when that wing is the mainsail in a tandem foil or main/jib system. The jib can then develop lift in the usual way.

The question to me is the C E of the sailplan, and the l/d of the wing/jib in higher winds.

Edit- We saw BOR from about 30' away the 2nd day of testing, and she really looked like a sloop. I suppose it's possible that she could go wing only, but I kind of doubt it.

[Paul B]

Originally Posted by Paul Scott

There was a piece in Seahose a n issue or so ago about some wind tunnel work on tall boys and jibs for downwind work that showed a real increase in performance,

It seems Marchaj was under the impression that the tall boy would help upwind in light air as well as downwind.

What we learned back in the 1970s was the use of a tall boy upwind was a losing proposition. I had to opportunity to sail with a couple of guys who still beieved this after everyone else figured it out. It was not fun to get slaughtered on the first leg of the day.

People like Marchaj did what they could with the tools available at the time. Marchaj also had some points of view that I think colored some of his results. I doubt you could go into a modern wind tunnel test and conclude a crab claw sail would be the best choice on any point of sail, in any condition, or conclude the use of a tall boy when beating in light air would be a benefit.

Of course this will never stop people who have no real-life experience from quoting these archaic texts as if the words had been etched on stone tablets by some diety.

[RHough]

Originally Posted by Paul B

People like Marchaj did what they could with the tools available at the time. Marchaj also had some points of view that I think colored some of his results. I doubt you could go into a modern wind tunnel test and conclude a crab claw sail would be the best choice on any point of sail, in any condition, or conclude the use of a tall boy when beating in light air would be a benefit.

The crab claw is an interesting idea, based on the observations of the flow on delta wings that hold on to high CL values at extreme AoA. Trying to duplicate that on sail boats has been ... um ... problematic?

The two luffs acting as the leading edges of a delta wing on landing approach has not been duplicated anywhere but in the pages of books AFAIK. But not for lack of trying.

[Perm Stress]

Originally Posted by CT 249

If the Marchaj test results are right, then everyone - Moth foiler sailor, America's Cup tri racer, round the world racer, windsurfer, speed record canting-rig proa sailor, all of them - is wrong. With due respect to Marchaj, maybe we should go with everyone else.

We have seen people try flip rigs, assymetric rigs, kites, captive kites, delta rigs, jib only rigs, cat rigs, rigs with endplates, side-by-side rigs in cats, boomless rigs, bi-pole mast rigs, lattice-mast rigs, gollywobbler-carting schooners, staysail-setting cutters, wishbone-flailing schooners, lunjstrom rigs, dyna rigs, aero rigs, lateen rigs, every other sort of rig imaginable - and time and time again, the bermudan sloop proves to be superior in many ways.

Considering the ingenuity that has gone into design of sailboats, it's a big call to say that the crab claw sail is the best, but that no one has ever been able to show it to perform better before a wide audience. It's an especially big call when boats and boards with crab-claw type sails have been tried and have failed to perform. I can recall a pic of one at a speed week in the late '80s, and boards with similar rig were tried (by De Vries, IIRC) at speed weeks. Performance was definitely lacking.

Interesting to see that according to the Proafiles site, "Other experimentors have been unable to duplicate Marchaj's test results".

One reason often used to support the prevalence of bermudan sloops and bermudan cats is (as the Proafile says) "the modern sport of yacht racing has developed around the triangular race course, a course that highly favors windward sailing efficiency." However, that doesn't apply to many ocean races (which is why we've had ketches win the Round the World race) or in speed records, or in many windsurfing events - so why would they have stuck to bermudan-type rigs if they were inferior?

The thing that makes me dubious about many rigs is that the claims are so over the top. For example, there's been some talk here about the test that 'showed' that a sprit rig gave something like a 30% increase in windward performance over a sloop.

If that was true, you could re-rig a Snipe with a sprit rig and it would go upwind with a 49er (which is around 30% quicker than a Snipe). Or you could re-rig a Hobie 16 with a sprit rig and it would be about as quick as a 25' wing-masted C Class cat upwind. Or you could re-rig a Catalina 22 with a sprit rig and it would beat a J/44 upwind and leave a Farrier F 27 trimaran astern at the rate of 30 seconds per mile. Or you could sort of chuck a spritsail on an original Windsurfer, and it would beat or scare the latest Formula Windsurfers (with twice the sail area and 1/3 the weight) every beat.

Call me a conservative, but somehow I can't see a simple re-rigging job turning a Hobie 14 into something that can wipe a 6' longer Olympic Tornado off the planet upwind. So it seems more logical to think that the 'problems' with the bermudan rig are greatly exaggerated, and that other rigs tests that are not replicated on the water are also incorrect.

Did not paid much attention to this post until recently I did recall one obvious and well known thing:

Depending entirely on crew skill and desire to squeeze every bit of speed, passage times of identical sailboats are different up to 30% (compare how you trim, steer, change sails in a race and when cruising just for pleasure with friends )

Than we could remember and put together few more facts:
1) Highly skilled sailors rarely experiment with exotic rigs (crab claw, junk, whatever... ) -they are busy enough with conventional and recognized boats. It is mostly the enthusiasts or true originals, ho care to bother with out of the mainstream boats.
2) In wind tunnel every model tested is, so to say, equally well designed and equally well "handled", so any possible pluses and minuses are generally same for all models and cancel each other
3) Current bermuda rig with all it's efficiency and adjust-ability is a result of about 100 years of continuous development and optimization, driven by desire to win races
4) The opinion of Americas' Cup winner is much more valued by any one, not deeply involved in making unortodox rig, as that of the autor

When we put together all the 4 observations, it pretty clear, that:
1) 30% increase in efficiency of any unortodox rig will be not enough to give it wide acceptance and recognition, because up 30% of it could be easily lost due to lack of skill, desire to push and accumulated knowledge of generations of racing sailors
2) To gain instant recognition it is necessary to have near to 50-80% gain in efficiency, with same simplicity and robustness in operation; it is highly unlikely for for all of this to happen in one time
3) Even if the superiority is clear, this would mean a necessity to rearrange all the well established sailboats industry (new style of masts, sails, hulls, design philosophy, etc...) -who will care to do it?


To sum up, up to 30 % gain could be realistic, provided equal skill in design, build and handling is available. at the moment, all these advantages are on the side of bermuda rig...

[ancient kayaker]

Haven't read the entire thread so I may repeat something here, but there are sound aerodynamic reasons why tall high aspect rigs work so well that they are all alone in open racing. Exotic rigs like crab claw and junk may have some advantrages in certain conditions but do not win races. The junk is designed to make optimum use of scarce materials and minimize crew, and it does it very well. The closer a sail gets to a wing the better it gets until it IS a wing then the taller it gets the better it gets ... of course if the hull is not good enough to let the rig show its stuff it's all moot. For a heavy, clumsy hull the biggest rag that will fit on it is the way to go. And all other things being equal the best crew/skip will win most if not all of the time.

[Doug Lord]

This is from post # 7 in the "Why so Stable?" thread under "multihulls" by Tom Speer. I thought it was extremely relevant to the discussion of sail aerodynamics,particularly, when you realize he is talking about THE WING. The comment highlighted in red is worth some serious thought!

"Really, the wing is not that much different from the soft sail rig. There's a rotating mast and a mainsail attached to that. For the wing, the mast is larger in chord and the mainsail is thicker. But the cross section topology is the same for both rigs - a teardrop shaped mast, a small gap, and a much thinner mainsail that is articulated to be cambered relative to the mast. The hinge points where the flaps attach to the main element/mast are just like batten cars that are fixed in position instead of being on a track. The flap ribs are just thicker battens. The covering is also a woven material, so USA 17 could be said to have a more traditional sail material than A5's molded 3DLs!

Despite its size, the main (forward) element of the wing can be considered to be a leading edge device for the mainsail, just as the mast is for the soft sail rig. Deflecting the flap one degree while holding the main element fixed relative to the apparent wind will produce more than 80% of the lift one would get by rotating the whole wing one degree while holding the flap fixed relative to the main element. Which also means that rotating the main element while holding the same orientation of the flap relative to the apparent wind will only change the lift by less than 20% compared to rotating the whole wing. So qualitatively, it's where the flap points that matters.

That's one reason why twist control on USA 17 is all done with the flap instead of by twisting the main element as was done with Cogito. It was a simpler and lighter way to go for a wing that size. This is another way that the wing is similar to the soft sail, where twist is all in the mainsail, too.

It may look different, but it really is a sloop."
__________________
Tom Speer