sail aerodynamics

[Rick Loheed]

Mast Rotation

I'll probably get in trouble here- aside from the fact that every case is a different problem in general, the fact that a sloop rig comes into it's own on heavier boats- whereas a single wing or well done cat rig works better for high speed- is no surprise to me. The game for lightweight high speed boats is more one of striving for max L/D simulteneously with max CL, whereas heavier boats tend to need Max CL. Speeds are lower, boat drag is higher, it's not as 'knife edged' of a design problem. Since the jib/main combination is widely adjustable, and high lift, it can sort of do both and you can usually change the headsail size.

For a nice wingmast discussion, and more on the aerodynamics of these things visit http://www.tspeer.com/ if you haven't already.

I don't have recent experience with the boats you mention, I sail a C&C 24, my sailing canoe, and an E-Scow, none have rotating masts but the sailing canoe sail is pocketed. The E-Scow of course is plenty fast.

However, I have had experience with rotating (but not wing) masts, both on an M-16 Scow as well as on small Catamarans. Overrotating allows the mast to bend more forward, tending to flatten the sail shape considerably in heavy air. Typically though, the M-16 carries more camber cut into the main sail (meaning there is more to remove) than a cat. Can these wingmast rigs bend laterally?

Here is a comparison using Javafoil of a simple 15% camber (pretty high lift) 40% max camber location Jib/main combination arbitrarily loaded to near Max CL, and a 20% Clark 'Y' wingmast shape based on methods from Tom Speer's wingmast paper. For an input Aspect ratio of 10 for each case, this simulation shows a Max Cl of 2.11 for the main jib combo readily achieved for the combination, whereas the wingmast gets to a fairly typical Cl of 1.2 max. before separation occurs. I did not look at the polars, I have to get to work- but the point here was about max CL. I need to read Martin Hepperle's web site to find out how he's implemented the aspect ratio corrections in a 2D section study, but for now I will assume it analyzes the center-span pressure distribution for a rectangular planform. It's the only quick multi-element 2D foil simulator I have...the VLM code would take longer and this has to happen over coffee this morning...

it should be a pretty fair apples to apples comparison and it's meant for illustration only.

 

[tspeer]

Brian, while I understand the theoretical efficiency of the headsail due to its cleaner leading edge, I'm still puzzled....

The theoretical effectiveness of the headsail is not due to its cleaner leading edge. It's due to its interaction with the mainsail. The mainsail gives a boost to the head sail, and the headsail reduces the lift of the mainsail. It's exactly the same as lee-bowing your competitor - the jib gets a lift from the main, and the main gets a header from the jib. But the effectiveness of the whole combination is increased, so there's a net gain.

The thin leading edge of the headsail is actually a disadvantage because there's no forward-facing area upon which leading edge suction can act. A mast with no separation zones would be a better leading edge than the thin leading edge of the jib - hence the effectiveness of wingmasts and pocket luffs.

There are two separated zones behind the mast - one on the lee side and one on the windward side. The jib reduces the effect of the leeward surface separated zone in a couple of ways. One is by reducing the wind speed at the mast, so the mast effectively sails in a lighter wind, producing less drag. The other way is by creating a favorable pressure gradient to the jib trailing edge that encourages the separated flow to reattach to the mainsail.

In high winds when the mainsail is operating at a lower angle of attack, the backwinding of the main by the jib, causing reverse curvature in the luff of the main, also reduces the windward separation zone. This is anagous to under-rotation of a wingmast.

So the jib reduces the drag of the mast over what you'd have with the same rig bare headed. This is in addition to the lee-bow effect.

 

[Rick Loheed]

Yes, exactly what the JavaFoil results show for the main/jib interaction. Well put again, Tom Speer!

There is also a nice discussion of this in Tom Whidden's book, 'The Art and Science of Sails" presented in more layman's terms, which I read over the holiday. Interestingly, according to Tom W. the highest incidence is not at the highest L/D either, but may in fact still be the fastest depending on the other mitigating factors. Not a huge surprise- but interesting. I experimented with that some yesterday in my C&C using my GPS to verify speeds. That was very interesting- not extremely conclusive, but Loads of fun!!....

The book is a bit dated- published in the early '90's, the computers shown were not equal to what you are using to view this post today...but the principles are the same.

 

[CT 249]

Very interesting, thanks.

Rick, some of those wingmasts do bend laterally. The point about max Cl seems good...a theoretical backing to the old "big jibs are powerful" rule of thumb.

Tom, I didn't realise the mast improved the main's efficiency, I was going off information like Marchaj's notes about things like "the adverse effect of the mast on the sail" and Bethwaite's words "if we take the mast away, and consider the performance of a sail set on a wire, such as a headsail, the situation becomes much better..."

Why has the view changed? Is it that the testing techniques have improved, or where some of the earlier theorists just using wrong principles? My Marchaj has him talking about air speeding up in the slot, which as you and Gentry point out, is the opposite of what happens. Or so I hope, as I feared (and I think Gentry says) aerodynamics is not an area for laymen.

As you say "the jib reduces the drag of the mast over what you'd have with the same rig bare headed. This is in addition to the lee-bow effect" why do A Class and C Class perform better without jibs? Were you just using the case of a mainsail of the same size and adding a jib, rather than redistributing a given area between the two sail? Is the drag reduction fairly minor? I assume it's actually fairly minor (although "minor" is indefinable) because otherwise boats with low foretriangles (and hence a smaller % of the mainsail with the advantage of the jib's effects) would be less effective than masthead rigs in some conditions, wouldn't they?

Aaaarrrrgghhh, it's all too complicated!

 

[gggGuest]

My fairly ill informed opinion has it that a single sail rig can operate at lower angles of attack than a sloop rig, just because of the interaction. Where a craft is power limited in going upwind this is not an issue, where a boat is low drag (Icenpay, C, A) then the higher pointing from the single sail wins.

Another theory of mine, with only the very barest of empirical evidence from feel, based on recent IC experience is that I suspect that s sloop rig like the current ICis much easier to keep in "the groove" upwind than a single sail boat. That would suggest that for craft about the borderline between the two a sloop rig might be faster for shifty and variable comnditions and the less talented helm, and the single sail faster for more talented helms and steadier conditions. Stevbe C's opiniopn on this would be interesting.

 

[Paul Scott]

Saildog-

My Grandad was the master of a few of the Guano Trade Clippers during the first part of the 20thC, and, unless the ship was loaded to sinking, they never went dead downwind, unless the conditions were huge and the waves were right, because being on any kind of a reach 'brought the wind forward', and all the sails were drawing, bigtime. Well, at least certain combinations of sail, depending on the conditions and the particular boat. Going dead downwind if I remember right was only done in big wind in certain seastates, pretty much with only the forward sails set, depending on the individual clipper. They really steered with the sails. He used to tell me about leaving a couple of the really high sails set so there would be some power while the ship was in a deep trough. The Southern Ocean. So cool.

Bob-

You are so right about the lift aspect. From what my Grandad told me, the forward sails were REALLY set up for lift.

Paul

 

[saildog]

Nelson's Logs

Lord Nelsons logs are actually available somewhere on the web, I wonder how much DDW they did and under what conditions.

 

[MAINSTAY]

Tom Speer said:
"The thin leading edge of the headsail is actually a disadvantage because there's no forward-facing area upon which leading edge suction can act. A mast with no separation zones would be a better leading edge than the thin leading edge of the jib."

I can hardly believe that a man of his reputed stature in this community, actually advocates that it would be better if the jib has a mast on its leading edge also. Has he never seen a slack luff in the wind shadow of a mast? Isn't this portion of the sail sufficiently forward facing? It seems this partion would produce more drive and less heel forces than any other portion of a sail.
Larry

 

[RHough]

Tom Speer said:
"The thin leading edge of the headsail is actually a disadvantage because there's no forward-facing area upon which leading edge suction can act. A mast with no separation zones would be a better leading edge than the thin leading edge of the jib."

I can hardly believe that a man of his reputed stature in this community, actually advocates that it would be better if the jib has a mast on its leading edge also. Has he never seen a slack luff in the wind shadow of a mast? Isn't this portion of the sail sufficiently forward facing? It seems this partion would produce more drive and less heel forces than any other portion of a sail.
Larry

Was there something about "no separation zones" that you didn't understand? The slack luff you describe is in a separation zone ...

 

[tspeer]

Take a look at Polhamus' leading edge suction analogy, expecially figure 2. Although the paper is concerned with planforms that have more sweep than a typical jib, the flow picture at a jib's leading edge is quite similar.

A sharp leading edge basically has no forward facing area except for the slope of the sail's camber. The low pressure peak at the leading edge gets pointed more to leeward than would be the case for a more rouded leading edge. Here's a physical way of looking at it. The air has to turn through a certain angle in order to follow the lee surface of the jib aft of the leading edge. If the air does most of the turning curving around the leading edge, that orients the suction more forward. But if the air flows past a sharp leading edge and does its turning around a vortex or leading edge separation bubble, then the low pressure of the turning is oriented at right angles to the sail's surface instead. This is still pointed forward relative to the boat, but you'd rather have it pulling forward and to windward instead of forward and to leeward.

This is the basic idea behind Polhamus' analogy. There has to be a low enough pressure over a large enough area to turn the air and produce a given amount of lift. You get the same lift in either case, but the sharp leading edge has a drag component that the rounded leading edge with attached flow doesn't.

The ideal shape would be thin everywhere except near the leading edge, like this Liebeck section:

As a practical matter, jibs with head foils have proven to be every bit as competitive as jibs with wire luffs or hanks. They are an example of a jib with a thick leading edge.

 

[brian eiland]

The thin leading edge of the headsail is actually a disadvantage because there's no forward-facing area upon which leading edge suction can act. A mast with no separation zones would be a better leading edge than the thin leading edge of the jib - hence the effectiveness of wingmasts and pocket luffs......There are two separated zones behind the mast - one on the lee side and one on the windward side.

But of course a mast with no separation zones would not be possible

A sharp leading edge basically has no forward facing area except for the slope of the sail's camber. The low pressure peak at the leading edge gets pointed more to leeward than would be the case for a more rouded leading edge. Here's a physical way of looking at it. The air has to turn through a certain angle in order to follow the lee surface of the jib aft of the leading edge. If the air does most of the turning curving around the leading edge, that orients the suction more forward. But if the air flows past a sharp leading edge and does its turning around a vortex or leading edge separation bubble, then the low pressure of the turning is oriented at right angles to the sail's surface instead. This is still pointed forward relative to the boat, but you'd rather have it pulling forward and to windward instead of forward and to leeward.

You're right Tom, that 'very thin foil' leading edge can be a real problem in the practical world as well. In gusty, shifty conditions it would be real difficult to maintain an ideal flow over the headsails, particularly at this leading edge. Originally I had thought the big round furling foils of Profurl units were less efficient than the oval or foil shaped sections of some other manufacturers. But as I explored the situation further I became more inclined to utilize a round-shaped furler foil on my twin-headsailed rig. This round shaped foil rotates more evenly, and could be made more robust. And it would act to cut down some of the 'sensativity' associated with a very thin leading edge. Some 'bluntness' at this leading edge actually transmits an advanced signal to the incoming flow.

To suggest that a mast section at the leading edge of a sail is not that detrimental is not to my liking. Granted mast sections have steadily declined over the years either by material advances and/or creative rigging support. But most are still pretty big 'obstacles' at the leading edge of an airfoil. On most cruising boats with fixed mast I'd still be willing to wager that close to the first foot of sail area behind the mast does not contribute to driving the boat forward. Okay lets say this might be as little as only 6 inches in the very best cases. One half a foot along the entire luff of the conventional mainsail is a lot of lost sail area, and particularly towards the top of a fractional rig where there is no help from a headsail. If one doesn't utilize a fat-head mainsail you might as well write off the top 10-15 percent of the mainsail area as a contributor of forward drive to the boat.

 

[Paul Scott]

I can't get the Polhamus link to come up, but this page does beg a few questions for me (if these questions have been covered in other threads, please point me to them)-

what size does the main have to be relative to the jib to provide enough upwash to put the jib into a effectively significant lifted condition (is this a ratio?), and what type of proximity does the main need to have to the jib (does it have something to do with Marchaj's Munk (biplane theory) derived 2.4 times chord?)? Is there a bright line of upwash past which the main/jib combination becomes desirable?

is there something magical about the jib (vertical lift? easier engineering?), or would a mast with a really thin section work just as well- for example, like the sections being used on some of the most recent class A cats (like a few Boyers I've seen in the last few weeks), which seem to have very little if any separation behind the mast?

Since the jib is in a lifted condition, and it seems, therefore, not having to deal with really small angles of attack, how much thickness for it's leading edge would be significantly negative, if the section was effectively shaped to reduce or eliminate separation, and if the section had a rounder leading edge as a result of the increased thickness, might that keep flow more attached with fluctuating angles of attack? (I'm thinking about the incredibly attached flow on some of the RAF's I've had on raceboards I've sailed that had better VMG to windward when sailed lower than you'd think prudent. And yes, I had about 20 telltales on some of them.)

Paul

 

[brian eiland]

Match Race, Sloop vs Square Rig

...a little diversion here...

How about a match race between two very different superyachts, one with a sloop rig, and the other with a 'modern' square rig.

"MV" vs "MF" (Mirabella V verses Maltese Falcon) unofficial challenge
http://www.yachtforums.com/forums/27531-post49.html

The World's Three Largest
http://www.yachtforums.com/forums/8815-post5.html

Square Rig Pointing (some aero questions here)
http://boatdesign.net/forums/showthread.php?t=5991
(regrettably this subject thread has been closed to additional postings)

This subject will likely generate considerable discussion on its own, so I thought it might deserve its own thread, HERE

 

[DanishBagger]

Square Rig Pointing (some aero questions here)
http://boatdesign.net/forums/showthread.php?t=5991
(regrettably this subject thread has been closed to additional postings)

Not anymore

 

[CT 249]

Very interesting Tom, thanks. I remembered Stearns saying years ago that the headfoil actually increased the jib's efficiency. Obviously the Liebeck foil has a different leading edge to a conventional mast but I assume you reckon the same effect would still apply when there was a conventional mast at the leading edge?

On the proven superior performance (in some applications) of cat rigs, I only just woke up to the fact that a cat rig (in A and C Class cats etc) is higher aspect, and that Tom tells us that the critical factor is the AR of the whole rig.

Like Paul, I've noticed that sailboard rigs seem to like being oversheeted (although the fact that the leading edge moves to windward can make it look as though the rig is sheeted more tightly than is in fact the case). That's irrespective of the effects on heading and vertical lift, as far as I can see. What that means, I have no idea!