sail aerodynamics

Same Old 'Slot' Misconception

Letters to the Editor
Yachting World magazine​

Let me open this letter by complimenting your magazine on being my absolute favorite of all the yachting publications. It is extremely well balanced, and provides a wealth of information in each issue. The presentation is also extremely good.

With this mind I want to next express my real disappointment in a portion of an article that appeared in your May 06 issue on the “Parasailer, The Ideal Atlantic Sail”. The problem appears under ‘the history of the hole’ when attempting to explain the ‘slot effect’, the Venturi effect.

Quote: “On a yacht, adding a headsail speeds up the air through the slot and reduces the pressure on the lee side of the main(sail). The main then tries to equalize by moving forward into the low pressure, creating drive.”

This is entirely UNTRUE. This has so often been repeated in sailing books and publications that it has become accepted as gospel. As one knowledgeable sailor put it, like the Dracula legend, “there’s no way to finally put a stake through the heart of that old explanation—it just keeps coming back to life”

Adding a headsail to the rig actually decreases the flow in ‘the slot’ between the two sails, which ends up cutting down on the mainsail’s drive contribution and increasing the headsail’s contribution. The pressure on the lee side of the mainsail is increased as a result of the slowed slot air, not decreased as the article claims.

If a reader is so inclined to look for more discussions on this subject I would invite them to visit a number of forum discussions that have occurred in reference to this matter at the BoatDesign.net forums.
A few of them:
Sail Aerodynamics, http://boatdesign.net/forums/showthread.php?t=457
How Sails Work, the slot effect: http://boatdesign.net/forums/showpost.php?p=5685&postcount=3
Cutter Rig Pointing, http://boatdesign.net/forums/showthread.php?t=5596
The Slot Effect, http://boatdesign.net/forums/showpost.php?p=40247&postcount=108
http://boatdesign.net/forums/showpost.php?p=23390&postcount=13
http://boatdesign.net/forums/showthread.php?t=623

It’s really difficult to believe the number of accomplished sailors who still do not fully understand the ‘slot effect’.

Brian Eiland
RunningTideYachts.com

 

Brian,
This is what I wrote to them on February the 18th:

"Dear Sirs,
In March 2006 number of your magazine, there is an article about the PARASAILOR chute (Page 44 to 47), where there is a wrong concept on the aerodynamics of sails. It is stated that air accelerates between main and a foresail, thus reducing pressure on the leeward side of the main and so increasing her lift. I'm afraid it doesn't happens like that. As matter of fact air flow slows in the slot, increasing pressure on the leeward side of main (This, in conjunction with downwash from foresail, is why a mainsail backwinds close to the mast). When a foresail is used, the mainsail experiments a detrimental interference from it (mainsail is sailing in the "bad air" of the foresail). The overall increase in sails force, when a foresail is used in conjunction with a mainsail, is due to the increase in the angle of attack of the air in the jib (upwash) induced by the presence of the main (jib is "sailing in the safe leeward position" relatively to the main), as well as more air is directed around the leeward side of the foresail, causing higher velocity (lower pressure) and thus more force. Venturi effect in the slot increasing the lift on the mainsail is an old "myth" stubborngly still wandering around, but long time discarded.
Regards,
Guillermo Gefaell
Pontevedra, Spain"

They did not published it yet.

 

Apologies if this has been covered previously, but I have a question.
Why is the positive effect of the main on the jib greater than the negative effect of the jib on the main? Why are the two effects not equal, thereby cancelling out? Or, why is it not dependant on the particular sail configuration (e.g. relative sizes of main and jib, overlap etc)? I could just about understand that there could be particular instances where the overall effect is positive, but i can't see why it is the general solution.
Can anyone explain?

 

Reasoning about the effect from the main on the jib or the jib on the main might be missleading. It is better to see them as a multi foil system. The system depends on the parts in a comlex way.

See also earlier postings in this thread.

Anders

 

Thanks Jam.
As I understand it the flow between the jib and main, in 'the slot', is slowed down so that the air flow on the leeward side of the main and windward side of the jib is slower. This means that the pressure to leeward of the main is not as low as it would have been if the jib was not there, so the main is not producing as much vaccuum lift. However, this negative effect is more than offset by the fact that the jib produces more lift than if it were operating in isolation. How is this? I can see that the jibs lift is increased, but why is it increased more than the mains lift is decreased? Please explain

 

How sails work

go here, and study the images and text;

http://syr.stanford.edu/SAILFLOW.HTM

Another point I would like to make comes from the turbine, propeller, and windmill world. With a rotating disk, more blades= higher 'solidity'. Higher solidity means that without a duct, air would prefer to go around a windmill with many blades like an axial turbine compressor wheel, rather than through it. That is why windmill designers prefer 2 blades- or even one if the forces and balance issues can be dealt with. Turbines are OK with that because they are pulling it in- creating a 'sink' in the flow ahead known as the 'vena contracta', which also happens through an unshrouded propeller disk.

A set of sails in a sloop rig behave as one foil, with effectively sort of a "Hanley-Paige' slot between the jib and the main. The Hanley-Paige slot was developed with STOL aircraft in mind. I say 'sort of' because the configuration of the sloop rig is highly adjustable- we are only really speaking of the windward 'system' here. When it is wide open, it is a different animal.

One successful approach to racing I used to employ with great success is to load up the sails until I see the leeward sidestay telltale on my M-16 Scow point up into the slot, then come up just enough to see it trail back again. This effectively proved two things- that the flow can actually reverse in the slot which at the time I found fascinating, and that by barely bringing it back I could be assured I was 'stepping on the gas' as much as the sail set could take, so long as the telltales on the leach of the main agreed. Because the Scow is hard to sail fast in low winds, I spent considerable time on the leeward side of the sail inspecting my real life 'wind tunnel experiment'!

 

"A set of sails in a sloop rig behave as one foil".

Which leads me to ask - when we look at aspect ratio, is the important factor the AR of the individual sails, or the whole rig?????

I have a yacht with a low-aspect genoa and high-aspect main. I can see intuitively that this may mean that the low-aspect genoa "wants" to operate at a different angle of attack to the mainsail, but is this correct?

Tom? Rick? Buehler?

 

Seems very correct to me?
The angle of incidence of apparent wind striking the luff of the low AR genoa will be significantly different to the angle of apparent wind striking the mast when considered in relation to a line drawn on a fore-aft direction on the hull.
When sheeting the two sails the luff area of genoa is generally more open than the luff area of the mainsail ( in the lower two-thirds ) which is tighter to the centre line. This lends support to the concept that AR of mainsail needs to be higher than the foresail as it necessarily must be more aerodynamically efficient to provide it's maximum potential drive.

 

The whole rig. The best indicator of lift/drag ratio is the wetted aspect ratio - the span-squared divided by the total wetted area.

 

Thanks.

Does that partly explain the success of assymetrics? They lower the aspect of the entire rig, perhaps down to the point where (according to Marchaj etc) there's a massive "spike" in the rig effectiveness at reaching angles. I'd imagine a skiff, fast cat etc may well go downwind at the 30-35 degrees apparent where Macharj's figures show the rig with an AR of 1 is extremely efficient (I've never bothered to really check the angles).

Does this massive spike (according to Marchaj) also partly explain the way assy boats really pick up pace at certain angles? Obviously attached flow and apparent wind are major factors, but the lobe for an AR of 1 at 35-45 apparent is incredibly distinct (in Marchaj) and it could perhaps account for a lot of the way that assys light up at certain angles. Symmetrical kites, with their generally shorter poles resulting in a higher AR (????) would have more even performance across the angles.

Is Marchaj correct re AR and angle of incidence? Much of his info seems correct to my gut feeling; I know high-aspect rigs (F16 cats, Canoes) really light up at tight angles and get quite ordinary at broad angles. It's always interesting to see a Laser (smaller rig, similar wsa, more beam, less LWL) hanging in with a Canoe on square runs and broad reaches in light winds.

However Marchaj's rigs are all so different (AR from 6 to 1/3) that they may be hiding trends, no?

Or is Marchaj old hat? Can he be slammed in the same way he dissed Manfred Curry?

 

When racing in any sport, pressing the gas as hard as possible is the right thing to do to win. It's trying to define 'as hard as possible' that is the trick. Monohull racers know this too, trust me....I have won six packs of beer off catamaran sailors who doubt it.

 

On a cat, it's probably defined as the point where the windward hull starts to fly. On a keelboat it's very different, because you have a direct tradeoff between lift and heel.

 

Strongest Boat, Smallest Mainsail

interesting observation

....excerpt from Yachting World on America's Cup...
But what was interesting was to see how tightly the new American boat can turn without losing pace. Just as impressive is the boat's ability to perform the America's Cup equivalent of a handbrake turn, carving to a halt before accelerating away once Dickson's foot is back on the gas.

Such nimble behaviour could well increase the speculation as to what lies beneath the waterline, but the clear waters off the Valencian coast are starting to reveal more and clarify part of the picture.
It seems more likely now that USA-87 has a 'conventional' rudder and keel configuration. The question now, is whether she has a trim tab on the back of the keel or perhaps a forward rudder.

Part of the reason for the widespread speculation is the position of the rig and the bow sprit that goes with it. But Shosholoza tactician Dee Smith believes that the reason for the difference is far simpler.

"BMW Oracle do not have anything strange under the water," he said. "All the rigs in the fleet are further forward because of the fat top mainsails. Also, the jibs are bigger, the boats are lighter and so the rigs have to go forwards.

"Last year we saw a lot of rigs moved forwards because nobody could change them in time for the sail development. We saw that the strongest boat in the fleet last year had the smallest mainsail, so what does that tell you about balance?"

That it continues to be the biggest key to Cup yacht design?

 

Don't forget the keel

With most of the emphasis being on the sails, it is important to give the keel its proper due when discussing what makes sails propel craft anywhere but before the wind. Although aircraft operate in 3 dimensions of space, they generally do not operate in two media of matter (air and water) like sailing craft and thus the keel becomes paramount in working with (or against) the sails to achieve optimum speed, balance and control.