Graham Cox wrote:

Um, what about ice yachts and land yachts?  They seem to sail to windward quite well, I think? Or is the resistance of their skates or wheels the equivalent of underwater appendages? (If you pull the centreboard up on a sailing dinghy, it certainly doesn't' sail to windward too well!).

Darren corrected my concept that the boat is sucked to windward by low pressure created by asymmetrical foil interaction, though I couldn't help contemplating that vacuum cleaners manage to suck quite effectively.  

Try those ice yachts on castors! Another example would be kites, many of which completely lose lift when the string is released - the conservation of momentum laws demand that an external force be applied to generate lift from a change in stream direction or velocity (such as gravity, skates, or wheels), and many kites lack the weight to unstall them without being anchored by the string.

Suction is indeed part of lift; a suck is blow if you turn 180 degrees. It's another way of viewing Bernoulli's Law, which is very real - carburetors come to mind. There are many examples of suction - which is really another way of saying pressure gradients - or the trading of pressure, volume and acceleration, in nature and in man-made items, from hurricanes to jet engines. You could say that tyres are 'inflated' by the suction of the lower pressure air outside them. 

As Darren says, tension isn't the strong suit of a gas, but if you really wanted helium running rigging, you could do so by following the universal gas law and keep the volume constant; this is the world of pneumatics and they stick to the rules by using hoses.

I can't resist - my turn for the soapbox:-

A paper dart will demonstrate the operating principle of a wing: it converts acceleration due to gravity (a 'downwards' force) into forward motion. If you could make a dart weightless, there would be no propulsive force and it would not fly. It's just as true for 787 - if it were made weightless, aerodynamics would be traded for aerostatics. A surprise for most beginner pilots is that on a balanced aircraft, what you do with the engine and propeller has no effect on speed, which - all things being equal - is proportional to the square root of the weight of the aircraft.

The corollary of aircraft weight in a sailboat is the underwater appendages. If you mounted a yacht on castors and put it on a frozen lake, the yacht could not go to windward. To generate 'lift', a wing or sail needs a force to act against. The rules of conservation of momentum would otherwise be broken. The lateral resistance, or lift, of the underwater portion of the yacht is necessary before the sail can generate lift.

An airfoil cannot generate lift; it's analogous to a single line from lines drawing of hull. Likewise, the streamlines presented in a 2 dimensional drawing do not exist in the real world; they just give clues to the behaviour of a complete wing or sail. One of the many factors in a sail is the effect of aspect ratio on vortices, which cannot be modeled in 2 dimensions. The same with circular flow - it's a mathematical construct to aid problem solving and does not exist in nature. So many of the descriptions of how wings and sails work take Joukokowski's brilliant mathematical approximation of airfoils (a description of the taste of beer) and present it as how wings and sails work (a recipe for brewing beer).

Stepping down from the soapbox... Arne and David's comments about fairing the yard into a sleeve on the keel reminds me of what we did to single surface hang glider wings in the seventies. By burying the leading edges in a sleeve approximately 30% of the length of the chord, we gained a performance improvement of perhaps 10%.  If ultimate windward performance is a goal, this sounds like an easy way of wringing a little more from a junk rig.

It surprises me how much interest this topic has generated now, suddenly. When I wrote about it two years ago, only Edward and Annie responded. I am glad, though, that others have made the same observation as I did, so I don’t have to re-write that thing ☺.

Anyway, on the sails with high-peaking yards, the formal head of the sail is just an extension of the luff. I haven’t bothered myself, but I have seen others do it  -  making a quite wide sleeve along the head of  their sail.

This should do two good things:

·         It hides the yard, which then becomes a kind of wing mast. This should cut drag.

·         The now rounded luff will make this section less vulnerable to stalling (?).

As the yard/head of these sails makes up a substantial part of the luff, it must be quite a drag factor, so this simple modification cannot hurt.

Arne

 

Darren Bos wrote:

 What is odd, is that folks have been suggesting that the actual good tack is the one with the mast to leeward.  This is a bit surprising as I'd of thought this was more likely to spoil the lift, as you could create a fair bit of turbulence on the low pressure side of the airfoil.  It might be that with the mast to leeward, the mast and sail combined present itself like a highly cambered airfoil?  The sail pressed against the mast might make the shape gentle enough that you gain more from the added camber than you lose form turbulence?  

It's not odd, as soon as you set hard-surfaced aircraft wings to one side, and look at junk sails and their masts. 

Flat sail, straight battens, mast to leeward: big separation bubble encloses mast - less drag than exposed mast on windward side. Mast to leeward likely to be the better tack - but both tacks not good, when compared with any cambered sail.

Flat sail, hinged battens, mast to leeward: small or non-existent separation bubble - much parasitic drag on both tacks.

Cambered panel sail, straight battens, mast to leeward: mast gets buried in the loose cloth of the panel, such that the airflow skips straight over the mast and re-attaches downstream of it. Little drag on the lee side, and since the air is near stagnant on the windward side, no serious drag here either. It may look bad, but it's not. Mast to leeward very likely to be the better tack.

Slightly cambered panels, combined with hinged battens: Mast only partially buried in the loose cloth, so some scope for drag here, but not too serious. Mast to leeward is still likely to be the better tack, but not to any marked extent. This is now my preferred option, being easy to set without unsightly excessive creasing, not needing HK parrels or any other unpleasantnesses in the way of tweaking lines, and providing good performance.

Graham Cox wrote:

My rather crude understanding of what makes a sail work to windward is that the wind has further to travel around the leeward side of the sail, so it has to "hurry" to fill the vacuum and thus sucks the boat forward.  The science of sail design and trim is to optimize this need to hurry according to boat design and circumstance.  Even a flat junk sail makes the wind hurry somewhat, due to angle of incidence and twist, plus, when the mast is to leeward, so are the battens, and the wind has to hurry a little more to get past them.  Kind of a comic book interpretation but it makes sense to me!

Graham, you have inadvertently triggered a near compulsive gripe of mine about how lift is commonly explained.  Sorry in advance for the long post.

Lift doesn't suck!  It is impossible to pull (suck) a boat forward with a gas (air), because one of the properties of a gas is that it can't exert a force under tension, which is too bad because I'd love a set of Helium lines for my rigging.  Also, the atoms of air don't transit an airfoil in equal time whether they go to leeward or windward.  You can see video of this here.  Further, the distance that the air travels over the airfoil doesn't matter either. You've probably seen an airplane fly upside down.  Either that airplane had a symmetric airfoil (same shape on upper and lower side) or it was asymmetric and was still producing lift working the wrong way around.

In the end the actual explanation of lift (drive for sailboat) is much simpler and can be looked at in two ways, which aren't mutually exclusive.  The simplest explanation is that according to Newton's third law, when you redirect air it pushes back with an equal and opposite force.  So lift is just redirecting air and the air pushing back.  Alternatively, you can look at the pressure differences above and below the wing created by redirecting the air and use their sum to figure out how hard the air to windward is pushing the sail along (be careful here to remember that the Leeward side is actually contributing more to lift than the windward because it is more important in redirecting the air).  Either way, you just need to keep in mind that all you are trying to do is redirect air while creating as little turbulence as possible.

How this relates to the bad tack is interesting.  Because the air to windward is under pressure, it is more willing to follow along the sail as it is constantly being pushed towards it.  However, on the Leeward side the pressure is lower and you have to be careful about how hard you try and redirect the air, otherwise it will stop following the surface of the wing (sail) and you will lose virtually all the lift you were creating.  What is odd, is that folks have been suggesting that the actual good tack is the one with the mast to leeward.  This is a bit surprising as I'd of thought this was more likely to spoil the lift, as you could create a fair bit of turbulence on the low pressure side of the airfoil.  It might be that with the mast to leeward, the mast and sail combined present itself like a highly cambered airfoil?  The sail pressed against the mast might make the shape gentle enough that you gain more from the added camber than you lose form turbulence?  It would be nice to collect the data from the boats that have wind instruments like Ketil to see what the differences are amongst a variety of sails/boats.

Now stepping down from my soap box.

For a short video explanation of lift look here.

To read Nasa's explanation look here.