The Physics Of Sailing

[grumpy_o_g]

True. but it's that little bit that little bit that points forward (if you're not too close to the wind) which matters.


Groan. Fundamental physics. If you don't change the momentum of the wind, then no force is acting. The trick is to change the momentum (direction in this case, rather than speed) by only a small amount. From (say) 45 degrees off the centreline to (say) 25 degrees off the centreline.

This is much to the chagrin of the guy receiving your dirty wind, who now has to pay off 20 degrees because of the wind shift you've created.

Now whether you use a bit of cloth to do this, or a jet engine (OP at work here!), or a big propeller in the air, really doesn't matter.

The problem I have with people talking about momentum when it comes to aerofoils is that it implies that the air is colliding with the sails and imparting momentum in that way. Downwind that's true but then the sail is not acting as an aerofoil. I'm sure you know that only too well but it's a little misleading if someone doesn't understand the physics behind it. On a close reach the majority of the force (though not by any means all) is due to the pressure differential between the upper and lower surfaces. The dirty wind is purely a side effect. If you change the velocity of any anything with mass you change it's momentum but the change in direction of the airflow is much more complex than a linear relationship with lift produced.

In the fact the component of the force acting on the sails that is due to pressure difference is produced at a molecular level and has no direct relevance to the momentum of the airflow (though there's an indirect relationship obviously). I'm not sure if you're posting tongue in cheek but it's speed of the airflow that is critical - the whole point of changing the direction of the airflow with the curved sail is to create a situation whereby the airflow will accelerate and therefore crate a low pressure area as per Bernoulli. If the airflow is at any direction other than parallel to the sail's surface you're seeing a significant drop in the performance of the sail.

 

[deuc02]

By god, i think i might just be right jeeves

 

[jimbaerselman]

The problem I have with people talking about momentum when it comes to aerofoils is that it implies that the air is colliding with the sails and imparting momentum in that way.

That's a billiard ball analogy. Air, being compressible, behaves more subtly. It weaves, rather than bounces, and starts weaving well before arriving at any surface

On a close reach the majority of the force (though not by any means all) is due to the pressure differential between the upper and lower surfaces.

Well windward and leeward surfaces! But yes, agreed. And what does that force do? It accelerates the air sideways compared to its original direction.

The dirty wind is purely a side effect.

lost me there. I find that being headed by a wind direction change behind another boat is a very dirty trick, and also proof that a direction change has occurred.

If you change the velocity of any anything with mass you change it's momentum but the change in direction of the airflow is much more complex than a linear relationship with lift produced.

The full story is told when you measure the velocity change due to a boat in two components, one along the original air flow, and one at right angles to it. This allows for the effect of all the micro-interferences (lift, turbulence, induced drag, boundary layer mis-behaviour, the cook sticking his head out of the gangway, Bernoulli, Reynolds). The air will have been de-celerated along it's orginal direction, but will have accelerated laterally. So it's moving in a new direction, but with a slower speed than before. Slower speed? Yes, it has transferred energy to the boat. That's why the boat moves. That's when Froude joins the club, in addtion to all the other guys who explain what the keel is up to.

In the fact the component of the force acting on the sails that is due to pressure difference is produced at a molecular level and has no direct relevance to the momentum of the airflow (though there's an indirect relationship obviously). I'm not sure if you're posting tongue in cheek but it's speed of the airflow that is critical - the whole point of changing the direction of the airflow with the curved sail is to create a situation whereby the airflow will accelerate and therefore crate a low pressure area as per Bernoulli. If the airflow is at any direction other than parallel to the sail's surface you're seeing a significant drop in the performance of the sail.

Well that last para expains why I like looking at boats as energy exchange mechanisms. You don't have to get lost in the detail of explaining what generates which force where and when, which molecules do what. The fact is that force must come from pressure differences, and pressure differences create forces. Either way, air momentum will change as a result.

Obviously some general knowledge of mechanisms helps when you're trying to get the best performance out of a boat. If you're an innovative designer, working on an America Cup defender, you'll be writing those text books.

Back to the much earlier point. I suppose some people find the idea of chaging momentum of the air a very simple concept, which tells no lies, and makes more sense of the detail. Others like to rummage through big tomes in search of the holy grail of full understanding.

I suppose being supported on columns of hot air changed my views. Never stopped spouting since . . .

 

[AngusMcDoon]

That's a billiard ball analogy. Air, being compressible, behaves more subtly.

Now during my fluids part of a mech eng degree so long ago that is was in the previous millenium, we were told that for steady state flow in open systems at speeds below about 0.8 Mach, air is to be treated as incompressible, because it virtually is.

Before anyone jumps on me, this is for open systems like aerofoils. Air is of course easily compressible in a closed system like my bike pump.

 

[mjcoon]

Now during my fluids part of a mech eng degree so long ago that is was in the previous millenium, we were told that for steady state flow in open systems at speeds below about 0.8 Mach, air is to be treated as incompressible, because it virtually is.

Before anyone jumps on me, this is for open systems like aerofoils. Air is of course easily compressible in a closed system like my bike pump.

It's what physicists call a perfect gas (OK, I did "applied physics" rather than engineering. A bit like that old joke about the chemical engineer being a chemist when with engineers and a engineer when with chemists!)

The point is really just that in the flows we are considering the pressure changes are tiny compared with atmospheric, so the density changes are correspondingly ignorable.

Mike.

 

[jimbaerselman]

Now during my fluids part of a mech eng degree so long ago that is was in the previous millenium, we were told that for steady state flow in open systems at speeds below about 0.8 Mach, air is to be treated as incompressible, because it virtually is.

Before anyone jumps on me, this is for open systems like aerofoils. Air is of course easily compressible in a closed system like my bike pump.

Correct for density calculations. But you're quite right, I didn't express it well. Perhaps I should have said "object aware", in that (subsonically) pressure waves transmitted forwards affect the air behaviour/flow before the object arrives. So "collisions" (billiard ball analogy) don't occur.