Wind Weight

Originally posted by Stefan Lloyd Originally posted by Scooby_simon Sailing upwind your sail area pushes you "up wind".  Nope. You seem to think I said something different to what I actually said. As you say, on anything but a run, the pressure difference arises because of the aerofoil effect of the sail. I never said otherwise. But if you think sail area doesn't come into it, try sailing upwind with a pocket-handkerchief and see how far you get. Why do you think yachts put up smaller sails upwind when it gets breezier? Force = pressure * area is just basic physics. Not opinion and not a subject for debate. Actually it is the definition of what pressure means. One pascal = 1N per square metre.

Yes sail area is VERY important because this controls the pressure difference between the front and back of the sail. 

This difference is a result of the difference in the speed of the air across the front and the back of the sail. 
So yes the density of the air is small factor(but as someone above explained with a worked example is was a small one for just simple "push")   We then come to the point where we need to decide what the wind extra "push" in the wind can provide as "thrust" via the sails by creating greater low presure area behind the sail(s).

So I agree that colder air is heavier - obvious.
BUT how much effect does this really make ?
how much more thrust(low pressure) do you get by running  slightly denser air around the sails.

I don't know how to calculate this and IMO it's going to be almost not worth worrying about.  Yes cold air is going to blow you over slightly harder when you are standing still (or sitting on the start line waiting to go).  But once sailing along it will provide a litte more thrust (a very little).  But not much.

What we need is someone who's done an aeronautical degree(or somesuch)  who knows the answer to this by calculcuting the differential lift if hot/cold air over a wing.....



 

Originally posted by Scooby_simon Sailing upwind your sail area pushes you "up wind".  Nope.

You seem to think I said something different to what I actually said. As you say, on anything but a run, the pressure difference arises because of the aerofoil effect of the sail. I never said otherwise.

But if you think sail area doesn't come into it, try sailing upwind with a pocket-handkerchief and see how far you get. Why do you think yachts put up smaller sails upwind when it gets breezier?

Force = pressure * area is just basic physics. Not opinion and not a subject for debate. Actually it is the definition of what pressure means. One pascal = 1N per square metre.

Edited by Stefan Lloyd

An anemometer is measuring the speed of the wind not the force.  I'd rather be hit by a fly at 30mph than a car at 30mph.  The difference is caused by the mass.  It seems perfectly reasonable to me that if we bend wind of greater mass we get greater force even though the wind speed is the same.

Now somebody said we'd be unlikely to be able to notice a 10% difference in force - I'd have to disagree.  When competing against other boats the difference in performance between boats can be in the order of 50m in a kilometer - about 2%.  When you are truly in tune with the boat quite small differences feel quite large, for instance what is the difference between an old jib and a new one - just a couple of seasons use can make the sail feel "off".

Finally what difference to the mass is caused by humidity?  Heavy wind is often wet wind?

Originally posted by Scooby_simon We don't use the "force" of the air to move us along; we use the pressure difference as a result of the speed differences across the front and the back of the sail(s)

And the pressure difference results in a force. Force = pressure x area. No force, no movement (Newton's 2nd law).

 

Originally posted by CurlyBen Wouldn't denser air have a greater pressure differential anyway though?

Yes.....

but there was some tongue in cheek

Heahache time in terms of actually "calculating" if colder = faster and hotter/higher/thinner = slower.....  you never know, you might find that thinner (but faster air) might reward the more skillfull.