Wind Weight

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

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.....



 

Aircraft cruise high (low dencity) and cold ... gives higher speed at design Mach No. for wing and reduced drag. Not sure it reads across to a 0.03 Mk No (20 kts) at sea level.

Dencer air = more molecules in contact with sail = more drag, another factor to consider.

Redback asks what difference moisture makes the the density of air. The answer is actually the oppopsite to what one would think - moist air has a lower density than dry air. Odd, but true - and here is the boring mathematical bit to prove it (remember Gas Equation from school):

Density = Pressure/RxTemperature

R= Gas constant, which for water vapour is 1.6 times dry air. Hence density of moist air less than that of dry air.

I'm impressed by you guys.