Does cold air make you sail faster?

Lower air temperature = greater air density = greater presure.

Not sure that this has a dramatic effect on sailing speed, but it could well explain why willies shrink in winter. Another one of life's little mysteries solved on Scuttlebutt.

<hr width=100% size=1>Think I'll draw some little rabbits on my head, from a distance they might be mistaken for hairs.
 
Agree with your first 4 paras.
But anemometers measure the difference in forces at each end of the lever of the arms holding the cups - isn't this why they can be reasonaly accurate in varying temps and humididties ?
I did like the idea of the fighting cups from and earlier poster !
Ken

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A sail works like an aircraft wing and the formula for calculating lift on a wing is: lift = 0.5 x air density x wing area x coefficient of lift x velocity squared.
I have no idea what the coefficient of lift might be with a sail, probably pretty poor like early delta hang gliders, but increased air density will increase the lift or power of a sail. Biggest influence is velocity 'cos that's squared.

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Just had a little think about all this cos I know air density has little to do with boat speed /forums/images/icons/crazy.gif -

The density of the air is proportional to air pressure and inversely proportional to temperature in degrees Kelvin ie 0C is 273 Kelvin. So if you are looking at big excursions of temperature say from 30C to 5C then you would get a decrease in density of around 10% which would result in an increase in driving force of the same amount for the same weight and shear.

But getting back to DrAlex's question as to whether the boat will sail any faster the above is all pretty much irrelevant. The boat will sail faster but only by a very, very small amount. Without getting horribly complicated (ie get into stuff I don't know about) that is easy to see.

The driving force is proportional to wind speed squared so if we increase the wind speed by 10% we get get a 20% increase in driving force and we know that a 10% increase in wind does not give us anything like a 20% increase in boat speed (unless in exceptional circumstances like a dinghy starting to plane). This is a wind speed change from say 10 knots to 11 knots - we know that for such an increase boat speed will only increase a very little, probably not noticable unless, perhaps, in a high performance race boat.

Now with an air density increase due to a temperature drop of say 30C to 5C, the driving force only increases by 10% so the increase in boat speed will be much less than the even very small increase in boat speed from a 10% increase in wind speed. Probably not noticable in any boat.

So, wrapping all that up yes the boat will go faster in cold winds, all else being equal, but not so much as would be noticable in my opinion.

Whew, that was a close call - but am looking forward to finding myself slayed when I get up in the morning though /forums/images/icons/smile.gif.

{Edit: I see some others are saying the same. What a relief I can sleep in peace}.

John

<hr width=100% size=1>I am the cat but I am only 6.<P ID="edit"><FONT SIZE=-1>Edited by Ships_Cat on 17/11/2004 12:14 (server time).</FONT></P>
 
re shrinkage- tie a piece of string to it the you can always find it no matter how cold and even wearing gloves.

Hi Argonut

I though modern sails were pretty efficient aerofoils now. It would be interesting to see the air flow over modern sails in a wind tunnel. Your equation wll only give the lateral force ( true lift) from the sail and not the vectored forwards force we're interested in for sailing.

<hr width=100% size=1>Just enjoy it.
 
I seem to recollect that the power required to make anything move faster increases as the cube of the speed increase. So to multiply your speed by, say, 2 you must increase power by 2 cubed, ie 8.

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From my microlight flying days I can report that in cold high-pressure conditions there is a noticeable effect due to the increased air density - take-off runs are shorter and climb rates quite dramatically improved. Pilots know this and reduce take-off weight accordingly in 'hot and high' conditions (ie low air density).

When a yacht has the wind forward of the beam the sail is essentially working as a wing, therefore 'lift' on the sail will be increased in cold high pressure conditions.

Going downwind things are a little more complicated; I can't do the maths, but I would expect upwind performance to be more noticeably improved while downwind there might not be much difference.

- Nick



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Dranex

No, the lift generated by a wing is resolved to the vertical simply because gravity works in the opposite direction but the actual force vectors produced by the airflow are not necessarily the same but commonly have a forward component which is part of the reason why gliders glide and sailboats sail.

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Yes, but you are talking about two different things.

The experience the plane has with lift is the same as if one considers the heeling of a yacht. In cold winds the yacht will heel more due to the greater force of the denser air at the same wind speed.

Without giving it any thought I would guess that a 10% increase in air density (temp drop from 30C to 5C) would increase heeling by something close to but less than 10% (other things happen when heeling like the presented sail area decreases). Same with a plane - without much knowledge of planes, maybe it can lift 10% more cargo.

But as previously explained in several posts this will result in only a small inconsequential increase in boat speed because you need a great big increase in driving force to increase the boat's speed. So, the upwind performance will not be noticably improved, and nor will the downwind either.

John

<hr width=100% size=1>I am the cat but I am only 6.
 
The performance of a helicopter is very much tied to air temperature. We have (rather confusingly) 3 different type of altitude:
1. the type that stops you hitting the ground
2. Pressure altitude - the altitude from the standard 1013 mb
3. Density altitude - the altitude measured from standard pressure (1013) at standard temp (15degC)

It is type three that is used for calculating aircraft performance and can make a huge difference to the amount of weight that an aircraft (10 -15% in extremis), so air temperature has a significant effect.

But boats work at the interface of water and air so changes in air temperature will have a negligable effect on sail performance. Now a change in water temperature and therefore density will have a far greater effect.....

In answer to your original question, I reckon that you are sailing faster because of the money well spent at the sail makers!

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Actually I think your wrong ;-)
You have forgotten to take into account the increase in fluid viscosity on the the surface of the water due to the cold. my calculations indicate that the stem to stern wave will form at a lower speed thus decreasing maximum hull speed.

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If we accept that a temperature drop of 25c increases density by 10% won't that also increase pressure on the anemometer cups, thereby causing a higher wind reading? In which case - how do you determine true wind speed in order to make a valid comparison between different temperatures, when the anemometer itself is subject to the same influences?

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Yes, but.....

As the temperature reduces, ice build-up on the hull will increase waterline length, and hence maximum hull speed. Also, the difference in temperature between water at the bows and the stern (caused by generation of heat from hull friction through the water) should lead to a small "tidal" effect, with cold water being drawn towards the warmer water at the stern, thus potentially offsetting the effect of the increased waterline, and erm.......... bugger.

/forums/images/icons/smile.gif

<hr width=100% size=1>Je suis Marxiste - tendance Groucho
 
Anoraxia

Yes, but that will lead to increased windage across the decks and greater drift to leeward, so not very helpful in the circs. /forums/images/icons/laugh.gif

<hr width=100% size=1>Je suis Marxiste - tendance Groucho
 
Re: Ahhh but...

if it's a pleated anorak the wind is funnelled and will increase lift...

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