Does cold air make you sail faster?

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dralex

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Stupid hypothetical question, but now supported with some anecdotal evidence.

I was sailing on Saturday in what felt like an air temperature of about 5 degrees with a sub zero windchill and the boat definately felt faster. It may well be the new sail, but are there any people who know the science well enough to say definatively if colder denser air does make a significant difference to the force in the sails for a given windspeed.

BTW- the sea temp was 13 degrees still.

<hr width=100% size=1>Just enjoy it.
 
Presume the air is denser so that for a given windspeed there is more force. I reckon I've got to start reefing in lower wind speeds in the winter. Interested if there is a table relating wind speed / temperature / force?

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Well, me school boy physics is a bit rusty, but here goes.

If the force is constant then all things being equal you ought to go slower 'cos as air gets denser as it gets colder, you are having to push through more dense mass and hence more resistant air. The increased resistance wouls slow you down.

Mind you I'd expect the effect to be more than cancelled out by any surfing down a 1/2 inch high wave.

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No - any effects are due to wind shear and it is this that is meant when said there is "weight" in the wind. Air density has very little effect as it does not change much over normal temperatures.

Wind shear normally occurs when the sea is colder than the air which does not seem to be the case you experienced - however, other things like sea breezes and other weather patterns can cause it.

In case not known, wind shear is the difference between the speed of the wind at sea level and at higher elevations - the friction of the sea slows the lower level down so you have higher wind speed at the masthead than at the deck. The higher windspeed further up the sail can have a big effect on heeling and on the desired twist in the sail (more twist higher up).

The shear can also be directional ie different direction higher up.

Trust that of some use and haven't spoilt what seems to be coming a potentially amusing thread.

John

<hr width=100% size=1>I am the cat but I am only 6.
 
Thanks John- it was a "sensible post"- so thanks for the information- I've heard of wind shear, but that explains it well. I do however enjoy non sensible replies.

<hr width=100% size=1>Just enjoy it.
 
Hate to disagree with one as esteemed as yourself

but as I understand it

difference in pressure due to temp is c 8% between freezing and 70F

However the absolute measurement surely is barometric pressure and 990 in summer or winter is the same.

However is possibly because a lot of winter sailing is around a high where pressure can be 1030 as opposed to summer sailing which tends to be around lows of 990 ish ie a difference of 4%, however as I understand it the relationship of wind weight to wind force is exponential rather than linear .. or is that the usual rational thought process or is the answer just blowing in the wind as the man on the roundabout munching magic mushrooms muttered?





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gan' away and Boyle yersel' ya wee Charlie and stap gibbering oan aboot ra Boltz mon constantly ...

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Yes, it does.

There was a long thread discussing the maths of this a while back, <A target="_blank" HREF=http://www.ybw.com/cgi-bin/forums/showthreaded.pl?Cat=&Board=ym&Number=347413>HERE</A>.

Essentially, air density is greater with lower temperature, lower humidity and higher atmospheric pressure. So an equivalent wind-speed will exert more force on the sails. Jimi is right, in an extreme case the addition force could be 20% extra comparing a cold, dry, anticyclone with a hot, damp, low pressure day.

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Been steamed plenty of times but not often esteemed.

The wind shear/weight thing is pure real stuff but really dunno about barometric pressure effect with the change in air density you mention. Here summer sailing tends to be in highs and winter sailing (but not for me I am a bit soft for that) more in lows as the centres of the westerly progressing highs tend to follow more southerly paths in summer. So here when more lows it is colder as is winter, so maybe air is less dense from barometric pressure but more dense from temperature.

Force of the wind on a sail or whatever is not linear with wind speed as you say but I think one would find (haven't checked) it is linear with the weight (in the density sense not wind shear).

In the end our boat sails faster in cold winds but because have the engine running to get home into the warm quicker.

John

<hr width=100% size=1>I am the cat but I am only 6.
 
Re: Does the Higher pressure

Squash the boat.. shortening the mast and therefore causing the sail to bag out?

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Re: Does the Higher pressure

Do'nt be silly, a lass of your experience should know that the key factor is whether you're sailing towards or away from the high pressure .. towards means you're sailing downhill cos the high pressure means lower sea levels than where you are.

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"Force of the wind on a sail or whatever is not linear with wind speed ..."

I believe the force on the sails is proprtional to the cube of the wind speed, but don't ask me why 'cos I can't remember where I read this and it would seem more logical to me that it was proportional to the square.

Dave

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by machurley22 on 17/11/2004 11:25 (server time).</FONT></P>
 
This seems logical but....we measure windspeed with anemometer cups. Therefore a denser wind will move the cups faster, thereby resulting in a higher measured windspeed. Or am I missing something obvious? If we could be sure that cups only spin at windspeed then no argument but clearly they can't because they rotate and the wind moves in a straight line. Also, for every cup facing the wind and being driven by it, there is another cup fighting it, albeit less effectively because it is presenting its convex face. There again, physics was never my strongest subject.

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Tempurature effects:
Say dropping from 25C to 5C (298K to 278K) will increase density by 7% at constant pressure.

Pressure effects:
Raising pressure from 980mb to 1030mb will increase density by 5% at constant temperature.

Humidity effects:
Not so sure but I think this is unlikely to exceed 50g of water per 1000g of air so could add 0.5%.

So air denisty could change by something like 12% between the two extreme conditions.

Pressure on the sails will be proportional to density and the square of wind speed. You get 9 times as much pressure from 30 knots of wind as you get from 10 knots. So an increase in windspeed from 20 knots to 21 knots will produce the same effect as these extreme changes in air pressure and tempurature. Maybe it is just windier in winter.

It gets even more complicated because your anemometer is actually measuring the force exerted by the wind on its cups so its readings will change with changes in tempurature and pressure at constant windspeed. Should be able to work out whether this makes it read faster or slower but brain overheating already.

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Re: Overheated brain and wind cups

wont the windcup readings be influenced by centrifugal force too? or is it centripetal??? can't remember.. or is it both???

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