kds
Well-Known Member
@cos the change affects both cups - the driving and the dragging.
Ken
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Ken
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Does cold air make you sail faster?
- Thread starter dralex
- Start date
PeterStone
Well-Known Member
You aren't arguing that the change in pressure is effectively cancelled out because it acts equally on all the cups are you? The greater the wind speed/pressure, the faster the cups spin. If we accept that colder air creates more pressure on the sails, it follows that it must create more pressure on the cups, which will cause them to spin faster. If we accept the idea that the pressure exerted by the wind alters as the temperature changes, then we have to accept that anemometers only read pressure applied by the wind and not true wind speed. So - the question remains - how can you read wind speed with any degree of accuracy?
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webcraft
Well-Known Member
Suggest you read Argo-Not's post . . . the 'driving force' you refer to is the same as lift for a plane, and has nothing to do with the heeling force . . .
Boats going to windward are driven by lift - i.e. by low pressure at the front of the sail, which is acting like a wing.
You are probably right, though - the increased density may also increase heeling which will increase drag and decrease lift, so maybe any increase is cancelled out.
Personally I'd rather sail slower somewhere warm.
- Nick
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Boats going to windward are driven by lift - i.e. by low pressure at the front of the sail, which is acting like a wing.
You are probably right, though - the increased density may also increase heeling which will increase drag and decrease lift, so maybe any increase is cancelled out.
Personally I'd rather sail slower somewhere warm.
- Nick
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dralex
Well-Known Member
But surely this lift will generate some heel as the force is perpendicular to the angle of the sail- the lift is resolved to forward motion by the keel or hulls resisting lateral movement. Think of the vector diagrams for the forces on sail and boat. The heel secondary to lift will be minimal compared to that of the wind force pushing directly on the windward side of the sail.
<hr width=100% size=1>Just enjoy it.
<hr width=100% size=1>Just enjoy it.
boomerangben
Well-Known Member
"changes in water temperature.. and therefore density"
I think not !
Are you suggesting that water density remains constant over a change in temperature? Its why the Nazi's were making heavy water in Norway during WW2.
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I think not !
Are you suggesting that water density remains constant over a change in temperature? Its why the Nazi's were making heavy water in Norway during WW2.
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Ships_Cat
Well-Known Member
<<<Actually I think your wrong ;-)>>>
Oh nooo, not me, I will go away and sulk for the day now
.
And I am even more wrong cos I am sure that the colder denser air means more drag on the boats windage. So maybe for some boats they go slower in the cold wind.
John
<hr width=100% size=1>I am the cat but I am only 6.
Oh nooo, not me, I will go away and sulk for the day now
.And I am even more wrong cos I am sure that the colder denser air means more drag on the boats windage. So maybe for some boats they go slower in the cold wind.
John
<hr width=100% size=1>I am the cat but I am only 6.
Ships_Cat
Well-Known Member
No, the driving force I refer to is the vector in the direction of motion of the sailboat. That is not the same as lift on a plane (if it were a plane would not be able to maintain level flight). So I think you must misunderstand me.
On a sailboat lift is angled forward of the perpendicular to the direction of motion of the sailboat, the perpendicular being the side (or heeling) force. These resolve into a resultant vector of angle between the lift and side force vectors.
The angle of the line between the ends of the lift and resultant vectors is the apparant wind angle and its length is the drag.
The angle of the line between the ends of the side force and resultant vectors is the direction the boat moves in and its length is the driving force.
Within the approximation I made (and I actually said that the heeling {side} force would be less than my 10% examples because of other factors, I did not say they would be equal) I think you will find that all resolves out to what I said.
In any event as set out in the example in my earlier post (and by others also), one can make practical comparisons of the effect of increased air density with those which we can easily measure due to an increase in windspeed, as the driving force, being that in the direction of travel, increases linearly with air density but to the square for wind speed. I think it is common experience that an increase in windspeed from 10 to 11 knots, as in the example, makes an unobservable difference to boat speed but the effect of such increase in wind speed is considerably greater than the effect of decreasing the air temperature from 30C to 5C (as in my example) so the increase in boatspeed for the increase in air density will be much less again.
So, whatever, in the end there is an insignificant increase in boat speed.
John
<hr width=100% size=1>I am the cat but I am only 6.
On a sailboat lift is angled forward of the perpendicular to the direction of motion of the sailboat, the perpendicular being the side (or heeling) force. These resolve into a resultant vector of angle between the lift and side force vectors.
The angle of the line between the ends of the lift and resultant vectors is the apparant wind angle and its length is the drag.
The angle of the line between the ends of the side force and resultant vectors is the direction the boat moves in and its length is the driving force.
Within the approximation I made (and I actually said that the heeling {side} force would be less than my 10% examples because of other factors, I did not say they would be equal) I think you will find that all resolves out to what I said.
In any event as set out in the example in my earlier post (and by others also), one can make practical comparisons of the effect of increased air density with those which we can easily measure due to an increase in windspeed, as the driving force, being that in the direction of travel, increases linearly with air density but to the square for wind speed. I think it is common experience that an increase in windspeed from 10 to 11 knots, as in the example, makes an unobservable difference to boat speed but the effect of such increase in wind speed is considerably greater than the effect of decreasing the air temperature from 30C to 5C (as in my example) so the increase in boatspeed for the increase in air density will be much less again.
So, whatever, in the end there is an insignificant increase in boat speed.
John
<hr width=100% size=1>I am the cat but I am only 6.
JOHNOO
Well-Known Member
YES! Ask any pilot or heating engineer about the density of cold air.
If you want to how it effects your boat send me a PM.
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If you want to how it effects your boat send me a PM.
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