Hot Air = Big Sails?

Hot Air = Big Sails?

Post by David » Thu, 12 Nov 1992 10:23:02


Hi,

This one may be a bit off the wall but should appeal to you physics types
out there. I do most of my sailing in Singapore. Our trades season just
started yesterday & usually lasts till about end Feb/March. Singapore is
sort of 100 miles north of the equator (hot, in other words).

I've noticed that for a given wind speed, I use much bigger sails here than
I do in Maui or elsewhere. In marginal conditions, 7.5s & 8.0s are not
uncommon among the bigger sailors here. I've had some experience sailing
in the UK and that seems to reinforce my opinion that air density has
something  to do with this. Does it make sense? Should air temperatures
affect density & thereby sail size? Note this is based on purely intuitve
notions, I haven't been running around measuring wind speeds etc.

Regards

Sukhdev Singh

 
 
 

Hot Air = Big Sails?

Post by Cris Han » Sat, 14 Nov 1992 01:26:55

Quote:

> Hi,

> This one may be a bit off the wall but should appeal to you physics types
> out there. I do most of my sailing in Singapore. Our trades season just
> started yesterday & usually lasts till about end Feb/March. Singapore is
> sort of 100 miles north of the equator (hot, in other words).

> I've noticed that for a given wind speed, I use much bigger sails here than
> I do in Maui or elsewhere. In marginal conditions, 7.5s & 8.0s are not
> uncommon among the bigger sailors here. I've had some experience sailing
> in the UK and that seems to reinforce my opinion that air density has
> something  to do with this. Does it make sense? Should air temperatures
> affect density & thereby sail size? Note this is based on purely intuitve
> notions, I haven't been running around measuring wind speeds etc.

I've heard people say they notice a difference and there is a difference in
air density sea level and here in Colorado (thinner up here), but that would
correspond to a larger sail size the higher you go.  As far as air temp. goes;
that also has some effect on density but I've never noticed any adverse effects
on sail size.  There have been days that it *feels* like there should be more
wind, but most of the time I've found it to be a case of wishful thinking
rather than actual lack of wind.  Not a real technical response, just my .02.

BTW; I spent 3 years in the middle of the Pacific (9 degrees north of the
equator), I've sailed on the northern coast of Oregon, Maui, and currently
the Colorado area.  I've never used a wind meter.  I just go by, as you say,
intuition.  I've only been windsurfing for about 5 years now but have logged
over 1,500 hours of sailing time.  If it feels like 4.5 weather, that's what
go with.

-------------------------------------------------------------------------------
Cris Hannu                       |  Windsurfing the high country.


-------------------------------------------------------------------------------

 
 
 

Hot Air = Big Sails?

Post by Ned Stoffel 213 » Sat, 14 Nov 1992 02:49:25

|> Hi,
|>
|> This one may be a bit off the wall but should appeal to you physics types
|> out there. I do most of my sailing in Singapore. Our trades season just
|> started yesterday & usually lasts till about end Feb/March. Singapore is
|> sort of 100 miles north of the equator (hot, in other words).
|>
|> I've noticed that for a given wind speed, I use much bigger sails here than
|> I do in Maui or elsewhere. In marginal conditions, 7.5s & 8.0s are not
|> uncommon among the bigger sailors here. I've had some experience sailing
|> in the UK and that seems to reinforce my opinion that air density has
|> something  to do with this. Does it make sense? Should air temperatures
|> affect density & thereby sail size? Note this is based on purely intuitve
|> notions, I haven't been running around measuring wind speeds etc.
|>

I believe the air density comes into the lift formula linearly while
velocity affects lift quadratically. In simple terms, a 10 deg. C
(or 18 deg. F) drop in air temperature results in a 3% increase in air
density, but a 1.5% increase in wind speed would have the same effect.
If is hard to believe that air density is enough to cancel out the
increased weight of your steamer or drysuit in cold weather.

Maybe the difference is that there is less wind when its warmer.
My observations seem to indicate that there is less disparity
between the official wind readings and the actual wind at water
level during cold fronts, and I suppose this has to do with the way
air currents naturally stratify vertically. The cold air winds seem
to hug the water while warm winds tend to ride above a stagnant
layer of cold air at sea level.

Ned

 
 
 

Hot Air = Big Sails?

Post by Gregory Russe » Sat, 14 Nov 1992 06:29:42

Another important factor is humidity - warm air holds more moisture, and
tropical areas tend to have higher relative humidity.  These factors add
up to more H2O in the air.  Air is 79% N2, 20% O2, for an average molecular
weight of about 25 grams/mole.  H2O is only 18 grams/mole, so a few percent
water vapor can also change the air density by perhaps 1% to 2%.  Thus,
there may be 5% lower density, and 5% less lift in Singapore than in the
Gorge, at the same wind velocity.  This still wouldn't account for the
difference in sail area (5.0 --> 5.3 m2 is more than 5% increase in lift).

To further complicate matters, only anemometers with rotating elements will
indicate true wind velocity, (unless you correct for temperature, humidity,
et cetera).  The simple ball and tube anemometers will actually measure
dynamic pressure, which is the same thing that sail lift depends on.  So,
the inexpensive wind meters are probably the best type for choosing sail
size, as they incorporate the air density into the measurement.  Once again,
this should only be about 5% factor at sea level, but the folks in Colorado
(especially on a hot, humid day - I know it's never humid there) will see a
dramatic difference between spinner type and ball and tube type wind meters,
and the ball and tube type should be more closely related to sail size than
the spinner type, as you go back and forth among Maui, Gorge, and Boulder.

Any pilots out there should be able to attach more accurate numbers to
this discussion.

Bye -

 
 
 

Hot Air = Big Sails?

Post by Richard Engelbrecht-Wigga » Sat, 14 Nov 1992 10:36:03

Quote:
>Should air temperatures
>affect density & thereby sail size?

The basic law for gases is Pressure x Volume = a constant x absolute Temp.

Room temperature is about 300 degrees Kelvin, whereas a temperature
difference of 40 degrees fahrenheir corresponds to a change of 25
degrees Kelvin.  So, if you hold everything else fixed, 40 degree F
air is about 10% denser than 80 degree F air.  Since force is
proportional to the change in momentum, and momentum is proportional
to mass, the colder air would extert 10% more force on a sail of
given size ASSUMING this denser air flows around the sail just as
the warmer air would.

Colder air tends to be lower in humidity, and that also slightly
increases the density of cold air, but I think that this is a small
effect compared to the temperature effect.

A 10% change is close to the limit of what untrained humans can detect
(in weight, size, pitch, speed, etc) without making direct, side-by-
side comparisions.  So, this physics doesn't seem to explain the
effect that you--and many others--seem to feel.

..Richard E+17

 
 
 

Hot Air = Big Sails?

Post by Kirk Lindstr » Wed, 18 Nov 1992 03:35:34

Quote:

>This one may be a bit off the wall but should appeal to you physics types
>out there. I do most of my sailing in Singapore. Our trades season just
>started yesterday & usually lasts till about end Feb/March. Singapore is
>sort of 100 miles north of the equator (hot, in other words).

>I've noticed that for a given wind speed, I use much bigger sails here than
>I do in Maui or elsewhere. In marginal conditions, 7.5s & 8.0s are not
>uncommon among the bigger sailors here. I've had some experience sailing
>in the UK and that seems to reinforce my opinion that air density has
>something  to do with this. Does it make sense? Should air temperatures
>affect density & thereby sail size? Note this is based on purely intuitve
>notions, I haven't been running around measuring wind speeds etc.

>Sukhdev Singh

----------
In Maui, people use smaller sail sizes for a given wind speed than we do
in the Bay Area.  The best I can figure is that the wind is much more
consistant.  I sailed Singapore one afternoon last February and i think
the wind speed reported is closer to peak rather than average.  It felt like
a 5 knot day with gusts to at best 12 knots.  I was on a rented pig with a soft
sail, old boom, no foot straps and "tie some rope to the boom" harness lines
and "sailed" (even slogged is kind) to the ships in the harbor.  The police
boat came out and gave me a good look-over (I guess to make sure I wasn't
smuggling *** in my harness, shorts, booties or helmet).  I waved and turned
back to shore.  It was like sailing in bath water!  Boy was that nice!

Kirk (hope to try again with some "real" Singapore wind) out
=> "We are what we pretend to be."  -  Kurt Vonnegut Jr.
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|   Engineer/Scientist, Hardware     |                                |
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Hot Air = Big Sails?

Post by Henrik Klagg » Thu, 19 Nov 1992 03:40:57

I might add that although humans are warm-***ed,  holding on to that
4.0 MegaSpeed in a frigging force-ten moving in from the atlantic at 8
degrees celsius is just like operating at the lower end of your usable
survival range. Reaction speed and power output simply seem to drop.

Cheers,
  "life begins at 40 knots" Henrik

 
 
 

Hot Air = Big Sails?

Post by David » Fri, 20 Nov 1992 09:51:52

Thanks for the comments. Actually, this issue really came to a head last Feb
when I entered a PBA event at Kuantan, Malaysia (150 North of Singapore).
There was no wind the entire event (unseasonal for Kuantan in Feb,it usually
blows 15-25knts daily at that time). The race officials anemometers (cup type)
did indicate about   9-11knts but nobody was planing much (btw it was blazing
hot!). This of course set off interminable arguments about the effect of
heat on wind...all inconclusive of course. In the end, no races were run as
the wind never hit the slalom minimum. The prize money was shared out (easiest
US$1200 that I've ever made).

Sailing in these conditions does have its upside...falling in is actually a
joy. (Kirk, those police boats are a very welcome sight when you are drifting
offshore with broken gear, they patrol mainly to make sure that ships don't
do any dumping of junk,gunk,oil etc in Singapore waters).

Sukhdev "still borrowing this id" Singh
MY3
Hotworks, Neil Pryde

 
 
 

Hot Air = Big Sails?

Post by Mitchel A. Sta » Fri, 27 Nov 1992 08:12:07

Wouldn't the wind speed as measured by a "cup-type" anenometer be affected in exactly the same way by air density?  So in hot, humid weather the anenometer would read lower for a given wind speed than it would in colder weather, so you'd rig a bigger sail.  That would mean that measured wind speed should correctly determine sail size, regardless of temperature and humidity.  I'd guess that most anenometers depend in a similar way on air density.  So would a sailor's estimate of wind speed which is based on

 whitecaps, blowing sand, etc.

--
Mitchel Stark   |\       Santa Rosa, CA
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