View Full Version : air temperature, wind speed and choice of sails
28th May 2013, 07:30 AM
I go Windsurfing at temperature of air from +3C to +40C
my NP 8.6 on Isonic 121 at 12 m/s, felt comfortable at +35C
but when +6C I feel overload. This objectively ?
28th May 2013, 10:25 AM
Air density decreases with temperature. For a given windspeed you need smaller sails in winter. From 35 to 5°C it goes from 1,14 to 1.27 so you get an equivalent "push" with a 7.7 sail (roughly).
Air density also drops with altitude. I've seen people windsurfing at lake Mont Cenis in the Alps (~2500m) with very large sails though it was quite windy. They told me that otherwise they woul not plane at all.
28th May 2013, 07:14 PM
Google the mass flow of air and you'll get it
28th May 2013, 07:52 PM
Wow, in 12m/s = +/- 23 knots, I'm on my wave board + 4.7 to feel comfortable and surely not a 8.6 nor 7.7.
Or is something wrong with my m/s - knots conversion ?
I assume that 12m/s is a peak value and that the water state is pretty flat.
28th May 2013, 08:15 PM
Nothing worong with your m/s - knot conversion. Fully cammed race sail have quite different caracteristics than wave or freeride sails and have a much larger wind spectre.
I sail with my 7,9m race sail (4 cams) in steady winds up to 12 m/s wind gusts up to 14-15 m/s (temperature from 10 deg to 20 deg C). Water is quite flat though.
29th May 2013, 10:22 AM
Farlo is spot on.
The power of a foil (windmill, fin, sail) is directly proportional to air density. From 35°C to 5°C, that power increases by roughly 10% (cold air is denser, heavier).
That 10% power surge might appear small but if wind speed and foil's efficiency are kept constant, the power of an 8.6 m2 sail (assuming a luff lenght of 500 cm) used at 35°C is roughly the same as that of a 7.7 m2 sail (assuming a luff lenght of 474 cm) used at 5°C. So, it's no surprise that you feel overloaded by using your 8.6 m2 sail at 5°C whereas you feel nicely overpowered at 35°C.
You can play around with numbers to calculate the power of a foil (i.e., a windmill) here : http://www.engineeringtoolbox.com/wind-power-d_1214.html
29th May 2013, 07:16 PM
What is more dense air 5 degrees 80% moisture or 30 degrees 80% humidity?
30th May 2013, 07:29 PM
Moisture shall be lighter than air to some extend, because clouds are in the sky water must go up there.
I'd bet cold air - either moist or dry - will be always heavier due to condensation (vapor replaced by mist).
Such formulas are misleading. With twice as much wind you should get the same traction from a sail four times smaller, which is obviously not the case. You may be comfortably powered with 8m² in 12 Kts but you will never fly with 2m² in 24 Kts. There are certainly other factors at play (like sail+sailor efficiency/drag ratio). A few years back there was a very good article from Jim Drake about balance of forces, but it is no more online.
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