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User 28th May 2013 06:30 AM

air temperature, wind speed and choice of sails
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 ?

Farlo 28th May 2013 09:25 AM

Air density decreases with temperature. For a given windspeed you need smaller sails in winter. From 35 to 5C 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.

Marko CRO169 28th May 2013 06:14 PM

Google the mass flow of air and you'll get it

BelSkorpio 28th May 2013 06:52 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.

Unregistered 28th May 2013 07:15 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.


Jean-Marc 29th May 2013 09:22 AM

Farlo is spot on.

The power of a foil (windmill, fin, sail) is directly proportional to air density. From 35C to 5C, 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 35C is roughly the same as that of a 7.7 m2 sail (assuming a luff lenght of 474 cm) used at 5C. So, it's no surprise that you feel overloaded by using your 8.6 m2 sail at 5C whereas you feel nicely overpowered at 35C.

You can play around with numbers to calculate the power of a foil (i.e., a windmill) here :

Cheers !


Unregistered 29th May 2013 06:16 PM

What is more dense air 5 degrees 80% moisture or 30 degrees 80% humidity?

Farlo 30th May 2013 06:29 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 8m in 12 Kts but you will never fly with 2m 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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