Wind speed difference

[Wayne]

Does anyone know the rough difference in wind speed between the tops of our masts and the water surface?

[Tom Soko]

Wayne,
I don't think there an absolute number or percentage difference.  From what I have read, at lower wind speeds, there is a greater difference between top and bottom of mast readings.  At higher wind speeds there is less of a difference.  I think it has something to do with wind being "attached" to the water surface, but I don't know the details.  The whole concept is above my pay grade.

[Tom Glennon]

About 48 feet!

[Stu Jackson]

There might be some info on this website:  http://www.arvelgentry.com/

Also suggest a google on sailing aerodynamics.

[dave davis]

The reference that Stu mentioned in the previous post about "arvelgentry" was fascinating. I could not understand any of it but it sure does blow away the old theories of how "lift" works on sails. It really says to forget all the old theories. It has nothing to do about the wind at the top of the mast vs the bottom. Fascinating! click on  "lift".
Thanks Stu for ruining my day
Dave

[Joe and Carol]

Long time ago I studied the relationship of wind speeds in the area of 50 to 100 feet above the ground to help land airplanes in various wind conditions.  Your question relates more to wind over water, so I offer the following answer:  Wind speed measured in heights above water increases logarithmically.  Heat produces wind, water produces humidity as well as heat, so the factor of air-sea moisture is a dominate factor in the relationship of wind speed at various heights above water. 

An equation for use is U(Z) -Us equals (U*/k[ln(z/z0) + Q(z,Zo,L)]     

Wind Speed relative to the surface speed as a function of height:  Friction (U*) is the square root of the kinematic stress and k is a constant.  ln(Z?Z0) is the logarithmic term and Q(Z,Z0 is the modification due to atmospheric stratification (L).  Atmosphere stratification can be neutral (Z/L = 0), no stratification and stability term is zero.  The friction velocity (U*) and the roughness length (Z0) are functions of wind speed, atmospheric stratification and sea state.      Note:  Just remember most surface water wind speeds are less in stable air and more in unstable air.

Simply put, one can notice more wind at the surface in rough than calm water (These are dramatic oversimplifications) or watching tree tops move when there is no apparent surface wind.  And we could think about the shape of sails, larger nearer the surface of water and smaller at the height of mast tops doing something to ease the results, if any, of different wind speeds from the base to the top of the mast?

Some sailboats read wind velocity at mast top height. The cups spin in as much clean air as possible to measure velocity.

So . . .why do you ask the question?  Is there something more here I can learn about sailing?

[Wayne]

Wow, that is quite a site.  I'm still reading--I just can't handle more than three articles at a time before my brain short circuits.  Really good information.
The reason I'm wondering is because my mast head reading wind instrument (I think) needs better calibration.  When white caps are just getting started on the water surface (about 10 kts, right?) my masthead is reading over 20 knots.  And my boat is sailing nicely on full sails, not up to hull speed yet.  So, I was hoping to get a rough guesstimate of the likely difference in wind speed between surface and the top of my mast, and at least adjust my sensor so that it is more in the ballpark.

[Ken Juul]

The easiest way might be to ask 3 or 4 of your neighbors the next time you are at the dock.  Take an average of their readings and adjust yours to match.

[Joe and Carol]

Some charts of wind profiles viewed showed with a calm wind of 3 knots at surface the mean wind speed at 16 meters height would measure 6 to 7 knots.  Hope this helps.

[Wayne]

Thats what I thought I had read somewhere . . . About a 4-5 knot difference in most conditions.
Thanks!

[Jon Schneider]

Wayne, one thing you might try instead of trying to compute the theoretical difference in wind speed between the sea surface and the masthead would be to use a hand-held wind meter and compare its reading to the apparent speed that your masthead wind instrument is telling you.  Of course, the two instruments won't be perfectly synchronized, but it'll give you a rough read of the gap in various conditions.  If you don't have a hand-held wind meter, stores such as RadioShack sell "non-marinized" ones for considerably less than you'd pay at WM. 

[Tom Soko]

Wayne,
Another thing you could do is to go out in a "no wind" condition.  Simply adjust your wind speed to your boat speed.