by Kenn Batt
Remember those race days when the trimmers had an awful time
trimming sails, the helmsperson, probably you, being so frustrated
because the boat felt better on one tack and not the other and the
tactician seemed to be out of phase with the shifts!! Then all other
things being equal you were most likely experiencing the effects of wind
shear.
What is wind shear you may ask? It is simply the change in wind
direction and/or speed with height. We can differentiate between
directional and speed shear, but generally speaking both occur
simultaneously and hence the term wind shear. We can also have
horizontal and well as vertical shear, but it is the vertical shear that
creates the most headaches for racing types. Be aware that the vertical
shear may vary rather dramatically in the horizontal!
How does wind shear occur? It all comes down to good old friction, the
closer you are to the surface of the sea or land, the slower the wind
speed and the more the wind direction will be veered (southern
hemisphere) in relation to the gradient or the friction-free wind. This
change in wind speed can be easily detected between the masthead and the
deck and even though the direction of the true wind changes ever so
little in the lowest 30 metres, it's the direction of the relative wind
or the wind felt by you and by the sails that can change quite markedly
between the deck and the masthead. It all comes down to the stability
nature of the air-sea(land) interface. Differences in speed range from
very little in unstable air (around 5%) to enormous amounts in stable
air (up to 300% ). From a directional point of view, differences range
from about 1 degree in unstable air to about 30 degrees in stable air.
So the basic rule at this stage is that you need more twist in your
sails on stable days and least twist on unstable days. More about this
later.
It is wind shear and not changes to the air density that causes what
is known as the "weight of the wind". Even though the air density will
change with a change in the air temperature, the change in the actual
value of the air density will be very small. It is the variation in wind
speed and hence the wind shear between the masthead and the water
surface that is largely responsible for this "weight" phenomenon.
When it occurs, it is this variation in wind speed that can lead to a
huge difference in the heeling moment of the boat. In stable air there
will appear to be more "weight in the wind" since there will be
generally a stronger wind speed at the masthead than at deck or sea
level. In unstable air, there is generally good mixing or overturning
taking place in the boundary layer so the wind speed is fairly constant
between the masthead and deck or sea-level and hence
the heeling moment
is less.
What's all this stability stuff? In a nutshell it all comes down to
the temperature difference between the sea and the air sitting on it! If
the temperature of the water is colder than the air then we have a
stable sailing layer. On the other hand, if the sea temperature is
warmer than the air temperature then we have an unstable sailing layer.
When the sea temperature equals the air temperature then we have a
neutral sailing layer. Simple eh!? Not really but this will do. I will
recommend further in-depth reading on this matter later.
Apart from temperature considerations, wind shear will generally be
associated with a developing sea breeze situation. It can be very marked
whilst this breeze is building and will drop away to nothing once the
breeze has reached maturity. Shear can also be marked when one is close
to a towering cumulus or cumulonimbus (thunderstorm) cloud and also if a
cold front is close by.
The direction of the wind shear (directional shear) can be
indicative of the direction of future shifts (permanent) and speed shear
by itself, will tell you that you can expect oscillating wind shifts.
What are the signs of wind shear? Sail trim will vary from one tack
to the next. The headsail trimmer has "a dog of a time" getting the luff
tufts to break evenly and the mainsail trimmer has a horrible time
getting the twist right. The helmsperson is very frustrated because the
boat feels better and faster on one tack (generally port) than the other
(generally starboard). The person calling the shots is probably out of
phase with the wind shifts. You will notice big differences between the
wind speed measured at the mast head to those sensed by you at deck
level. This all adds up to one lousy day on the water!!
What's happening here is that the top of your sails are being lifted
on one tack (generally port) and headed on the other (generally
starboard). The sail tufts will react very differently from one tack to
the other.
What are the remedies? Before the race attempt to measure the actual
air and sea water temperature. If it works out that shear will be a
problem, be vigilant and be aware of the signs and be prepared to make adjustments
very quickly. On the lifted tack you will have to have more
twist in your sails. On the headed tack your sails will require less
twist. Jim Marshall from Ockam suggests that "one should be very careful
not to eliminate too much twist. Lack of twist is a sure way to slow
the boat down too much." Be prepared to adjust your targets from tack to
tack since the boat will be faster on one tack than the other. Also be
aware that your instrument readings, especially the apparent wind angle,
will be "over the top". In the case of the apparent wind, it will be
very wide on the lifted tack and very narrow on the headed tack.
Jim Marshall also suggests that you note the following: "The
Velocity Prediction Program (VPP) assumes no wind shear and an average
amount of wind speed increase with height when it calculates polar
performance. However, the actual amount of wind shear (directional and
speed) and therefore the effective wind angle and the total amount of
force the boat is feeling from the wind field can vary dramatically from
the program's assumptions. Therefore when you recognise the presence of
wind shear, you need to compensate for its effects".
I trust that this article has alerted you to the problems that are
associated with wind shear. I recommend that you get your hands on a set
of Ockam-U course notes (Jamie McPhail is the agent here in Sydney) for
further in-depth reading on this topic as well as many others. These
notes are excellent and have helped me write this. The North-U-Fast
notes by Norths, Weather at Sea and Wind Strategy, both by David
Houghton, and available from Norths and Boat Books respectively, are
also great sources for further reading. Remember that these publications
have not been southern hemispherised, apart from Wind Strategy, so make
the necessary adjustments!!
NB wind speed and direction sensors are usually at the masthead so
there will be a big difference at times between what you read on the
instruments to what you sense coming across the deck. You can place more
faith in your instruments during unstable conditions and least faith
during stable conditions.
Smart sailing!!
References:
Houghton, D and F. Sanders; 1988: "Weather at Sea", International Marine Publishing Company.
Houghton, D; 1992: "Wind Strategy 2nd Edition", Fernhurst Books.
North Sails Inc; 1990: "The North U. Fast Course", North Sails Inc.
Ockam Instruments; 1989: "Ockam U", Ockam Instruments Inc.
© Copyright Commonwealth of Australia 2008, Bureau of Meteorology (ABN 92 637 533 532)
