and gradient are universally existing natural phenomena
caused in part by the earth's boundary layer.
We define wind shear as a change in wind direction with
altitude, and wind gradient
as a change in wind speed
with altitude. Shear and gradient are ALWAYS
present, but not always
to a noticible extent. In the spring (during calibration season), with
over cold water, the effects can become quite pronounced.
If you don’t believe in wind shear, the next time
tacking upwind on a nice sunny calm spring day, take a look at the
each tack and you’ll see what we’re talking about. You can also deploy the 'ShearOmometer'.
Wind shear changes wind angle (both apparent and true) but it does not
invalidate the wind direction solution. Because the top
of the mast is pointed in the same direction
and going the same speed as the hull, the solution
However, wind shear prevents tack-to-tack
comparison of wind angle and boatspeed.
|The photo on
the right is of a 64’ boat on the east cost with
well-calibrated instruments. Note that heading and wind direction are
both 288º (i.e. zero true wind angle), and yet the boat is
trucking along at over 9 knots! The jib is trimmed on port, but the top
is plastered against the mast.
Not a good day to calibrate.
There are a lot of non-believers out there, who, once
they get it; realize that knowing when the instruments
tell them that wind shear and gradient are present, use the information
to help them control the boat better.
The effect of
wind shear and gradient
Wind shear has a major effect on boat performance; a few
degrees can cause considerable change in performance tack to
tack. When the wind is twisted, the sails have to be trimmed
each tack in order to get the best performance. Boatspeed, heel angle
and helm feel
will also be different. Unless the
underlying cause is understood and factored into the way the boat is
handled, the crew can become frustrated.
For the "fat" tack (wind twisted to weather), sail tops must be let out
and jib cars moved aft. Boatspeed will be above
target. On the "skinny" tack, the opposite holds true. For
high gradient situations, you will not be able to meet your target
speed. Once you have determined your new target, you should factor it
in when going onto the new tack.
Wind gradient (change in wind speed with altitude) is more subtle. It
generally manifests itself as the inability to meet
your target speed, sometimes dubbed "popcorn wind".
Instruments provide the means for discovering and quantifying wind
shear and gradient. The amount of shear can be determined by comparing
true wind angle (not wind
direction) tack to tack.
There's more shear
can tell you
Shear indicates wind direction aloft that will eventually mix down to
the main part of your sailplan. This means that it predicts the future
wind direction. Knowing the shear gives a clue about which way to
go to be on the favored side of the fleet when the new wind
arrives. Here's how
to find out how much shear you have.
Adjust out wind
shear or not?
Many instrument systems have a means to "adjust" out shear (Ockam
included). Many sailors think they should use this adjustment to
force wind angle to be the same on both tacks. However, this masks a valuable piece of
information that can be used to improve performance. Instead, the shear
should be determined and used to guide
the proper sail trim and helmsmanship. When you know boatspeed will be
low and by how much, and that the helm will feel dead, and that the car
should be placed there,
bliss will return, and performance improves.
The WTP apparent wind display is actually back-calculated from
their true wind solution. They do this because they want the
adjustments applied to the true wind solution to "fix up" the apparent
wind. In this process, any wind shear and gradient information is
obliterated. The sails see the apparent wind that the masthead
sensor sees; retro-apparent is a fiction that prevents you from knowing
the sails actually see.