Apparent wind is a very simple concept that continues to mystify
many people who have been sailing for years. It is the combination of
two winds: the one derived by the boat moving through the air and the
wind produced by nature—the true wind. Thus, it is the wind you feel
on the boat. Cigarette smoke, telltales, and the electronic wind
direction indicators of boats all show the apparent-wind direction. We
often get the comment from people on their first time out, "You said
we sail within 40 degrees of the wind when close-hauled, but the wool
on the shrouds indicates we're sailing almost into the wind." This is
their first experience with apparent wind on a sailboat.
Imagine yourself standing up in a convertible. It is a calm day, so
there's no true wind. As the convertible starts forward, you will
begin to feel a breeze on your face that increases as the speed of the
car increases. At 10 mph you will feel a 10-mph breeze on your face.
This is apparent wind.
Now imagine yourself in the same car heading north and there's an
easterly wind of 10 mph blowing. This nature-produced wind from the
east is what we call true wind. It is hitting the right side of your
face. As the car starts forward you will not feel two different winds,
one on the side and one on the front of your face, but a resultant
wind coming from an angle forward of the true wind. What you feel is
the apparent wind.
By drawing a parallelogram from the boat speed and the true wind,
you can determine the force and direction of the apparent wind. (Refer
to Figure 1.) Let's say your boat tacks through 80 degrees. That means
the true wind is 40 degrees off your bow. If, for example, the boat
speed is six knots and the true wind is 12 knots, measure off the
units as boat-speed wind and true wind in Figure 1. Then draw a
parallelogram, the diagonal of which is the apparent wind. By
measuring the length of the diagonal, you can determine the speed in
knots of the apparent wind.
In this example (see Figure 1), the apparent wind (the diagonal)
measures 17 knots and bears 27 degrees from your heading versus 40
degrees for the true wind. Notice how the direction of the apparent
wind changes with the true wind in the subsequent diagrams. (For the
purposes of these diagrams we'll keep true-wind speed and boat speed
constant, which would only be the case at different points of sailing
if the boats were different sizes.)
Four points are obvious from these diagrams. First, the apparent
wind is always forward of the true wind (unless the true wind is dead
ahead or astern). Second, as the true wind comes aft, the apparent
wind lessens in velocity. Third, when the true wind is well aft, a
small change in true-wind direction makes a large change in
apparent-wind direction. And fourth, when the boat is on a beam reach
or close-hauled, the apparent wind is of greater velocity than the
true wind.
The first point is important when considering when to jibe. Since
it is desirable to sail at a slight angle to the wind rather than dead
downwind, you may not be heading to your desired destination and will
have to jibe to reach it. It's important, therefore, to determine the
direction of the true wind and the angle your heading is making with
it. If you know you are steering 20 degrees from dead downwind on one
jibe then you will be on the same point of sail when you are 20
degrees from dead downwind on the other jibe. The point of jibing
should come when your destination bears 40 degrees off your bow from
your present heading. The key, of course, is determining the direction
of the true wind. By glancing at your telltales and at the wind signs
on the water—like streaks and ripples—you can judge about how far aft
of the apparent wind the true wind is.
A more positive way of determining true-wind direction is by
heading off momentarily until the apparent wind and the true wind line
up, i.e., dead downwind. The difference between the new heading and
your former heading, 20 degrees in the example above, when doubled (40
degrees) is the number of degrees through which you'll jibe.
The second point—as the true wind comes aft, the apparent wind
speed lessens—is obvious if you have ever seen power boats head
downwind. Sometimes they cruise along at the same speed and direction
as the true wind. Their exhaust hangs around the boat in an enveloping
cloud (one reason we sail!), and the apparent wind is just about zero.
This reduction of the wind speed you feel on the boat and thereby
the force of the wind on the sails can lull you into forgetting the
difference which becomes evident when you round a mark and start on a
beat. You may have started your sailing outing on a run, so you had no
idea of the apparent-wind strength on a beat. Or the wind may have
increased during the run. Either way you must consider the possibility
that you may have to shorten sail on a cruising boat when you come up
on a beat, and it is easiest to change jibs while still on the run.
Let's say a boat is going nine knots in a 16-knot breeze. If dead
downwind, the apparent wind is the true wind minus the boat speed, or
just seven knots. This doesn't feel like much wind and the force on
the sails is relatively light. When the boat starts beating, she may
slow down to six knots, but the apparent wind increases to almost 21
knots. You would assume that, since the apparent wind is now three
times greater than the downwind velocity, it exerts three times the
force against the sails. Wrong. The force of the wind quadruples
as the velocity doubles (the square of the velocity), so the wind
force is nine times greater on the close-hauled course than on
the run in this case. Couple this with the increased heeling moment of
the close-hauled course, and the boat may very well be overpowered. So
you should have the forethought to shorten the sail during the run.
The third point we made about the apparent-wind diagrams was that
when the true wind is well aft, a small change in the true-wind
direction makes a large change in apparent-wind direction. (Compare
Figures 2 and 3 with Figures 4 and 5.) In our example, a 30-degree
change in the true-wind direction made a 38-degree change in apparent
wind. In comparing Figure 4 with Figure 5, we find that a 16-degree
change in the true wind makes a 28-degree change in apparent wind.
This, among other things, is what makes steering dead downwind so
difficult. A small swing by the lee actually results in an exaggerated
swing of the apparent wind by the lee. This can cause oscillation as
the apparent wind swings madly back and forth from one side of the
boat to the other on relatively minor changes in heading. At worst, it
can force an inexperienced helmsman into a flying jibe.
The fourth point was that when a boat is reaching or
beating, the apparent wind is of greater velocity than the true wind.
You are, in effect, "making your own wind." In iceboating this is an
important part of the resulting high speeds. Some iceboats, for
instance, can reach speeds five to six times the speed of the wind and
can attain speeds of 120 knots in 24 knots of wind. The faster the
boat goes, the higher the wind velocity it creates. Only because of
the lack of friction can these high speeds be attained. A normal
sailboat is limited in speed by hull resistance, skin friction, and
wave-making drag, so it cannot take full advantage of the increased
apparent-wind velocity. A planing sailboat is more apt to get up on a
high-speed plane while on a reach rather than a run just because of
this apparent-wind increase. Even so, the faster a boat is to
windward, the more close-winded (able to head close to the wind) it
must be.
In the first set of wind diagrams, everything remains constant
except the direction of the true wind, which moves farther aft in each
subsequent diagram. Now let us change the boat speed and the wind
velocity, keeping the true-wind direction at 45 degrees off the bow.

Figure 6:
Study this diagram and you'll
begin to see that in a puff, the apparent wind comes aft. |
Notice in Figure 6 that the wind speed was initially 10 knots and
the boat speed four knots. The dotted extension of the true wind line
indicates a four-knot increase or puff. So we see a basic axiom: "In a
puff, the apparent wind comes aft." To be correct, this necessitates a
constant speed on the boat's part. Generally, however, by the time the
boat picks up speed the puff has passed, so the axiom holds true.
We already know that we point higher in order to reduce heeling
when hit by an overpowering gust. This axiom shows another reason to
do the same thing. As the gust hits, the apparent wind goes aft
causing more heeling and less drive and changing the angle of
incidence—the angle the apparent wind makes with the sails—so that the
sails are now improperly trimmed unless you head up or ease the sheets
or traveler. This change in apparent-wind direction is important to
remember even on light days. When you have a three-knot breeze, the
wind velocity in a puff is apt to be more than double the regular
breeze. When it is blowing 15 knots, gusts may get to only 20 or 22
knots, or about a third higher. Thus, the change in apparent-wind
direction aft is often greater on light days than on heavy ones.
The dot-dash lines in the diagram show the resulting change in
apparent wind when the wind dies suddenly. With the boat speed
remaining constant and the wind velocity lowering to six knots, the
apparent wind goes forward. This is one of the reasons that small
catamarans rarely carry spinnakers. If they do carry a spinnaker, it
is a flat, asymmetrical sail much like a reaching genoa. The hulls
have very little resistance to the water, and the cats sail downwind
almost as fast as the wind, making it very difficult to keep a
spinnaker drawing. If the wind dies for a moment, the spinnaker
collapses and it is very difficult to get it filled until the boat
slows down. Therefore, small cats, much like iceboats, tack downwind
by jibing. By sailing from reach to reach, they pick up greater speed
and make up the extra distance sailed.