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INTRODUCTION
Any time
you picture a body of water larger than a puddle, one of
the first things that comes to mind are the rolling waves,
or the surf lapping at the shore. Well, at least that's
what happens with me... But waves are much more than
tranquil anomalies, they can be dangerous and destructive
to any vessel in almost any body of water. Understanding
how waves are made, how they normally behave, and how to
predict their response to weather can make the difference
between a smooth, comforting time on the water and a
bumpy, frightening experience.
The first rule of waves, especially in the open ocean,
is that there are no rules. Kind of a hypocritical
statement considering the intent behind this article, but
it is a hard, cold fact. There are simple physical factors
that makeup the "normal" wave, but within the
forces of nature, there a myriad of other factors that
need be considered into the equation. Regardless, an
understanding of what makes a "textbook" wave
can be of considerable merit to the sailor. What we will
examine here are the laboratory examples of wave creation.
How, in a perfect world, waves would behave. In reality,
alternating weather patterns, varying water depths,
opposing currents, fetch obstruction and a multitude of
other factors may change the way waves in a particular
area react.
ANATOMY OF A WAVE
Keeping all
of that in mind, we'll throw out reality and concentrate
on theory for a few minutes here. There are three factors
that make up waves:
- Wind speed
- Length of time the wind has blown
- Distance of open water that the wind blows over;
called fetch
All of these factors have to work together to create
waves. The greater each of the variables in the equation,
the greater the waves. Waves are measured by:
- Height (from trough to crest)
- Length (from crest to crest)
- Steepness (angle between crest and trough)
- Period (length of time between crests)
There are theoretical limitations, however, for each
variable. If there is a limited fetch, say 10 nm to land,
and the wind is blowing at 36 knots, the waves will be 7'
high no matter how long the wind blows. Whereas for a 36
knot wind with an unlimited fetch blowing for 56
hours can create waves of 63'.
|
Wind
|
|
Wave
|
|
|
Speed (knots)
|
Fetch (nm)
|
Height (feet)
|
Length (feet)
|
|
30
|
20
|
7.2
|
183
|
|
30
|
50
|
10.5
|
270
|
|
30
|
200
|
13.5
|
360
|
|
40
|
20
|
12
|
264
|
|
40
|
50
|
18
|
423
|
|
40
|
200
|
24
|
392
|
|
50
|
20
|
18
|
382
|
|
50
|
50
|
30
|
570
|
|
50
|
200
|
42
|
719
|
The table
above demonstrates the relationship between wind speed and
fetch.
Both in
theory and in reality, waves are never created in one
uniform height. Waves fall into a systemic pattern of
varying size. Therefore, in order to classify wave height
we determine the significant wave height, which is
the average of the highest 1/3 of the waves in a system.
This is how weather reports will specify wave height. Once
you have the significant height, it is simple to determine
the theoretical average height, the highest 10% and the
highest wave sizes in a given area. Mathematically
speaking, it's simple arithmetic based on predetermined
ratios:
|
Average height
|
.64:1
|
|
Significant
height
|
1:1
|
|
Highest 10%
|
1.29:1
|
|
Highest
|
1.87:1
|
To
determine any one of the wave sizes, take the significant
height and multiply it by the numerator in the applicable
ratio. For example, if the significant height is 10', the
average height is 6.4' (10 x .64),
the highest 10% of the waves will be 12.9' (10 x
1.29), and the
highest waves will be 18.7' (10 x 1.87 ).
We can also determine the speed or period of a wave
mathematically by multiplying 1.34 times the square root
of the wave's length. [This number originates from many
years of scientific research on wave speed.] Hence:
- A 40' wave travels at 8.48 knots (
1.34 x sqrt of 40 [6.33])
- A 50' wave travels at 9.48 knots (
1.34 x sqrt of 50 [7.07])
- A 100' wave travels at 13.4 knots (
1.34 x sqrt of 100 [10])
TYPES OF WAVES
Waves take
their time to develop; they don't spontaneously erupt from
the ocean. It takes a certain speed of wind to blow over a
certain distance for a considerable length of time to
create lasting waves.
There are three different types of waves that develop
over time:
Ripples appear on smooth water when the wind is light,
but if the wind dies, so do the ripples. Seas are created
when the wind has blown for a while at a given velocity.
They tend to last much longer, even after the wind has
died. Swells are waves that have moved away from their
area of origin and are unrelated to the local wind
conditions -- in other words, seas that have lasted long
beyond the wind.
The definition of swells can be a bit confusing when
you understand that waves never actually go anywhere.
The water does not travel along with the waves, only along
with the current -- two mutually exclusive elements of
water animation. If two people stand at either end of a
long rope and undulate their arms up and down in an equal
rhythm, waves will develop along the length of the rope
that appear to move from one end to the other. The
rope fibers aren't actually moving at all, other than up
and down. This is exactly what is happening with waves.
The speed, or velocity of the wave is measured by how long
it would take a wave to pass a given point crest to crest
-- say a line drawn on the ground beneath the rope. There
is a slight movement of the water particles within a wave,
but we'll get into that in a little bit.
Waves can be further described as:
A non-breaking wave, is a "normal" rolling
wave. A breaking wave is one who's base can no longer
support it's top and it collapses. Depending on the size,
this can happen with considerable force behind it -- 5 to
10 tons per square yard. Enough force to crush the
hull of a ship. When the ratio of steepness of a wave is
too great, it must break. This happens when a wave runs
into shallow water, or when two wave systems oppose and
combine forces. The steepness ratio is expressed as the
height to the length. A 1:24 is a long, shallow swell
found in deep waters. A 1:14 and up is a wave that is too
steep to stay together. This can also happen if the wind
quickly grows strong and actually blows the top (crest)
off the base of the wave. Wave characteristics also change
in shallow water. Imagine if the rope that we talked about
earlier was lowered to the ground so that the troughs of
the waves hit the floor. This gives you some idea what
happens when a wave hits shallow water, only the height
and period won't change, just the length and hence the
steepness (as the length changes, so does the height to
length ratio). Once the ratio gets high enough (like
fractions, the closer together the numerator and
denominator, the higher the fraction -- 1:1 is the highest
[that would be a wave at a right angle with the length
exactly as long as the height.]) the wave will break.
|
Water Depth
(feet)
|
Wave height
(feet)
|
Wave length
(feet)
|
Period
(seconds)
|
|
150 +
|
15
|
360
|
8
|
|
75
|
15
|
270
|
8
|
|
30
|
15 (breaks)
|
210
|
8
|
|
15
|
15 (breaks)
|
153
|
8
|
WATER MOVEMENT
Water
particles within a wave have different patterns of
movement based on whether it is a breaking wave or not. In
a normal wave, there is an orbital movement of the water
particles. This is best demonstrated by a cork floating in
the water. As the wave rises, the cork spins in place
(pushed by the orbital motion). This is a very passive
movement, whereas the lined particle movement of a
breaking wave is very aggressive -- hence much more
destructive.
The images above demonstrate the orbital motion of a
cork floating in the water as a wave passes from right to
left. The cork's position actually never changes other
than a slight rotation.
WIND AND WAVES
The
interrelationship between the wind and the waves is so
important to skippers that a completely new classification
system was designed as a guideline incorporating both wind
speed and the wave conditions most readily found at those
speeds. This system, called the Beaufort Scale, was
developed in 1805 by Admiral Sir Francis Beaufort of the
British Navy. It is a guideline for what can be expected
in certain conditions and a weather classification system.
It assumes open ocean conditions with unlimited fetch.
|
Force
|
Wind
Speed
|
Description
|
Sea
Conditions
|
Waves
|
|
0
|
0
|
Calm
|
Smooth,
like a mirror.
|
0
|
|
1
|
1 -
3 knots
|
Light
Air
|
Small
ripples, like fish scales.
|
1/4'
- 1/2'
|
|
2
|
4 -
6 knots
|
Light
Breeze
|
Short,
small pronounced wavelettes with no crests.
|
1/4'
- 1/2'
|
|
3
|
7 -
10 knots
|
Gentle
Breeze
|
Large
wavelettes with some crests.
|
2'
|
|
4
|
11
- 16 knots
|
Moderate
Breeze
|
Increasingly
larger small waves, some white caps
and light foam.
|
4'
|
|
5
|
17
- 21 knots
|
Fresh
Breeze
|
Moderate
lengthening waves, with many white caps
and some light spray.
|
6'
|
|
6
|
22
- 27 knots
|
Strong
Breeze
|
Large
waves, extensive white caps with some spray.
|
10'
|
|
7
|
28
- 33 knots
|
Near
Gale
|
Heaps
of waves, with some breakers whose foam
is blown downwind in streaks.
|
14'
|
|
8
|
34
- 40 knots
|
Gale
|
Moderately
high waves of increasing length and edges of
crests
breaking into spindrift (heavy spray). Foam is
blown downwind
wind in well-marked streaks.
|
18'
|
|
9
|
41
- 47 knots
|
Strong
Gale
|
High
wind with dense foam streaks and some crests
rolling over.
Spray reduces visibility.
|
23'
|
|
10
|
48
- 55 knots
|
Storm
|
Very
high waves with long, overlapping crests.
The sea looks white, visibility is greatly reduced
and
waves tumble with force.
|
29'
|
|
11
|
56
- 63 knots
|
Violent
Storm
|
Exceptionally
high waves that may obscure medium size ships.
All wave edges are blown into froth and the sea is
covered with patches of foam.
|
37'
|
|
12
|
64
- 71 knots
|
Hurricane
|
The
air is filled with foam and spray, and the sea is
completely white.
|
45'
|
Aside from
just wind speed, the temperature is also a factor in
creating waves. Warm air (which rises) moving over water
has a less acute angle of attack on the surface than does
cool air (which sinks). A cold front moving across open
water will create much steeper waves and hence create
breakers sooner than a warm front moving at the same
speed.
Also, a change in wind direction over existing waves
can create confusion and hence larger waves. If a wind has
been blowing northeast over an open body of water for
three days and suddenly switches to northwest over that
same body of water, new wavelettes will form within the
existing system of waves. The energy of both systems will
multiply to create larger waves.
When a wave system meets a current flow one of two
things can happen. If the wind and current are both going
the same direction, it tends to smooth out the waves,
creating long swells. If the current and wind are moving
in contradicting directions, it will create much steeper
and more aggressive waves.
MAKING SENSE OF THE MUMBO JUMBO
So, what
does all this mean? Why is it important to know how waves
are made? Well... You can determine several things from
waves.
One of the things you can tell based on waves, is boat
speed. This assumes that your vessel is a displacement
ship, like a keelboat, and not a planing one like a
speedboat. When sailing a displacement vessel, the boat is
constantly displacing a large chunk of water as it moves
along. The heavier the boat, the deeper the trough it
carves through the water. Now, along with the physics of
waves we discussed above, we can add that the faster a
wave travels, the longer it is. As a boat's speed
increases, the number of waves that it pulls along the
hull decreases until the boat is actually trapped between
the crest and trough of a single wave that it has created
itself moving through the water.
We know, from above, that the speed of a wave can be
determined by the formula of 1.34 times the square root of
the wave length. Since a displacement boat, traveling at
top speed, is trapped in between the crest and trough of
it's own wave, we can also determine theoretical hull
speed with this same formula using the boat's L.W.L
(Waterline length or length on waterline).
[It's important to note
here that L.W.L. is not the same as L.O.A. (Length
overall) which is what most people use to describe a
vessel. A 22' Catalina (22' being it's L.O.A.) does not
necessarily have a 22' L.W.L. Check out the Boating Safety
Course Chapter
3 for more information on L.W.L. and L.O.A.]
A boat with a L.W.L. of 30'
has a theoretical hull speed of 7.34 knots. (1.34 x sqrt
of 30 [5.48]) Now, very rarely will a boat ever reach her
theoretical hull speed. This happens in the most perfect
of conditions. However, since at that top speed the boat
is trapped in a single wave, at lesser speeds, there will
be more waves along the hull; proportionately so. If there
are two waves on the windward side of the vessel then the
boat is traveling at 1/2 theoretical hull speed. If there
are three waves, then the boat is traveling at 1/3 that
speed. You can fairly reliably judge your boat's speed by
counting the number of wave crests on the windward side
between bow and stern and divide that number into your
theoretical hull speed. Using this method, you can create
a Dead Reckon plot without a speedometer.
Theoretical hull speed (trapped in one wave)
1/2 Hull Speed, two waves along the windward side
1/3 Hull speed, three waves along the windward side
You can also quickly spot shoals by watching the waves.
A shallow area will create breakers in the middle of
otherwise normal seas -- this can help you further
estimate your position by finding the shallow area on your
chart and taking a bearing to that area.
Understanding the relationship between wind speed and
fetch can also help you plan your trip and avoid
uncomfortable situations. Limiting the fetch you sail in
will limit the maximum size of waves that you encounter
for a given wind speed, and thus further ensure that you
don't encounter a situation beyond your experience level.
|
Wind speed
|
Theoretical
Max Wave Height
|
50 % Fetch
|
75% Fetch
|
100% Fetch
|
|
7 - 10 knots
|
2 feet
|
3 nm
|
13 nm
|
25 nm
|
|
17 - 21 knots
|
8 feet
|
10 nm
|
30 nm
|
60 nm
|
|
28 - 33 knots
|
20 feet
|
22 nm
|
75 nm
|
150 nm
|
|
41 - 47 knots
|
40 feet
|
55 nm
|
150 nm
|
280 nm
|
|
56 - 63 knots
|
63 feet
|
85 nm
|
200 nm
|
450 nm
|
By limiting
the percentage of fetch (based on the theoretical max) you
can considerably limit wave height that you will
encounter. The theoretical max wave height above is based
on an unlimited fetch and wind duration. By selecting a
fetch in one of the three columns to the right of that
max, you can adjust the theoretical max by that
percentage. For example, in a 17-21 knot wind, with a 60
nm fetch and an unlimited wind duration, you can encounter
8 foot high waves. If you limit that fetch to 10 nm at the
same wind velocity, you will encounter 4 foot high maximum
waves. (50% of 8 feet ).
You can determine wind direction by watching the
ripples on the surface of the water (Swells may be running
contradictory to current winds). Keep an eye out for
shifting winds this way as the smaller waves are the
greatest natural indicators at sea of wind direction.
Also, using the Beaufort Scale, you can roughly determine
wind speed based on the wave conditions -- for instance,
white caps generally form at around 12 knots of wind.
KEEPING YOUR SHIRT DRY (HANDLING WAVES)
One last
area to cover before we close this opus, and that is how
to handle waves encountered at sea. Whether large rolling
swells, or choppy breakers, the surface activity at sea
can be one of a skipper's most challenging obstacles. The
following are some guidelines on how to deal with waves.
In order to avoid big waves:
- Avoid shallow water. Not only does shallow water
create breakers that move at more destructive and
higher velocity than "normal" waves, but due
to the sinusoidal movement of a wave, the actual
nominal sea level is hard to determine. In reality,
the nominal sea level is slightly below the center of
trough and crest. In shallow water, you are more
likely to run aground, even if the chart says you're
okay. Not to mention this can be a bumpy ride.
- Don't go upwind in big waves. This particularly
applies to sailors who when tacking upwind will be
crossing large waves at a relative angle of 45º to
the bow. For sailors working upwind with escalating
winds, plan to make your upwind progress before the
winds build and sail downwind with the large waves
later.
- Use land as a natural breakwater. Sailing in the lee
of an island will create smaller waves as the wind
comes off land and then toward your vessel.
- If the waves are so big that you can feel them
pushing your boat sideways, or backward -- turn into
the wave. Try to hit the crest at a perpendicular
angle and head off again just as the crest reaches the
bow. This minimizes the surface area that the waves
can push upon. Sailors be sure to keep steerageway by
heading off once you're on the crest.
- You can estimate the height of waves by knowing your
eye height above the water. If, when standing at the
helm, your eye is 10' above the surface of the water,
waves just at the horizon line will also be 10' in
height.
- When sailing in big seas, the true challenge is to
pay attention. In reality you can never predict what a
wave system will throw at you. Waves can suddenly come
up from sideways, or a large threatening wave may pass
quietly while a small one might break violently into
your boat. Being alert and reactive is your best bet.
Don't concentrate on one area too much, but rather the
whole picture.
- Watch the weather reports. Not just for the day you
are planing to sail, but the week before as well.
Remember that a sudden shift in wind pattern contrary
to what was happening for several days can create much
larger waves in that area. You may be looking at a
great fishing spot and NWS tells you that there are
only 10 knot winds from the southwest there, however,
what you don't know is that the winds were blowing 10
knots from the northeast for the past five days --
hence the significant wave height in that area is
double what it should be for the conditions...
You can get real time wave reports over the internet
from the National Weather Service at: www.nws.fsu.edu/bouy
Above is an example buoy report from the internet. It
clearly shows the date, time, sky conditions, weather
description, temperature, wind direction, wind speed,
speed of wind gusts, water temperature, significant wave
height, wave period and visibility in miles.
Once you have a better understanding of wave systems,
it can make your time on the water much more enjoyable.
Being able to maintain a slight degree of control over
what you will face while venturing out will help maintain
your comfort level. Whether out for a short jaunt or a
circumnavigation adventure, take care, keep a keen eye and
enjoy.
Jay Holben is a Director/Cinematographer living in Los
Angeles. Fairly new to skippering a sail boat, Jay is a
recent graduate of BoatSafe.Com's Boating Safety course
and the Coastal Navigation Course. Jay is also a freelance
writer for American Cinematographer magazine.
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