8

Waves

Types of waves
Ripples When the wind is light, ripples appear on a smooth water.   Once the wind is gone, so are the ripples.  If the wind gains on strength, ripples will grow into seas.
Seas These are larger waves when the wind is stronger, and they don't go away when the wind stops.  Instead they develop into swells.
Swells Large waves that are not related to the local wind conditions, they move away from their area of origin.

There are three factors that affect the formation of the waves:

  1. Wind strength: measured in knots or on the Beaufort scale.
  2. Duration: amount of time (in hours) that the wind has been blowing over a certain body of water.
  3. Fetch: distance of open water over which the wind has been blowing.

For each combination of the three, there is a maximum wave height. 

There are many guides to tell you the factors, among them is the American Practical Navigator by Nathaniel Bowditch.  You can find it there on page 444.

Some basics

The following diagram should help you understand a typical wave form:
  • The top of a wave is called a crest.
  • The bottom of a wave is called a trough.
  • Wave length is measured from crest to crest.
  • Wave height is measured from trough to crest.
  • The time interval between the passage of successive crests is called a period.
  • Wave is always steeper near the crest than near the trough.

waves.gif (1641 bytes)

Waves can also be divided into breaking waves and nonbreaking waves.   Nonbreaking waves have orbital motion at the crests, and breaking waves have a lined motion (the breaking crests go down at a steep angle).  The breaking waves can be damaging to a small boat.  Usually, the breaking waves are more common along the coastline, while the nonbreaking waves are common on an open ocean.  However, depending on a weather the waves can differ considerably.

Weather forecasts

Weather forecasts will give you a "significant wave and period".  What does it mean?

Even when you are sailing in sustained winds, the waves won't be uniform.  Some will be larger, some smaller.  The significant wave height and length is a system consisting of those differing waves.  It is determined by averaging the highest one-third of all the waves.  Therefore, if you hear that the wave heights are 10 feet, you can be sure that you'll see some waves nearing 13 feet, and others below 6 feet.   The following is the relationship between the significant wave height and some other heights:

Type of a wave height Relative height ratio
Average

0.64
Significant

1.00
Highest

1.87

If you are uncomfortable with the wave heights, you can "reduce" them by limiting fetch.  In other words, you can choose to sail closer to the shore.   The tables in Bowditch (the book I quoted earlier) will help you find the distance from the shore with wave heights suitable for your preferences.  There certainly is a limit to this, but it's very good to know what you are up against, and whether you can make it less stressful.

Another place where you will find important information regarding wave heights and periods are real-time buoy reports.  The website for all US weather buoys is www.nws.fsu.edu/buoy. 

Causes of abnormal wave forms

If a storm is moving in slowly, or is stationary, the waves are able to move away - this is where the Bowditch tables are true.  There are, however, moments when the wave's periods are shorter.  It's usually the case with a fast moving storm.   The waves simply don't have time to move away, and their periods are shorter as a result.  Sometimes the waves pile up before a fast moving storm, forming at times a prominent wall of waves, especially before a hurricane.

Another cause is shallow waters (less than half a wave's length).   The wave's forward movement is slowed by less room, where the wave takes on a different shape, and cannot move as fast. (* For those who have already seen this lesson, please note that it's not bottom friction as I've indicated earlier, but less room - I was corrected by Mr. Fabrice Ardhuin, who is a wave specialist - my thanks to him for helping me out!)  This often results in a wave to become unstable and turn into a breaker.  The phenomenon of the increase of the wave height on shallow waters is called shoaling, and if the waves form a wall of breakers, they're called a surf (yes, surf board and surfing derive from this name).  There are times when the waves pass through a shallow water and don't break - it's common after large storms.  It is then called a ground swell.

Mr. Fabrice Ardhuin has also indicated that another reason for formation of abnormal wave forms is waves traveling on opposing currents, that can lead to "freak waves" (the east coast of South Africa, in the Agulhas current is notorious for them).

The last common cause of abnormalities is the fact, that depending on the weather, the waves from different wave systems cross each other at different angles.   The troughs and crests of the waves can then cancel or amplify each other, producing flat areas, but also large seas.  It happens because the leading waves slow down as they multiply - produce flats, and the waves behind catch up - producing larger seas.

You can find average seas information (for the US waters) at www.nws.noaa.gov. 

Lesson 9: "VHF Channels (Canada)"

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