Waves

Waves• A wave is a disturbance that propagates

through space and time, usually with transference of energy.

• A mechanical wave (eg. a water wave) exists in a medium.

• Waves travel and transfer energy from one point to another, often with little or no permanent displacement of the particles of the medium (eg. the water does not move, rather the shape of the water changes).

http://en.wikipedia.org/wiki/Wave

How do waves form?• Waves result from distant winds.

• Three factors influence the formation of "wind waves":

• wind speed, distance of open water that the wind has blown over (fetch), length of time the wind has blown over a given area.

• The greater each of the variables, the larger the waves.

http://en.wikipedia.org/wiki/Ocean_surface_wave

Dangerous Waves

• A non-breaking wave, no matter how high, will not capsize a conventional boat with good stability.

• Breaking waves are the dangerous waves.

http://www.cruising.sailingcourse.com/heavy_weather.htm

Breaking Waves

• Can easily capsize a boat if the height of the wave is equal to or greater than 60% of the length of the boat when hit end on.

• When hit beam on, the breaking wave has only to be higher than the width of the beam of the boat to capsize the craft.

http://www.cruising.sailingcourse.com/heavy_weather.htm

Breaking Waves

• A breaking wave is one whose base can no longer support its top, causing it to collapse.

• A wave breaks when it runs into shallow water, or when two wave systems oppose and combine forces.

• When a wave is too steep, breaking is inevitable.

http://en.wikipedia.org/wiki/Ocean_surface_wave

Options in Big Seas• Make for a windward shore.

• Reduce sail/run with bare poles.

• Surf with the wind.

• Use a drogue.

http://www.cruising.sailingcourse.com/heavy_weather.htm

Options in Big Seas

• Heave to.

• Use a sea anchor.

• Hoist a small sail at the stern.

• Lie a-hull.

http://www.cruising.sailingcourse.com/heavy_weather.htm

Planning for Waves

• Wind/wave analysis charts are available from many providers.

• Show wave heights and direction.

Reading Wave Products

• Wave height over a period of time is usually expressed as significant wave height.

• This figure represents the average height of the highest one-third of the waves in a given time period.

• The largest individual waves are likely to be about twice the reported significant wave height for a particular day or storm.

http://en.wikipedia.org/wiki/Ocean_surface_wave

Find Additional Information on Waves and

other Marine Topics at MarineEncyclopedia.com

http://www.waterencyclopedia.com/Tw-Z/Waves.html

Image Credits• 1. The Great Wave, Katsushika Hokusai

• 3. http://en.wikipedia.org/wiki/Wave

• 5, 7, 11. Photos, Darusha Wehm

• 10. http://entertainment.howstuffworks.com/surfing6.htm

• 12. NOAA http://www.photolib.noaa.gov/bigs/wea00816.jpg

• 15. NOAA http://www.photolib.noaa.gov/bigs/wea00810.jpg

• 18, 19. Weather products from NOAA http://www.opc.ncep.noaa.gov/

• 20-35 Weather products from PassageWeather.com

WavesA presentation to the Bluewater Cruising

Association Weather Group

by Darusha Wehm

March 12, 2008

C This work is licensed under the Creative Commons Attribution-Noncommercial 2.5 Canada License.