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# Waves

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A presentation to the Bluewater Cruising Association's Weather Seminar

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### Waves

1. 1. 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
2. 2. How do waves form? • Waves result from distant winds. • Three factors inﬂuence the formation of quot;wind wavesquot;: • 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
3. 3. 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
4. 4. 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
5. 5. 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
6. 6. 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
7. 7. 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
8. 8. Planning for Waves • Wind/wave analysis charts are available from many providers. • Show wave heights and direction.
9. 9. 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
10. 10. Find Additional Information on Waves and other Marine Topics at MarineEncyclopedia.com http://www.waterencyclopedia.com/Tw-Z/Waves.html
11. 11. 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