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Old 07-24-2007
chris_gee chris_gee is offline
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Theory and practice are not divorced.
The original question was about breaking waves. All waves break in shallow water ie at the shore.
In deep water the wave characteristics depend on fetch ie over what distance the wind has been blowing, duration and wind speed. As these increase so do wave height period and wavelength.
For a wave to break the relationship in deep water of 1/7 holds.
For practical coastal purposes a F8 fresh gale blowing for 30 hours over 400 miles gives a significant (average of the highest third) wave height of 8m and a period of 10 seconds with a wavelength of 150 m. I suspect the average coastal sailor would avoid going out in that forecast let alone knowing a gale has been blowing for 30 hours. Nevertheless he should be able to handle F8 and the waves will not be breaking.
However against a strong current or in shallow water the wave period will shorten and the waves steepen, and may well break, say on a bar. Hence if it is bad it is better to head offshore rather than through shallow water and against a current.
i don't agree that a 12' swell every 12 seconds is bad news, the wavelength is 216 m and you would hardly notice it.
I haven't heard of the waveheight in feet = period rule of thumb but it doesn't sound right. An 8' wave at 4 seconds would be 24 metres apart and at 8 sec 128m. The former you may wish to avoid but the latter is hardly a problem.
Period and wave height inmetres may be equal but only with strong gales over a long time and fetch see http://www.bom.gov.au/bmrc/pubs/tcguide/ch4/ch4_3.htm.
In shallow water you can get a nasty chop particularly with wind against tide but the wave height will be quite low and the period very short. That may be unpleasant but not dangerous in a reasonable sized boat.
A rule of thumb is to stay home if the forecast windspeed is greater than the boat length in feet. As mentioned a 32 ' boat should be able to handle 30 knots adequately reefed, because intended or not it is likely to encounter such winds. However the seastate depends on fetch current depth and wind duration which are local conditions.
A 5m ocean swell should not be a concern, assuming a reasonable depth because to reach that height requires time fetch and quite a long period unless you are talking hurricanes.
I daresay you can get water over the bow in 8' but the period is likely to be less than 5 seconds and the wavelength short.
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