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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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Old 07-23-2007
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Steep/Breaking Waves

When trying to determine wave steepness, data suggests H/L ratio of 1/7 indicates breaking waves. Bouy wave data gives height and seconds but not length (distance) between waves. Are seconds and lenght interchangeable as a practical matter? I know there are many variables other than wave height and periond but what is the proper/practical interpretation of wave data to determine when one should go or sit under a tree?
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Old 07-23-2007
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Check out this link, zero in on the "wave steepness" and "wave speed" paragraphs. Doesn't look like seconds and length are interchangeable.

http://darter.ocps.net/classroom/klenk/ms/Fwave.htm

One of the many variables is current. wind opposing current will steepen the waves a lot. Ask anyone who's crossed the Gulf stream when current and strong wind were opposite.
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Old 07-23-2007
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Nolatom is correct. Inland or coastal, fetch plays a big role as well, and the swell and the wave trains can run counter to each other, with the current being a third element: hence, "confused seas" and a pretty horrible ride.

Buoy data often includes "period" (the time between crests) and with that and the wind speed, one can guesstimate the length.
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Good input and a great reference site but still confused. Reference says "Steepness = Height/Length A wave length of 70m will cause a wave to break when it exceeds 10m (1:7)" When I do the math S=H/L S=10/70
S=.14) When reversed to S=L/H you do get 7 which I assume is the 1:7 breaking point. What am I missing here? How do you guesstimate the length and more importantly, other than experience, how do you interpret bouy data or forecasts (height and period) to make the go/no go decision? I know there are many variables but am trying to get a general idea about what is considered acceptable wind waves for a 30 to 35 foot S/V doing coastal cruising.
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Old 07-23-2007
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Quote:
Originally Posted by tsingtao View Post
am trying to get a general idea about what is considered acceptable wind waves for a 30 to 35 foot S/V doing coastal cruising.
Seas to 8' broad reaching or 3-4' close hauled. Divide by 2 if your spouse is aboard.
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.14 is 1:7.
You can estimate the wavelength by L=1.5 period squared. So ten seconds is 150 and the critical height is 21.4 m roughly 70 feet. That depends on the water being on the water being more than half the wave height deep.
If it were 5 seconds it would have a critical wave height of about 5.5 m.
Of more importance to you is that in shallower water waves slow down so decrease in wave length and increase in height and break at a depth of 1.3 height generally that will be approaching shore. So if the waves were 3 m they would break in 3.9 m a wave height and depth you would want to avoid, but usually evident by breaking waves.
A 5 m ocean swell is not a concern because the wavelength is long so the slope is more gradual. However in shallow water and against a current a smaller chop can be unpleasant because it is steeper and much closer.
Probably you want to look more at windspeed, water depth and current. You may set a limit of say 30 knots, which should be manageable provided it has not been blowing for some days over a long fetch and the water is reasonably deep. At that point you would probably be looking at storm jib and double reefing because you could expect higher gusts to say 45.

Last edited by chris_gee; 07-23-2007 at 07:50 PM.
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Quote:
Originally Posted by rljend View Post
Seas to 8' broad reaching or 3-4' close hauled. Divide by 2 if your spouse is aboard.
Excellent!! I'll take practical advice over theroretical, any day...
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Chris,
Thank you. That is what I was looking for. All the stories about "breaking waves" and couldn't figure out how to "forecast" at what height and period that would occur. Locals (San Francisco now, previously San Diego) say don't go out if the wind waves are square (6 X 6) or if they're predicted to be 4 feet or more. I had 6 foot wind waves at 8 seconds on the trip up from San Diego, not comfortable beating into but not scary or out of control either. Being new to cruising just trying to relate marine forecasts and bouy data to what it is going to be/feel like when ya get there. Thanks for the help.
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Old 07-23-2007
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tsingtao-Don't know if you have been out the gate yet, but you can expect some very big waves and high wind in the channel under the bridge. Don't let that discourage you though, if it is not in the forecast. Many times it will be 25+ knots in the Bay with big swells under the Bridge and when you get out the Gate, its flat and 10 knots. Also, if no one told you yet, refrain from going out the gate on an ebb tide, or more correctly, trying to come in on one. Sailing backwards is one of the great wonders of the Bay.
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Thanks for the tips. Came in on a flood tide with zero wind, 1 foot swell, glassy sea. Couple of "whirlpools" just past the south tower but other than that it was like a lake. Know I'll have more exciting weather in the future but nothing can match the excitement of sailing (opps HAD to motor) under THE bridge for the first time.
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