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 Seas and Swells question		  New Topic  Topic Locked
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Russ.Johnson
Commodore

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USA
833 Posts
Initially Posted - 11/21/2007 :  00:32:24  Show Profile
You can tell I haven't been sailing and have too much time on my hands.
This is something I've wondered about for awhile but never bothered to ask.

1) On the VHF weather channel it forecasts "Seas" 2-4' and "Swells" 6-8'.
I think Seas are short period wind-driven waves (chop).
And Swells are long period ocean tide-driven waves.
Do I have that right?

2) I also wonder about how wave height is reported.
In the sample below, would this be a 2' wave or a 4' wave?
Just wondering.
Russ
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Champipple
Master Marine Consultant

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USA
6855 Posts
Response Posted - 11/21/2007 :  05:55:27  Show Profile  Visit Champipple's Homepage
They are measured peak to trough and the measurment is an average.

Check out here...

http://en.wikipedia.org/wiki/Swell_%28ocean%29
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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts
Response Posted - 11/21/2007 :  08:13:33  Show Profile
On point 1... The "seas" are indeed the "local" wind-driven waves, which are relatively short-period. "Swell" is waves generated by weather at some distant location--possibly thousands of miles away. As the big, steep wind-driven waves in that location propagate across the ocean (in an arc, as when you drop a stone in a puddle), they get longer and longer, also increasing in speed. So, the report might indicate two or even three intersecting swells with different heights and periods, created by storms in different parts of the world. Since intersecting waves are additive at their peaks and troughs, this can create a rather confused feeling for the sailor! (Up, down, <b>DOWN</b>, up, <b>UP</b>, down, up, down, <b>UP</b>...) As the swell enters shallower water, it slows, making it shorter and steeper again until it gets too steep and turns into breakers.

BTW, tide has nothing to do with swell... the tide itself is two <i>very</i> long waves following the moon around the globe.

(I recall hearing a tsumami, generally caused by underwater quakes, can travel across the open ocean as a very long, almost imperceptibly low wave at speeds like 1200 mph until it reaches shallow water, where it slows down and rises up.)
Edited by - Dave Bristle on 11/21/2007 08:26:31
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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts
Response Posted - 11/21/2007 :  08:36:18  Show Profile
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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts
Response Posted - 11/21/2007 :  10:12:47  Show Profile
Cool, Don! That shows the effects of reflection--the thing that makes inlets so chaotic... Fortunately, the beaches of the world don't reflect waves very well.
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John Russell
Master Marine Consultant

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USA
3444 Posts
Response Posted - 11/21/2007 :  15:22:20  Show Profile
That looks like the channel of Sandusky Bay. On a good day. BEFORE the stinkpots start to mix it up!
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piseas
Former Treasurer

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USA
2017 Posts
Response Posted - 11/21/2007 :  16:02:18  Show Profile  Visit piseas's Homepage
Now I know why they call Don, Dave and Duane, "Master Marine Consultants".
You guys are phenomenal. You should all three collaborate and write a book.
Steve A
Edited by - piseas on 11/21/2007 16:06:33
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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts
Response Posted - 11/21/2007 :  16:51:24  Show Profile
You actually think Don and Duane would collaberate with a stinkpotter?? ...although I have been sailing for longer than either of them has been <i>alive</i>. (...not continuously.)
Edited by - Dave Bristle on 11/21/2007 16:56:40
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piseas
Former Treasurer

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USA
2017 Posts
Response Posted - 11/21/2007 :  20:02:35  Show Profile  Visit piseas's Homepage
Dave, you are one stink-potter I am proud to know, Unless you are one of the two who created a wake so huge that it caused my dog, Buddy, to fall off the bow and take a swim.
Steve A
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dmpilc
Master Marine Consultant

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USA
4593 Posts
Response Posted - 11/22/2007 :  10:50:02  Show Profile
Excellent graphics, and thanks, Russ, for raising the question.
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.