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Old 19 September 2001, 02:24   #1
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Sea State

Hi there

Not being a person afraid to ask a dumb question, here goes again...

I have been looking at the inshore weather forecast and see the sea state described as 'smooth, moderate, slight etc. I have a mental picture of this BUT, (you are ahead of me here aren't you?) what exactly does this mean? Where can I find the actual definitions of the sea states?

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Keith Hart (SBS)

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Old 19 September 2001, 07:15   #2
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Hi Keith,

I am quoting from Peter White's book "Powerboating The RIB and Sportsboat Handbook"

Wind Speeds

Beaufort Force General Description Speed (Knots)
0 Calm Under 1
1 Light 1 - 3
2 Light 4 - 6
3 Light 7 - 10
4 Moderate 11 - 16
5 Fresh 17 - 21
6 Strong 22 - 27
7 Strong 28 - 33
8 Gale 34 - 40
9 Severe Gale 41 - 47
10 Storm 48 - 55
11 Violent storm 56 - 63
12 Hurricane 64 and over


I think you should get a copy of the book. I found it very useful.

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Dimitris
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Old 19 September 2001, 09:23   #3
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Or... Smooth..... time to look for the nearest pub on route and have a beer or two
Moderate...........Take your wown beer beacuse it is going to take longer to get there.
Rough.........It is going to hurt and everone on board is going to moan at you.
My advice is to keep your friends and family and only go out in the "smooth forecast"

Alan P
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Old 19 September 2001, 09:52   #4
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There is a diference between sea state and wind speed.
The values Dimitris has given are wind speed against the Beaufort scale
Sea state while often linked with wind speed does not always work that way. Conditions like wind against tide can cause a rougher sea state and wind with tide can make things smother.

Other factors like, if it has been rough recently can also affect sea state toghther with the type of botton and depth of water.

But to answer you initial question I dont know where it states smooth as XXX and rought as YYY etc.

I will do some digging and find out what equates to what

Regards Gary
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Old 19 September 2001, 11:46   #5
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Ah, perhaps it was not such a dumb question after all then. I imagined that there would be some sort of classification eg:
Smooth = no waves
Slight = waves up to 10cm
Moderate = waves up to 50cm
etc etc.

I was thinking along the lines of the classifiaction of my little boat (class C etc.) which started the thread 'inshore'.

I've had a look at the met weather forecast site but no luck there. I suppose we may have an answer here within a couple of days.

Awaiting with baited breath.

Keith Hart (SBS)
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Old 19 September 2001, 11:48   #6
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I can't find this information either! It's not on the Met Office web site, so I've emailed them -- I'll let you know what they come up with.

John
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Old 19 September 2001, 13:23   #7
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Funnily enough I was reading about this last week whilst studying an inshore waters forecast! If you have a stanfords chart the info is printed on the back (or it may be an Imray one).

Keith is correct that the sea state relates to wave height in metres. I can't recall the exact definitions but seem to recall that rough was 4m or so. That'd be fun enough for me!

Alan
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Old 19 September 2001, 13:55   #8
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First of all, I don't know the answer either. But I would like to know.
Also, when reading nautical magaines I sometimes read about Sea State 3 or Sea State 5 or whatever. So there seems to be yet another classification.

A while ago, while looking for material for my site I stumbled across a great series of photos showing sea states at various wind speeds (F0-F12). The photos were on the site of the National Weather Service, but I can't find them now.

Luckily, I saved them on my hard drive and have now uploaded them. You'll find them here:

http://hem.bredband.net/sansej/temp/bforce0.gif
http://hem.bredband.net/sansej/temp/bforce1.gif
http://hem.bredband.net/sansej/temp/bforce2.gif
http://hem.bredband.net/sansej/temp/bforce3.gif
http://hem.bredband.net/sansej/temp/bforce4.gif
http://hem.bredband.net/sansej/temp/bforce5.gif
http://hem.bredband.net/sansej/temp/bforce6.gif
http://hem.bredband.net/sansej/temp/bforce7.gif
http://hem.bredband.net/sansej/temp/bforce8.gif
http://hem.bredband.net/sansej/temp/bforce9.gif
http://hem.bredband.net/sansej/temp/bforce10.gif
http://hem.bredband.net/sansej/temp/bforce11.gif
http://hem.bredband.net/sansej/temp/bforce12.gif

(Sorry for posting all the links, but the server does not allow listing of the directories.)

The above descriptions are only valid at open sea, where there is enough of distance for the waves to build. Also there is a minimum of time the wind needs to blow. There are diagrams that can be used to predict wave height given wind speed, time duration and stretch of open water.
And, as Gary already said, there is a number of other conditions that influence the sea state.

I know this is not an answer to your question, Keith, but I hope it is a step in the right direction.

Sasa
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Old 19 September 2001, 14:03   #9
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Sea State

Here is my attempt to reduce things to a basic description; I can't figure out how to lay out a table in here (it was looking great till I previewed it) so I hope that the information will make some sense!

The info is laid out as follows: first is the Sea State number followed by the description for that state, then an approximate wind strength for that sea state and the final figure in brackets is the average wave height. I did not attach a wave height for states 0,1 and 3


0 Calm Glassy, force 0
1 Calm Rippled, force 1 and 2
2 Smooth wavelets, force 3 ( 2 feet )
3 Slight, force 3 to 4
4 Moderate force 4 to 5 ( 5 feet )
5 Rough force 6 ( 9 feet )
6 Very Rough, force 7 ( 14 feet )
7 High, force 8 ( 23 feet )
8 Very High, force 9 ( 31 feet )
9 Phenomenal, force 10-11-12 ( 54 feet )

The sea states do not exactly co-incide with the wind strengths (that would be too easy) and the wave heights are the suggested average for the Nominal Sea State, obviously with a fairly significant difference in windspeed covering a given sea state, the wave height will be far less at the bottom end of the range than the top.

What I have put here is trimmed to the minimum (always risky!) but perhaps gives some idea of what is involved, I found an excellent table with the relevant data laid out very clearly at http://www.seathree.demon.co.uk/seastate/index.htm
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Old 19 September 2001, 15:06   #10
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Allan,

Your link summarizes very nice sea states.

Keith, sorry for giving you a misleading or half answer. The Book is nice though.

Allan P, any particular brand of beer that goes with sea states ?


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Dimitris
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