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Author Topic:   Measuring wave height

David Jenkins posted 10-07-2002 10:30 PM ET (US)
Are six-foot seas measured from the bottom of the trough to the top of the crest, or are they measured from an imaginary line that is halfway between trough and crest? In other words, when you are at the bottom of the trough in six foot seas, are the tops of the crests six feet above your deck, or are they 12' above your deck?
kingfish posted 10-07-2002 10:51 PM ET (US)
Waves are measured from "trough to crest".
And with all due respect to various perceptions of wave heights, I am pretty nuts and I've been around the snotty kind of waves we get in the Great Lakes for 55 years, and I can tell you that to even be out in water where the waves are half as high as your boat is long, unless you could run the trough you'd be lucky to make 3 or 4 miles an hour and return to tell about it.
jameso posted 10-08-2002 08:26 AM ET (US)
David,
With your 15 you want to stay out of 6 foot seas. PERIOD
Also ofter overlooked but just as important to a small boater is the frequency. Also other factors, see the 'following seas' thread.
Jim Armstrong
David Jenkins posted 10-08-2002 08:34 AM ET (US)
Thanks for the safety tip, Jim. Actually, I was not planning my next trip, I was trying to determine the validity of someone's comment in http://continuouswave.com/ubb/Forum1/HTML/003609.html in which the poster said, "When your in a 17 ft boat, at the bottom of the trough looking up at 6 footers, they seem like their 12 ft."
DaveH posted 10-08-2002 09:03 AM ET (US)
David:
We did this topic in July. Through our new search engine, I found my old post:
The actual method of reporting sea characteristics engineering-wise is the significant wave method. Waves can amplify and cancel out each other as we all know, the highest one-third of all wave heights is used to determine reported design sea height:
* Most frequent wave=0.5 wave height
* Average waves = 0.61 wave height
* Significant waves = 1.0 wave height
* Highest 10% = 1.29 wave height
per US Naval Oceanographic Office,1958
There are many indicators and statistical charts at my disposal which I won't bore the
Forum with, but ideal wave height:
* Is ALWAYS measured crest to trough.
* Is affected by fetch, wind speed, wind duration, and depth of water.
* Will crest when the height divided by the wavelength (H/L) is 1/7 or about 120 degrees.
* Is affected by the bottom when the bottom is less than the wavelength divided by 2
L/2). Anything greater than L/2 is "deep water".
kingfish posted 10-08-2002 09:19 AM ET (US)
When you're in a 17' boat at the bottom of the trough looking up at a six-footer, it *does* seem like they're lot bigger, especially in tight frequency as I presume the storm in question was producing. Lazy swells of six feet with a slow frequency are no where near so big a deal.

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Author	Topic: Measuring wave height
David Jenkins	posted 10-07-2002 10:30 PM ET (US) Are six-foot seas measured from the bottom of the trough to the top of the crest, or are they measured from an imaginary line that is halfway between trough and crest? In other words, when you are at the bottom of the trough in six foot seas, are the tops of the crests six feet above your deck, or are they 12' above your deck?
kingfish	posted 10-07-2002 10:51 PM ET (US) Waves are measured from "trough to crest". And with all due respect to various perceptions of wave heights, I am pretty nuts and I've been around the snotty kind of waves we get in the Great Lakes for 55 years, and I can tell you that to even be out in water where the waves are half as high as your boat is long, unless you could run the trough you'd be lucky to make 3 or 4 miles an hour and return to tell about it.
jameso	posted 10-08-2002 08:26 AM ET (US) David, With your 15 you want to stay out of 6 foot seas. PERIOD Also ofter overlooked but just as important to a small boater is the frequency. Also other factors, see the 'following seas' thread. Jim Armstrong
David Jenkins	posted 10-08-2002 08:34 AM ET (US) Thanks for the safety tip, Jim. Actually, I was not planning my next trip, I was trying to determine the validity of someone's comment in http://continuouswave.com/ubb/Forum1/HTML/003609.html in which the poster said, "When your in a 17 ft boat, at the bottom of the trough looking up at 6 footers, they seem like their 12 ft."
DaveH	posted 10-08-2002 09:03 AM ET (US) David: We did this topic in July. Through our new search engine, I found my old post: The actual method of reporting sea characteristics engineering-wise is the significant wave method. Waves can amplify and cancel out each other as we all know, the highest one-third of all wave heights is used to determine reported design sea height: * Most frequent wave=0.5 wave height * Average waves = 0.61 wave height * Significant waves = 1.0 wave height * Highest 10% = 1.29 wave height per US Naval Oceanographic Office,1958 There are many indicators and statistical charts at my disposal which I won't bore the Forum with, but ideal wave height: * Is ALWAYS measured crest to trough. * Is affected by fetch, wind speed, wind duration, and depth of water. * Will crest when the height divided by the wavelength (H/L) is 1/7 or about 120 degrees. * Is affected by the bottom when the bottom is less than the wavelength divided by 2 L/2). Anything greater than L/2 is "deep water".
kingfish	posted 10-08-2002 09:19 AM ET (US) When you're in a 17' boat at the bottom of the trough looking up at a six-footer, it does seem like they're lot bigger, especially in tight frequency as I presume the storm in question was producing. Lazy swells of six feet with a slow frequency are no where near so big a deal.