How do you actually estimate...........

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asteven221

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the height of waves. I have been messing about in boats for over 20 years and to be honest couldn't tell you to any degree of accuracy what height the waves are. So.... like I suspect many people do - I make it up. If I want my crew to be calm I just say that they are tiddlers of about a foot and not too worry. If on the otherhand I want fellow yachtsmen to know of my near death experience fighting a F9 then I tell them the waves were the height of 2 x double deckers. (That's a common measurement yardstick, like large areas are always compared to the size of Wales!) Oh easily... maybe more...couldv'e been well I would say even about 15 metres! This provides me with a lot of credibility as a yachtsman and even gives me hero worship from newbies. /forums/images/graemlins/grin.gif

I happened to be at a dinner event recently where Sir Chay Blyth was the speaker. He mentioned wave height and the fact that people almost went on about the huge waves in the Southern Ocean. He said (and I think this is a fairly accurate quote) "when people say that the waves in the Southern Ocean are about 120 feet, they are talking bollocks". He said the wave height was nothing like what people say and went on to explain what the waves were actually like in terms of shape etc..... Unfortuantely I forget the rest as I was getting pissed by that time. What a guy though. Serious respect for what he's achieved.
 
I'm not sure either, but I figure that when I start climbing up a hill then going down the other side, rather than bouncing around, it's getting a bit much and I want to go home...

The worst I've been out in was on a 38 footer and we were climbing the hill for about 6 boat lengths. I got thrown across the cabin and ended up in hospital with suspected broken ribs - they weren't but bloody'ell it hurt when I moved the wrong way! /forums/images/graemlins/shocked.gif
 
The other think to consider is pitch.

We were caught in a F7-8 off Cap la Hague and while the wave were about 40' it was more a 40' swell than breaking waves and we just went over them in an AWB like a cork.

However when we got to Cherbourg outer harbour and initially felt safe the wave were minute but about 40 pitch and the stern was up while the bow was buried.
 
The problem is that you need to know how far away the wave is that you are estimating, and that just substitutes one problem for another. With only waves to go by, there is no frame of reference.

I remember the first time I walked any distance in a desert, with very clear air, seeing mountains and thinking "those look to be about 1000m high and about 15km off", after 3 days walking towards them I realized they were nearly 3-4000m high and 50km away. I "knew" all along, having a map, but my eyes kept on telling me different!
 
Shall we have a guess the wave height thread?
th_b832980c.jpg
 
Easy. I just take the figure someone's given and halve it.

There was an interesting programme a few years ago about freak waves. It seems that the distribution of wave heights does not follow normal statistical values but something more like those of quantum theory so that waves of exceptional height occur not infrequently, akin to quantum tunnelling. There were satellite radar maps of wave heights which seem to support this observation.
 
Caught out in 50kts wind just SW of Brest where the shelf rises, and was looking up at whitecaps at what I estimated to be at least 30 ft above my head. Thankfully not breaking. Didn't feel unsafe utill I went below to sleep, then it felt awful. But even then, I couldn't say what the real hight was - just bl**dy big!
 
It’s very difficult to judge the height of waves from the deck of a small boat because you don’t have a stable reference point. I’ve been in heavy weather and alongside other vessels and found that this provides a better impression of wave height – but more on the lines of “oh [--word removed--], look at that!” rather than “that wave seems to be 20’ high!
I have an old book by a US Navy captain, William J Kotsch, in which he gives tables of wave height under various conditions:
First he gives wave height for different wind speeds with unlimited fetch. For example:
Wind speed 12k, wave height 5 feet. Wind speed 19k, wave height 12 feet. Wind speed 39k, wave height 36 feet. At 50k the wave height is 50 feet!
He then gives wave heights for various wind speeds blowing for various lengths of time. For example, in 10k a wave would be 2’ after 5 hours and still be that height after 50 hours. However, in 20k the wave height would be 5’ after 5 hours and 9’ after 50 hours of blowing. At 50k the waves would be a mere 19’ after 5 hours but 50’ after 50 hours.
Then he gives wave heights for different fetches – blowing 40k a wave would be 8’ with a 10 nm fetch and 34’ with a 1000nm fetch.
He gives minimum, average and maximum wave heights for the tropical trade wind belts and then it gets really complicated with various combinations of wind strength, fetch and duration of blowing. Fascinating stuff!
The really interesting part is the variations in wave height in a wave train: He calls the highest one-third of all waves in the train ‘the significant wave height’ and gives this a value of 1.00. He than tells us that the average of all waves would be 0.64 of this figure, the highest 10% would be 1.29 and, I can see your eyes glazing over, the highest wave would be 1.87!
So if you’re out in a 30 knot blow, with unlimited fetch, and it’s been blowing like that for 4 days the significant wave height will be 20’, but there will be the occasional wave of over 37’ in there. That’s the one that always hits when I’m on watch!

An interesting subject – I think I might put this data on my website.
 
I agree with all said before. It's extremely difficult to judge wave height...and where do you 'measure' from? I was always under the impression you measure/estimate from the bottom of the trough to the top of the next wave? Or am I totally wrong here? I've been in quite a few storms in both grey funnel liners and small sail boats and seen waves break right over the funnel of the former. In Oct/Nov of 2006 I was caught in Biscay with 60+ knot winds and at that time I estimated wave height of around 60 feet based on the height of my mast.

So....as per the OP....what exactly is the right way to measure wave height, or is it all just guestimation? /forums/images/graemlins/confused.gif
 
[ QUOTE ]
It seems that the distribution of wave heights does not follow normal statistical values but something more like those of quantum theory so that waves of exceptional height occur not infrequently, akin to quantum tunnelling.

[/ QUOTE ]

Blooming heck! /forums/images/graemlins/shocked.gif
 
Wave height can't be a simple prediction of wind speed alone, as it is influenced by other factors such as tide. Eg. Wind against tide gives higher waves? Or does it just give steeper waves?

And why, in a big sea, do quite large ships close by look like they are at the top of a mountain?

Orientation and perception can play strange tricks on the mind when at sea. I just tell people they were great "big" waves.
 
There was an excellent article in Yachting World, June 2005, entitled "100ft waves -fact or sailors fantasies"
Most of the evidence from ships is related to the region off South Africa where the Agulhas Current flows and 60ft waves have certainly been observed there.
Closer to home, 17m (56ft) waves have been observed from oil rigs in the North Sea.
The article also refers to a satellite programme called "Wavewatch" which regularly observes 25m waves (~82ft) in the Southern Ocean. There is a "hanclap" effect when two of these monsters meet an anybody in the close vicinity would be in serious trouble as thousands of tons of water drop from a great height.
The bottom line is that there is no theoretical reason why 100ft waves can't exist when you get winds of 100mph and a good long fetch.
I just don't want to be there to confirm it!!!!!!!
Here is a sample of the seas off Iceland:
http://www.youtube.com/watch?v=mPBaqh3dcVM&feature=related
 
Isn't it reassuring to know that pilot boat drivers are the same the world over!

Two speeds ~ dead stop or flat out.

Waves ~ what waves?

Wind ~ what wind?

Yacht entering a harbour ~ no worries pass by at 30+ knots, 50' off ~ no problems to me!!!!

Thinking back to my training days, my instructor said that the best way to recognise a pilot boat at night was "white cap" over a "red nose" because they're all drunken bar stewards ~ how true was that?

Peter.
 
[ QUOTE ]
My recollection is that wave height is officially half the trough/crest height.

[/ QUOTE ]

Wave height is the vertical distance between trough and crest. Estimate is just that - best method is to compare to what you have in front of you, such as knowing your height of eye from your cockpit and extrapolate/interpolate as necessary.
 
My recollection was wrong - wave height is trough to crest. Attached diag shows how variable a number this (H) is, as is the period (T) between waves.

wavezero.gif


It's from a web page which is quite an interesting read about how wave height is measured by the Australian Manly Hydraulic Institute (here)

Obviously wave height is too butch a topic to be monitored by a bunch of Sheilas at the Womanly Hydraulic Institute
 
On the Pacific coast of the USA, the CG forecasts, transmitted on the VHF WX channels, include wave height, direction and period (also the separate components of cross-swell). And you can call them and ask the current height, outside harbours with bars. 8ft or more is liable to mean a closed harbour.

I was surprised to find I consistently underestimate wave height. So my stories of 'whoppers' might even be true! /forums/images/graemlins/shocked.gif/forums/images/graemlins/shocked.gif
 
Many times the extreme waves seen in southern ocean are result of circumnavigation of globe ... very little land mass to stop it ... plus usually an amalgam of swell and wind wave.

The worst as most will know is the short sharp wave or the confused where two waves systems meet / wind meets waves / sheer drop of seabed ...

Bay of Biscay is famous for dangerous waves because of the continental shelf forcing UP the long drawn out waves that travel the atlantic ... also it's ability like North Sea to create dangerous conditions quickly.
You can have dangerous broaching waves just outside Cowes harbour on the east side ... approaching Chichester Harbour Tide beacon ... examples of seemingly safe "inshore" areas that have caught out many a boater ...

For me it's not height that's the problem - it's the type and shape of the wave that is ...
 
Hi Folks,

This my first reply and indeed first message on the forum.
I was prompted to reply because the subject is very close to my heart.
My job involves marine operations offshore, here in Portugal, and we are almost totally dependent on suitable sea conditions in order to carry out our work.
Therefore, good forecasting is very important to us and we depend on a few sites for forecasts ( of course they may create their forecasts from the same original information).

we use

https://www.fnmoc.navy.mil (you must enter the https:// bit or it does not open)

This is the US Navy site and the parts we need are open to the public. In particular www3, the swell section. We find the forecasts are generally very accurate.

www.hidrografico.pt

Look at the "boias ondografico". There are several wave rider buoys off the coast. We use the one off Povoa de Varzim, North of Porto. You can get up to date (slight time delay) data from the buoys, including a graph of the wave heights. (I just looked at it and the peak wave was 5.26m at 1100)

www.winguru.cz/pt

Although this site is essentially for windsurfers, we find it quite accurate for the waves . The wind forecast is usually way below actual - we x2 to get the real wind speed.

Oh yes - peak to trough is correct

Have fun
 
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