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Old 25-10-2014, 14:40   #1
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Wave Height

I keep reading posts where people talk about the sea state. They might say the waves were X meters and so on, which got me to wondering how can they tell the difference between 1 and 2 meter wave height and between 3 and 4, or whatever. After using Google, I found that there are chartplotters that will tell you the sea state, but is that calculated by the device, or is it just a forecast sent from a satellite? How accurate can that be? Are people just eyeballing it?

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Old 25-10-2014, 14:46   #2
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Re: Wave Height

Well... I think eyeballing it is the norm. The height is from the trough to the top.... so looking afield it's not hard to estimate....
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Old 25-10-2014, 15:03   #3
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Re: Wave Height

Mostly eyeballing it. Knowing your height of eye in various locations on the boat and looking at wave crests. For example, standing in my cockpit my eyes are 8' above the water and I can either see over the waves or not. This works up to a point, I am not climbing the rigging just to see how high the waves are. On my boat 12' is about the limit of accuracy, beyond that they are just big and it becomes a geometry problem to figure out so I just log it as whatever is on the weather fax reports.
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Old 25-10-2014, 15:19   #4
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Re: Wave Height

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Originally Posted by salmonlv View Post
I found that there are chartplotters that will tell you the sea state, but is that calculated by the device, or is it just a forecast sent from a satellite? How accurate can that be?
If a chartplotter is telling you the sea state, that info is coming from the weather forecasts, which can be pretty accurate between *now* and a couple of days from now. The forecasts can be off quite a bit as if you are near shore, since the bottom contours can make a big difference.

There are a few boats that have three-axis accelerometers connected to sophisticated electronics that can measure wave height, but the vast majority of us just eyeball it (as has been described above).

Remember that the official "wave height" number given in the forecasts and buoy reports is the "average of the tallest one-third" waves, so there will be many waves larger than that. The big waves are the ones we remember!
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Old 25-10-2014, 15:19   #5
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Re: Wave Height

I will sometimes use the freeboard of a nearby vessel as an aid to estimating wave height. This only works if the waves are far enough apart that the vessel is fully in the bottom of the trough and not "bridging" between two crests.

Another good trick to use near shore, is to find a piling that is exposed to the wavetrain. If you are really lucky you might find a piling with water level heights in feet or meters stenciled on.

I think most people have a natural tendency to overestimate wave height. I know I am always humbled by actual measured heights compared to what they appear to be.

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Old 25-10-2014, 15:40   #6
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Re: Wave Height

IPads with GPS have a gyroscope in them. No one knows what to do with it.... But if you can write and ap the gyroscope should be able to work out a wave height, shouldnt it?
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Old 25-10-2014, 15:55   #7
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Re: Wave Height

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IPads with GPS have a gyroscope in them. No one knows what to do with it.... But if you can write and ap the gyroscope should be able to work out a wave height, shouldnt it?
Hmm... no sure if that would work but then again, there are many people a lot smarter than me

However I have used a (borrowed) precision air pressure sensor that worked quite well at measuring heights down to about half a metre or so. Of course the waves / swell had to be over a metre or two to make it meaningful.
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Old 25-10-2014, 15:58   #8
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Re: Wave Height

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For example, standing in my cockpit my eyes are 8' above the water and I can either see over the waves or not..
That makes sense to me. Thanks.
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Old 25-10-2014, 16:20   #9
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Re: Wave Height

Most people measure with exaggeration.
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Old 25-10-2014, 17:42   #10
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Re: Wave Height

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Most people measure with exaggeration.
That's what I was thinking, but didn't want to say it.
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Old 25-10-2014, 19:30   #11
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Re: Wave Height

I do know this much....the longer you have been out there and the more oceans you cross the smaller the waves are.
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Old 25-10-2014, 19:37   #12
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Re: Wave Height

I have an aviation GPS but never taken it on the boat. Aviation gps, obviously, gives altitude.

Anyone tried one on a boat and seen anything in terms of altitude?
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Old 25-10-2014, 19:41   #13
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Re: Wave Height

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Most people measure with exaggeration.
I think most people also forget how wave height is calculated. Just because you see a 12' wave, does not mean the wave height is 12'. On top of that they tend to call it 14' or 16'. Last time I was a few 14' waves and quite a few 12' waves, the official wave height (from the buoys) was 8'.
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Old 25-10-2014, 19:42   #14
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Re: Wave Height

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Originally Posted by ontno1 View Post
Most people measure with exaggeration.
In the bar I much prefer to exaggerate that be in the real thing
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Old 25-10-2014, 20:04   #15
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Re: Wave Height

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I have an aviation GPS but never taken it on the boat. Aviation gps, obviously, gives altitude.

Anyone tried one on a boat and seen anything in terms of altitude?
Yep, but my experience was that the resolution was way too big and the response too slow either that or I was really sailing in 200 foot waves with a period of 2 minutes

Others may have better GPS units!

The precision air pressure sensor I mentioned before was near instantaneous and had a resolution of around 2 foot.
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