How do you measure the height of a wave?

[Thresher]

Judging the height of waves must be difficult from the cockpit. I was once off the west of Ireland thinking that these waves were half way up my 10 metre mast but there was no danger, they were not threatening. That would make them 5 metres. I'm reluctant to tell people that because it sounds like BS. When people report the conditions they were in and the size of the waves, how can we possibly know?

 

[Thresher]

Judging the height of waves must be difficult from the cockpit. I was once off the west of Ireland thinking that these waves were half way up my 10 metre mast but there was no danger, they were not threatening. That would make them 5 metres. I'm reluctant to tell people that because it sounds like BS. When people report the conditions they were in and the size of the waves, how can we possibly know?

I'm sorry, I should have said. How can they possibly know.

 

[KevinV]

Judging the height of waves must be difficult from the cockpit. I was once off the west of Ireland thinking that these waves were half way up my 10 metre mast but there was no danger, they were not threatening. That would make them 5 metres. I'm reluctant to tell people that because it sounds like BS. When people report the conditions they were in and the size of the waves, how can we possibly know?

That would make them 2.5m - 2.5 up, 2.5 down

 

[VicS]

That would make them 2.5m - 2.5 up, 2.5 down

Wave height is the difference between the crest and the trough
You are thinking of amplitude

 

[penfold]

Does the horizon disappear when you're standing in the cockpit? On most AWBs that means the swell is at least 2m. If it disappears when standing on deck that's >3m. Above that is mostly academic.

 

[LittleSister]

You send the least favoured member of the crew up the mast, and tell them to stop when they reach the height at which they can see the waves beyond the nearest at all times!

I don't think there's actually an accurate and reliable method of doing it. (See the recent thread on trying to do it using electronic kit.)

Somewhat like you, I've been in a huge but non-threatening swell. In my case it was in the western Channel, about midway between Brittany and Cornwall. The waves towered above us when we were in the troughs, and it seemed to me to be approximately the height of our masts (lowish aspect ratio schooner rig on a 39 footer). It was a smoothish, even, long swell - very strange experience but not scary. On top of the crests we could see for miles. In the troughs we couldn't see anything but the face of the approaching wave and the back of the retreating one. there could have been a big fishing boat or other vessel in the next trough and we wouldn't have seen it.

 

[KevinV]

Wave height is the difference between the crest and the trough
You are thinking of amplitude

Ah, well said!

 

[ChromeDome]

- and a picture of said method:

 

[Biggles Wader]

I tend to use Nelsons column and how many double deckers would fit in the trough as a reliable measure.

 

[fisherman]

I was haulng gear in (smooth swell) waves at 47ft, from the sounder. No danger except had to be careful if the gear hitched, but only so as not to part out. Been in much more trouble in 8-10ft waves.

 

[Daydream believer]

I tend to use Nelsons column and how many double deckers would fit in the trough as a reliable measure.

Well being a single hander & having sailed in fog for hours without sleep, I have seen my late grandad & Elvis Presley pushing a supermarket trolley. But I am b..gered if I have seen a double decker bus in the middle of the English Channel. So how on earth is one going find one out at sea to measure a wave with that system.

 

[veshengro]

I tend to use Nelsons column and how many double deckers would fit in the trough as a reliable measure.

I've been in waves higher than a double decker bus, but minus it's bodywork. Late summer 1962 bound for Havana Cuba, from London with two ex London Transport bus chassis' complete with engines still attached as deck cargo. Both chained down on the foredeck. The Cubans would put open wooden bodies on them and use them as worker transport in the Sugar Industry.
Caught on the fringes of Hurricane Carla and both chassis eventually went over the side together with a long section of the portside deck rails. Next time you are waiting for bus and wondering where it has got to, there are two a couple of hundred miles North East of Cuba.

 

[Roberto]

Judging the height of waves must be difficult from the cockpit. I was once off the west of Ireland thinking that these waves were half way up my 10 metre mast but there was no danger, they were not threatening. That would make them 5 metres. I'm reluctant to tell people that because it sounds like BS. When people report the conditions they were in and the size of the waves, how can we possibly know?

First thing would be to agree to talk about the same thing.
Height of "a" wave can only be guessed/estimated/measured by a human observer specifically there in that specific moment (let aside remote measurements), after a transient period that specific wave will disappear and no one else will ever meet her again. So yes anyone can say I have seen a wave Xm high.
Otoh, more interesting estimations of a given sea state, the dynamic complex of all waves in a given area, can be obtained by values like Hs significant wave height.
If you have seen a single 5m high wave and if it was the highest, statistically the significant wave height of the sea state would have been a mere 2.5m. If one accepts some assumptions about the statistical distribution of wave heights, it should happen about one wave out of 800.
If you reckon the average height of 1/3 of the highest waves is 5m, then Hs=5m (by definition), if you wait there could be a single wave 10m high.

A bit like wind speed, one thing is Beaufort, another is max speed, peak gust etc etc. Take a 5Bft with violent gusts, one could experience 40kt max wind speed but if the 10min average is around 20kt it will remain a 5Bft; of course at the pub those 40kt peak can quickly become an 8Bft

 

[johnalison]

I do what everyone else does. Think of a figure that might just correspond with the wave height, then double it.

 

[Buck Turgidson]

Part of the problem of trying to sight the height of waves is that the boat is pitching and healing. I’ve given up worrying about it.

 

[Chiara’s slave]

Well being a single hander & having sailed in fog for hours without sleep, I have seen my late grandad & Elvis Presley pushing a supermarket trolley. But I am b..gered if I have seen a double decker bus in the middle of the English Channel. So how on earth is one going find one out at sea to measure a wave with that system.

That’s odd. I seem to recall that the Daily Sport found a double decker on the moon. On that basis, I’m sure there must be some at sea too.

 

[Blueboatman]

I’ve been quite tired singlehanding but not hallucinated ( yet) enough to see a fleet of buses marching across the horizon

Think I read as a kid of an early circumnavigator- Alain Gerbault perhaps ?- who leapt up onto the mast and clung on watching a wave sweep the length of his boat below him , the boat ‘disappearing ‘ below him
That’s a big enough wave for me !

 

[Dellquay13]

I usually look at the sounder at peaks and troughs and work out the difference as the wave height, being aware of taking readings at similar heel.
Then multiply it by the (fisherman’s one that got away) constant to make it sound more dramatic

 

[MikeBz]

I tend to use Nelsons column and how many double deckers would fit in the trough as a reliable measure.

Or in really extreme conditions, football pitches.

 

[Farmer Piles]

Well being a single hander & having sailed in fog for hours without sleep, I have seen my late grandad & Elvis Presley pushing a supermarket trolley. But I am b..gered if I have seen a double decker bus in the middle of the English Channel. So how on earth is one going find one out at sea to measure a wave with that system.

Its the equivalent of a blue whale on top of a football pitch next to an Olympic sized swimming pool. Easy!!