How do you measure the height of a wave?

[Farmer Piles]

I do not know the specific place, but the height reached by water is not a proof of wave height, a powerful wave against a large obstacle can send its water several times higher than its own height.

Beautiful images of tall lighthouses disappearing into water/spray

I passed very close to this lighthouse on my way back from France on a boys' boat trip; we stopped at Ushant for a night. Luckily it was flat calm for us.

 

[oldmanofthehills]

Sorry! I disagree.

Sometimes you have absolute proof of the height of the wave.

"On 11 March 1861 at midday the lighthouse on Eagle Island,[5] off the west coast of Ireland was struck by a large wave that smashed 23 panes, washing some of the lamps down the stairs and damaging the reflectors with broken glass beyond repair. In order to damage the uppermost portion of the lighthouse, water would have had to surmount a seaside cliff measuring 40 m (130 ft) and a further 26 m (85 ft) of lighthouse structure. "

I have seen pictures of wave breaking up over the 120ft cliffs at Sennen in West Cornwall on 8th Mar 2008, but that did not mean that waves were 120ft as can be seen in photo merely that the surge and spray were directed upwards by the waves. I think it was F9 down there or maybe F10.

Sadly my old boat was swamped on its Somerset moorings on the night of 8th March 2008, probably at HT when the boats rise above the shelter of the marshes - but very much doubt waves in Upper Bristol Channel were that big as a mere 3m wave in the creek might swamp a boat restricted by trots.

A 215ft wave approaching land would be a like Tsunami and destroy an entire region and there seem no report of the widespread destruction of inland W Ireland in 1861, though the East Coast storms of February caused great loss of boats and sailors

 

[Chiara’s slave]

Measure this then. It's said that 100ft waves occur there, this one is pretty big, maybe 75ft?

 

[Bouba]

Measure this then. It's said that 100ft waves occur there, this one is pretty big, maybe 75ft?

If that’s 75 then 100 foot is beyond nightmarish....imagine being in a boat

 

[Sandy]

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?

It is only if the depth reads zero metres do I worry! I read depth under the keel.

 

[V1701]

Quickly, before it breaks...

I'll get my coat...

 

[Slowboat35]

How to measure a wave?

Perhaps use a traditional Britannia Wave Rule?

er, I'll get my coat...

 

[Roberto]

Measure this then. It's said that 100ft waves occur there, this one is pretty big, maybe 75ft?

This is how it can appear on radar.

 

[Farmer Piles]

I have a mate who regularly surfs that wave at Nazare north of Lisbon. Andy Cotton; the most easy going guy you could meet but has to have a screw loose to even contemplate trying to surf waves like that. He has been hospitalised more than once. They wear inflating lifejackets a bit like our boat ones but they have dump valves so that they can dump the air if they are about to be hit by another wave if they have wiped out. It gives them a chance to dive down and try to mitigate the force of the wave - but they still get hammered. They usually have three or four gas cylinders so that they can repeat the process until they can be rescued by a jetski. The risk is huge for those guys too as after a wave has passed the water is so aerated that the jetski impellor struggles to produce enough thrust.
There is a canyon in the continental shelf there that focuses the energy of the waves to make them so big. You can see it on Google maps satellite. I think that I am a hero if I surf waves with a ten foot face.

 

[Roberto]

There is a canyon in the continental shelf there that focuses the energy of the waves to make them so big.

An interesting video about the formation of the wave

 

[Bouba]

How far out to sea do have to be to be safe?

 

[Biggles Wader]

How far out to sea do have to be to be safe?

Minus 50 yards.

 

[Bilgediver]

l once stood by an oil rig on board a supply ship for about a week waiting on weather to swap out two anchor chains each 3500 ft long. I was told later that there were periods when those on the tig could not sea any part of our vessel as we sank into the troughs and were some times concerned as they waited for us to come back into view.
Just a wee bit of rough water !