How deep is my trough?

[rlea]

Trying to get my head around this one. I'm sailing in a calm area with 4m of depth happy in the knowledge that I draw 1.8m. Ahead I can see the sea building to wave height I estimate at 2m. My question: Is the trough also 2m which would mean if I sailed on I would only have 0.2m under me?
Really interested in your reply.

 

[AngusMcDoon]

2m waves in 4m of water would not be much fun for anyone other than surfers.

 

[DJE]

Trying to get my head around this one. I'm sailing in a calm area with 4m of depth happy in the knowledge that I draw 1.8m. Ahead I can see the sea building to wave height I estimate at 2m. My question: Is the trough also 2m which would mean if I sailed on I would only have 0.2m under me?
Really interested in your reply.

Wave height is normally quoted as vertical distance from crest to trough. So 2m waves increase depth to 1m above mean at the crest and reduce it to 1m below mean at the trough. So max depth will be 5m and miniumum will be 3m. But 2m waves in 4m of water are likely to be close to breaking so I think I'd be avoiding the area.

 

[maby]

I suspect that the answer is more like 1m - you can only judge the height of the peak with reference to the trough in front of it so, if it appears to be 2m, I would assume that the peak is 1m above mean sea level and the trough is 1m below - the water has to come from somewhere and there is actually very little horizontal movement of water in waves that are not breaking - it is just see-sawing up and down.

 

[Hadenough]

I would take the view that the trough is going to reduce the depth in just the way you describe and potentially leave you with just 0.2 under the keel. Also need to take into account the plunge effect of being "dropped" from the 2 m high wave. Particularly evident when crossing bars. I always play safe for more depth when there is a sea running.

 

[jordanbasset]

Like others think it is one metre, but also like others in such conditions would stand well off the area

 

[maby]

Like others think it is one metre, but also like others in such conditions would stand well off the area

The famous loss of the brand new yacht off Margate about 16 months ago was due to this - the skipper had done his sums and concluded that he would have more than a metre clearance, but had not taken into account the 2 metre waves - bounced the keel along the bottom for a few hundred yards till it cracked at the hull joint.

He quite rightly reported that he had been on a couple of sailing courses including Dayskipper and at no time had anyone mentioned taking wave height into account when calculating clearances. OK - it should be common sense, but the Dayskipper syllabus is 90% common sense, so why do they miss this particular nugget of advice out?

 

[lw395]

Wave height is always a guess anyway, they are not all the same.
When you fall off a wave, a yacht will sometimes plunge into a trough rather deeper than its nominal draft.

Personally I would find half metre waves probably scary with less than 2m under the keel.

 

[WilliamUK]

Two things...

...as an occasional surfer I can say for sure that wave heights aren't set in stone. I've never been out in 2m breakers, but on a day of 1m breakers I was wiped out and dragged along behind a 1.6m-ish breaker. I'd feel uncomfortable allowing for waves smaller than 1.5 to 2 times the predicted or observed height.

...not all people quote wave heights the same way. Some (Hawai'ians especially) have a weird way of measuring the back of a wave that results in a number WAY smaller than the actual height.