Wave heights: what's a lot?

[BelleSerene]

Having a discussion with a colleague about a group Round the Island sail at the weekend (12-14 Oct).

The weather forecast couldn't look better to my eye.

He points out these forecasts
http://www.windguru.cz/int/index.php?sc=47904
http://www.windfinder.com/forecast/sandown_isle_of_wight
, specifically the wave heights, and is suggesting staying within the Solent - which isn't exactly my style.

I would read this as typical swell, but given the wind strength nothing to worry about. But I'm not accustomed to reading wave heights in a forecast.

Can our illustrious fellow forumites enlighten me?

 

[Seajet]

I'm not used to those forecasts either, but if I'm reading it right they're mentioning a 2metre-ish wave height over the weekend ?

That does seem a bit high for a jolly, though I couldn't at a glance see any wind speed to explain, which may indeed mean swell - but what preceding wind caused it, and where ?!

Needs more study than my quick glance but on the face of it your chum may be onto something...

 

[AntarcticPilot]

It's going to depend what kind of wave-height they are reporting.

The usual statistic that is measured is "Significant Wave Height" or SWH. That can be measured fairly easily from satellites, so we know it on pretty much a global basis. It is reckoned as being 4 times the standard deviation of the elevation of the surface - which is difficult to visualize!

It is also reckoned as approximating the wave-height that a trained observer would estimate.

Another wave-height sometimes reported is the RMS wave height, which is approximately the SWH / 1.4

Snag with any wave-height is that the elevation of the sea surface is not a normal distribution, and wave-heights substantially larger than the SWH can readily occur, and are likely to do so in areas where different wave patterns meet, giving interference effects. "Monster" waves several times the SWH from peak to trough have recently been found to be much more common than used to be thought, as well.

 

[Boat44]

Its all relative!

What you dont know is the wave period. ie how close together the wave crests are from one wave to the next. The closer they are the steeper the wave the choppier the conditions.

So actual wave height is only a rough indication of the conditions; and fairly meaningless without the period. Or local knwoledge of what that wave period is likley to be; eg the difference in waves between sailing in the Solent and in the channel in the same wind/tide conditions.

A 2m swell whose period equals the length of your boat is going to be rather unpleasent; but a 2m swell with a 100m period is hardly going to be noticed!

For places like the Solent you are better to make an assessment on wind strenght, direct and tidal flow (wind over tide conditions); and judge this against your prior knowledge.

Sat and Sun look like great days for a sail to me!

 

[BelleSerene]

Interesting, thank you.

And a 2m swell with a 6 second period? (I don't know the speed of waves in seawater so I can't convert period into wavelength which is what you're talking about)?

 

[BelleSerene]

Generally wave speed is a function of wavelength √(9.8 wavelength) (wavelength in m, speed in m/s), but this assumes that the wavelength is more than twice the depth. Where it isn't you need to allow for the depth with (spreadsheet format, wavelength and depth in m, result in knots):
=SQRT(9.8066*wavelength/(2*PI())*TANH(2*PI()*depth/wavelength))*3600/1852

Thank you John - I may see why you worry about chaos theory and schroedinger's cat!

Let's say then that I'm a bear of very limited brain - either that, or just that I distrust the assumptions in the formula (the depth varies wildly after all).

Speaking as a practical man: how would you feel about 2m wave height and a 6s period round the S side of the island? Non-plussed, as I was, or sagely wary if you want to avoid giving your crew a stomach-muscle workout?

 

[Roberto]

Interesting, thank you.

And a 2m swell with a 6 second period? (I don't know the speed of waves in seawater so I can't convert period into wavelength which is what you're talking about)?

for practical purposes, a 6s period gives an average wave length (open sea) of about 50m


also, 50m wavelength means that shallow water effects should be felt at about 25m depth, with wave becoming steeper as depth decreases below that level

 

[greggron]

Naive question

Perhaps a naive question here, but I don't appear to be able to find the answer elsewhere and I know it has caused long discussions mid-channel before.

Is wave height measured peak to trough or from neutral to peak (as in amplitude (The maximum extent of a vibration or oscillation, measured from the position of equilibrium)?

 

[KellysEye]

The forecasts are significant wave height which is the average height (trough to crest) of the one-third highest waves valid for the indicated period. Also there are usually noticeably higher waves every seventh or so, and as said then a monster comes along. But not in the Solent. You can see them when well offshore on the horizon rising much higher than the waves on swell height at the time. Obviously you are on somebody else's horizon so you get them too. Lovely.

 

[John Barry]

Thank you John - I may see why you worry about chaos theory and schroedinger's cat!

Let's say then that I'm a bear of very limited brain - either that, or just that I distrust the assumptions in the formula (the depth varies wildly after all).

Speaking as a practical man: how would you feel about 2m wave height and a 6s period round the S side of the island? Non-plussed, as I was, or sagely wary if you want to avoid giving your crew a stomach-muscle workout?

You'd get a 6s period from a 55m wavelength in 30m of water. For an inexperienced crew that may be lumpy. Depends on the windspeed and direction, of course. On a run you'll get a nice corkscrew effect...

My original post was unclear, and should have said:
For a wavelength <2 times the depth the formula for wavespeed is
√(9.8 wavelength/(2pi))
For a wavelength >20 times the depth the formula for wavespeed is
√(9.8 depth)
For wavelengths between 2x depth and 20x depth you need the full formula
√(9.8 wavelength/(2pi) x tanh(2pi x depth/wavelength)
to convert the results from m/s to knots multiply by 3600/1852
The full formula works for all wavelength / depth combinations.

Sorry about that - lesson for the day: don't rush the 'submit button'.

 

[Neil_Y]

This shows you where the swell is coming from and swell heights will peak around midnight Friday up channel (Brighton)http://magicseaweed.com/Brighton-Marina-Surf-Report/1347/

Check Bournemouth and hover over the line on the graph with cursor this will show you the wave/swell front progression and height in coloured images below graph.

Doesn't look too extreme and its a peak rather than a building swell over a number of days.

 

[Resolution]

What does it mean in practice?

Having a discussion with a colleague about a group Round the Island sail at the weekend (12-14 Oct).



I would read this as typical swell, but given the wind strength nothing to worry about. But I'm not accustomed to reading wave heights in a forecast.

I took some office colleagues round the Island (anti-clockwise) last month in spring tides, and watched the WindGuru wave height forecasts for Swanage beforehandwith some apprehension due to some strong winds in the preceding few days.
The last wave forecast before setting off from Yarmouth was 1.3m at 6 seconds, which sounded much better.
In the event we had 15 knot westerlies and the sea state (wind with tide) was what I would describe as slight. Downwind from the Needles was fast and easy. Important for you to note that even then there was enough motion for each of my colleagues to feel sea-sick down below. The attempt to make coffee fizzled out rather quickly.
Had this been wind against tide then I am sure the waves would have kicked up enough to be troublesome.

 

[Boomshanka]

Wave heights: what's a lot?

Isn't there something in the heavy weather books that gives a wave height to LOA percentage (35% comes to mind), beyond which you might come a cropper? I think the crew would potentially fail long before the boat though. As others have said, wind against tide in the Solent is far more of a problem, and I'm sure you already know to avoid the Shingles bank!

 

[DJE]

Wave heights: what's a lot?

Isn't there something in the heavy weather books that gives a wave height to LOA percentage (35% comes to mind), beyond which you might come a cropper? I think the crew would potentially fail long before the boat though. As others have said, wind against tide in the Solent is far more of a problem, and I'm sure you already know to avoid the Shingles bank!

That would be for breaking waves. Breaking waves with a height equal to the beam of the boat are often reckoned to be capable of causing a capsize. Personally I don't think a long wavelength 2m swell round the back of the island will be anything to worry about unless you have nervous novices on board.

 

[haydude]

Having a discussion with a colleague about a group Round the Island sail at the weekend (12-14 Oct).

The weather forecast couldn't look better to my eye.

He points out these forecasts
http://www.windguru.cz/int/index.php?sc=47904
http://www.windfinder.com/forecast/sandown_isle_of_wight
, specifically the wave heights, and is suggesting staying within the Solent - which isn't exactly my style.

I would read this as typical swell, but given the wind strength nothing to worry about. But I'm not accustomed to reading wave heights in a forecast.

Can our illustrious fellow forumites enlighten me?

Wave height does not matter much on its own and must be considered together with period and direction. Normally in coastal areas the wave period is short giving uncomfortable motion and if sailing against the waves, slamming on modern AWBs.

Around the Isle of Wight a 2m wave with 6" period can become more uncomfortable than a 6m wave in the open ocean with a 15" period. But if the wind picks up to a gale just south of the Isle of Wight and starts whipping 4m waves with a 4" period, that can become lethal to your yacht's safety.

You will find as well that in challenging weather it is best to sail far from the coast, especially cliffs, where waves break and bounce back creating an unpredictable chop. Brighton Marina's entrance, being protected by a break water and near cliffs exemplifies this condition perfectly.

Wave direction in respect to your direction is the other most important factor. The further forward from the stern the most uncomfortable the motion with two critical points: 1) beam: it is at best very uncomfortable and can be dangerous, as already said, with breaking waves higher than the boats' beam and 2) bow modern AWBs with flattish hull cannot take waves on the bow very well, at best they slam and at worst (with a short period and high waves) they fall off with a BANG! and dislodge internal fittings, or delaminate.

If you are sailing within the protection of the Solent you would rarely see challenging wave conditions.

 

[fisherman]

Some interesting numbers from 'Naval Architecture of Small Craft' 1957. Wave disturbance at a depth of one third of wave length is 1/8 wave height (crest to trough at the surface). At 1/2 WL it is 1/25, and at 2/3 WL, 1/70.

Wave speed depends on wave length, and is expressed as V squared = 1.8L, V is speed in knots and L is length in feet.

As to comfort/safety, I have happily hauled gear in 47ft swells, and given up quickly in 10ft chop.

 

[AntarcticPilot]

You will find as well that in challenging weather it is best to sail far from the coast, especially cliffs, where waves break and bounce back creating an unpredictable chop. Brighton Marina's entrance, being protected by a break water and near cliffs exemplifies this condition perfectly.

Often, the least comfortable bit of a day's sailing for me is as I return to my berth along the Clyde ship channel. The channel is bounded on the north by shallow water (dries at extreme low water, but I've never seen it dry), and on the south by a series of harbour walls, wharves and other hard structures. The result is a very short, confused sea. The waves aren't very big, but make for a very uncomfortable, unpredictable motion, especially with the wind from the west, which gives the longest fetch.

 

[onesea]

Often, the least comfortable bit of a day's sailing for me is as I return to my berth along the Clyde ship channel. The channel is bounded on the north by shallow water (dries at extreme low water, but I've never seen it dry), and on the south by a series of harbour walls, wharves and other hard structures. The result is a very short, confused sea. The waves aren't very big, but make for a very uncomfortable, unpredictable motion, especially with the wind from the west, which gives the longest fetch.

Reminds me of between the landing stages in the Mersey. Brick walls either side a little bit of traffic strong currents a bit of wind all the things you need to make waves close and vertical with big holes behind them...

I would see how much of the wind comes through to make the swells, the forecasts at the moment are not exactly precise...

 

[MM5AHO]

I've noticed K5 buoy (off Western Isles) reporting 19m wave height once. That's the highest I've noted.
In Sept 2011 I was west of Islay and wave height was about to the spreaders. So I turned back!

 

[Amulet]

At the risk of dumbing down this erudite thread, you might want to look at the Met office glossary of sea states:

Smooth: Wave height less than 0.5 m
Slight: Wave height of 0.5 to 1.25 m
Moderate: Wave height of 1.25 to 2.5 m
Rough: Wave height of 2.5 to 4.0 m
Very rough: Wave height of 4.0 to 6.0 m
High: Wave height of 6.0 to 9.0 m
Very high: Wave height of 9.0 to 14.0 m
Phenomenal: Wave height more than 14.0 m