Waves

[DangerousPirate]

I wonder what size waves you consider large and bothersome, and how do I use the wave forecast when planning passages to my advantage?

Currently looking at tomorrow's forecast where they predict 1 m waves every 4 seconds coming from the wind direction right on my beam. Yesterday I had some waves at the end of my passage, but not sure how big those were, so can't really predict if it's going to be the same. But it was rough.

Looking at a 10-16 hour passage tomorrow and not sure if that is expected to be very bothersome or not. The boat is a fin keel, 30ft. What do you think?

 

[johnalison]

I can’t say that I have ever tried to assess waves with that precision. So much depends on the shape of the waves and the course one is trying to make that size in itself doesn’t really mean much to me. i suspect that the only way to make a useful assessment would be to follow a known source over a period of time and learn to relate the results to one’s own experience, much as we do with wind forecasts. In the sailing I have done, the effect of currents on wave shape have often dominated the actual wave size and this correction would need to be applied to a general forecast for it to make sense.

 

[DangerousPirate]

I can’t say that I have ever tried to assess waves with that precision. So much depends on the shape of the waves and the course one is trying to make that size in itself doesn’t really mean much to me. i suspect that the only way to make a useful assessment would be to follow a known source over a period of time and learn to relate the results to one’s own experience, much as we do with wind forecasts. In the sailing I have done, the effect of currents on wave shape have often dominated the actual wave size and this correction would need to be applied to a general forecast for it to make sense.

Oh? So the wave height doesn't matter that much? What factor should I be looking for so that i don't pick days to fight waves again.

 

[dunedin]

Lots of factors. Yesterday was very light winds by recent standards, and inside the sheltered Clyde the waves on Saturday were pretty small (I know as I went out for a sail due to the pleasant conditions).

As well as the wind, tide is a big factor - against wind and tide you will get nowhere slowly. When the tide is against the waves it can get extremely bumpy. Around the Mull of Kintyre, Mull of Galloway and North Channel the tides are often very strong, which can result in very uncomfortable (occasionally dangerous) wave conditions, even in relatively modest winds.

 

[dunedin]

PS. Looking at Predict wind for the waters off Mull of Kintyre for tomorrow Monday it is currently showing a complete screen of orange, red and dark red from 5 different weather models!!! At noon UK Met Office showing 26 knots gusting 37 knots.

Inshore waters forecast mentions F6-8 for near Isle of Man and F5-7 for North Channel, albeit due to reduce slightly.
But don’t forget that near large headlands like Mull of Kintyre often gets winds 1-2 Forces higher than the general area forecast.

 

[DangerousPirate]

PS. Looking at Predict wind for the waters off Mull of Kintyre for tomorrow Monday it is currently showing a complete screen of orange, red and dark red from 5 different weather models!!! At noon UK Met Office showing 26 knots gusting 37 knots.

Inshore waters forecast mentions F6-8 for near Isle of Man and F5-7 for North Channel, albeit due to reduce slightly.
But don’t forget that near large headlands like Mull of Kintyre often gets winds 1-2 Forces higher than the general area forecast.

Where do you get your weather forecast? I don't see that. I checked windy and xcweather. But yes, it was a very pleasant sail. I don't know when the tide turned, but around 7 or 8 o clock it became rough, maybe that's when the tide changed.

 

[Sandy]

I wonder what size waves you consider large and bothersome, and how do I use the wave forecast when planning passages to my advantage?

Currently looking at tomorrow's forecast where they predict 1 m waves every 4 seconds coming from the wind direction right on my beam. Yesterday I had some waves at the end of my passage, but not sure how big those were, so can't really predict if it's going to be the same. But it was rough.

Looking at a 10-16 hour passage tomorrow and not sure if that is expected to be very bothersome or not. The boat is a fin keel, 30ft. What do you think?

I find anything that I'm looking up at from the helming position as 'large and bothersome'. Again a 10.1 metre fin keel boat.

In August 2021 I left Peterhead into a six metre swell, but the wavelength was long, however my crew mate went a whiter shade of pale. After we cleared the Skerries and turned south it was a very pleasant run, his usual colour returned.

Crossing Biscay in August 2022 the maximum we had was three metres breaking waves, all very comfortable. It's a confused sea that is uncomfortable.

 

[dunedin]

Where do you get your weather forecast? I don't see that. I checked windy and xcweather. But yes, it was a very pleasant sail. I don't know when the tide turned, but around 7 or 8 o clock it became rough, maybe that's when the tide changed.

UK Met Office Inshore waters - Inshore waters forecast and strong winds - Met Office
and Predict Wind App (which compares various models)

 

[Ardenfour]

Looks like you have a Nic 30. When I had mine we came through some pretty lumpy stuff between Arran and Bute, was never cause for concern. just get reefed down in good time, the boat will cope .

 

[DangerousPirate]

Looks like you have a Nic 30. When I had mine we came through some pretty lumpy stuff between Arran and Bute, was never cause for concern. just get reefed down in good time, the boat will cope .

Been there, done that. I am just confused about waves and wish I could predict the sea state better.

 

[AntarcticPilot]

Been there, done that. I am just confused about waves and wish I could predict the sea state better.

The problem is that sea-state depends on non-local factors. The waves you see at one place may have their origin on the other side of the North Sea (or other equivalent body). They are affected by fetch (that is, the distance over the sea that the wind has passed over), the depth, and the tide. There may be other factors I've forgotten! The longer the fetch, the bigger the waves, the lower the depths the "choppier (i.e. shorter) the waves are and if the wind is blowing against the tide, the crests will be closer together than if the wind is blowing with the tide. All these factors can change over short distances.
The wave height given in forecasts is what is known as Significant Wave Height, and it measures the peak-to-trough height of the waves. Fine and good - but it doesn't measure the predominant wavelength, which is actually what makes the difference between a leisurely oceanic swell that may have a large SWH but a wavelength many times the length of your boat or a short North Sea chop with much lower SWH but a wavelength about the same as the length of your boat. The former is barely noticeable; the latter is horrible! Other local factors are where wave-trains from different directions meet - a common factor around headlands, which can result in nasty but quite local confused seas.

 

[Supertramp]

Completely agree with dunedin about wind, tide and headlands in the Irish Sea. Even without a strong wind, the swell built up from days of strong winds from the SW will persist and can roll across the wind waves creating lumps and holes. Even a slight or moderate wave forecast can give a very uncomfortable ride at the wrong times and near tide races and headlands.

Hitting slack water off headlands is usually the safest approach. It also tends to be less confused offshore and in deep water. With a "double headland" passage it might go better to cross to NI if the wind is favourable allowing a much easier crossing down the Irish Sea when the wind turns SW. Plenty of sheltering spots on the Irish coast. Don't underestimate Bardsey or the Stacks if you end up near them further down.

 

[Pye_End]

Currently looking at tomorrow's forecast where they predict 1 m waves every 4 seconds coming from the wind direction right on my beam.

Beam on seas, even of significant size, are fine ( so long as there is a breeze to stiffen up the boat). Bashing into a sea is another matter - especially if the seas are 'bigger than the wind' - ie if your boat cannot devlop enough power to keep the speed up. The short time interval doesn't help as it means steeper waves. Quartering seas can be uncomfortable and will test those without iron stomachs, and hard work on the helm.

 

[oldmanofthehills]

I can’t say that I have ever tried to assess waves with that precision. So much depends on the shape of the waves and the course one is trying to make that size in itself doesn’t really mean much to me. i suspect that the only way to make a useful assessment would be to follow a known source over a period of time and learn to relate the results to one’s own experience, much as we do with wind forecasts. In the sailing I have done, the effect of currents on wave shape have often dominated the actual wave size and this correction would need to be applied to a general forecast for it to make sense.

I agree. My dear navigator was greatly worried scurrying in a small boat from Scilly to Cornwall by the 2.5 to 3m waves until I pointed out that they were not breaking and just gently lifted the stern with only the occasional plash onto our stern cabin. Would probably have been fairly ok heading into it, if a bit slow.

2m waves off the tip Arran this summer turned us right on our ear and caused great displacement of crockery charts tools and such like, and some damage. A semi-race plus we were running broadside to the rubbish

 

[Yngmar]

It all depends. We've sailed in 3-4m Atlantic rollers that were spread out 12-15s apart without great discomfort (and dolphins jumping out of the back of the waves), while in the Mediterranean a steep chop over 1m is often seriously uncomfortable, as they're under 4s apart and almost square!

The angle matters greatly too. Upwind it's going to slow you down, get you wet, stuff the bow in and in extreme cases require handsteering so the boat doesn't fall off the top and crash into the through. On the beam, you'll roll, but it can be fine if there is enough wind to keep the boat heeling to one side. Waves on the beam with little or no wind are awful as you'll roll through the center and if unlucky, it can build up like a pendulum until stuff goes flying around. On the quarter is often the most uncomfortable, as the boat does a sideways lurch as first the stern and then the bow gets lifted, while also rolling. Going down waves, comfort is great and even in that silly Medicane we were surfing down big waves in what felt relatively safe, as the wind, current and our own boat speed of 8 knots was stretching them out relative to our boat.

And the landscape matters too. Sailing past a steep cliff, you'll get reflected waves that make a confusing sea, so you're better off staying a few miles off. Any current or tide pushing the other way can make very steep waves and get you thoroughly wet. Shallows will cause breaking waves that can be very dangerous.

For predicting, we look at the wave view on Windy, which gives you two swell directions (so you can tell if it'll be cross seas) and the interval (in seconds). You can also work out if you'll have distant swell coming from a different direction than the wind waves, which again create cross seas. And in the Med, Home - SeaConditions is quite reliable and will also give you the current forecast.

 

[Neeves]

I have always found it difficult to define wave heights when at sea.

Jonathan

 

[geem]

I think I read that the weather model predicted wave heights are only 5/8 of the maximum wave heights you are likely to see. So when sailing you are likely to see the occasional big one. As others have said, wave height is not a major issue normally its usually a short time interval between them thst makes it imteresting.
Sailing in places like the Irish sea rarely sees large rollers. However, off the Western Approaches or along the Southern Irish coast we have seen significant rollers. These usually cause no issues. Wind blow waves are a different matter. They often cuase a short steep chop that can be over laid on top of a large swell. If you then add local conditions such as tidal effect or strong adverse currents you can often create very unpleasant conditions. So in summary when predicting conditions for a particular stretch of water its ground swell, wind blown waves and local effects. If you have 2 of 3 of these from different directions seas state will be less than comfortable.
In the Caribbean sailing conditions are often lively. Wind blows from the East and ground swell comes from East with waves blown waves from East. Sea's are predictable with 15 to 25 kts of wind normal and stronger squalls. Sea is often 2 to 2.5m. Wind is usually on the beam or just ahead as current pushes West so you have to head up a bit. Spray is often flying but it's warm. Perfect conditions for a nice fast sail.

 

[AntarcticPilot]

I have always found it difficult to define wave heights when at sea.

Jonathan

You and the whole oceanographic community! Significant Wave Height is well-defined statistically and is (fairly) easy to derive from satellite altimeter data, and it corresponds pretty well with wave heights reported by trained and experienced ship-borne observers. But it a) doesn't say anything about the distribution of the heights and b) doesn't look at the wavelength of the predominant frequencies. The wave height we see on small craft is quite different to that seen by an observer on a ship. We're indifferent (more or less) to wavelengths substantially longer than our boat's length; we're affected more by short wavelengths with steep wave crests. But a ship would be more bothered by much longer wavelengths.

The statistics of waves is very definitely an area with active research; only a few years ago it began to be understood that the distribution of wave heights differs substantially from a normal distribution (in the statistical sense). and that so-called "rogue waves" are much more common than previously thought and also much more likely in certain well-defined areas of the oceans - I recall that the Agulhas Current was one area singled out.

 

[bignick]

Currently looking at tomorrow's forecast where they predict 1 m waves every 4 seconds coming from the wind direction right on my beam. Yesterday I had some waves at the end of my passage, but not sure how big those were, so can't really predict if it's going to be the same. But it was rough.

What forecast are you looking at? A 4-second period would be considered extremely short for a significant height of 1m.

 

[Buck Turgidson]

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