Understanding Waves

There is something extremely wild and rugged about big waves crashing up a windswept beach. The force and power of the swell as it silently rolls towards the land before curling its top and sending white water smashing and crashing, then fizzing like champagne as it rushes up the sand, greedily grabbing little shells with its foamy fingers and dragging them back in to the ocean depths.

Sailors have a love hate relationship with this incredible force that transforms a soft glassy mirror calm sea in to a turbulent angry watery body, capable of making your life miserable and smashing your boat to smithereens.

As with all things in life your perception is your reality. A wave that a terrifies a sailor will be pure ecstasy for a surfer. I can’t change your perception, but I can help you get to know waves a little better, and give you a little insight in to what makes them who they are.

Do you have an issue with waves in your life? Do you want to get along better? Let’s get it off your chest and tell me all the things you don’t like about waves.

OK I’ll start:

Waves can be annoying. They can act like careless noisy toddlers, spilling your drinks, wetting the bed and throwing items all over the floor. Or they can be like a terrible hangover and make you feel sick – all day.

Waves can unpredictable. One minute they can be calm and gentle, then they can be irregular, confused, wild and angry.

Waves can be dangerous, they can act like a big bully, trying to roll your boat over, make you go too fast or too slow, they can crash over the deck threatening to wash you or your stuff over the side, they can fill your boat up with water and try and sink it.

Waves can be scary – described as rogues or freaks – the kind of people you want to avoid in general day to day life.

But surely there must be something you like about them!?

Waves can also be fun. You can surf down them and make your boat go fast – like on a sleigh ride. They can be beautiful, you can watch them crashing on the beach and fall asleep to the sound of little waves lapping at the hull. They can make you feel like you are alive when a light spray splashes up as you sail along. Sailing without waves would be like driving without corners. Comfortable, but pretty boring.

Well, if you are planning on spending any time at sea, waves are a part of life. It is important that you and waves learn to get along a little better…

What is a wave?

A wave isn’t actually a tangible thing, but rather a force that is moving through the water. They are generally made by the wind which blows across the surface of the water creating friction. The water inside a wave moves in a circular motion as the energy passes through it. There is no forward movement (unless the top is breaking).

They are measured by length – the distance between crests and height the distance between the crest and the trough.


Generally speaking, the stronger the wind, the longer the period of time over which it blows and the longer the fetch (body of water where it blows over) the bigger the waves will be.

A big rolling wave can also be described as an ocean swell. Long after the wind has stopped blowing the wave force it has generated can continue travelling for hundreds of miles as a swell.

So the surface of the water can be glassy calm but there can still be a big swell rolling through which could have been generated from a storm many miles and days ago somewhere else in the Ocean.

Weather forecasts will generally give you the swell and sea state. Sea state relates directly to the wind that is blowing in your location at a particular time. It can be described as small ripples – calm, smooth or slight, wavelets with breaking crests – white horses, moderate waves with spray – rough or very rough foaming crests with the tops being blown off – high, and then when the waves start blowing in streaks and flying up in the air – high or phenomenal, you know its really windy…!

Forecasts might also mention “Significant wave height” this is calculated as the average height of the highest 1/3 of the waves. So some waves will be even higher (and some lower) than the significant wave height.

Waves – or the force traveling through the water can move at some speed. In doing some research according to an excellent article on Boatsafe.com a 40′ wave can travel at a speed of 8.48kts, and a 100′ wave at 13.4 kts… (not sure that I am keen to see a 100′ wave any time soon…?)


In the photo above you can see an example of some rough waves – but no swell. There isn’t enough fetch (distance) to create a swell, but the waves are big enough to make your life very miserable and downright dangerous if you are in a dinghy trying to get on board your boat.

The Beaufort scale correlates the affect the wind has on the surface of the water. You can also learn to tell how windy it is by what the waves are doing.


What makes waves different

Wind blowing over a body of water creating waves as in the picture above are one thing, but there are many other factors which make waves different and more difficult to handle.

Wind against tide. We get this one a lot in Lyttelton Harbour. The prevailing Easterly breeze blowing straight up the harbour blowing against the outgoing tide. The two conflicting forces make the waves stand up higher with steeper faces and guarantees a wet and wild ride.

Similarly when the wind and tide are travelling in the same direction, the friction against the water is less, and the sea can be smoother.

Rising warm air has less of an effect on the water than colder or heavier air. Cold air creates much steeper waves than a warm front will.

Current in narrow channels can create some interesting waves – especially when the swell or wind is blowing in the opposite direction to which the current is travelling. Current can also create eddies like big whirlpools and standing waves when the seabed is an unusual shape, pushing the force of the fast flowing water upwards. Like here in the Karori rip in Cook Strait.


You can also get conflicting swells – so for example an Easterly swell combined with a Southerly swell and a Nor Westerly wind can create a very confused sea, and where the two swells meet you can get a much larger wave than you were expecting. This can make your voyage feel more like a ride in a washing machine.

The seabed also has a big effect on waves. When big swells travelling over deep oceans reach an area of shallower water, the force of the wave is pushed upwards, causing them to pile up and creating steep faces and breaking tops. This is also what causes waves to break on a beach. This is what makes places like the Bay of Biscay, and Foveaux Strait here in NZ so rough – as those areas are much shallower than the deep ocean from where the big swells have originated.

Then you get waves caused by underwater disturbances – like a tsunami. These can travel at very high speeds and across entire Oceans where they can then cause a huge amount of damage when they reach land – but I believe that if you encounter them in deep water then you are OK, but correct me if I am wrong.

You also get man made waves called wake which is usually caused by inconsiderate other boaties who have no clue of the damage they can cause when the water that is displaced by their boat zooming around gets pushed out in both directions perpendicular to their course. When your boat gets hit by one, particularly if you are in a dinghy or a kayak or tied up alongside other boats you will learn to really dislike them and their creators.

Another thing to understand about waves is that it isn’t necessarily the size of the wave that is the problem – but rather the distance between the waves, the shape and steepness of them, whether they are breaking and the trough on the other side that can really cause the damage.

Huge ocean swells spaced over a long distance can be ridden over quite comfortably, but a smaller steep breaking wave can cause all sorts of destruction. The base of a wave – or the measurement from one trough to another trough, can only support a certain height. Once that height is exceeded the wave becomes too steep and it starts to break and collapse forward. The estimated formula is 1:7 – if the height of the wave is greater than 7x the length then it may break. So if the height of the wave is 1m and the length of the base of the wave is 7m of less, then it is going to be quite steep and start breaking. (Not a very creative example but I am terrible at maths so it works for me)


So understanding waves is all very well you say, but how does it effect me?

Well if you are in a boat – and if the waves get big enough you are going to be concerned about three major things (other than getting wet and feeling seasick, you might be concerned about that too):

Rolling, Pitching & Yawing

Rolling occurs when the wind and waves are on the beam – or on the side of the boat, coming at you at 90 degrees to the direction in which the boat is heading. A recent study found that a breaking wave with a height of just 30% of the hull length of your boat can potentially be big enough to roll it, and a wave 60% of the length of your boat can most definitely roll it right over. So for Wildwood an 8.8m boat, you’d only need from a 3m high breaking wave to potentially roll her – if hit beam on.

Of course some boats are more susceptible to rolling than others, and you can learn more about that with stability curve examples in this post.

Pitching is caused when the wind and waves are right ahead or dead astern, and your boat is hobby horsing up and down like a bucking bronco. You need a much bigger wave to do a pitch pole – which is where the bow digs in to the trough of the wave or the next wave in front and the boat rolls stern over bow.

And Yawing occurs when the waves in a following sea are travelling faster than the boat. When a wave comes up behind and starts to break, the crest hits the stern and starts swinging the boat off course, the boat starts sliding down the front of the wave and then the bow ends up in the slower water travelling in the opposite direction at the bottom of the wave. This can turn you around cause you to broach and possibly capsize.

Check out this video I found on YouTube:

So with this knowledge, you now know that you shouldn’t allow the boat to get side on to breaking waves, nor allow a following sea to start pushing you down the face of a steep wave.

To avoid doing this you can firstly check the weather forecast, the sea state and any nasty currents and tide factors and try to avoid being out in conditions which might be unsuitable for the size of your boat.

But if you happen to already be out there and caught up in some ugly waves you can deploy some seamanship techniques to help your boat cope with the wave conditions.

Instead of punching head on in to steep waves you can try to take them on a slight angle tacking back and forth and adjusting your speed to suit the waves to get the more comfortable ride. Even better – try not to sail upwind, check the forecast before heading out.

If you are sailing in to the waves, try to take them head on as you go over the crest to avoid being hit sideways, and then steer off once you are over the crest.

You could sail in the lee or shelter of some land to try and reduce the area of fetch of the waves. Similarly you want to avoid sailing close to land where the wind and waves could blow you on to the lee shore – so give yourself more searoom if the wind is blowing you towards rather than off the land.

Avoid sailing in shallow water. As mentioned before the waves can be shorter and steeper in shallow spots.

You can deploy a drogue – which is towed from astern and holds your stern in to the waves – preventing you from surfing down the wave face and broaching at the bottom.

If you can go fast enough – like in a speed boat you can position yourself on the back of a wave and travel at the same pace.

Or you can deploy a sea anchor which keeps the bow in to the waves while you sit and wait for some more suitable conditions.

You can heave to, or stop the boat and wait it out – check out this post to learn how.

Keep a good look out all around you. Waves can come from any direction. Keep checking the forecast too and get an idea for what might be heading in your direction and how that will affect the sea state.

Check out this fabulous book: Heavy Weather Sailing. And practice the techniques that are most suitable for you boat and the conditions you encounter.

Hard to get a good pic of waves!

There is one final thing you need to know about waves. They are very camera shy. No matter how enormous and scary they appear to the naked eye at the time, they look absolutely tiny on camera when you look at your photos later while you attempt to explain to your friends how big the seas were, and they shake their heads and say “It doesn’t look THAT bad…?!”


13 thoughts on “Understanding Waves

  1. Lovely article Viki, like a road with out corners, very good, One thing I have heard and can be witnessed is the set of seven rule, when waves combine to produces a larger wave in a group, I am a real believer on this and it seems to be fairly true when watching the boats jump about at Magazine bay in a southerly too. I’m sure you have experienced this on your coastal passages. Here is an interesting link https://www.livescience.com/33624-waves-ocean-sets-seven.html

    Liked by 1 person

  2. What a great post, Viki! I haven’t had great internet in a while so haven’t been reading blogs – this was a really fun post to come back to! And have to agree with you about your final point on camera-shy waves!

    Liked by 1 person

  3. Great post! An interesting fact is that the amplitude or height of waves varies with the third power of wind speed (given sufficient fetch and water depth), which means that a doubling of the wind speed can produce waves 8 times the height! Even more impressive is that the power of the sea (wave energy) varies with the fourth power of wind speed (maximum sea condition), which means that doubling the wind speed can produce waves with 16 times the power! This is why storms and hurricanes have such destructive potential – going from a Near Gale (~30 knots) up to 60 knots of wind can increase wave height 8 times and wave power 16 times! (again, maximum sea condition)

    Liked by 1 person

  4. Pingback: RYA Offshore Yachtmaster | Astrolabe Sailing

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