The Physics of Surfing for Beginners – Part II

In Part II of the series on the physics of surfing for beginners, we continue the conversation on the dynamics of waves and surfing.

This blog breaks down the physics of breaking waves, bathymetry, onshore and offshore winds, paddling speed, and tidal changes, and why this is good information for any new surfer to know.

It also provides some practical advice for beginner surfers to think about the next time they paddle out.

If you missed Part I of the series, you can read it here.

How does wind affect waves?

The wind energizes our oceans in a continuous cycle. Wind can add more energy to the waves already in motion, making them bigger and bigger – but the direction the winds are blowing makes a difference in surf conditions.

Winds blowing “offshore” refer to winds blowing directly out to sea. This props up the waves and changes how they break.

If it’s shallow water, the waves break with more intensity. If it’s a very windy day and the waves are choppy and disheveled, offshore winds can actually help clean them up which makes for better rides.

If there are light offshore winds and good groundswell forecasted, that means surfing conditions should be good.

Winds blowing “onshore” refer to winds blowing from the ocean towards land. This makes waves collapse more quickly since they are being pushed over from behind with the wind. When onshore winds are strong enough, they can make waves very choppy, meaning your chances for long rides are slim to none.

Why do you have to paddle?

Surfers need to paddle so that they are moving at a rate of speed and momentum sufficient to match the waves. If you are not moving at the same speed of the wave, i.e., if the wave is accelerating faster than you, it’s not going to pick you up and take you with it.

Paddling is all about the surfer generating enough kinetic energy so that by the time the wave hits them from behind, the wave and the surfer become one, now traveling together towards the shore.

As a beginner surfer, you have to get used to conserving energy during the paddle out so that by the time there’s a good wave coming in behind you, you can tap into that energy reserve and paddle hard to catch the wave.

You should be paddling as fast as possible so that you are propelling yourself forward with enough momentum for the wave to take you, rather than roll over you. Slow, shallow paddling makes it less likely for beginners to catch waves.

Why do waves pull you back as they get near?

Because the waves come a-rollin’. If you have ever seen cars driven through flooded streets, you may have noticed an undulating ripple effect occurring as the wheels moved through the water.

The same principle applies to the waves we surf. As waves travel across the open ocean, the undulations – the continuous rippling of crest and troughs – are caused by the waves moving the water in a circular motion, similar to how a wheel rolls in a constant circular motion as it moves.

The circular motion is why waves curl – the wave energy rolls from the water beneath the surface up to the surface and then back down again.

As the wave goes from its trough to crest (i.e., as the water builds up and the wave visibly gets bigger), whoever is in front of it gets sucked up into the curling front of the wave, lifted onto the wave’s crest, pushed forward as the crest passes by, and then lowered down to pretty much the same place it started off in.

This is fancy talk that goes to our earlier point – if you want to catch a wave to ride it, you have to paddle hard, not only to match the momentum of the wave but to counteract the natural force of the wave that is pulling you backward, into its building face.

If you do nothing as the wave approaches, you and your surfboard still get sucked backward.

If the wave behind you is almost at its breaking point, that is a situation where you not only get sucked backward but then can be thrown over the falls, which may or may not be fun. (Again, ocean waves are constantly rolling in a circular motion towards land – put a sticker on a soccer ball, let it roll, and you will see the idea of how waves suck surfers in and then back over the top).

Keep in mind that when the waves are bigger, the force generated by them is much more powerful, meaning the backward pull gets stronger.

Why does the nose of a surfboard have a curved front edge?

Shapers back in the day applied the same principle to surfboards that have been (and still are) applied to boats.

Having the nose slightly elevated off the water allows for the water to travel more quickly underneath the surfboard than on top.

This generates an upward lift (a.k.a. hydroplaning) that helps the surfboard glide over the water. This effect is key for surfing because it allows the surfboard to move more quickly with less drag.

How do low tide and high tide affect waves?

Short answer: the tide makes a difference on the break of a wave because of the change in water depth.

Tidal change is caused by gravity and the earth’s rotation during its daily cycle.

“Low tide” means shallow water and that the waves break farther out from shore.

“High tide” means deeper water and waves that break closer to shore.

Depending on what’s laying on the ocean floor at the spot you’re surfing, a rising tide (one coming in) or a receding tide (one going out) will make the waves tend to break somewhat better or somewhat worse than usual.

Keep in mind that surf forecasts usually mention whether a surf break offers better conditions at high or low tide. This is helpful information to look for and know in advance if you are trying to have the best surf experience possible.

Looking at an example of how all these forces work together…

Let’s say you show up to surf when it’s low tide with onshore winds and not a lot of swell. In these conditions, there will be too little water depth and not enough energy to generate waves for stand-up surfing. There’s not going to be enough momentum in the water for surfers to ride.

Even for beginners who are just starting to learn how to surf, understanding these basic principles of physics will help make sense of where to go when to go, and what to expect when you get there.

If you haven’t already checked out HOKALI’s blog on how to read a surf forecast, definitely give it a read. Now that you know the laws of nature at play in producing the conditions that make for great surf, you will know what to look for in the surf report and you can begin to figure out what spots are your go-to breaks for whatever kind of day it is.

If you are a scientist at heart and interested in learning more about the science of surfing, especially how to predict when the waves will be good, check out Professor Butt’s Surf Science: An Introduction to Waves for Surfing.