It’s a funny one to think about, but do you actually know why your board shoots you forward across the face of a wave? Most people don’t think about their board in this way, even the sort of person who does take note of tail shapes and concaves, is often not aware of the actual physics that are taking place under their feet.
Now to start with, Gravity of course plays a big roll. When you paddle into a wave and drop down the face, it’s Gravity pulling you down and making you accelerate, and even when you angle across the face of the wave, gravity still plays a roll. But if Gravity was the only force in play, you should go fastest when you run straight into the beach. In reality the opposite is true, and your board can travel way faster by setting a good trim line across the face of the wave, and the reasons for this lie in the way your board interacts with the flow of water on the wave face.
It’s a tricky concept to get your head around, but when a wave rolls through the ocean, it doesn’t actually move the water much. A rubber duck floating on the surface would pretty much bob up and down as the waves passed. However, from the perspective of a surfer riding the wave, this effectively creates a flow of water running up the face of the wave.
When we set our rail and ride across the wave, the rail of our board interrupts this flow of water. Some of that water will flow around the soft, rolled rail of your board, and it’s this flow that grips onto the board and stops it slipping down the face.
The rest of the water hits the bottom of the surfboard, turning the flow, and is pushed backwards through the tail. Now by applying Newton’s 3rd Law of Motion (every action causes an equal and opposite reaction) we can see that turning this flow of water will not only generate lift, but also forwards thrust in direct proportion the the volume of water that is displaced. This is why most modern surfboards incorporate some form of concave bottom, to help channel the water backwards. It’s also why it’s so important to learn to lock in your rail, and keep a high line on the wave face.
The image above shows cross sections of 3 surfboards on a wave. Surfboard 1 at the top of a wave has a 90° angle to the effective flow, this means that lots of the water is re-directed backwards, creating lots of speed. Surfboard 2 at the bottom of the wave has a much softer angle, so most of the water will flow past the rail, meaning that the board will be going much slower. Surfboard 3 is in the middle of a bottom turn, also re-directing water, and therefore also generating extra speed.
This is why good surfers spend so much time learning to put their board on rail, as by doing so they can create more speed, and therefore do bigger turns. So next time you look at a photo of a surfer with their board on rail, remember, since water weighs 1kg per liter, for every liter of water flying off the board, they’re creating 2.2lbs of lift and thrust, setting them up for their next maneuver!
Finale Some insightfull in surfboard physics!! Could you shed some light on why a surfboards turns when it is on edge. Is it because of the rocker line in the water?
Thanx.
Robert
LikeLike
Hi Robert,
In short, yes. When on rail the board will follow the curve of the rocker that’s engaged in the wave face.
I’ll try and do a longer piece on that in the future.
HK
LikeLike
Sorry, insight.
LikeLike
Sorry, finaly, i hate self correction. I’m dutch and so are the language settings on my device.
LikeLike
is it important to stand using one knee or try and just place 2 feet on board ? Thanks 🙂
LikeLike
Hi Angel,
The ideal would be to place both feet at about the same time. But this is requires a lot of “skill specific” strength and flexibility, so many people use a knee when they’re learning, and that’s ok. Just try to keep practicing the “Pop-Up” on the land so you can perfect the quicker, cleaner method as soon as possible.
HK
LikeLike
Why do the water flow backward not forward? Is it because of the rocker not touching the wave face?
LikeLike
Why would the water flow backward not forward? Is it because the rocker is not touching the wave face?
LikeLiked by 1 person
So it’s a little hard to answer here, but I have another blog in the pipeline to hopefully explain better. The short answer is that some water would shoot forwards, however it would go forwards at about the same speed and direction as the surfboard is travelling, so you wouldn’t really get a chance to see it. Most of the water is re-directed backwards because even when just trimming across the wave the board is actually angled slightly downhill into the flow, the water then takes the path of least resistance and flows out the back of the board.
LikeLike
So what happens on a noserider longboard when the surfer is on the nose? What balances out his mass? Is it the low pressure under the rear of the board which is caused by the flow of the water over the lower surface of the board?
LikeLike
Hi Dries,
Noseriding is mostly about getting water on top of the tail to counterbalance your weight on the nose. That’s why you’ll see most good noserides are set-up with a stall manoeuvre. There’s a few design tweaks that are often made to longboards to help with noseriding though, so maybe I’ll try and do a blog on that in the future. Thanks for the inspiration.
LikeLike
Hello Harry and thank you for all the info you share with us. It is great. I looked for your blog in pipeline to read a little more about the flow underneath the surfboard but I didn’t find it. Could u send me a link where I can find your writings ? Thanks again
LikeLike
Hi Manuel,
Thanks for your kind comments. Having something in the pipeline is an English expression that means you have something planned but not finished. Unfortunately that is still true, as I have been very busy at work and not had time to write anything on here for a while.
LikeLike
Hi Harry,
This is one insightful post. Actually I’m surprised there’s so little information about this subject online and you nailed it with this post. I was wondering, how the physics work while paddling and during the popup *before* you drop down or stick the rail in the face. If you can describe this process I’m sure it will be eye-opening.
LikeLike
Hey Shahar, glad you found the information useful. The physics when the board is being paddled is actually a lot easier as the board works like a boat in displacement mode. I wrote a separate article about it here: https://hjmknight.com/2015/07/20/hull-speed/
LikeLike