# What Makes Surfboards Go

Surfers or surfboards redirect the forces of planing.

Planing is often associated with high-speed boating, but in fact, any partially submerged relatively rigid object in a flow will experience planing.

The phenomena has been explored extensively in water-craft hull design. Savistsky was one of the first to fully explore and approach the subject empirically and, to a lesser extent, analytically - his work remains the basis for much of what followed in the field.  Below is his simplified illustration illustrating the forces at play during planing.

Here, Savistsky, in cross-sectional view, illustrates a simple plank moving horizontally over some water (or other liquid surface). (But because such considerations are relative, you could equivalently view the above illustration as water flowing past a stationary plank.) If we consider the case in which the plank is moving, the means by which it is moving is assumed to be via some external power source, say an inboard or outboard engine, or perhaps the plank is being pulled along, or maybe its means of propulsion is from sail.

The key point in the diagram however is that the forces of planing generated by motion through the fluid, or by the fluid flowing past the the object, are up and back (back as in the direction of the flow relative to the plank, generally in the direction which you would associate with the term ‘drag’.  An engineer would say that the resulting force of planing will have both ‘lift’ and ‘drag’ component.)

But this diagram needs to be altered for surfing. Surfers generally don’t have a outboard engine (though admittedly now kites and sails are often used.) The fix however is simple, just rotate the diagram.

Surfboards derive their means of propulsion by getting in the way of flow.

The inclined surface is now the face of the wave, the water moving both upward, as well as forward. And now it can be seen that the planing forces have a component in the direction of that forward motion. (The diagram here doesn’t include the forces that arise from the interaction with the forward movement of water, which is actually decelerating, see “The Decelerating Wave-form”.) But more importantly, there is no need to assume the existence of an external power source like an outboard engine. The flow is the result of the upward acceleration of water of the shoaling wave – and the propulsion comes from the surfboard surfaces redirecting the forces of planing.  And it is this redirection of the forces of planing that makes surfboards go, at least under most conditions.  In fact, it is what separates the sport from other more ‘gravity reliant’ sports, like skiing. Skiers don’t move by getting in the way of snow, however surfboards move by getting in the way of flow.

How these forces are redirected is up to the surfer, and will depend on how he chooses to present the bottom surfaces of his surfboard to the flow. In particular, its quite possible to redirect them so that there is a component of force in the transverse direction. (To visualize this, think of your flatten hand subject to a jet of water. What direction would you hand tend to move if you angled it relative to the on-coming jet?)

Gravity plays a crucial role in the dynamics.

A surfer’s connection to his board is tenuous, unless he’s strapped in, or has grabbed a rail. Under most circumstances, if orientated correctly, gravity will keep him on his surfboard. This isn’t to say, that at other times, other forces aren’t contributing.

However, gravity also plays additional roles.  It is used as a counter force against the forces of planing, keeping the surfer in the upward flow on the wave face. In addition, it is also used as a means of acquiring additional kinetic energy, by sliding down the wave-face, much like a sleigh sliding down a hill. And though gravity plays a crucial ever-present role in the dynamics, for the most part, gravity itself isn’t what allows a surfer to move transversely across the face. Those forces come from the redirecting the forces of planing (as described above.)

Modern surfing styles tend to utilize gravity as an additional source of kinetic energy (dropping then climbing then dropping etc.) far more than the more classical styles in which ‘trimming’ (moving transversely) was emphasized. But even in the modern style, trim is utilized extensively, especially when the surfer is able to get himself in the “pocket” or “barreled”. Climbing and dropping isn’t really an option is these situations, all of his propulsion comes from the redirection of the forces of planing.

The principal source of propulsion for the surfer/board is gravity.

1.  If the sum of the forces pushing the surfer/board toward the top of the wave does not exceed the force of gravity, the surfer/board will continue to move along the "moving slope" of a wave.
2.  If the sum of the forces pushing the surfer/board towards the top of the wave exceeds the force of gravity, the surfer/board will move up the face of a wave.
3.  If gravity exceeds the sum of the forces pushing the surfer/board toward the top of the wave, the surfer/board will move down the face of a wave.
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thanks kcasey for posting (all three related threads) interesting food for thought

I appreciate all 3 posts KC…

Interesting blog stoneburner.  … and thanks for reading the posts.

I think waves are what makes surfboards go

the better the wave most likely the faster any surfboard will go

boards and waves go togethor

ie they gotta fit

to make it fun

or its not…

The surfer makes the board go with subtle changes in position of the fins, rail and bottom surface in relation to the force of the wave.

I think waves push the surfboard towards shore too. A broken wave (whitewater) will push a board or anything bouyant to shore. When we ride the unbroken wave face we play with several forces. There’s the wave moving towards shore, gravity with the board falling from the top towards the bottom of the wave, and the forces we generate with the fins and the edges of the board.

If we do nothing but go straight in, we can only go as fast as the wave is moving towards shore. When we use the wave face and the interaction of the board and the wave, we can go much faster. Learning how to push the board against the force of the wave is the key. The wave is always changing and we have to be able to put our boards into the right spot, at the right time and with the right amount of force to get the maximum reaction. That skill keeps going higher and higher as the kids get better and better. I think most of us reach a point where our bodies are too old to have the reaction time, the agility, and balance. The young kids will always have that advantage, more flexibility, greater power to weight ratios, fearlessness, better balance.

Add to that the differences a board’s design makes to how it will interact with a wave plus how each person rides each board and you get endless variables.

P=1/2 ro v² is a part of Bernouilli formula. Those formulas can only be applicate to constant flow. Because of speed direction variation and Re number at contact, surfboards are not in constant flow by far, this formula is completly wrong here !

It’s pretty ridiculous to use mechanics basis every where to explain or justifiy surfboards without a bit of respect of science and knowledge.

Landlocked?..not surfing…too much time to think ?

really funny…Kcasey, Stonney and Huck…to much work , to far inland. Get your ass down to the beach and go surfing…Really!!! Go surfing…Take Lamat with you…get your nose out of the books and go SURFING !!!

Kcasy you have a mistake in your premise. Water does not move in a wave or push the board a wave in energy passing thru the water the same as a sound wave does not move the air. The water is relatively motionless. Surfing is the manipulation of the energy pushing the wave and gravity.

… " WHAT makes surfboards ’ go ’   ? "  *

let’s see…

cheap price , pretty colours , ‘what the pros ride’ / impressionable buyers , and people not wanting to spend money

the same things that make surfboards return , really …

( …for repairs … )

[ * ‘go’ =  out the door of the shop , for those that need the explanation  [ ?! ]

until the wave breaks

http://www.swaylocks.com/forums/decelerating-wave-form

While your list of bullet points is correct, the idea that gravity is the source of propulsion is erronious. Gravity is a constant force, which is always there (it is also the reason you can sit in your chair), the “source of populsion” in surfing is the wave (specifically, the flow of the water in the wave). The wave (and secondarily the surfer) is the source of energy when it comes to surfing.

Saying that gravity is the source of propusion is the same as saying that your weight is the source of propulsion. Kind of a silly thing to say…

The water certainly moves in a wave. How else would flat water become a wave if it didn’t move up?

Imagine you are at the shore of some ocean, and you’re standing in waist deep water. The waves that day are small.  As a small wave passes by you notice the water level rising. At its peak it rises to the level of your chest, and then, as it passes, the water level drops back down. What you would have just experienced was water in motion -  it went up and then it went down.  Water in relative motion is a ‘flow’.  (It also moved slightly forward in the direction in which it was traveling, and then back, but the up and down motion suffices to make the point.)

I have often encounted, in oceanography texts, or in the literature, statements along the lines that the ‘net’ movement of water during the passage of a deep water wave is virtually zero. The key term  being ‘net’.  If you encounter an author that leaves out this term, or fails to imply this term is some other way, I suggest you contact them and point out his/her error.

No problem for me i surf as i want, about 4 to 5 time for week,  i don’t spend so much time in books, i just do it when i am paid to read them to my studdents LOL.

A snow skier is propelled/pulled down the slope of a mountain by gravity.  By adjusting the angle of descent, the ride can be longer or shorter.  The skier slides down the slope because gravity exceeds the friction of snow (frozen water).

The wave is a slope of water.  The wave is a moving slope.  As  long as the face of a breaking wave does not closeout, the surfer can change the distance/duration of the ride by adjusting the angle of descent down the moving slope of the wave’s face.  Gravity pulls/propels the surfer down the face of the wave.  The surfer/board slides down the slope of a wave’s face because gravity exceeds the forces pushing the surfer/board toward the top of a wave.

Very different from sitting in a chair – unless of course, my chair is very slippery and sloped downward toward the ground.  Then gravity will cause me to slide out of the chair.

Unrelated to my response to ObProud:

The kinetic energy in a wave is the up and down motion of the water as the waveform passes through it.