Surfing??

[Hans Friedel]

Do anyone know the principles behind surfing?

If I am going down wind I can easily feel the extra energy from the waves even if the waves are small. But what are the mechanics behind it?

Hans

[tspeer]

There was an excellent paper by John Letcher on this at one of the Chesapeake Sailing Yacht Symposia:
Letcher, John, "Surfing- Motion of a Vessel Running In Large Waves", Third CSYS, Anapolis, MD, 1977.

Also, see:
http://www.curtin.edu.au/curtin/cent...gscience99.pdf

[innomare]

I would think you could consider surfing simply as sailing downhill.
The hill may not be very high, but it's moving forward.

[tspeer]

It's anthing but like sailing downhill. The wave isn't just moving forward, the water is accelerating. "Local gravity" - gravity plus the acceleration - is always perpendicular to the water surface. This is different from a hill, where there is a component of gravity that is parallel to the surface.

And then there's the orbital velocity of the water. If the wave is round-crested, the current at the crest is in the direction of the wave, but isn't moving as fast as the visible movement of the shape of the water. In the trough, the current is in the opposite direction to the wave motion.

So if the wind or the motor is producing enough thrust to move the boat at a given speed in flat water, when the boat approaches the crest it will accelerate because it's moving into a favorable current. Then as it passes the crest, the speed through the water seems to increase because its moving into an increasing head current. This is also why the boat is slow in the trough - it is fighting the adverse current from the orbital velocity.

If surfing was like sailing over hills, the boat would have its slowest velocity at the wave crest and would be moving the fastest in the trough. But that's not how it is.

[Hans Friedel]

Thanks Tom for a great reply

Hans

[mistral]

Tom explained you even why flat-bottomed light boats surf better; they have most of their hull submerged in rapidly accelerating fluid on the wave's crest, while a long-keeler will have a huge part of the keel buried down where water particles have less acceleration, 2 meters under the wave's crest

Mistral

[CDBarry]

The reason surfing works is that the body surfing is initially at a non-zero velocity relative to the wave.

Thus it acelerates less than the water when caught by the wave, so it sees down in a slightly different direction that the wave face, (which is level with respect to the net of gravity and acceleration) and there is a forward component to the buoyant forces - yes, the boat falls down the wave face. This is why extreme surfers need to be towe in to monster waves, to get enough velocity before being caught,

(And yes, this is from Letcher's paper.)