Re: [Phys-l] surfing

["John SOHL" <jsohl@weber.edu>]

Hi Carl, 
 
It has been a while since my surfing days, but here goes:
 
The key issue is that you need to go down hill as fast as the hill catches up to you. In other words, you need to be on a steep part of the wave so that you have a gravitational force that speeds up the board as it "falls down the wave front." 
 
If you get too high on the wave  you slow down and the wave goes under you and you lose your forward velocity. If you get too low on the wave it will catch up and sort of "scoop you up." The problem is, when you are low and the wave catches up you need to be prepared to ride it or you will either fall over or be surprised by the wave's speed and it will pass under you. 
 
The above description is very crude but it catches the general idea. In actual practice there are a lot of very interesting force vectors (much like sailing) where you need to edge the board into the wave to carve a path and provide the needed forces. 
 
To address your other questions think about the description above. An open ocean wave is typically of very long wavelength as compared to the amplitude. The result is that you don't have much of a "down hill" region on the wave to work with. I'd be very surprised if that would work. 
 
I'm no expert on rogue waves, but my understanding is that waves with massive amounts of energy are often of very small surface amplitude when in the open ocean. (Less than a meter if I remember it right.) So that wouldn't work for surfing. 
 
I have quite a bit of ocean kayaking experience and have experienced very large waves in Alaskan seas. As long as you are not near shore the waves are not an issue because of the long wavelength and the resulting shallow slopes. The biggest danger there is not uniform waves but chop. 
 
In all cases above, I am referring to the wave amplitude above the sea surface. The actual amplitudes are much larger but that doesn't effect a small craft such as a kayak or surf board when in deep water. 
 
Hope that helps and I'm late for a meeting... 
 
John 
 
 
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John E. Sohl, Ph.D.
Professor of Physics
Weber State University
2508 University Circle
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> > > Carl Mungan <mungan@usna.edu> 7/9/2009 9:23 AM >>>
I'm curious about the physics of surfing (for no particular reason 
other than curiosity... and note that I don't surf so I don't have 
any physical experience to help me build intuition). Here are a few 
initial questions that come to mind. If anyone sheds light on these 
questions, they might lead to other interesting questions.

1. It appears that surfers like to catch a wave just before it 
breaks. Why do breaking waves couple momentum better to a flat 
surfboard?

2. Given that breaking waves occur in shallow water (presumably 
because the lower water is slowed down by the bottom and so the 
higher water rushes forward and breaks), would it be possible to surf 
in deep water on "ordinary ocean waves" (in the absence of a net 
water current or air wind)?

3. I've heard of "rogue waves" occuring in deep water. Could one surf on them?

Carl
-- 
Carl E Mungan, Assoc Prof of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu     http://usna.edu/Users/physics/mungan/