Re: [Paddlewise] Wave Speed - (totally trivial question!)

From: Peter Osman

>
> Can anyone tell me if all (sea) waves travel at roughly the same
horizontal
> speed (relative to the current of course). I've been trying to observe if
> this is the case for some time and am coming to the conclusion that they
do
> but am not sure if they speed up when approaching shore?
>

I knew that I had this written down on a card somewhere, and have finally
found it (reference unknown):

"For waves in water deeper than half the wavelength,

    Period (secs)  Wavelength (metres)   Speed (knots)
        4                         25                                12
        5                         39                                15
        6                         56                                18
        7                         76                                21
        8                       100                                24
        9                       126                                27
      10                       156                                30
      11                       189                                33
      12                       225                                36

So Wavelength equals 1.56 times the square of the Period,

and Speed in knots is three times the Period in seconds.

When the depth is less than half the wavelength, the length decreases, the
height increases and the period remains the same (i.e. the wave goes
slower). At a depth of one tenth the deep water wavelength the height
increase is marked, the wave is unstable and ready to break."

So you can sit out at sea bobbing up and down and time the period (easy
enough). Working out the wavelength might be more than you want to attempt,
but multiplying the period by three to get the speed is not too difficult.

Then you can turn towards the shore and see if you can paddle at wave speed!


Allan Singleton
Hamilton NZ

And I hope the table formatting survives.






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