Math!

[dmksails]

Ok, school was a looong time ago.

I'm looking at the NOAA website that shows sea conditions at various bouys across the globe. Neat site. I see that the bouy off the Oregon coast is showing a wave height of 5-ft and a average period of 6 seconds.

Question - what would the distance between wave crests be?

[dwoodall]

Here is a handy little calculator from the fine folks at hyperphysics (one of my favorite web sites for such things). BTW, this is not a high school subject. I didn't run across this until I took a fluids course eons ago. I taught AP Physics a few years ago and mentioned this, but we didn't do the math because the derivation of this thing is a beast having to do with energy transfer.

Wave Motion

I vaguely remember the way the formula looked, but not the numbers. All I really remembered about it is was it had to do with the depth of the water and the wave motion looks mostly like a sin wave.

Good luck bouncing around.

Regards,
Doug.

[Ziggy]

Originally Posted by dmksails

Ok, school was a looong time ago.

I'm looking at the NOAA website that shows sea conditions at various bouys across the globe. Neat site. I see that the bouy off the Oregon coast is showing a wave height of 5-ft and a average period of 6 seconds.

Question - what would the distance between wave crests be?

Assuming the buoy is in deep water, the relationship is:

wavelength = gT^2 / 2pi, where g is acceleration due to gravity (9.8 m/s^2), T is the wave period, and pi is the ratio of circumference to diameter of a circle (3.1415926....), or:

wavelength = 1.56 * T^2

So if the wave period is 6 s, the wavelength is about 56 m or 184 feet.

It gets more complicated in shallow water (depth less than wavelength / 4)

[Mirage35]

WOW! This is what I love about this forum: you can ask an obscure question like this and somebody will have useful information.

[StuM]

About 184 ft or 56 meters. And height is irrelevant.

A couple of useful conversions:
Speed of wave in knots = 3 x Period.
Length of wave is 1.56 x Period Squared in meters, x 5.12 in feet

It all comes back to the same equation as used for Hull Speed
Speed of a wave = 1.34 x Sqrt(length of wave)
and period is length/speed.

[tkeithlu]

To clarify the completely correct but slightly confusing formula above, wave length in feet equals 5.12 times wave period squared.

[dmksails]

Thanks - exactly what I was looking for!