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post #16 of Old 09-30-2007 Thread Starter
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Originally Posted by chris_gee View Post
I don't follow your friend's comment that with the typical wavelengths of the Pacific a 35' boat can ride the crests most of the time.
Oceanwatch gives these figures. "Typically, the period of a long swell wave is 20 seconds, its length 624 m (2,040 ft), and its speed, 31.2 m/sec (102.3 ft/sec); a swell wave has a period of 10 seconds, a length of 156 m (510 ft), and a speed of 15.6 m/sec (51 ft/sec); and a wind sea wave has a period of 7 seconds, a length of 76 m (246 ft), and a speed of 10.9 m/sec (35.7 ft/sec). Waves in bays have rather shorter periods of about 3 seconds, and are 14 m (45 ft) long, with a speed of 4.7 m/sec (15.4 ft/sec); and ripples on ponds have periods of 0.5 seconds, and are 0.4 m (1.3 ft) long, with a speed of 0.8 m/sec (2.6 ft/sec)."
Just taking the ocean swell and windwaves wavelengths of 156 and 77m the wavespeeds are roughly 31 knots and 22 knots.
You would need a rather fast boat to travel at those speeds most of the time. Logically you would be on the rising part of the swell for just over half the period going downwind pretty much regardless of the boat length.
In shallow water you may be able to stay on the face at 9-10 knots for a proportion of the time, more so if the boat is designed to surf easily.
That reasoning of boat length in relation to wavelength seems to me to apply mainly in short choppy conditions in shallow water where the longer water line boat is supported on more than one wave rather than ploughing up and down each one.
Hi Chris,

Great analysis, and I reached a similar conclusion - which would contradict my friend's Steve's assertion, that's why I asked this forum. I can believe that bigger boats might give a more comfortable ride (all things being equals) but I did not buy the crest-to-crest story for the ocean.

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