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post #9 of Old 11-05-2012
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Re: Why Waves Look Bigger From a Boat


Firstly I agree that 70 foot waves are improbable/unlikely. Many times I watch videos of rescues where the seas are reported as 60 or 70 ft and when you do simple comparisons with other elements (boats, aircraft, etc.) you can see that it is not true. It is also no coincidence that the graph posted stops at 55 feet. Anything above that is seriously abnormal.

The graph that you posted leaves out one vital element (IMHO) and that is duration of the wind. We were caught in a strong blow in the South Pacific, a condition known as a squash zone. Winds of 75 knots gusting 85 but the storm system moved very fast by comparison to normal tropical depressions (35 knots compared to some storms that travel at under 10) so the duration of "our" blow was just 24 hours compared to sustained blows of 4 days or more.

After 18 hours of 75 -85 wind speed we estimated the wave heights at 35 to 40 feet so considerably smaller than the 55+ feet suggested by the graph. I'm not contesting the graph, I think it is probably correct for sustained high-wind conditions.

We gauged the height of the waves by observing how much wave as behind and ahead of our boat when going up and over the swells and figured that the surface of the wave was probably about 60 feet but that is not its height. It is the hypoteneuse of the triangle if you will, much longer than the vertical.

The problem with 35 foot waves is when "just the top" breaks and becomes a foamy roller, it can be up around 10ft of white water which on your local beach is a really scary-sized wave and it hits the boat with astonishing force. I know, we got hit twice.

Thanks for the geometry lesson, most interesting.

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