I have seen crew on the boat I have been on report waves as 8 feet high, when
the wave did not come above the boat's freeboard, and crew on other boats in
the same water at the same time report waves of 20 feet. All in water that
won't support 8 foot waves without breaking.

The method for getting accurate wave height data from a boat is to know the
height above the waterline of some points at different heights. You then
stand on one and stretch or crouch until you find yourself looking across
the tops of the waves when the boat is in the trough. It usually takes a
few waves to get an average but it's quite striking when you get the right
position. You then measure from eye to feet and add it to the height known.
That's the average wave height.

The first time you do this, you'll probably find yourself lying flat on the
deck trying to get the right angle. Like I said, waves always look a lot
higher than they really are.

It's almost impossible to measure the height if individual waves. However,
if you know the average wave height, you can then proportion by eye and get
a reasonable estimate. I still wouldn't trust my own estimates just looking
quickly without going through the above procedure. There are just too many
illusions and too few reference points.

I've often been amused to have even fairly experienced sailors say that the
waves must be eight feet. I usually don't point out that our eyes are
perhaps six feet above the surface and we can still see all the tops when we
are down in the trough.
--

Roger Long



"Steven Shelikoff" wrote in message
...
Roger Long wrote:
I've been kind of interested in some of the posts about Lake Erie
conditions to see references to 12 foot waves. I know the waves are
shorter and steeper there due to the lighter water and shallow depths.
Twelve footers would considered pretty big however, even on the ocean.

I've been pretty involved in past years with marine safety and accident
investigation projects and this gave me a chance to look into wave height
reports. There is a pretty consistent tendency for even experienced
seaman to over estimate wave heights by about 100 percent.

Years ago, when I used to fly tests for the US Navy, part of the test
report was the sea state. Even though it's not the same as wave height,
sea state is closely related to wave height, wind speed, etc. It's more a
"roughness" measure and is determined from the air by the density of
whitecaps and goes from 0 to 9, but we usually wouldn't conduct tests in
anything higher than 5. It was usually a coordinated test, me in the air
and some surface and/or subsurface assets. The number I came up with from
200 to 1000 feet up was almost invariably at least one lower than what the
boat came up with bobbing on the surface.

Steve