Paul wrote:
"BrianH" wrote in message
...
Matt O'Toole wrote:
On Fri, 02 Mar 2007 08:05:53 -0500, Roger Long wrote:

Shaun Van Poecke wrote:

If the waves are big enough to worry about this, the
real issue is breaking waves. Any time you are in
water less than about twice the depth of the average
size waves going by, there is the possibility of one of
the few largest waves of the day breaking on that spot.
If you think a wave twice the size of the average
breaking hard on your boat would create more excitement
than you care to deal with, you should keep at least
twice the average wave height under your keel and a
little more when passing over isolated ledges.
Shallow water may cause steep waves, but prevents them
from building beyond a certain point. So you don't have
to worry about the occasional huge wave in shallow water
like you do in the open ocean.

"You don't have to worry"??? Last summer in the entrance to
Laguna di Marano, northern Italy, a returning professional
fishing boat was slewed and rolled by an extra large breaking wave and
dropped on the bottom while inverted, smashing in the wheelhouse and
killing two men. A third, who was on deck, was thrown in the sea and
survived.

This sounds like a case where deep-water swells ran onto a shallow bar or
entrance. In this case, when the swells start to feel the bottom they slow
down, getting closer together and steeper, and taller -- not a good place to
be, of course. On the other hand, when you are surrounded by shallow water,
any large deep-water waves will quickly lose their energy (often by
breaking) and become smaller. And as Roger said, wind-driven waves can not
build beyond a certain height in shallow water.

This whole area is shallow, around 3-6 metres out to at
least 2nm off-shore. The channel is dredged to about 5
metres, on average. The fishing boat was slightly out of the
channel, converging on it and well off from the entrance, in
about 3 metres depth. The wind was fresh and directly on-shore.
It sure shocked us sitting in harbour wondering if we should
be going out.