* Shaun Van Poecke wrote, On 3/2/2007 6:17 AM:
When sailing in shallow waters, what do you allow for swell/waves? oviously
this would depend a bit on the bottom, the boat, and the type and size of
waves/swell, but are there some good rules of thimb for this?

ie. if you are sailing through waters 10' deep and there's a 3' swell, do
you consider it as being 7' waters?

No - the wave height usually used is "significant wave height" which
is defined as "the average height (trough to crest) of the one-third
highest waves valid for the indicated period."

Solving the simple geometric problem leads to a depth of 8.5 feet.
However, things can get complicated real quick. For starters,
"significant wave height" allows for the probability of some waves
being higher. In addition, if the ocean swells are deflected by an
island, you can easily create interference patterns with occasional
waves twice the height of the normal waves. Anyone who has sailed
behind Block Island when there are ocean swells coming in has felt this.

And then we get to the issue of surface waves interacting with the sea
bottom, which is a whole study unto itself. You certainly don't want
to be in a position where you're in breakers. Personally, I don't
like being in real skinny water when there are any waves at all.
Trying to infer the last few inches of clearance from roughly
understood data is futile.

If this stuff is of interest to you, read Van Dorn's text on
Oceanography; I think I might pull it out today since its pouring rain
here now!