Shaun Van Poecke wrote:

When sailing in shallow waters, what do you allow for swell/waves?

Half the wave is down and half the wave is up so, if you are cutting it real
close and just worried about bumping the keel, you subtract half the height.
That's a little nuts though because bigger waves come along, soundings are
scattered, old mooring blocks and other stuff gets left on the bottom, etc.

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.

--
Roger Long