Actually, the rise and fall of water over a shallow plug or area does cause
a "suction" effect and will pull you toward it.

See for yourself, just watch the water over a shallowly submerged boarding
ladder, piling or rock. The structure truncates the wave structure
underneath the surface and only allows higher frequency components of the
wave to exist above the shallow, hence the faster waveforms in the water in
the decaying response waveform and the lower frequency components in the
forcing function. In other words, the water drains faster than it fills (or
has a different waveform) and a relative "vacuum" is formed.

BC


"Navigator" wrote in message
...
I've noticed something strange. I had suspected it for sometime but just
a little while ago my suspicion was confirmed. We had sailed to a small
island and moored in the only cove. Chart indicated no hazards, just
sand with small shell bottom. Dropped the anchor (Sarka) in about 4m of
water and laid out about 6:1 rode as per usual. The depth under the keel
indicated 5m. Went to bed. During the night I woke up with a strange
feeling of uncertainty. Perhaps the boat's motion had changed but
nothing obvious. Peering into the blackness, the loom against the stars
indicated we had not moved. Nevertheless, the feeling did not go away. I
flipped on the depth sounder and it read 4m but then started to chnage
-then 3.5 then 3.0 then 2.5 and that is when I reached for the engine
key. But it stopped rising at 2.3 and we need 1.8m. With little tide
left all would be well. As I watched it slowly increased again. I
deduced that we were ever so slowly swinging over an uncharted rock.
Intrigued I snorkeled out in the morning (Brrrrrr) and saw the rock. We
had laid just enough rode to place out keel right over the center of a
2-3 m wide rock platform with sand and deep water all around it.

Now why did we just end up over a rock? My suspicion is that boats seek
out rocks -is this possible? If so, why do boats seek them out?

Did I wake up because the small swell was accentuated as we swung over
the rock and I sensed it in my sleep?

Any similar experiences?

Cheers

Interesting idea. What if the wavelength is much longer than the
structure -does that have an effect?

Cheers

Bob Crantz wrote:

Actually, the rise and fall of water over a shallow plug or area does cause
a "suction" effect and will pull you toward it.

See for yourself, just watch the water over a shallowly submerged boarding
ladder, piling or rock. The structure truncates the wave structure
underneath the surface and only allows higher frequency components of the
wave to exist above the shallow, hence the faster waveforms in the water in
the decaying response waveform and the lower frequency components in the
forcing function. In other words, the water drains faster than it fills (or
has a different waveform) and a relative "vacuum" is formed.

BC