In article et,
W. Watson wrote:
I've been browsing in some physics books, two in particular. "Physics
Simplified" by Epstein and "Mad About Physics" by Potter and Jardonski (sp?).
The idea of both books is to pose puzzlers and then explain their answers. Both
have more than a handful of questions that apply to boating. One question
involves undertow that is created by a sharp turn in a river or stream. Another
concerns flow from a narrow channel into a wide channel and vice versa.

I've certainly observed an effect like this at bends in a river. What happens
is that the river undercuts the bank on the outside of the bend, and piles up
loose material on the inside, creating a sandbar. You have to watch for downed
trees hanging off that outside bank! But there's also likely to be circulating
water just off the sandbar, as the main current passes by closer to the
outside of the bend. What can happen is that you come along at a good clip,
you see the bend ahead, and you say, "Might get pushed into the bank--might
hit strainers--stay well to the inside!" So you head to the inside of the bend,
you think staying out in the center far enough not to run into the sandbar.
But what happens is that the bow gets into the circulating water behind the
sandbar while the stern is still in the main current, being carried downstream.
The result is that the boat turns suddenly and unexpectedly. Usually it's just
an embarrassing thing, not dangerous, but it can be puzzling as you think
there's a smooth flow of water around the curve. The right course seems to be
to stay just in the downstream flow in the middle of the river, neither
getting swept to the outside nor getting turned by the different speeds of
flow.