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Re: the Kelvin Wake again



At 12:19 5/30/00 -0700, Leigh wrote:
...
Kelvin wakes pertain only to displacement boats.
...
Does the turbulent wake *look* different, apart from
having a variable angle?

Certainly. The turbulent wake has a turbulent surface;
it "breaks" or is "foamy". The Kelvin wake is a laminar
phenomenon that propagates as a coherent wave from a
boat moving with constant velocity....

I also recall a speedboat wake as being just a continuous line in each v -
must check if this is observed.

I expect it is incoherent and will not propagate for
long distances. The Kelvin wake of a planing speedboat
probably persists long after the turbulent wake has
dissipated....
Leigh

I have now looked up the result in a book. The opening
angle of the wavelet fronts is 109.5 degrees, independent
of boat velocity. The book, "Bores, Breakers, Waves, and
Wakes" by R. A. R. Tricker, first published in 1964 by
Mills & Boon Limited, London, has a very clear derivation
which should be quite accessible to brighter IB students.
The downside is that the book itself is not very
accessible. I couldn't find it at six booksellers' web
sites that I tried.



I checked the effect yesterday during a water ski outing.
When the boat was making about 28 mph, the principal wake seemed to
have an included angle of about 45 degrees.

The water passing the stern closes in and mounts to a height of two
or three feet (i.e above the freeboard) and then parts in a distinct
wave to either side.

These waves seem to propagate to quite a good distance.
One bobs at anchor to wakes from boats travelling several hundred
yards away.
Pulling a skier in a big circle risks upsetting him with ones
own wake.

I recall reading grumbles from power boaters about the discomfort of
boating through wakes, when other power boats are present.(!)

At some marinas, there is a 'no-wake' requirement close to jetties,
so that boats approach at below 'hull-speed' (the limiting speed in
question.)

At higher speeds, the wake is bigger and more disruptive.


brian whatcott <inet@intellisys.net>
Altus OK