Re: [Phys-L] floating fiasco physics

[John Sohl <jsohl@weber.edu>]

> From John Denker:
"So we see that the oft-used analogy to a paddle wheel is not entirely apt.
If we don't look too closely, the overall effect is similar; however, the
details are different, and the underlying physical mechanism is quite
different."

Yep! That right there is the reason I put "paddle wheel" in quotes. There
are other effects as well. Turns out that hull design is wickedly complex
and subtle. On many sailboats the prop is a good bit in from the stern so
you get the effect in forward as well, but, as John notes, it is different
than when in reverse.
Lots of weird little things contribute from the shape/position of the keel
and rudder to the overall hull shape. Even the amount of growth (barnacles,
moss, weeds, etc.) can change the effect.
One fluid dynamics effect is that the fluid speed right at the surface is
effectively zero and then increases as you get further from the surface.
(Ever wonder why fan blades get dusty?) Add to this the fact that any water
currents near the dock will change the flow of the prop wash and where/how
it impinges on the hull. The result of all this is that the general
behavior of prop wash is easy to anticipate, but the actual effect is
slightly different every time you dock.

Getting complacent is your enemy.

On some sailboats the prop is off center. That adds yet an additional
variable to the entire process! Try maneuvering a 50' catamaran into a
tight berth when one engine isn't working. (Add some wind and water
currents in different directions for some real fun.)

Yea. Real paddle wheels are a lot simpler.

John
- - - -
John E. Sohl, Ph.D.
WSU Brady Presidential Distinguished Professor of Physics
Weber State University
1415 Edvalson St., Dept 2508
Ogden, UT 84408-2508

Office: TY 326
Office phone: (801) 626-7907
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