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Re: [Phys-L] floating fiasco physics

Fun analysis John. As I have vastly more experience with boats than
airplanes, I'd like to point out another fact about steering. Rudders are
utterly worthless if you are not moving or if you don't have water flow
going over the rudder. Some of this can come from the ship screws (props).
But the point is that rudders (and to some extent a keel) doesn't do much
unless the boat is already moving. That makes steering even more
unintuitive. Obviously, this issue doesn't turn on at some speed but
changes with speed from zero steering to more and more useful steering
ability.

Another issue is prop-walk. If there are two screws that are counter
rotating this is less of an issue, but you get an additional vector thrown
into the mix from the "paddle wheel" behavior of props. Not only does a
prop thrust water backwards, it also provides a sideways force (I've seen
this in aircraft prop wash as well). This force can be welcome when going
into or out of a dock, it can also be very annoying. For larger sailboats
this is an issue that all skippers must adjust for when docking or
undocking and is part of standard training for any captain.
- - - -
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
cell: (801) 476-0589 (Text me, I don't answer the phone if you are not in
my contacts.)


3) One annoying thing about steering a boat is that in the classical
arrangement, you don't have a way of pushing the bow in the direction
you want to turn. Instead you use the rudder to push the stern in the
direction you /don't/ want to go.

As long as the angles are small and the off-course distance is small,
this is only a minor annoyance. OTOH it also means there is a coffin
corner; that is, if the boat is too far outside the envelope, trying
to fix it just makes things worse. You have to proactively stay out
of such situations.

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