RE:[Paddlewise] wave stuff

Robert wrote:

>>>>Matt Broze wrote:

>>Actually in deep water the angle of the wake stays the same at all speeds,
38 degrees 56 minutes (at least until the wave barrier is broken and you
start to plane--and you don't do that in a human powered kayak).
Therefore
knowing the angle won't tell you much about the speed of the kayak, but
a
photo's measured angle might be able tell you about the depth of the
water
(at least if the angle is greater than 38 deg 56' and you know some of
the
other parameters in the picture). <<

I imagine this angle is related to viscosity or how much molecules of a
particular substance are attracted to each other (I can't remember the
word for this, cohesion?). If I were paddling in mercurey would the
angle be more obtuse and in gasoline more acute?

Robert<<<<

 I believe this angle is independent of density and viscosity of the fluid
(but have not yet found this stated in a textbook so I hedged a bit here by
using "believe"). This wake angle is the result of one wave train
(transverse or following waves) keeping station with the speed of the hull
(therefore getting longer the faster the boat moves) interacting with the
bow (or divergent) waves radiating away from the disturbance created by the
boat's bow (much like when a stone is dropped into water making a circle of
waves). The most visible "wake" is at the point where these two different
wave trains intersect (here the wave crests reinforce each other making the
very noticeable steep waves that we call the wake). Since both sets of waves
are traveling in the same medium their relationship to each other should
remain the same irrespective of the density or viscosity of the fluid.

If density or viscosity made a difference the texts I have read would
probably have hedged and said the angles were slightly different in salt
(denser) and fresh water or in cold (more viscous) and warm water. They
don't hedge here so I am pretty confident I'm right about this even though I
have never seen it stated anywhere in a reliable source (or even in an
unreliable source for that matter).

Matt Broze
http://www.marinerkayaks.com


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