"Bruce In Bangkok" wrote in message
...
On Sat, 12 Dec 2009 14:01:13 -0700, "Bob La Londe"
wrote:
This has been bouncing around in my head as well since you forced me to
think a little more. The reason I think a sharp edge is more efficient is
basically the same as why a pad is more efficient at optimum speeds. The
sharper edge will break the surface tension and shed water weight and
reduce
resistance much sooner than a rounded edge. The average boater will never
notice the difference, but guys who are always running on the ragged edge
(oxymoron not intended) and looking for the best performing prop to run
right at redline will notice the extra 1 or 2 miles per hour.
Rather then "surface tension" I suspect (if the effect is not large)
that it is simply a matter of surface area. Exactly the same reason
that the small flat bottom is faster then a vee. At any speed it would
take more "lift" to raise an inclined (vee) surface then one at right
angles to the force, thus any rounded corner would act much the same
as the vee, for that portion of the pad, and reduce total lifting
force, by some percentage.
That is certainly part of the equation now that I think about it. Surface
tension has an affect as well. I'm sure the answer is not one dimensional.
However.... I have read that the multiple, sharp edged, chine designs
actually increase speed because the sharp edged chines increase the
amount of air that is trapped and forced to flow under the boat but
whether that works to decrease drag by reducing the viscosity of the
water or by reducing the wetted area I don't know.
That could be both.
Air cushion is definitely a factor in many designs but I am not sure that it's
the primary factor in multiple chines. Seems I read somewhere that multiple
chines was to increase lift at various speeds until the boat is at optimum
cruising speed. Basically the multiple chines help it come out of the hole
faster. I seem to recall and I think you mentioned it in passing earlier in
this thread that it also helps with stability and steering control. If you
have ever run a flat bottom boat that is adequately powered I'm sure you
know that it planes out almost immediately. The way it was explained to me
is that multiple chines are a compromise. Gives you more lift like a flat
bottom, but as they come out of the water they cease to create drag.
Now I'm tempted to start a whole new thread on air cushion designs.
Tunnels. Twin tunnels. The big M shape of the front 1/3 to 1/2 of many of
those modified V-Pad bottom boats. We could talk about lift, cavitation,
kiting, Miss Budweiser doing end over end flips, LOL. My experience with
tunnels is more limited, but I did have a Baker Custom Boats twin tunnel for
a few years. Awesome ride in light chop.
I think we can both agree that the less boat you have in the water (in
general) the faster it will go.
** Thought I better throw this in: **
Chine walk can be deadly scary in such a boat if not properly setup or if
driven by somebody who hasn't taken the time to learn to drive their rig.
The biggest safety move for anybody else reading this is to SLOW DOWN
gradually. A sudden loss of throttle can be almost as dangerous.
Especially in traffic in a busy system. Increase power (like I did) and
it's a whole new rig to learn to drive.
Left that last bit in because it bears repeating.
Bob La Londe
www.YumaBassMan.com