On Sat, 12 Dec 2009 14:01:13 -0700, "Bob La Londe"
wrote:
"Bruce In Bangkok" wrote in message
.. .
On Fri, 11 Dec 2009 20:44:31 -0700, "Bob La Londe"
wrote:
"Bruce In Bangkok" wrote in message
...
I wish that you could point me at a url showing this "speed pad" as I
can't seem to visualize it, nor can I find more then the words, i.e.,
has a speed pad. I am somewhat familiar with the hulls of both three
point hulls (the old speed boat hulls with the sponsons) and more
modern open ocean racing stuff, but the speed pad defeats me
Most modern high performance fiberglass bass boats made today have a pad
style bottom atleast to some degree. Fast Cat (tunnel) and a few shallow
V
and pocket tunnel flats boats being the exceptions.
http://www.bwbmag.com/output.cfm?id=1122481 "When we think of a bass
boat,
we typically envision a modified V hull, with a pad bottom, and more often
than not, an extended transom/built-in jackplate. Pretty cool stuff,
right?
"
Kinda goes to the argument that it's a pretty common thing if that's what
"we typically envision."
Here is an aluminum one, but I do not like their design. Most are
flatter,
although a lot of Rangers have a semi V pad like that. I mostly do not
like
the sandwiched or layer construction for aluminum as it can trap water.
Their V pad might be the most efficient design for some setups, or
possibley
the best compromise: http://www.freepatentsonline.com/6666162.html
Here is a picture of the inside the hull for one under construction. The
outside view would be much more defined.
http://media.photobucket.com/image/b...lassonhull.jpg
I am finding that there is an amazing dearth of pictures online of the
bottom of bass boats. I am finding myself headed out to the shop to take
pictures of the bottom of my Basscat.
Here is a view from the transom:
http://www.yumabassman.com/pictures/shelf-001.gif
O.K. I see what you are talking about now. Essentially a flat bottomed
boat once you reach the optimum speed.
Essentially. Yes. A narrow flat bottom boat. Low resistance and tends to
put motive force towards going forward rather than up. Speed and power
available being major factors of course.
I do not have the math skills or the engineering background to explain it
all, but I think it works like this. The narrow pad creates less resistance
than a V of similar surface area within the displacement envelope primarily,
but not exclusively because of the surface tension of the water. There are
other other forces such as angle of pressure, total pressure against forward
movement etc. The idea is the boat comes up and then flattens out. The
edge of the pad will break the surface tension of the water and shed the
water much more quickly than than a straight V will, and it will have less
surface area touching the water than a shallower V design or than a flat
bottom boat. More of the extra power is used to push the boat forward
rather than to lift the boat out of the water. I discovered this first hand
when I went from a 225 (around 215-220 HP) to a 250 Pro XS (260-270 HP) on
the boat I pictured. At 70+ I need less trim angle than I did at 68 (WOT)
with the old motor.
As speed increases water pressure against the bottom of the boat
increases and increases lift (if that is what you want to term the
force acting against the bottom of the boat). If "X" lifting force is
applied to a flat surface, at 90 degrees to the force, the entire
force is applied to the surface at 90 degrees to the surface. If the
same force is applied to a surface at other then 90 degrees only a
part of the force will be applied at 90 degrees to the surface. Thus
the same lift, derived from forward speed,applied to a vee hull will
result in less force being applied to raise the boat "up" then if the
same lifting force applied to a flat surface.
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.
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.
The difference is certainly much less than the simple large gains from a pad
style bottom vs a V bottom.
That is a simple to demonstrate. Hold your hand out of the window of a
moving auto. If you hold your hand flat there is a large force applied
to the hand. Turn your hand on edge and there is little force applied.
There are a lot of dynamics involved. Some can be engineered away, but
since I am not an engineer I have to visualize and maybe experiment.
** 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
Bob La Londe
www.YumaBassMan.com
Cheers,
Bruce
(bruceinbangkokatgmaildotcom)