In article , says...

"Bruce In Bangkok" wrote in message
...
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
m...

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.

Several designs do trap air by a number of methods. One you see that is
fairly common is that they make the front portion of the boat in a big
rounded M if you were to cut it in cross section about 1/3 back. My Bass
cat kinda has that look also. Lots of boats do. It certainly works well
towards eliminating spray over the bow. The guys at Basscat have even
warned me that excessive trim can cause the boat to kite with a net effect
similar to chine walk. This is caused by air though rather than the falling
off due to balance or prop torque like regular chine walk.

I am sure multiple chines do something as lots of designers use them. I
always thought it was for stability and steering control at various speeds.
Also to increase lift at a particular speed from the flat bottom of the
individual chines. I suppose they could trap some air also, but I don't
think that is the primary reason for them.

There are some tunnel hull designs that do a much better job of providing
air cushion. One I had the chance to play with for a couple years was a
Baker twin tunnel made by Baker Custom Boats. It was another 20 footer but
only had an EFI 200 on it. It would consistently run right at 65 GPS
**regardless of the load (within reason)**. The basic setup was about 500
pounds heavier than my Basscat. Since the Basscat was originally powered
with a much more modern 225 and only ran 3-5 MPH faster that is saying
something. Can't really compare since I put the bigger motor on the kitten
now. The big problem with tunnels is the extra weight and lost flotation
for similar durability of construction. No big deal when you are running
flat out, but makes a difference when sitting still and fishing or running
water to rough to run flat out. None of the boats I have direct experience
with are paper thin, shave every ounce, race boats.

I wish I still had the Baker for the sake of this conversation, because if I
recall correctly the center hull/deflector also was kind of a pad design.
It was the best handling boat I have ever driven at speed. It steered like
a car at any speed above idle.

As a side note. The tunnel hull took light chop a lot smoother at speed
than any of the pad bottom modified Vs, but when it gets rough the pad-V can
slow down a little and run like a V hull. The tunnel has to stay on the
high end of its speed range and run wave top to wave top hoping not to spear
one, or find safe harbor.

Everything is a compromise.

Anyway... I guess its time to start doing some basic math and figuring out
the weight of my theoretical sharp pad aluminum pad-v. LOL. Who knows. I
might build it someday.

Its late. I know I'm gonna look at this post tommorrow and wonder... what
was I thinking. LOL.

Bob La Londe
www.YumaBassMan.com



The article I read years ago studied the air that runs along the dozens
of chines on the old Viking ships. The theory was that as the boat moved
through the water, air bubbles would get trapped along the angles formed
by the lap strake type construction.