Barry Palmer wrote:
The poster is adding another factor, side load, when the hull is forced by a
sail but we are referring to hump generating capability, where it should be
stated that rationality would assume side load were low or negligble unless
otherwise stated.
However, the aspect of the Hobie Cat "planing" on its outboard topsides (sounds
weird, I know, to a monohull sailer) is another story. Isn't a Hobie faster in
a broad reach, where side loads are small, than with one hull flying, where
side loads are great, close to the wind?
Not to forget, hobies have no centerboard and do not go to
windward as well as boats that do, except during a few perilous
seconds while control may be maintained.
While one one hull, they slide sideways (upwards?) as well as
foreward, and steering gets wonky too. I think it's a control
problem and some one is gonna get it tuned just right one day.
Perhaps a few square inches of skeg / keel / lifting foil in the
right place would work out? At speed, a little surface area could
do a lot.
The world in't over, yet. Advances are still to be made.
The 'bump in the water' idea doesn't go away, it just looks like
a square or maybe triangular wave, instead of a sinusoidal dent.
The harmonic content in the waves created contain energy left
behind by the boat as it passes, as well as the tonic wave left
behind a displacement or a planing hull.
Planing lift is a result of the reaction to moving the water out
of the way of the hull, which happens at a rate of accelleration
determined by the speed of passage.
The faster you plane, the less water need be moved, but the lift
is proportional to the 4th power, I believe, of the speed, or the
cube of the accelleration. Or was that the square of the
accelleration of the water. Ahhh, all that math was so long ago,
so unused since.
Gotta go now, to deliver meals on wheels. G'day, eh?
On a hobie at the tilt, the water is pushed only one way, partly
sideways, partly downward, not out both sides and down without
the 'recoil' energy which pushes a displacement hull foreward,
but is left behind by a planing boat.
Only the 'bottom' side of a hobie hull planing on one foot is
wet, reducing friction synergistically to the energy system
examined.
Terry K
Barry Palmer wrote:
Mathematical modelling, even with things as simple as earth surface
gravity,
always involves a hypothesis regarding the physical principles involved.
If
the apparent results line up with the hypothesis, the "theory" could be
said to
be correct.
In the case of Hobie Cat versus 45 footer, one only has to look at the
theory
in a different way, one that takes slenderness into account. Obviously, as
a
given hull becomes finer, less wavemaking potential exists, and load
carrying
ability of the hull diminishes. As the extreme of a knife edge in the water
is
approached, load carrying capability approaches zero, and friction with the
water becomes the dominant drag force, making wave drag less relevant, as
friction becomes greater and greate compared to wave drag.
A hobie is a different study. The side of the hull may be
considered as part of a system most easily understood as one hull
lifts clear of the water, balanced somewhere between the
sideways, upwards and downwards forces acting on the sail and the
keel effect of the more deeply submerged lee hull.
The balancing act works because it uses the keel area of the side
of the hull as a planing surface. The lift from planing is not
vertical, but presents a vertical component in a vector analysis
of all the forces at work.
If you have ever ridden one, you can see the flat water behind
the lee hull. A hobie on one hull is not in pure displacement
mode any more. The narrownwess of the hull means it does not make
a large hole in the water, requiring energy to move a lot of
water aside. A hobie does not climb over it's bow wave, it slices
like an edge on a ski on ice. It works more like a skipping
stone.
I do not know why someone has not built a catamaran with flat
bottom rails angled so as to enable a more stable balancing act,
with perhaps 10 % of the weight still on the windward hull,
supported by the flat portion on plane.
There would still be a possibility that a lump in the water would
launch the windward hull towards orbit. It would still require
twice the level of concentration as driving a car on only two
wheels.
The opposite extreme is found in hovercraft, where there is little friction
(in
smooth water), and the body making the wave is broad, and wave making
theory
comes into play, with friction of minor concern. (High drag in rough
water is
a problem with hovercraft, which may be overcomeable, but not at all
explored
by the industry.)
To analyse the wave making problem on my surface skimmers, I took a
different
approach from classical marine analysis, and got roughly the same results.
I
assumed the craft influences a semi-circle section of water under it and
gives
that body of water a downwards force, the vertical momentum of which is the
dynamic lift on the hull, as lift from displacement diminishes as forward
speed
builds. Whadiya know, it works for broad barge-like hull bottoms with no
friction.
After all, the theory of earth surface gravity breaks down when you start
to
get away from the earth's surface, yet we use the theory in many
engineering
endeavors. (When you leave the planet, Neutonian physics "theory" breaks
down.)
Subject: Hull speed theory?
From: (JAXAshby)
Date: 7/20/03 5:52 AM Pacific Daylight Time
Message-id:
it ain't any theory whatsoever. It was merely a "scientific" explanation
given
to 19th century British naval brass to "help" them understand why putting
2x
the power in a boat didn't make the boat go 2x the speed.
The "theory" sounded scientific and it had numbers in it and it was called
a
theory so the brass accepted it.
most sailboats built in the last several decades will easily exceed
"theoretical" hull speed. in fact, a deep vee hullled Hobie cat
(displacement
hull by any standard) at 18 feet will under many points of of sail easily
pass
a 45 foot sailboat (also a displacement hull for most every 45 sailboat
out
there).
Barry Palmer, for
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