[Rhodes22-list] Re: Pointing

Thu Sep 23 10:25:39 EDT 2004

Slim,

Actually, all of the foils can stall out, both in the water & in the air.
An object does not have to be a certain shape to generate lift.  To prove
this to yourself, stick your hand out of the car window.  If you hold your
hand at an angle to the air flow, do you feel a force?  That's lift!  Is
your hand shaped like an airfoil?  Even a flat plate can generate lift if it
is held at an angle of attack to the fluid flow.  The fluid does not have to
be a gas, like air, either.  It turns out liquids obey the same laws of
hydrodynamics as gases.  The only differences between gases and liquids show
up in the defining equations as terms for density & viscosity.  Liquids are
usually more dense and more viscous than gases as the same temperature &
pressure.  Without going into the physics, what this means is that dense
liquids will produce the same amount of lift force/unit area at a lower
fluid velocity than gases.  Or alternatively, at the same fluid velocity,
liquids require less surface area to produce a given amount of lift force.
For example, at room temperature & pressure, the density of air is about
0.076 lb/ft^3 vs. water at about 62.4 lbs/ft^3, a factor of about 800X.  So,
the keel only needs to have about 1/800 the surface area of the sails to
generate the lift forces required to resist leeway under sail.  Water is
also much more viscous than air.  This has the effect of making the
underwater foils much more forgiving or less prone to stalling out than the
sails.  This is a good thing because it makes sailing much easier.  If your
underwater foils stalled out as easily as your sails; then, every time the
boat lifted in a wave or every time you moved the rudder blade, these foils
would stall out & quit generating lift.  However, at a sufficiently high
angle of attack, even your underwater foils will stall out & quit generating
lift.  This happens most frequently with the rudder blade.  If we define the
angle between the tiller & the centerline of the boat as the angle of attack
of the rudder blade; then, the rudder blade is starting to stall out at an
angle of about 30 degrees & completely stalled out at an angle of about 45
degrees.  At angles greater than about 45 degrees, the rudder blade behaves
more like a water brake or drag device than an underwater foil.  So, unless
you are trying to slow down the boat, putting the tiller over more than
about 45 degrees off the centerline is counterproductive as far as steering
goes.

People cite the analogy of airflow moving faster over the curved surface of
the top of a wing vs., the straight bottom surface as causing a pressure
difference between the top & bottom surfaces & that's what causes lift.  In
the middle 1700's, a Swiss mathematician & scientist named Daniel Bernoulli
did a mass & energy balance on all the forms of energy contained within a
moving fluid.  These days, mass & energy balances are fundamental to
engineering calculations.  But, in Bernoulli's time, this was a completely
new & creative approach!  Bernoulli found that, if you keep a running tally
on all the forms of energy in the fluid as it flows from place to place;
then, total energy is conserved.  The energy can change form - i.e. kinetic
energy can be traded off for pressure &/or potential energy & vice versa;
but, the total amount of energy remains constant.  Bernoulli expressed this
idea in the form of an equation that now bears his name.  Bernoulli's
equation is one of the 1st things students learn in any class on fluid flow
or hydrodynamics.  Naval architects, aeronautical engineers, & chemical
engineers have it tattooed on the inside of their eyelids so they see it in
their sleep!  Macroscopically, one of the things Bernoulli's equation
predicts & experimental measurements have verified is that there is a high
pressure region on the windward side of a sail, a low pressure region on the
leeward side of a sail, & greater air velocity on the leeward side vs. the
windward side - hence the common analogy cited above.  The difference
between these two air pressures, multiplied by the surface area of the
sailcloth over which the pressure difference is acting, is a force, which we
call "lift".  Although Bernoulli's equation is correct, it doesn't provide
much insight into what's actually going on, physically.  Physically, what's
actually happening is Newton's Laws of Motion are at work, as always.  The
air flowing over the sail is being forced to change direction by the shape
of the sail.  Since the air has mass & Newton's Laws state that it doesn't
"want" to change direction, forcing the airflow to change direction requires
that work must be done.  The only source of energy available to do this work
is the kinetic energy of the moving air itself, so that's where it must come
from.  Macroscopically, we observe this work as an increase in the air
pressure on the windward side & a decrease in pressure on the leeward side
of the sail.  The speed of the windward side & leeward side airflows adjust
themselves in response to these new pressures.

So, what the heck is stalling out?  Well, back to Newton's Laws again.
Remember the fluid flow does not want to change direction.  Forcing the
fluid to change direction too abruptly will cause the more or less orderly
flow of molecules to break down into a more chaotic pattern.  The fluid
molecules sort of get in each other's way when they are forced to change
direction too abruptly & go bouncing off in random directions.  This process
turns the kinetic energy of the fluid flow into random molecular vibrations
or heat.  We call this process "turbulence".  Bernoulli's equation doesn't
"care" what form of energy we convert the fluid's kinetic energy into, heat
is just as good as pressure.  So, at the onset of turbulence or stalling,
the pressure difference across the sail goes away in favor of a slight
temperature increase in the airflow.  Again, this has been verified
experimentally.  Around the turn of the 20th century, a British physicist
named Osborne Reynolds came up with the concept of a dimensionless parameter
which could be used to predict the onset of turbulence under any set of
fluid conditions.  This dimensionless parameter is now called the "Reynold's
Number" in his honor.  (NOTE: In engineering, one of the highest honors is
to have a dimensionless number or fundamental defining equation named after
you!)  The Reynold's Number is given by:

Re = (L * V * ro) / mu

Where:
Re = Reynold's Number
L = Characteristic Dimension Or Length Of The Flowing System (ft)
V = Fluid Velocity (ft/sec)
ro = Fluid Density (lb/ft^3)
mu = Fluid Viscosity (lb/ft-sec)

Note: all the physical parameters that go into this calculation must be in
units that cancel each other out, hence the term "dimensionless number".
For any given physical geometry, there is a certain critical Reynold's
Number above which the fluid tends to become turbulent.  For example, for
fluids flowing in pipes, the L parameter is usually the inside diameter of
the pipe & (Re)critical = about 2100.  Note that the fluid viscosity appears
in the denominator of this equation.  i.e., more viscous fluids like liquids
tend to resist the onset of turbulence better than less viscous fluids like
gases.  Again, this tends to make the underwater foils more resistant to
stalling out than the sails & this is a good thing!

There, that's probably more than you ever wanted to know about foils &
stalling out!  hopefully, I answered your question.

Roger Pihlaja
S/V Dynamic Equilibrium

----- Original Message -----
From: "Steve Alm" <salm at mn.rr.com>
To: "Rhodes" <rhodes22-list at rhodes22.org>
Sent: Thursday, September 23, 2004 3:26 AM
Subject: [Rhodes22-list] Re: Pointing

> Peter,
>
> Hold on, thar!  "Lift" from the keel, CB and rudder?  The underwater
> appendages are symmetrical with the hull and cannot provide any lift.
They
> only serve to prevent lee way, or to provide lateral resistance.  That
part
> I agree with.  Brad might have a better description, but lift happens when
> air (or presumably water) has to travel farther around one side than the
> other, creating a difference in pressure between the two sides.  Lift is
> created by the curved shape of the sail or airplane wing and will stall if
> not going fast enough.  The keel, CB and rudder do not have that kind of
> shape.  I'm with you on the rest as far as pinching vs. pointing goes, but
> it's the sails that stall out, not the keel, CB or rudder.
>
> Slim
>
> On 9/22/04 7:58 PM, "Peter Thorn" <pthorn at nc.rr.com> wrote:
>
> > Hello Ed,
> >
> > If you verify that you're able to point your R22 35 degrees off the true
> > wind, I certainly would like to visit Lake Hartwell to see that.
Perhaps
> > it's the apparent wind, the combination of the boat's velocity across
the
> > bottom combined with the true wind direction, that's making you think
you're
> > pointing so close. On a reasonably fast boat like R22, the apparent wind
> > angle can move quite a bit forward.  In an extreme example such as
iceboats
> > (that travel many times the true windspeed) the wind indicator points
almost
> > straight forward.
> >
> > Are your headsail sheets led to tracks at the foot of the cabinhouse
roof?
> > That, I think, would certainly improve pointing.
> >
> > It's good to be aware of the difference between pointing and pinching.
> > Sailing too close to the wind can cause the underwater foils to slow
down
> > then stall.  That's pinching.  When the keel, cb and rudder stop
producing
> > lift, the boat will start to produce a lot of leeway, or sideways drift.
It
> > is very difficult to detect leeway when aboard the boat that's making
all
> > the leeway.  The bow points higher, so the skipper might think he's
pointing
> > pretty high because the sideslip is so hard to feel.  To avoid this
> > condition, foot off and keep the boat moving.  After regaining speed,
head
> > up a little.
> >
> > If you have a GPS you can verify your pointing angle by measuring your
> > heading (not the direction the bow is pointing), tack to the other tack,
> > measure heading again and divide the angle difference by 2.  I think
someone
> > mentioned this not too long ago on the list.
> >
> > I too have wondered about the diamond board, and would guess Phil Rhodes
> > original cb is pretty hard to improve on.   A while back Roger wrote a
very
> > scientific sounding comparison, do you recall that?
> >
> > Perhaps you Lake Hartwell guys should conduct on-the-water pointing
trials
> > and settle the issue.
> >
> > PT
>
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