[Rhodes22-list] Re: Pointing

Thu Sep 23 17:53:47 EDT 2004

Brad & Slim,

I'm sorry, I didn't mean to scare you! :)

Roger

----- Original Message -----
From: "Steve Alm" <salm at mn.rr.com>
To: "Rhodes" <rhodes22-list at rhodes22.org>
Sent: Thursday, September 23, 2004 2:40 PM
Subject: Re: [Rhodes22-list] Re: Pointing

> Roger and Peter,
>
> A musician should know better than to talk physics -- especially around
> here.  I'll shut up now.  8-)
>
> Slim
>
> On 9/23/04 8:25 AM, "Roger Pihlaja" <cen09402 at centurytel.net> wrote:
>
> > 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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> >>
> >>
> >
> >
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