[Rhodes22-list] Re: Pointing

[Peter Thorn]

Roger,

That was great!  A wonderful, science-filled explanation of  foils, stalling
and the technical woes that befall skippers who pinch.   Previously, my
knowledge about stalling sails was merely that the outside telltales stopped
flowing aft.  Good job.

I'd like to reprint your stalling missive in our sailing club's (Carolina
Sailing Club, Raleigh NC) newletter, with proper attribution of course.  OK
with you?

In Bill's defense, it's pretty clear to me that you haven't cruised the East
River lately.  Motoring back and forth across the current there would be a
death defying act, exposing R22 broadsides to harbor tugs with 4' bow waves,
full-throttled Egg Harbors and the like. <G>

PT

----- Original Message ----- 
From: "Roger Pihlaja" <cen09402 at centurytel.net>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Thursday, September 23, 2004 9:25 AM
Subject: Re: [Rhodes22-list] Re: Pointing


> 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
> >
> > __________________________________________________
> > Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
> >
> >
>
>
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