[Rhodes22-list] Re: Pointing

[Steve Alm]

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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