Hi, other Dave S.--
Your hunch is correct; laminar flow in water disappears at a pretty low for
the waterline lengths and foil lengths we're discussing. After I wrote my
comments earlier, I went back through the thread and found the link to Bryon
Anderson's excellent article, which explained everything I was trying to
cover in a better fashion, with diagrams too. He mentions 5 knots as the
approximate speed at which we lose laminar flow. That's oversimplifying,
but it gives you an idea. But laminar flow has a lot to do with the NACA
profile--some profiles intentionally move the maximum depth aft in order to
maximize laminar flow; the idea is to keep the lift working for you as long
as possible. My last plane was a Cessna 177RG, which had a laminar flow
wing designed to keep the flow 'attached' for 70% of its surface or better
at speeds of about 170 mph. It took a long time getting off the ground, but
was incredibly efficient in the air. It was very different from my friend's
175 (we traded for a while so he could get his commercial license), which
had a fat high-lift wing that got you off the runway in a third of the
distance but only provided 2/3 the top speed for the same power and fuel
burn.
Just because a flow goes turbulent, we're still interested in it and it can
still perform some useful work. Besides, it's kind of a necessary evil; you
can't just provide the part of the foil that gives you laminar flow and then
remove the rest <grin>. The whole profile works together.
Now, you mentioned vortex generators... They sell dimpled surface material
for airplanes to put on the surfaces where the flow starts to detach, under
the theory that the dimples (like the ones on golf balls, except standing
proud of the surface) generate mini-vortices that help keep the flow (albeit
turbulent) attached. Makes me wonder what this would do on the aft faces of
a keel. I notice my BMW has raised bumps all over the edges and strut for
the side-view mirrors, to cut wind noise by keeping the flow attached so it
can't escape and whistle.
But let's talk for a minute about porpoises. Capable of 25 knots
underwater, and without a turbulent boundary layer... I guess they don't
understand Reynolds numbers. Apparently they have a paper-thin outer skin
with a thin spongy layer below that covering their real skin. One theory
says they can detect turbulence and adjust their body shape to reduce it by
controlling this soft layer. Dr. Kenneth Davidson studied them heavily and
figured their speed was more due to their streamlined shape and the smooth,
oily skin, but later research does suggest that the softness is apparently
as important as the skin. But I wonder why my inflatable isn't quicker...
Anyway, since I don't see a mechanism for the porpoises to control this fat
layer, maybe it's a passive thing. I don't think I'm ready to plaster my
keel with neoprene to try it out, but it makes me wonder if the flexible
layer couldn't react to impending turbulence and change shape just enough to
keep the flow attached.
Dave Shaddock
-----Original Message-----
From: [email protected]
[mailto:[EMAIL PROTECTED] On Behalf Of David Shugarts
Sent: Monday, March 17, 2008 12:14 PM
To: [email protected]
Subject: Re: catalina27-talk: Keel Fairing
Hi, Dave--
That's all true, more or less, but what I have a strong hunch you will find
is that these foils do not give us laminar flow at our speeds and angles of
attack.
In other words, going to windward, I believe you would find that we are
nearly always in some form of turbulent flow, at some point in the keel
section, unlike aircraft, where we do get laminar flow most of the time,
over most of the wing.
(BTW, the ratio of Reynolds numbers is 13:1, water versus air. Don't hold me
to it, but I believe this is the practical consideration when modeling
foils.)
These days, there are inexpensive underwater cameras that could perhaps show
us what our keels are doing. It isn't a fair comparison, but I get a good
look at my rudder and it always looks like it's in some degree of turbulent
flow going to windward. (I have the old rudder, which is an anachronism.)
Regards,
Dave S.
PS-I am very familiar with the root versus tip design concept for beneficial
stall behavior in aircraft, and we could throw in wing twist if we wanted to
complete the picture. And let's not even get started with Whitcomb winglets,
stall fences and stall strips, not to mention vortex generators.
On 3/17/08 12:31 PM, "David Shaddock" <[EMAIL PROTECTED]> wrote:
The Cessnas and other aircraft sometimes use different foil shapes at the
root and tip in order to make sure the inner part of the wing (closer to
the
fuselage) stalls first, making the aircraft dive and regain speed while
still providing some control out at the wingtips to avoid a spin. This
isn't an issue with sailboats.
But our keels can still stall--the keel provides windward lift if it
doesn't
stall, at the expense of some leeward slip. If the keel stalls, you lose
the lift and you see a lot more leeward slip/skidding.
There are so many NACA profiles that it's hard to imagine anyone using
something that's NOT a NACA profile--they have tested and published
results
for hundreds of them, with some having only a tiny variation from others.
But those tiny variations can make measurable differences, especially
since
we're operating our profile in a medium 800 times denser than air. I have
a
book I used for aircraft design purposes that's got everything they had
published through about 1990. At any rate, selection of the ideal profile
for a sailboat involves knowing the aspect ratio as well as the target
speeds. For example, there is a concept called the lift/drag bucket--a
high-lift keel profile provides a lot of drag, but might be a worthwhile
price to pay if you're trying to achieve the best VMG in light air,
because
at low speeds the drag doesn't hurt as much and adding lift while
minimizing
leeward slippage pays off. For higher speeds, a lower-lift profile works
better because when the boat is moving faster through the water, you'll
get
a resultant increase in the actual lift windward and have less drag to
worry
about--but overall you'll see more leeward slippage.
A bulb at the bottom of the keel offers two things--for one thing, it
minimizes the tip vortex (which adds a great deal to drag), but mainly it
helps provide a lot of mass at the extreme draft, which provides more
righting moment. If the rules allow, you can carry more sail because of
the
extra righting, and you'll heel less which means more sail upright and
working for you (although heeling may increase your waterline length on
some
hulls and raise your speed). If the rules don't allow added sail, you can
take advantage of the increased righting moment by cutting weight out in
other areas and you'll accelerate faster.
The profile Tim has picked out for his sportboat is a good one; at the
speeds he might be getting on a planning boat, he could probably have done
well with a narrower profile, too, but this way he's covered for a wide
range of conditions.
A lot of this information is in Steve Killing's book on Yacht Design and
also in Skene's Elements of Yacht Design--but the later publications of
that
are much more informed than the early ones).
Dave Shaddock
-----Original Message-----
From: [email protected]
[mailto:[EMAIL PROTECTED] On Behalf Of David Shugarts
Sent: Monday, March 17, 2008 10:52 AM
To: [email protected]
Subject: Re: catalina27-talk: Keel Fairing
Hi, Tim--
I think your summation of it as "like a Chevy" is a pretty good analogy.
To
go back to the source, I have now heard Frank Butler answer a number of
sophisticated questions with what sure sounded like naivete to me, so I
have
a hunch that our factory keel section was a "oh, whatever" decision at the
time. Then these better keel sections would naturally be an improvement,
but
only because the bar was set so low.
It would be interesting to hear from an expert here, because I just feel
as
though the designers of the cool toys are way beyond NACA foils. Or
perhaps
they really are more about the bulb than the keel section itself. For
instance, if we could hang a heavy lead bulb on a carbon fiber keel, we
would probably do it, and we might find that ANY keel foil would be fine
for
the purpose.
BTW, this link: http://www.hanleyinnovations.com/glossary.html, shows a
few
cases where the NACA 0012 was used in aircraft, but it also shows that
some
venerable aircraft (e.g., the Cessna 150/152) had one foil at the wing
root
and another at the tip (in other words, more sophisticated). Notably, the
B-17 Flying Fortress had it as the root foil (love that airplane!).
Regards,
Dave S.
On 3/17/08 12:03 AM, "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> wrote:
I don't profess to have any knowledge whatsoever when it comes to fluid
dynamics, I have just been going on threads on SA and bits and pieces of
knowledge that I've read from different designers.
I think that as far as high performance (e.g., sport boats, hulls that
will plane
off the wind) sailboats are concerned, a bulb on a keel foil is pretty
much the
name of the game. Certainly heavy displacement and cruising boats will
look toward other keel configurations. But the NACA foils offsets have
pretty
much been determined to be the go-to configurations for fast keel struts
in the sportboat world. There are a few arguments over whether a 0011
section
might be faster than a 0012 seciton (with a resulting decrease in
strength/robustness
to loads, etc) for example, but the 0012 shape seems to be the chevy
pickup when
it comes to most foil sections below the waterline.
These are fairly simple shapes. Pretty easy for an amateur to cut with a
hot wire,
or for a CNC machine to do it.
(http://www.youtube.com/watch?v=q7uvq4RlhHM)
I can certainly imagine that areonautical designers would have the need
to
come up
with more complex shapes for specialized, shape-specific demands,
executed
at high speed with enormous G-force loads in the atmosphere, and new
materials and production techniques would allow for a huge amount of
variability when it comes to foil offsets these days.
But these are just simple symmetrical foils shapes that you can order up
and get made pretty cheaply on-line...I just ordered a 54" piece of
spyderfoam cut to NACA0012 sections,
for about a hundred bux incl. delivery. It's a dream-world out there now
for home boat (or aircraft) builders!
tf
My ears perk up here. First, I confess ignorance. Are boat keels based
on
NACA foils, and do they apply to water, as opposed to air? Perhaps there
was
a series of NACA foils intended for water? I just never paid attention
to
that part of things, although I studied NACA airfoils for my own
purposes
many years ago. I vaguely recall a factor called Reynolds Number that
would
govern foils in various media, such as air and water. Can you elaborate?
Regards,
Dave S.
PS--I was just a layman studying the foils at the time, but I went
through
them all pretty carefully. It seemed to me that they were kind of
empircal
in nature. I got the impression that the great virtue of a NACA foil,
for
an
aircraft designer of the 1930s or 1940s, was that it was thoroughly
tested
and predictable. However, it seemed as though a lot of developments of
later
decades, such as the Clark-Y, not to mention variable sweeps and tapers,
variable chords and foils in a given wing, etc., began to favor
departures
from the NACA foils (except when mere predictability was the goal, as in
vertical stabilizer foils). So, although I later got into aviation
writing
and was constantly looking for NACA foils, I didn't find many in the
wings
of light aircraft. In my time, we saw NASA come out with the GAW-1, and
I
have always assumed that later, composite aircraft designers were free
to
work with an infinitely variable foil in mind.
On 3/16/08 8:40 PM, "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> wrote:
but they also value every advantage they can get.
key words^, huh?
nice explanation, Chris.
So I guess Compu-Keel is still around?
http://www.compukeel.com/
odd because you get NACA foil specs on-line for free...but I guess all
class
legal keels cant be derived from NACA sections.
tf
