27 Jun 2005 14:16:01
R Lee
What makes a fin "loose?"

OK, all you amateur (and professional) fin experts out there. Here's
one for you.

What are the physics and fluid dynamics that make a wide, low aspect
ratio, swept fin more "turn-y" than a high apect ratio, narrow vertical
fin? Or is this just something that we have all accepted and taken on
faith?

Airplanes use swept wings in large part because of compressiblity
issues when approaching trans-sonic speeds. They use low aspect ratio
wings in fighters to increase the roll rate. Neither of these factors
appear to play a role in windsurfing. We are not concerned about roll
rate, although we do have to consider the amount of torque we can
produce with our feet/weight to keep the board level in countering the
upsetting force generated by the fin's lift.

I have always favored relatively high aspect ratio, slightly swept and
fairly narrow fins like the Curtis Ride&Jibe. I find these fins very
resistant to spin out, very controllable, turn-y, and fast. I've tried
"wave" fins and if the area is adequate, they seem to do fine as well.
I don't, however, perceive them to be any easier to turn than the
others, and in fact if the fin isn't big enough, I have a tendency to
spin out as I enter the jibe. This doesn't seem to happen if I'm
under-finned with the high aspect ratio fin.

I should mention that most of my sailing is in the Gorge, so I'm
usually on relatively small gear. Because of the opposing current, I
also don't have a huge need to be extremely efficient upwind so these
are not major factors in my preferring the high aspect ratio fins. I
am able ride and "slash" on swells, maybe not as much as Mike F...
hehe. It's not that I'm a great sailor or on any kind of exceptional
gear. But why do most high wind sailors (at least in the Gorge) seem
to prefer "wave" fins?



27 Jun 2005 18:51:13
LeeD
Re: What makes a fin "loose?"

Wave fins high surface area for low speed, but short in length so it
doen't lift too much, the windward rail or the tail when overpowered.
Why does gravity work?
Why is the sky blue?
Why...don't matter, it just does and is.



27 Jun 2005 23:43:11
Dan Weiss
Re: What makes a fin "loose?"

My sense is that it relates directly to how quickly pressure builds on the
fin. Swept fins (wave sweep) tend to build and release pressure more
gradually than high aspect race fins. One indication of this might be to
consider how difficult high aspect fins can be to reengage after spinning
out, but how wave fins often let go and recover in an instant. Moreover, a
deeper fin with the same area and similar section imparts a great deal more
torque to a board. This is analogous to the role rate you mention in the
context of airplanes and, of course, windsurfers are concerned about such
things as the first metric considered is often fin length or span. That
force alone might contribute to the impression that high aspect fins sail
more stiffly, and we select fins on a more detailed level with exactly this
roll rate in mind.. But to say everything is about high versus low aspect
oversimplifies the matter. Fin twist greatly contributes to the feel of a
fin. I think it is generally understood that fins work the same, but
achieve their results differently though different designs. It's not any
wonder that you find some longer or more upright fins to be more skatey or
loose. If you can control it a more upright fin should turn more sharply.
But swept fins do dampen the many effects inherent in a turning a board
through chop; and we are human after all.

-Dan
"R Lee" <[email protected] > wrote in message
news:[email protected]
> OK, all you amateur (and professional) fin experts out there. Here's
> one for you.
>
> What are the physics and fluid dynamics that make a wide, low aspect
> ratio, swept fin more "turn-y" than a high apect ratio, narrow vertical
> fin? Or is this just something that we have all accepted and taken on
> faith?
>
> Airplanes use swept wings in large part because of compressiblity
> issues when approaching trans-sonic speeds. They use low aspect ratio
> wings in fighters to increase the roll rate. Neither of these factors
> appear to play a role in windsurfing. We are not concerned about roll
> rate, although we do have to consider the amount of torque we can
> produce with our feet/weight to keep the board level in countering the
> upsetting force generated by the fin's lift.
>
> I have always favored relatively high aspect ratio, slightly swept and
> fairly narrow fins like the Curtis Ride&Jibe. I find these fins very
> resistant to spin out, very controllable, turn-y, and fast. I've tried
> "wave" fins and if the area is adequate, they seem to do fine as well.
> I don't, however, perceive them to be any easier to turn than the
> others, and in fact if the fin isn't big enough, I have a tendency to
> spin out as I enter the jibe. This doesn't seem to happen if I'm
> under-finned with the high aspect ratio fin.
>
> I should mention that most of my sailing is in the Gorge, so I'm
> usually on relatively small gear. Because of the opposing current, I
> also don't have a huge need to be extremely efficient upwind so these
> are not major factors in my preferring the high aspect ratio fins. I
> am able ride and "slash" on swells, maybe not as much as Mike F...
> hehe. It's not that I'm a great sailor or on any kind of exceptional
> gear. But why do most high wind sailors (at least in the Gorge) seem
> to prefer "wave" fins?
>




28 Jun 2005 11:36:53
R Lee
Re: What makes a fin "loose?"

Twist in a fin would seem to favor the higher aspect, narrower fin. It
should be easier to twist a long narrow object than a short fat one,
right?

What do you think the effect of sweep is in relationship to dampening
the board in chop?

Robert



28 Jun 2005 11:38:00
R Lee
Re: What makes a fin "loose?"

I keep thinking that Wardog and Bill Kline will chip in here.



28 Jun 2005 12:10:40
ScottG
Re: What makes a fin "loose?"

Wave fins, via the swept tip, are intended to flex and twist.
That part of the fin is also more flat (not foiled like the base of the
fin) so the swept part provides more lateral resistance than actual
lift.
Pointers tend to be foiled all the way to the tip, so the entire fin is
providing lift.
Also, wave fins tend to be made in polyester, so they flex more, but
pointers are usuallly stiff G-10.

R Lee wrote:
> Twist in a fin would seem to favor the higher aspect, narrower fin. It
> should be easier to twist a long narrow object than a short fat one,
> right?
>
> What do you think the effect of sweep is in relationship to dampening
> the board in chop?
>
> Robert



28 Jun 2005 12:31:23
R Lee
Re: What makes a fin "loose?"

Huh... interesting...

My wave fins I think are made of G10. At least they are the same
material as my Ride&Jibe fins. My Ride& Jibe fins are quite flex-y.
Maybe that's why they seem to work well for me. My wave fins are
G-surf, also by Curtis, but I think it is foiled all the way to the
tip.

Robert



28 Jun 2005 17:05:02
Martin Frankel
Re: What makes a fin "loose?"

Two guesses:

1) Maybe it really is roll rate. A "turny" fin lets you roll the board
from rail to rail, or in and out of a turn, quickly and effortlessly.
Try fanning an 85cm formula fin in the water, then try the same thing
with a wave fin. Just curious, what led you to conclude that roll rate
doesn't matter for windsurfing fins?

2) When the board is sliding sideways, a swept fin may twist more,
reducing the angle of attack of the fin and making it less likely to
stall. Straight fins have such a short chord that they will not be
induced to twist as much, so they would spin out sooner.

Martin



28 Jun 2005 20:04:45
R Lee
Re: What makes a fin "loose?"

1) Now that is very interesting. You might be onto something there.
I discounted roll because roll of the board is very limited. Let me
take a devil's advocate position. As we pressure the rail, the board
must pivot at a point (in the roll axis) somewhere between the center
of resistance of the fin and of the (roll axis) of the board. We
aren't really "fanning" the fin through the water, but pivoting it. On
the other hand, most of us have had the experience of standing on a
long board with and without the centerboard down. The centerboard down
state is much more stable.... hmmmm. Good hypothesis!

2) According to most pundits, when our boards spin out, the foil has
not stalled, but cavitated. The loss of lift is related to sucking air
down from the air/water interface, not from hydrodynamic stall.

Robert



29 Jun 2005 15:29:27
AD.
Re: What makes a fin "loose?"

On Tue, 28 Jun 2005 20:04:45 -0700, R Lee wrote:

> 2) According to most pundits, when our boards spin out, the foil has not
> stalled, but cavitated. The loss of lift is related to sucking air down
> from the air/water interface, not from hydrodynamic stall.

Just a minor correction - I think the word you were after was ventilation
rather than cavitation. Cavitation is where the pressure gets low enough
for the water to 'boil' - normally need much higher speeds for this eg
with propellors, turbines etc.

--
Cheers
Anton



28 Jun 2005 20:41:36
R Lee
Re: What makes a fin "loose?"

Oops, you're right.

Robert



28 Jun 2005 20:48:02
Scott
Re: What makes a fin "loose?"


> Wave fins, via the swept tip, are intended to flex and twist.
> That part of the fin is also more flat (not foiled like the base of the
> fin) so the swept part provides more lateral resistance than actual
> lift.
> Pointers tend to be foiled all the way to the tip, so the entire fin is
> providing lift.

The lift of a symmetric foil (like a fin) comes purely from the angle of
attack relative to the plane of symmetry of the foil and has nothing to do
with the shape of the foil itself. Because of leeward drift the angle of
attack on a fin is always non-zero. That is what provides lift.

Scott




29 Jun 2005 08:17:10
Peter
Re: What makes a fin "loose?"

> 2) According to most pundits, when our boards spin out, the foil has
> not stalled, but cavitated. The loss of lift is related to sucking air
> down from the air/water interface, not from hydrodynamic stall.

Ever spin out when hitting a lull & pushing upwinds? I think _that_ spinout
must be stall, not ventilation/cavitation.

Not sure, but I've read that windsurf fins can indeed cavitate... but maybe
that was Finian's attempt at world speed record ;)

Peter




29 Jun 2005 09:15:52
Dan Weiss
Re: What makes a fin "loose?"

I think low angles of attack (more sideways versus more straight ahead -if I
have that right) as you describe when pinching in a lull could be either
stall or ventilation. Stall makes a lot of sense, but remember that certain
foils generate a lot of lift even at slow speeds. When the degree of
pressure gradient from leeward side to windward side exceeds the water's own
ability to flow past the fin's windward surface the chance for ventilation
remains pretty high. Maybe an example of where ventilated spinout is more
likely to occur is when pinching in light air and disturbed chop. On the
other hand, the water in the chop moves itself and I could mistake stall
caused by a sudden change in pressures as the ventilation I describe.

But I don't think cavitation ever happens on windsurfing fins. Cavitation
is the sudden vaporization of the water caused by extreme pressure drop.
The force of these bubbles collapsing against the lifting surface was a
major problem for designers of submarine propellers. The concussion of
collapsing vapor bubbles would pit the surface of the screws and make them
very inefficient and loud, at the least. We don't travel fast enough yet to
create cavitation on our fins. Even Finian doesn't, and I doubt he or any
windsurfer ever will, since the only time (at least to my knowledge)
cavitation occurs in a marine environment is with powered craft able to
generate foil speeds well in excess of 60 knots. Power boat propeller
screws come to mind, and this concern is illustrated by the careful foiling
and twisting of these props. Cavitation also occurs on the stepped bottoms
of racing power boats, but this is done intentionally as I understand it in
an effort to reduce drag.

-Dan
"Peter" <[email protected] > wrote in message
news:[email protected]
>> 2) According to most pundits, when our boards spin out, the foil has
>> not stalled, but cavitated. The loss of lift is related to sucking air
>> down from the air/water interface, not from hydrodynamic stall.
>
> Ever spin out when hitting a lull & pushing upwinds? I think _that_
> spinout must be stall, not ventilation/cavitation.
>
> Not sure, but I've read that windsurf fins can indeed cavitate... but
> maybe that was Finian's attempt at world speed record ;)
>
> Peter
>




29 Jun 2005 06:27:20
ScottG
Re: What makes a fin "loose?"

You are incorrect.
The anle of attack helps, but so does the shape fo the foil.
Try sailing a perfectly flat fin if you would like to test this theory.
You would have to make your own, becuase no one makes them, since they
dont work as well as foiled fins, which provide much more lift than
flat.

Perhaps I was incorrect in saying the swept tip is flat and provides no
lift, but it does not provide nearly as much lift as if that same area
were incorproated into the heavily foiled section.
BTW, I was speaking about really swept wave fins, not the slightly
swept B&J pointer fins (which tend to be more foiled all the way to the
tip).

Scott (removethis) wrote:

> The lift of a symmetric foil (like a fin) comes purely from the angle of
> attack relative to the plane of symmetry of the foil and has nothing to do
> with the shape of the foil itself. Because of leeward drift the angle of
> attack on a fin is always non-zero. That is what provides lift.
>
> Scott



29 Jun 2005 15:36:31
Bob Jacobson
Re: What makes a fin "loose?"

Swept fins stall first at the tip, while vertical fins begin stalling at the
root. Further, vertical fins with elliptical planform tend to stall all
along the length of the fin at nearly the same time. So you can feel swept
fins beginning to let go, and you can pull them back before you fully spin
out. High-aspect vertical elliptical fins have low drag, and can work at
high angles of attack, but tend to let go all at once, making spinout
recovery difficult. Swept fins are less efficient, and don't point as high,
but have more manageable turning and spinout characteristics. The center of
effort of the swept fin is closer to the board, which makes it easier to
tilt the board to initiate a turn. All the above assumes the fins are equal
in surface area, and similar in cross section.

In reply to the original question, I believe what makes a fin loose is a
combination of aspect ratio and sweep: Low aspect-ratio swept fins will feel
looser. Fin flexibility is a more difficult topic. Flexible fins can either
washout as they flex (less angle of attack at the tip), or can actually
increase the angle of attack at the tip, depending on the materials and
lay-up. In general, stiffer fins will probably give more efficiency and
predictable performance up to the breakaway liimit.

"ScottG" <[email protected] > wrote in message
news:[email protected]
> You are incorrect.
> The anle of attack helps, but so does the shape fo the foil.
> Try sailing a perfectly flat fin if you would like to test this theory.
> You would have to make your own, becuase no one makes them, since they
> dont work as well as foiled fins, which provide much more lift than
> flat.
>
> Perhaps I was incorrect in saying the swept tip is flat and provides no
> lift, but it does not provide nearly as much lift as if that same area
> were incorproated into the heavily foiled section.
> BTW, I was speaking about really swept wave fins, not the slightly
> swept B&J pointer fins (which tend to be more foiled all the way to the
> tip).
>
> Scott (removethis) wrote:
>
> > The lift of a symmetric foil (like a fin) comes purely from the angle of
> > attack relative to the plane of symmetry of the foil and has nothing to
do
> > with the shape of the foil itself. Because of leeward drift the angle of
> > attack on a fin is always non-zero. That is what provides lift.
> >
> > Scott
>




29 Jun 2005 12:49:08
R Lee
Re: What makes a fin "loose?"

See this is exactly what I'm talking about. "Everyone" knows these
"facts" to be true... But what is the basis of these beliefs? Are we
really dealing with foil "stall" at high speeds? Why should a swept
fin be less efficient? Why are the turning and spinout characteristics
better? I personally find that I sense impending spinout better with
my bump and jump fin than the wave fin. I also find that I can recover
just as easily.

I do think we are on to something about the center of effort being
closer to the board making the board more free rail to rail.

Robert



29 Jun 2005 17:39:15
Dan Weiss
Re: What makes a fin "loose?"

NACA foil research is a great source for finding consistent understandings.
Swept fins are not always less efficient, but usually that efficiency comes
at comparatively high speeds where control issues make greater demands on
foil design than efficiency metrics. As for "why" this general rule applies
to the foils themselves, I think the answer for most is that science proves
it so.

That you can sense impending spinout with one fin over another might have to
do with the dimension, construction and longitudinal stiffness differences
between each of your fins. My experience holds that longer, moderately
swept fins with a soft tip give nice feedback, just like many down-the-line
wave fins.

-Dan
"R Lee" <[email protected] > wrote in message
news:[email protected]
> See this is exactly what I'm talking about. "Everyone" knows these
> "facts" to be true... But what is the basis of these beliefs? Are we
> really dealing with foil "stall" at high speeds? Why should a swept
> fin be less efficient? Why are the turning and spinout characteristics
> better? I personally find that I sense impending spinout better with
> my bump and jump fin than the wave fin. I also find that I can recover
> just as easily.
>
> I do think we are on to something about the center of effort being
> closer to the board making the board more free rail to rail.
>
> Robert
>




30 Jun 2005 15:10:05
R Lee
Re: What makes a fin "loose?"

Have Wardog and Bill Kline not been posting on this group? Their lack
of input on this subject is interesting.