Fins size Vs Sail Size

Fins size Vs Sail Size

Post by Tim Sercomb » Fri, 20 Dec 1996 04:00:00


Hi all

I've often wondered about what fin size to use for a given sail size.
Everyone knows that as the wind strength increases (sail size decreases)
and fins get smaller .... but why?

I can understand that a bigger fin is needed in marginal conditions to
get you planing earlier, but after that?

Lets assume that you are fully powered up and the water is perfectly
smooth (ideal situation). Also assume that you sail at the same speed
whether on a 7.0 or 4.0 or anything between. Then isn;t the sail
producing the same amount of lift? And therefore the fin size could be
the same? I know that a smaller fin is easier to control in chop and
creates less drag therefore tend to be faster. Is the sail producing
more lift at lower speeds (ie greater board:wind speed ratio) and
therefore you need a larger fin to counter-act this or am I missing
something else?

Anyone know a solution to this, as I have been puzzled for sometime over
this thought.

Happy sailing

Tim

 
 
 

Fins size Vs Sail Size

Post by Rob Darm » Fri, 20 Dec 1996 04:00:00

I'm not claiming to know the answer to this, but here is my guess.

If you are sailing perpendicular to the wind, then the direction of
your apparent wind is a function of board speed and wind speed.  

        |wind due to board speed (25 kts)
        |
        |                / apparent wind when wind = 20 kts
        |             /             / apparent wind when wind = 30 kts
        |         /        /    
        |     /    /
        V/
         <____________ true wind

If you compare two boards going the same speed,  and direction, but
with different true wind speeds, the board with less true wind is
actually pointing higher upwind than the board with more true wind.
Pointing upwind requires more lift, and thus a bigger fin.

Sound reasonable?

Rob Darman

 
 
 

Fins size Vs Sail Size

Post by Booker C. Ben » Fri, 20 Dec 1996 04:00:00

-----BEGIN PGP SIGNED MESSAGE-----

- - In order to add to the half-baked science often posted in this
group and waste some time I will attempt to explain basic hydrodynamics.

 Lift on a foil ~ surface_area * (velocity)^2

- - meditate on this over and over and over.... If you really understand
this equation and then draw the vector force diagram of windsurfing
you'll understand most of the "science" posts in this group.



Quote:
>Hi all

>I've often wondered about what fin size to use for a given sail size.

- - The only way to figure this out is trial and error. It totally depends
on your sailing style, body weight, board design. It's too bad you can't
rent fins to work this out. I have a collection of about 20 fins of which
I really only use 4-5.

Quote:
>Everyone knows that as the wind strength increases (sail size decreases)
>and fins get smaller .... but why?

- - Because you are sailing faster...

Quote:

>I can understand that a bigger fin is needed in marginal conditions to
>get you planing earlier, but after that?

>Lets assume that you are fully powered up and the water is perfectly
>smooth (ideal situation). Also assume that you sail at the same speed
>whether on a 7.0 or 4.0 or anything between.

- - This is just not true, even in an ideal situation. Since the velocity
is squared, even a small change in velocity creates a large change
in force.

Quote:
>Then isn;t the sail
>producing the same amount of lift?

- - No, because you are not sailing the same speed. You are probably
sailing at roughly the same efficiency (i.e. % of true windspeed ).
Your arguement assumes that sailing with smaller sails has a dramatically
lower efficiency. Why would this be true? Everything that contributes
to drag increases as you increase the size of the foil.

- - There's a whole bunch of other effects involved ( end plate,
form drag, angle of attack (both horizontal and vertical)...  )  
but you don't need these to explain the shinking fin size.


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Fins size Vs Sail Size

Post by Q » Fri, 20 Dec 1996 04:00:00

Quote:

>Hi all

>I've often wondered about what fin size to use for a given sail size.
>Everyone knows that as the wind strength increases (sail size decreases)
>and fins get smaller .... but why?

[SNIP]

>Happy sailing

>Tim

While ya'll are responding to this please tell what size (area and length)
fin you use with what size (area and type (wave. slalom, ...) and
board!!   I have some christmas/birthday money I want to spend on fins to
match my boards/sails.  (I get the money by telling everyone, "give me
money so I can buy WS stuff"  saves them shopping)

To answer the question:
          __
         |\
direction  \     | <board
of travel   \    |
without fin     /|        <----- WIND
               / |        <-----
         sail>/  |
             /   |
                 |

The wind acting on the sail gives a resultant push that is not in the
desired direction of travel.  Therefore, the fin balances the force of the
sail that is perpendicular to the board and desired direction of travel.
That is why you need a larger fin for a larger sail.  Not as large as the
sail though because water is dense and therefore "hard" at speed (belly
flop ananlogy).

Mach 10 and Windy Too.                    KevinPG

NCSU Chemistry
NCSU Windsurfing
Raleigh, N C, USA

 
 
 

Fins size Vs Sail Size

Post by Gary Wo » Sat, 21 Dec 1996 04:00:00


Quote:

>Hi all

>I've often wondered about what fin size to use for a given sail size.
>Everyone knows that as the wind strength increases (sail size decreases)
>and fins get smaller .... but why?

<snip>

Tim, I'm the farthest from a 'techno' that you will find, but thought I'd
attempt to cloud the issue yet more.  Remember that board shape and sail
form also plays a major role in fin selection, or rather, they make the
fin much more noticable.

Over the past year, I have shifted towards fins that are much smaller than
those I used to sail on.  Part of the reason is one new board, but also
smaller fins just seem to work better in all but the most marginal winds.
This assumes a *good* fin, of course.

FYI - On the newer board, I am now using a fin designed for around a low
5's sail (Finworks 330 - says 5.0-5.8) with my 6.5.  It is a great
combination.  Newer boards seem to need small fins, regardless of the
specs printed on the fin itself.

 
 
 

Fins size Vs Sail Size

Post by NLW TFW » Mon, 23 Dec 1996 04:00:00

There should be a TUNNN of posts on this topic from last spring. Check out
deja news for more than you could ever want to know on this topic. Some 30
posts never reached any conclusions except that it's a personal thing.
Mike \m/
Never Leave Wind To Find Wind

 
 
 

Fins size Vs Sail Size

Post by s1tur.. » Tue, 24 Dec 1996 04:00:00


writes:

Quote:
>FYI - On the newer board, I am now using a fin designed for around a low
>5's sail (Finworks 330 - says 5.0-5.8) with my 6.5.  It is a great
>combination.  Newer boards seem to need small fins, regardless of the
>specs printed on the fin itself.

I had thought it was just my poor technique, but I find that my Mistral
Explosion handles better with the smaller of my fins than with the
"correct" one.  
SamTurner
Tampa, FL
 
 
 

Fins size Vs Sail Size

Post by STOLA » Mon, 30 Dec 1996 04:00:00

Isn't it neat that our nervous systems are pretty good analog computers
that can automatically balance out the various forces of sail, fin, and
board,  and then give appropriate commands to the servo systems of bone
and mucle  so we can sail at top speed in a more-or-less straight line?
All the while attempting to look cool to people on shore (i.e., not
spinning out).

Sail forces and fin forces vary with velocity and angle of attack, but
because the media are different, as well as foil design, the relationip
between sail and fin forces changes with board speed, and not in a linear
fashion. As sailors, our job is to keep those forces in balance, which we
do by sheeting in or out, shifting our weight, foot steering, and raking
the sail fore or aft.  At a given point of sail and wind speed, there is
an optimal angle of attack for sail and fin that will produce the maximum
lift and minnimum drag.  While I'm certain that all these factors
theoretically could be computed to give the optimum sail and fin size, in
the real world we're stuck with empirical methods and rules of thumb.  For
example, with a 5.5m sail and a 9' slalom board and 16-20 knots of wind, a
14.5 inch blade fin seems to be about right.  
Bob Jacobson

 
 
 

Fins size Vs Sail Size

Post by Volker Wedemeie » Tue, 31 Dec 1996 04:00:00

Quote:

> I've often wondered about what fin size to use for a given sail size.
> Everyone knows that as the wind strength increases (sail size decreases)
> and fins get smaller .... but why?

> I can understand that a bigger fin is needed in marginal conditions to
> get you planing earlier, but after that?

> Lets assume that you are fully powered up and the water is perfectly
> smooth (ideal situation). Also assume that you sail at the same speed
> whether on a 7.0 or 4.0 or anything between. Then isn;t the sail
> producing the same amount of lift? And therefore the fin size could be
> the same?

Yep, given your ideal conditions, I think you would be right. But with a
fully powered 4.0 sail you are faster than with a fully powered
7.0 one. I guess the reason is, that you just cannot go 40 km/h in 26
km/h winds with a 7.0 sail, but you can well go 45 km/h in 70 km/h winds
with a 4.0.
One could say that sails get more efficient (and thus produce less
lateral drag in respect to forward force) when the wind gets stronger.
That's why you can use smaller fins with smaller sails.

Hang loose,

            Volker

 
 
 

Fins size Vs Sail Size

Post by andreas mack » Mon, 06 Jan 1997 04:00:00

Quote:

> While I'm certain that all these factors
> theoretically could be computed to give the optimum sail and fin size, in
> the real world we're stuck with empirical methods and rules of thumb.  For
> example, with a 5.5m sail and a 9' slalom board and 16-20 knots of wind, a
> 14.5 inch blade fin seems to be about right.

Weight, as well as the kind of sailing you do also play a big role.  At about 200#, I
tend to need larger sails and fins.  The above combo in the mentioned conditions would
result in a slog-fest for me, unless the sail was a very powerful (lots of draft) one,
in which case the blade fin is probably not that good a choice.  If I'm not trying to go
upwind, I carry a 6.5m sail with a 120l 9'6" and a 14.5" blade.  To get serious upwind
ability, especially in marginal conditions, a 17" blade seems more appropriate to me.  
Lighter sailors may have a problem with all the tail-dancing the board is likely to do
with that set-up.

Once again, as so often in this sport, there's little in terms of proven rules.  So have
fun experimenting..

 
 
 

Fins size Vs Sail Size

Post by Ian Knigh » Wed, 08 Jan 1997 04:00:00

Quote:


> > I've often wondered about what fin size to use for a given sail size.
> > Everyone knows that as the wind strength increases (sail size decreases)
> > and fins get smaller .... but why?

> > I can understand that a bigger fin is needed in marginal conditions to
> > get you planing earlier, but after that?

> > Lets assume that you are fully powered up and the water is perfectly
> > smooth (ideal situation). Also assume that you sail at the same speed
> > whether on a 7.0 or 4.0 or anything between. Then isn;t the sail
> > producing the same amount of lift? And therefore the fin size could be
> > the same?

The 4.0 is producing more lift.  The lift is limited by your counterbalancing
force, ie how far you get your weight outboard in your normal
sailing stance.  Because the centre of effort of a large sail is
higher a smaller total lift force from the large sail can provide  an
identical  moment about the mast foot.  Your sailing stance provides
the equal and opposite moment and you  probably don't change stance
much for large or small sails.

The fin loading with a smaller sail is thus also higher as it must
balance the transverse component of sail lift.

But with more lift from the small sail you should be going faster
,if not for the chop?, and at the higher speed the required lift,
 which is still more than needed with the large sail, can be provided
 by a smaller fin with a larger angle of attack. A small fin saves on
 drag and gives a less twitchy ride.

cheers Ian