"magic numbers" for fin sizes

"magic numbers" for fin sizes

Post by John Broo » Wed, 06 Oct 1993 04:26:05


        In a 1993 issue of Windsport (the Canadian magazine), there was an
article called "Fin Magic" which, in short, stated that there is a simple
was to calculate the optimum size of fin for a given sail (someone else
posted on this earlier).  For the purposes of the article "size" is the
surface area of the fin.  This measure of size is claimed to be best as
it allows aspect ratio to be ignored.  The fin sizing method assumes that
there should be a constant ratio of fin to sail area which will result in
a "magic number". A survey of good sailors was conducted to determine the
magic number.  The result was that for a givensail size, in m, the optimum
cross-sectional area of the fin, in square cm, can be obtained by multiplying
by 41.  For a 5.5 m sail this yields 225 square cm.  They claim a magic number
of 41 is applicable for sails of 4.0 to 6.0 m size and 45 is applicable for
sails 6.5 to 7.5 m.

        Intrigued by the simplicity of the concept, I traced 6 of the
fins I was familiar with and calculated their area.  I was then able to test
the formula using my experience using a range of sails with these fins.
The good news is that, yes, I obtained a fairly consistent ratio, however,
my personal "magic number" was different and ranged from 52 to 55 for 4.5 to
6.2 m sails. This is a significant difference, for example, for a 5.5 I would
be using a 290 square cm fin rather than a 225.  This is the difference between
a 12 in blade and a 13.5 in blade!!  A quick look at recommended fin sizes in
brochures and articles also suggests 41 is too small.

        Drag out some graph paper and try it for your sails and fins.  I
would be interested in your findings.  For reference; I use 8'6" and 9'
dedicated slalom boards, fully cambered slalom sails and a mix of pointer
and blade fins for mostly flat water sailing in chop.  I weigh 168 pounds

                                John Broome, Ottawa, Canada

                        (saw first snow today, how depressing)

 
 
 

"magic numbers" for fin sizes

Post by Kirk Lindstr » Wed, 13 Oct 1993 00:18:16

Quote:
>    Intrigued by the simplicity of the concept, I traced 6 of the
>fins I was familiar with and calculated their area.  I was then able to test
>the formula using my experience using a range of sails with these fins.
>The good news is that, yes, I obtained a fairly consistent ratio, however,
>my personal "magic number" was different and ranged from 52 to 55 for 4.5 to
>6.2 m sails. This is a significant difference, for example, for a 5.5 I would
>be using a 290 square cm fin rather than a 225.  This is the difference between
>a 12 in blade and a 13.5 in blade!!  A quick look at recommended fin sizes in
>brochures and articles also suggests 41 is too small.

>                            John Broome, Ottawa, Canada
>----------

I think these mag articles are written for the intermediate sailor where
blade fins are NOT recommended since you need more speed to make them
work well.  Blades are more EFFICIENT so the "number" will be higher.
You might add an aspect ratio factor to your equation.  Also need to
consider if you are slalom sailing or racing upwind.  I can get by with
a 15.5" blade on my 9'3" CS rigged with a 6.7 Race sail but I went to a
17" blade for going upwind with the same gear.  At the other end, I have
a 11.5" LD (swept blade) fin that I can sail with my 5.6 and 8'8" RKT
(epoxy with soft rails and pin tail) for wave/swell sailing in ebb tides
but I like to use a 14.5" Blade with a 8'11" CS board and the same 5.6 sail
for going upwind fast.  I also have a lower aspect ratio blade that has
the same surface area as the 14.5 but is 13" - this I use with the 5.6/8'11"
on ebb days at Coyote where I want to go fast, go up and down wind, and
have fun in the channel swells.  It is slower than the 14.5" blade, but more
fun.

Kirk out
8'8" ASD epoxy RKT, 8'11" & 9'3" ASD epoxy CS, Malibu & 8'8" ChallengeFlex
(The Mailbu is for sale plus 5.6, 6,1 and 6.7 Sailworks Sails)
Wt 213#, Ht.  6'0", Usually sail on SF Bay, Cailf.


 
 
 

"magic numbers" for fin sizes

Post by John Broo » Fri, 15 Oct 1993 05:07:44

Quote:
>>        Intrigued by the simplicity of the concept, I traced 6 of the
>>fins I was familiar with and calculated their area.  I was then able to test
>>the formula using my experience using a range of sails with these fins.
>>The good news is that, yes, I obtained a fairly consistent ratio, however,
>>my personal "magic number" was different and ranged from 52 to 55 for 4.5 to
>>6.2 m sails. This is a significant difference, for example, for a 5.5 I would
>>be using a 290 square cm fin rather than a 225.  This is the difference between
>>a 12 in blade and a 13.5 in blade!!  A quick look at recommended fin sizes in
>>brochures and articles also suggests 41 is too small.

>>                                John Broome, Ottawa, Canada
>>----------
>I think these mag articles are written for the intermediate sailor where
>blade fins are NOT recommended since you need more speed to make them
>work well.  Blades are more EFFICIENT so the "number" will be higher.

             """"""""""""""""""""""""""""""""""""""""""""""""""""""""

Quote:
>you might add an aspect ratio factor to your equation.  Also need to
>consider if you are slalom sailing or racing upwind.  I can get by with
>a 15.5" blade on my 9'3" CS rigged with a 6.7 Race sail but I went to a
>17" blade for going upwind with the same gear.  At the other end, I have
>a 11.5" LD (swept blade) fin that I can sail with my 5.6 and 8'8" RKT
>(epoxy with soft rails and pin tail) for wave/swell sailing in ebb tides
>but I like to use a 14.5" Blade with a 8'11" CS board and the same 5.6 sail
>for going upwind fast.  I also have a lower aspect ratio blade that has
>the same surface area as the 14.5 but is 13" - this I use with the 5.6/8'11"
>on ebb days at Coyote where I want to go fast, go up and down wind, and
>have fun in the channel swells.  It is slower than the 14.5" blade, but more
>fun.

>Kirk out

I generally agree with your comments but more efficient performance should mean
a smaller fin would be required which would give a lower number, not a higher
 number.

Anyways I would rather be sailing than playing with numbers.

                                        -John
                                ( now only 17 days to Hattaras !!)

 
 
 

"magic numbers" for fin sizes

Post by Bill North » Fri, 15 Oct 1993 23:33:05

:                                       -John
:                               ( now only 17 days to Hattaras !!)

It looks like you will be in Hattaras the same time that I will be. Who else
will be going there the first week of November and where will everyone
be sailing ?

        bill  

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