## Sail Size, Fin Size and Stuff

### Sail Size, Fin Size and Stuff

Hi there, I've got a question.

Recently, there have been a long discussen about fin and lifting and a while
age a discussion about sail size and fin size. If I remember right, peoples
opinion is: large sail --> large fin, small sail --> small fin. Right?
The next point is that people argue that a larger fin causes more lift
and a small one more spin outs. Right again? The conclusion would
be: optimal fin size = f(sail size).

Ok, I thaught that this sounds reasonable,. However, while thinking more
and more about the line of arguments, I more and more doubt that.
Usually, I change the sail if the wind conditions are different. I.e.
light wind --> large sail, strong wind --> small sail. The consequences
are that I have allways almost the same power (while planning) and therefore,
the same downwind pressure. That means that I allways need the same fin.
Actually I have only one fin for my slalom board and I use this for
4.0 - 6.5 sails without problems. I'd rather say, the fin size is determind
by you weight (!), the surf board, the sail quality and you sailing
skills.

Any other opinions on that (probably lots of objections).

-- Ralf

### Sail Size, Fin Size and Stuff

Quote:
(Ralf Salomon) writes:

|> Recently, there have been a long discussen about fin and lifting and a while
|> age a discussion about sail size and fin size. If I remember right, peoples
|> opinion is: large sail --> large fin, small sail --> small fin. Right?
|>
|>   [...]
|>
|> Usually, I change the sail if the wind conditions are different. I.e.
|> light wind --> large sail, strong wind --> small sail. The consequences
|> are that I have allways almost the same power (while planning) and therefore,
|> the same downwind pressure. That means that I always need the same fin.

Yes, good point, I went through exactly the same reasoning.

However, I carried it a little bit further, because after all
experimental evidence strongly suggests that you are better of
with different fin sizes.  I think there is one more factor to
take into account, and that is the angle of attack of your sail.

I think that as the wind increases and you use smaller sails,
you also don't need to sheet in as much to get the same speed
(suppose that speed does not change a lot).  Therefore the ratio
of forward thrust to sideways thrust increases, and you don't
need as much fin.

If, instead, you go faster as the wind gets stronger, then the
explanation is even simpler: smaller fin x higher speed = same
sideways thrust.  ---Luigi

### Sail Size, Fin Size and Stuff

Quote:
>The next point is that people argue that a larger fin causes more lift
>and a small one more spin outs. Right again? The conclusion would
>be: optimal fin size = f(sail size).
>Ok, I thaught that this sounds reasonable,. However, while thinking more
>and more about the line of arguments, I more and more doubt that.
>Usually, I change the sail if the wind conditions are different. I.e.
>light wind --> large sail, strong wind --> small sail. The consequences
>are that I have allways almost the same power (while planning) and therefore,
>the same downwind pressure. That means that I allways need the same fin.
>Actually I have only one fin for my slalom board and I use this for
>4.0 - 6.5 sails without problems. I'd rather say, the fin size is determind
>by you weight (!), the surf board, the sail quality and you sailing
>skills.
>Any other opinions on that (probably lots of objections).

yeah, opinions:
1.  let's assume he's right -- same downwind pressure, since force x wind
or something like that (forget bernoulli-lift on curved sail) approx.
2.  but:  smaller sail means you go faster (more wind), so lots more lift
by water flowing faster past fin surface.
3.  so:  faster wind means smaller sail and equal pressure, but
faster, so more lift AND more drag of fin thru water (drag force
varies approx. as vel. squared).
4.  but if you use smaller fin, lift can be the same and less drag

conclusion:  spend time sailing, not changing fins unless you are
competitively racing.

but then again, what do i know?

--
Bob Rouda
signature under construction
meanwhile:

### Sail Size, Fin Size and Stuff

Quote:

>Hi there, I've got a question.
>Recently, there have been a long discussen about fin and lifting and a while
>age a discussion about sail size and fin size. If I remember right, peoples
>opinion is: large sail --> large fin, small sail --> small fin. Right?
>The next point is that people argue that a larger fin causes more lift
>and a small one more spin outs. Right again? The conclusion would
>be: optimal fin size = f(sail size).
>Ok, I thaught that this sounds reasonable,. However, while thinking more
>and more about the line of arguments, I more and more doubt that.
>Usually, I change the sail if the wind conditions are different. I.e.
>light wind --> large sail, strong wind --> small sail. The consequences
>are that I have allways almost the same power (while planning) and therefore,
>the same downwind pressure. That means that I allways need the same fin.
>Actually I have only one fin for my slalom board and I use this for
>4.0 - 6.5 sails without problems. I'd rather say, the fin size is determind
>by you weight (!), the surf board, the sail quality and you sailing
>skills.

Smaller fins have a lower drag coefficient.  This means you can reach
a higher top end for the same amount of force exerted by the lift
in your sail.

Just a comment, I use the same 10 Inch wave fin from 6.5-3.2 on the same
board.  Sometimes a board fin combination just works right, for me this
combination has tremendous range.  However, I have found that when I'm
slightly underpowered a large fin is easier to rig than a larger sail
and will get me planing.  Conversly, I quite often switch down a fin size
when I'm slightly overpowered, rather than rig a smaller sail.  This seems
to keep my tail walking to a minimum and gives me more control.  Emperically
it definatly makes a difference on my board, as does the style of fin.
My wave fin loosens the board up considerably, my small bump pointer
is better going up wind and for recoveries from spin out but makes the
board considerably less jibey.

Quote:
>Any other opinions on that (probably lots of objections).

I think square inches of surface area is definatly the major factor
in a fin.

Quote:
>-- Ralf

Craig's .02

8'10" Bailey jump, 9'9" Sailboards Maui
Wt 155#, Ht 6'3", Usually sail on high desert lakes near SLC in Ut
Go short or go home

### Sail Size, Fin Size and Stuff

Quote:
Goudie) writes:

I have the complete opposite experience...fin size matters greatly for
each sail.  I don't think that boardspeed remains a constant as you change
to smaller sail in higher wind.  A larger fin with a smaller sail gets
overpowered, which feels like the upwind rail rising out of the water.
And as you've already noticed...you can't plane easily on a too small fin.

### Sail Size, Fin Size and Stuff

Quote:
: Goudie) writes:

: I have the complete opposite experience...fin size matters greatly for
: each sail.  I don't think that boardspeed remains a constant as you change
: to smaller sail in higher wind.  A larger fin with a smaller sail gets
: overpowered, which feels like the upwind rail rising out of the water.
: And as you've already noticed...you can't plane easily on a too small fin.

AND OF COURSE all this dicussion is valid mainly for slalom boards and
big sails. I use my wave board with 3.2 to 5.5 with the same fin (10 1/2'').
I need to use a 13 1/2'' on my slalom board to be fast with a 6.0 (beaten
everybody when I tried the Chicagoan spots, I don't pretend to be *this*
fast though, nor very interested in beating everybody, I just dislike
being passed).
And my 13 1/2'' is just too big for 4.6 or even 5.0.
And of course 4 years ago, I used to think that fins didn't make such a
difference, that the better sailors were working on details because
they were thinking too much of themselves.

Changing fins instead of changing sails is an interesting idea!
I have some difficulty to believe it but I'ld like to try it!

SO, WHEN IS IT GOING TO BLOW???
Paul

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Paul Soudais
University of Illinois at Chicago

Gear: Custom wave board, Adagio, UP and North.
Spots: Normandy, France and Lake Michigan
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