## Question on Fin Size

### Question on Fin Size

Hi,

I notice that big sails require big fins and small sails require small
fins. What is the principals behind this? Anybody?

Thanks,

### Question on Fin Size

Quote:
>Hi,
>I notice that big sails require big fins and small sails require small
>fins. What is the principals behind this? Anybody?
>Thanks,

Because the fin makes sure that the board is going forward and not sideward.

With a bigger sail the wind presses the sail and board harder to the side.
A concave or sword has the same function as a big fin.

Chris, Holland.

### Question on Fin Size

Quote:
>Hi,

>I notice that big sails require big fins and small sails require small
>fins. What is the principals behind this? Anybody?

>Thanks,

There was an article in Aeronautics and Astronautics about 25 years ago
with two useful thoughts.
1) Imagine there is no gravity.   Then a sailboat (or board) could be
just two foils, one for the air and one for the water, plus some way to
hook them together and control the angle.  Each would generate lift.
If they work at equal speeds, the water one should be smaller by the
2/3 power of the density ratio, or a factor 80.

2) Imagine there were air animals (those which naturally are at rest
relative to the air, as we are with the water).  They would find it
just as unreasonable to go straight up-water as we do to go straight
up-wind.

The normal state of motion for this craft would be to be moving at the
average velocity of the wind and water.  By putting one foil transverse
to its fluid and feathering the other, you could approach being
stationary with respect to one fluid.  A beam reach would be the
fastest point of sail.  It would be possible to tack upwind or upwater
if you wanted.
Ross Millikan

### Question on Fin Size

Quote:

>>Hi,

>>I notice that big sails require big fins and small sails require small
>>fins. What is the principals behind this? Anybody?

>>Thanks,

>Because the fin makes sure that the board is going forward and not sideward.

>With a bigger sail the wind presses the sail and board harder to the side.
>A concave or sword has the same function as a big fin.

>Chris, Holland.

I don't think this is true.  I think (therefore I am) that the sideways
force on the board is about the same, assuming that you're not over- or
underpowered.  Normally you select a sail such that you can hang your
body weight out over the water.  Your weight is the same, the booms are
in about the same position, so the force in the sail holding you up, and
thus pushing sideways, is about the same.

I believe that with a larger sail the wind is lighter, therefore you are
going slower, and therefore you need a larger fin to provide the
necessary lift.

You'll note that fin size is not a function of the sailor size.  I think
that the sideways force of the sail on the board through the feet
strictly depends on sail size (and position) while fin
lift depends on speed.  I
suspect that talent also comes into play, as well as chop conditions.
When Meg MacKenzie straps on that weight vest and pushes 50 mph, her
sail to fin size ratio is probably higher than for the rest of us.

Just my \$0.02 worth.

John

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