It's far from simple. In the upper sections, leech control is critical
to determine how much draft is right. As wind velocities at the top are
higher and if angles of attack are set to be optimal by twist control
then the upper secion should be slightly flatter. This is possible
enitirely due to the stiffness of modern fabrics that will properly
support their leech as the wind pressure increases. Have a look at the
North web site for some photos of their racing sails and try to find
full upper sections.

Cheers MC

DSK wrote:

It is the simple fact that a well cut sail will be
relatively deeper in the upper sections.

You are 100 percent wrong with this observation.

Take a look at an albatross sailing on the air currents
above the ocean. If you perchance to see one end-on
you will note their wing tips flare upwards and outwards.

Picture one of the bird's wings placed vertically and you
will see how incorrect your trying to close the leech
of your sail is. This a common fallacy with those who
fancy themselves racers. They are too busy looking
at all the other racers doing it wrong and can't see
folly and inefficiency even when they look right at
it.

Another little mother nature test is try feeding some
seagulls sometimes. The few being fed will put out
the word and others from all around will come
barreling in. They approach at high speed. Then
they draw their wings in a nice curve effectively
shortening their wing span and they feather and
stall. This inefficiency acts like a brake. What
you do with a sail with a closed leech in lower
winds is stall the damned thing out.

S.Simon


"The_navigator©" wrote in message ...
It's far from simple. In the upper sections, leech control is critical
to determine how much draft is right. As wind velocities at the top are
higher and if angles of attack are set to be optimal by twist control
then the upper secion should be slightly flatter. This is possible
enitirely due to the stiffness of modern fabrics that will properly
support their leech as the wind pressure increases. Have a look at the
North web site for some photos of their racing sails and try to find
full upper sections.

Cheers MC

DSK wrote:

It is the simple fact that a well cut sail will be
relatively deeper in the upper sections.

Simple Simon wrote:
You are 100 percent wrong with this observation.
What, twist control is wrong?

Bwhahahhaha

Cheers MC

So why is your leech closed?

Cheers MC

Simple Simon wrote:

What
you do with a sail with a closed leech in lower
winds is stall the damned thing out.

In any event, in light air, a big round section with a relatively
flat entry will keep attached flow and build up much more power
than a draft forward conventional airfoil shape.

The navigator© wrote:
There we disagree, in ligh airs the angle of attack of the leading
section must be close to zero to attach flow.

??? How do you get this out of what I said above?

Hence it should be full.

No. To keep flow attached (actually, to re-attach flow after passing the
mast) the leading section should be flatter further aft. Furthermore, at low
Reynolds numbers, the lift/drag ratio is better for the sort of shape I'm
describing.

This is one case where theory and practice seem to mesh pretty well. I read
a lot about new methods of sail trim and watched some of our competitors,
and found that what looked wrong (a deep round section) was actually quite
fast.




Surely you would agree that aeronautiucs shows that deeper foils
generate more lift at low speeds....

Again, how do you get this out of what I said?


I can see the topping lift, whih is presumably straight, and the
distance from the battens to the topping lift does not appear to
increase towards the middle of the sail.

You are forgetting the roach...


No I'm not, I'm looking at the angle of the battens. When a sail is twisted,
the battens take a series of angles as you look up the sail. Not exactly
rocket science.


Looking closely, I can see what might be leach ribbons flowing.
That's a good sign.

Yes they are, and it is.

Depends on the boat and the conditions. Some boats like the top telltale
flying all the time, some like it curling back some of the time, some more.

Usually boats that like to pinch (as opposed to boats that do better footing
off) want to have the upper leach ribbon curling back at least 25% of the
time, although in light air you have to be careful that the upper section
isn't stalling.




Why do you have a backstay antennae and no wire to it? Is the
wire lead up the mast and to the antennae at the top?

Well spotted. The wire is off.


OK, that's allowed

It's a good idea until you go offshore. Means that the connections don't
'weather' until they are needed.

You need some No-Ox.

Fresh Breezes- Doug King

Let me try to get this straigt. You are saying that a flat sail
generates more lift in lower winds than a full sail?

Cheers MC

DSK wrote:

No. To keep flow attached (actually, to re-attach flow after passing the
mast) the leading section should be flatter further aft. Furthermore, at low
Reynolds numbers, the lift/drag ratio is better for the sort of shape I'm
describing.

You are saying that a flat sail
generates more lift in lower winds than a full sail?

The word here is "lift."
And we see that MC doesn't know what it means!

RB

Loco, I was offering critique of Navvie's 'perfectly' set & shaped main.
For
some reason AOL will not let me look at your pictures.

For the leach, I like to look at the leach ribbons and the overall
twist.

Doug,
Oh sorry. BTW my tells are flying.


S/V Express 30 "Ringmaster"
Trains are a winter sport

Look more carefully. There are ribbons at each battenb and they are all
flying at that moment. You simply can't see twist from that camera angle
so the twist must be sufficient to keep the top ribbons flying...

Correct.


S/V Express 30 "Ringmaster"
Trains are a winter sport

Take a look at an albatross sailing on the air currents above the ocean.

I don't go out that far.


S/V Express 30 "Ringmaster"
Trains are a winter sport