Im a bit confused about this inset clew idea that Gaastra ( and probably many others ) have brought out this year. i know its old tech blah blah so spare me.
But, what im curious about is on the picture of the 2011 vapor they claim that the inset clew increases control and provides higher performance,
this isnt the normal inset clew where the leech of the sail simply protrudes outwards above the boom and continues upward like normal, this one is as if they have got the normal outer clew position and simply moved it in 30cm
Now in reference to the center of pressure on the sail, if the clew is closer to the CoP then the moment arm ( in this case the boom ) is shorter and therefore you have less leverage over the sail. wouldn't this decrease control and leverage over the sail and therefore decrease control ability?
For example, an aeroplane rotates around its lateral axis by using the elevator ( in the case of a sail this is the boom and sailor pulling on it ) if the elevator is a long way from the pivot point of the lateral axis or center of gravity ( approx 1/3 the chord line from the leading edge of the wing, or in a sails case the luff ) then only a little amount of effort is needed to provide a significant amount of movement its just the simple leverage action really, and vice versa.
so what im getting at and i hope some sailors with experience using inset booms or perhaps some sail manufacturers could shed some light as to whether its just sales hype or does it actually make a difference??
OK so let me get this right -
normal reduced-boom length sails have the leech protruding above the boom and the foot (roach? is that the word I remember form old school windsurfing?) comes up to meet the clew grommet, so it does not stick out past the boom. Yes I am familiar with that
You are saying some of the new ones are just like a conventional sail, but the clew has a big rectangle cut out to move the clew grommet inwards, so BOTH the lower leech and upper roach are poking out past the boom?
I love a discussion about moments, torque and levers and stuff, but I can't see it makes a difference when our harness lines are centred around the CoE
.....unless I missed something? (I may need a quiver of these new sails to test for a year so as to report properly 
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You could say it increases control as less back hand movement is required to change the sheeting angle. That is true, but it is much like a F1 car vs a normal car - much more control for experts but unusable unless you have tons of experience and are very sensitive to small corrections. So "more control" may be misnomer whereby it is TRUE but not necessarily GOOD for many sailors?
@ Mav
Have you thought about not being able to grab the boom all the way to the clew anyway nor having the need... but on the other side shorter boom is more stiff and thus providing more control.
This is the way I see it.
, but I can't see it makes a difference when our harness lines are centered around the CoE Good point so i guess if we adjust our harness lines accordingly it will still be as easy to sail, but we will have less fine control over the sail and therefore having decreased overall control )
The impression I get, is the effect on the sail of wind loading, having the boom further in allows all the back of sail behind the inset to twist off, this also has a leverage effect, along the battens, helping to keep the back section straight. In effect the wind load is trying to make the sail a slight "s" shape.
With the insert at the end of the clew, wind load is trying to make the whole sail concave, when the rear of the battens succumb, the draft moves aft and the sail gets back hand heavy
So with the inset inboard the sail should maintain it's shape better in strong gusts.
I guess theoretically if you set the batten tension and the clew far enough in, you could get the sail to decrease in lift as the wind increased. This would give enormous range, but would be very inefficient in strong winds.
Decrepit is spot on.
The centre of pressure of an aerofoil moves towards the rear when the camber (thichness) is increased.
For a sail, that means, the centre of pressure will move towards the rear as the wind gets stronger because the sail tends to belly out. i.e the camber increases.
This was really noticeable on the old no batten dacron sails which were quite elastic and changed shape a lot with variations in wind strength.
The problem was reduced by the newer monofilm sails with battens.
The monofilm didn't stretch much and the battens maintain the shape in light breeze.
The whole philosophy of different clew hole positions is to try and get the sail to change shape in such a manner so as to keep the centre of pressure as close to the same position as possible over a wide range of wind speeds and sail loading.
An inboard clew hole does this for the reason Decrepit said above.
The trailing edge deflects outwards which has the effect of moving the centre of pressure forwards in stronger winds.
A lower clewhole will do a similar thing as it allows the trailing edge of the upper section to flex out a little with the effect that the centre of pressure moves forwards. It gives the aerofoil (sail) what is called a "reflex camber".
Anything which moves the center of pressure forwards in gusts gives the sail a light feel because the backhand pressure remains constant as the wind gets stronger, which is totally opposite to what you would expect in a gust.
It makes for a very comfortable ride in strong or gusty winds.
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The inset clew is a good idea. Its been adopted industry wide since NP reintroduced it to the sport back in 2004 with the RS5 sails.
The previous generation of NP race sails (EvoII) took the concept further and enabled a useful amount of reflex to be generated, which as mentioned before, shifts the centre of pressure forward. We see this as standard on sails from most of the other brands that followed (with or without "special batten tensioners").
The benefits I'm enjoying:
-Shorter, stiffer booms for the same sail size.
-significantly more stable profile- the centre of pressure doesnt shift back in the gust
The closed version of the clew in the latest sails provides more control over the leach tension in the foot, which has effect on the twist and profile stability there, providing even greater wind range and performance when done right.
All the above noted
but why did NP dispense with the idea for the new Firefly and it works better??? 
