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 )



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?

Mark _australia said...

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 Correct, i was also talking about some sails that have only the upper leach protruding past the boom and the foot of the sail is still level with the boom.

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? YES - and my issue is, the COE remains further back in relation to the "true" clew postion ( where the clew would have been if it were a normal sail) but the boom is attached further in therefore providing less leverage than if it were further out in the old position

I love a discussion about moments, torque and levers and stuff < Me too!, 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
.....unless I missed something? (I may need a quiver of these new sails to test for a year so as to report properly )



You could say it increases control as less back hand movement is required to change the sheeting angle.True indeed, however while less movement is required, more force is required to move it the same distance and in my experience more force reqd means less fine control, eg if you have a really light back hand pressure sail/harness line setting then its easy to make small adjustments to the sail position but if your over powered then i find its allot harder to make small adjustments due to allot of force being required. 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?

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.

MavericK040 said...



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.

whaaaaaat?


Anyway, reckon Decrepits on the money with the flex off.. I would add that the battens take up where the boom ends and the 'clue' or the back of the sail stays practically in the same spot. Just like bird feathers, they can get pretty light weight at the leech because there is no lift at the leech, no need for any bones or booms. The leech is just making the air release cleanly.

Also less boom flex, and a shorter boom. Why did it take so long to work that one out?



In picture form.

pweedas said...

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.

Slowy was backing up a lot of these points the other day when raving about the Evo III.

nebbian said...

Sik forward Mr. Duck

I beg to differ.

Obviously the 3/4 point of a pushloop

(and Mr Panther, I though't yo'd reply to the Ezzy comments )

All the above noted

but why did NP dispense with the idea for the new Firefly and it works better???

Mark _australia said...

All the above noted

but why did NP dispense with the idea for the new Firefly and it works better???

You could argue that they just extended the 'cut' about 600mm further vertically, as they don't run a 'cross batten' on the fly they don't really have any other option; 2010 fly was the same.

I'm a fan, the SWAT's I run now have a pretty massive cut out and the resulting short boom keeps it stiff and out of the way when busting a move