Jib Sheeting Angle
Julian Bethwaite – Saturday, 7 January 2017
(Click on diagrams to enlarge)
Just before Christmas, I spent some time in Mumbai India, coaching 9ers.
Last time I was in Mumbai was 3 years ago, and the place has improved dramatically, there is a very positive energy in the air, and I was only accosted by a beggar once in 5 days. Quite extraordinary!
Lots of questions, feel, balance, sheeting angles, what the wires do. Due to the Nationals being scheduled in Chennai for the 29ers, we had 8 49ers and a few FXs in Mumbai, so it ended up being very 49er centric. The sailors were from Chennai, Bhopal and Mumbai, mostly, some Navy and Army node sailors also.
The English did leave India with English as the binding language, but it’s not the mother tongue. There are 2000 different languages/dialects across India, and that makes some of the answers more interesting in that you must get the idea across to a body of people in a manner they understand in my limited Australian.
On the first day, we laid a boat on its side and started to explain what each control did and the importance of batten tensions, but a question that came back time and time again was jib-sheeting angle.
A 49er/FX is a lot more complicated than a 29er in that you are allowed an adjustable (not while racing) jib tack length. The 29er is a lot simpler, in that it must be shackled on in a fixed position.
So to keep this simple, I am going to focus on the 29er, but the 49ers are exactly the same!
A jib is, for all intents and purposes a triangle, and that makes life very easy, in that it’s very easy to find the centre, what we refer to as the CoA (Centre of Area). You simply bisect say the luff, and you draw a line from the clew to the middle of the luff, do the same thing with the foot, draw a line from the head to the middle of the foot, and where those lines cross is the CoA.
This is a 29er jib, done to scale, it is accurate.
The black line shows the edges of the jib, and the cross lines are the battens.
The blue lines are lines drawn from the apex(s) of the triangle of the sail to a point that bisects the opposite side
The green dot just above the bottom batten is the CoA and I have given you measurements so you can actually go and measure this on your 29er jib.
The red dot 91mm above is the Centroid! I’m cheating in that I am using a 3d program to draw this, so I asked the machine to find the Centroid.
The difference is because of the roach in the leach, and I use the green position.
Now we have the Centre of Area.
That is NOT the CoE (Centre of Effort). I have just trawled through all my father’s books, Manfred Curry’s “Theory of Wing Section” but the late, great Prof Marchaj probably diagrammatically describes the shift from CoA to CoE the best, so I am poaching 2 of his drawings.
On the right is what my father would have referred to as Piedo Tubes, used heavily in the aircraft industry to sense air pressure, you will still see them today normally toward the front of the Airbus or Boeing, but they are equally useful in measuring the pressure differential across a sail.
This one gets to the guts of it, again courtesy of Prof Marchaj.
Without getting too involved as to why, if you measured the area under the curves and then found the medium point at which there was equal differential in front as behind, you would find you are somewhere between 30-35% back from the LE (Leading Edge).
When you first tack, let’s say you do a bad tack, the Centre of Effort will be approximately where the Centre of Area is, in the middle of the sail. But as soon as you start to move forward and start getting some meaningful flow across the sail, that CoE sucks forward. On an old fashion type of sail, a heavy boat, with a big fat knuckle forward, it may suck forward as much as 25% (of the chord) so it will end up 25% back from the Leading Edge.
On a 29er or 49er, which have reasonable size sails and are modern in their design, it finds a happy place about 1/3 back or 33%.
So in the following drawing I have drawn a Cyan line horizontal (because the water is horizontal and it will force the air to “mostly” flow horizontally) that runs through the CoA.
I have then measured the length of that line, divided it by 3, measured back that distance and the Cyan dot is “a close approximation” of the CoE.
So the Red Dot is the Centroid and the Green Dot is the CoA.
The Cyan Dot is the CoE.
If you go get your 29er jib, find a point 60mm above the centreline of your bottom jib batten, measure back 424mm horizontally, you will end up about 25mm (1”) above the batten and that is your CoE.
Extend a line from the clew, thought the CoE, and that should be your “normal” jib sheeting angle. Draw a line from the clew through the CoA and that should be a “light-air” jib sheeting angle.
For convenience I have extend them forward to the luff!
That is the range of jib sheeting angles that you should be using in 95% of sailing situations.
The line through the CoE is the easier one to explain!
Notionally ½ the pressure in the jib will be above that line and ½ will be below it, so in the normal gust/lull sequence as increased pressure rolls or passes across the headsail, the load will remain relatively constant, therefore the leach is not going to hang open, the foot is not going to round (fatten) up overly and it should result in maximum acceleration and minimum “stalling”.
Just going one stage further, as that gust-lull rolls across the rig, you are going to ease the mainsheet, and that in turn eases the forestay tension. Therefore you get forestay sag, but at the same time, because your jib has roach, the leach will hang open a little bit more, compensating for the forestay sag. That hanging open of the leach also increases tension in the jib sheet (as a proportion) along the foot, so it will maintain constant camber over the lower section of the jib (increased load will naturally try and fatten up any sail, stretch, etc).
And all of this happens in about 1/3 of a second, automatically.
It’s the reason we put roach into the jib.
In lighter airs, you don’t want your leach to open up, and because you are seeking power, rather than being fully powered up, an increase in wind speed will allow flow to remain attached around a jib with greater camber, so by sheeting “down the leach” as the gust rolls across the sail, the leach will tighten and the lower part of the sail will fatten up marginally.
So you get more power and provided you don’t ease the mainsheet “overly” (30-40mm is fine) there will be no meaningful sag in the forestay so you can tolerate a tight leach.
Again, because there is roach, the leach will blow open a little, and as the forestay sags a little you will get a deepening of the upper jib, which complements the fattening/deepening in the lower jib and then we can get into all the added benefits that has for the mainsail, and again all this happens in 1/3 of a second automatically.
At the extremes, if you end up in light air and lumpy water, the drag from the lumpy water masks the drag from the overly fat jib, and the extra power may be needed to punch through the slop, so you go even steeper with the jib-sheeting angle.
If it’s blowing “oysters off the rocks” (or “dogs off chains”) then going flatter on the jib sheet angle will allow better control of the lower jib, that you can “drive” off and allow the upper leach to hang more open, allowing de-powering!
But this is less than 5% of the normal sailing situations.
In a weeks’ time I will get into Feel and Balance.
There is a lot of the above that is of critical importance to Balance.
Having the boat Balanced or In Balance is possibility the single most important thing you can do.
Unless the boat is in Balance you can’t possibly hope to have any Feel.
That’s actually not true, but your Feel will be masked!
The best way I can think of describing Feel is if a motor car had no feed-back through the steering wheel, then you can still drive it, but a) you can’t drive it well and b) it’s a lot less fun!