Why are swing rig jibs smaller than conventional sails?
What would be the problem with making a conventional set into a swing rig?
On a swing rig if the jib surface is too large compared to the main an inverse twist (instability) may occurs . Generally it is agreed that for a stable conditions, the jib should not go above 26% - More or less a ratio of 75 / 25 .
In the conventional rig the interelation with the main does not exist anymore being the jib free to move independently. In this case in a class M, for instance, you may go up to 38/40 % of surface dedicated to the Jib.
To note that there is no restriction on classical rig setup, the splitting between main and jib may be also be of 85 % for the main and 15% for the jib. Obvoiusly ‘tuning’ is another story.
Claudio
Why is the swing mounted further forward than a conventional?
NevC,
simple, just observe the drawing and notice the jib CE movements.
Swing , produces a shift backward of the CE of the jib influencing the overall sail’s CE composed by the Main + jib.
Claudio
Thankyou Claudio, this is the best explanation I have seen so far…
Is there an easy way to explain how a different ratio would effect the stability of the swing with say a standard 60/40 conventional ratio sail set used as a swing rig ?
Just curious, and trying to understand the advantages of using a swing over a conventional setup.
Cheers, Neil
DIV,
in the class M the swing rig is limited to 75/25 . More then 26% on the jib it will render the swinging unstable like for the Wind Vane when the front surface is too large compared with the back surface from the hinge point.
Certainly a ratio of 60/40 will never work on a Class M and probably elsewere to.
Classic set up is very good and better performing then the swing, but cannot be interchanged as easy as for the swing where in fews seconds you can change the full rig. This, is for me, the only advantage over the Classic.
My M are all Classic, see some on Wingsail tread.
Ciao
Claudio
Claudio,
Your great drawings clearly show the aft shift of the swing jib CE. However, there will also be a shift foward of the mainsail CE, which will counterbalance this to some extent.
It seems to me that the larger factor influencing this issue is the simple fact that the swing rig jib is a smaller percentage of the total sail area than seen on a conventional sailplan. Therefore, the combined CE of main+jib will be further aft, necessitating stepping the mast forward to achieve a balanced rig.
Am I missing something here?
Bill
Well since my last post I’ve done some research to satisfy my curiosity, and this is how I se it…
The swing is made with a smaller jib to move the centre of ballance of the RIG rearward to allow it to swing rather than weather vane back to centre, and this change in C of B results in a C of E further rearward which requires the mast to be moved forward to restore the natural C of E position of the boat.
If I got the theory wrong, please explain…
Otherwise, I think I got the hang of swing rig basics. 
Cheers, Neil
Assume for simplicity the “running” sailing where the swing rig is almost orthogonal to the hull center line. Both CE will, with respect to this centerline, almost close to the mast position.
In the Classic this is not occurring since the jib remains indipendent and further ahead helping the total CE to advance much more then on the swing rig. This advance need to be compensate somehow by recessing the mast of a couples of centimeters. Tuning will do the rest.
This is my understanding. Not to forget that the CE identified geometrically on Classical sail’s drawings do not correspond to the real position where the CE is moving far forward once in the water. This is an additional reason to step back the mast foot.
Claudio
I agree that this in fact does occur & undoubtably plays a role.
However, my main point is that the different proportions of main & jib in the swing vs classic rigs will cause the combined CE to shift aft with the swing rig, simply because the main is now a larger proportion. The last I heard, the simple way to approximate total combined (main + jib) CE is to determine the CE of both sails, draw a line between them, then place a mark on that line that is proportional to the relative area of each sail. For example, if the main is 2x the jib (67%/33%) the CE would be marked 2/3 of the way back towards the CE of the main. If someone bothered to “run the numbers” on a couple hypothetical cases, I’m guessing this factor will outweigh the aft shift of the swing rig jib. I don’t know, maybe I’m still missing something??
First the ratio as already mentioned in a class M (0.5161 m?) is 74 % in the Main and 26% in the jib . Different value will ruin the dynamics.
In the classic you can start with the same ratio but not limited to, in fact you can go up to 38 % for the jib and this of course will imply the repositioning of the mast compared with 26% jib.
Now the way to calculate statically the global CE position is well knone.
Everything change when the swing rig will bring back the jib CE while in the Classic the jib CE will move ahead.
Make a couple of sketches you will understand .
I stop here
Claudio
Thanks everyone, I think Div and I are getting the correct ideas now.
Its one thing to run a rig and tune to what you think, but having this extra background allows a more considered approach to tuning and making swing rigs to suit sailing situations.