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Old Nov 04, 2012, 01:26 PM
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Sussex,GB / Azores
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Why Jib boom on Swing rig?

On swing rigs, why is the jib not tied directly to the front boom but has its own separate boom?
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Last edited by Markx; Nov 04, 2012 at 02:15 PM.
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Old Nov 04, 2012, 03:12 PM
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I would think that it is to allow accurate control of the position of the clew, for draft control and for leach tension.

John
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Old Nov 04, 2012, 05:43 PM
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But wouldn't that be pretty spot on without the boom anyway?
It only needs a small gap.
I haven't tried it but I'm just wondering if there is any point on a non-racing boat, This one will be used for towing out fishing lines in less than favourable conditions so I want to keep it simple as it might be lost.
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Old Nov 04, 2012, 05:55 PM
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IMHO.....
Without using the jib boom to "leverage" the forestay tension, the jib luff would have a tendency to fall off as winds increase. Fastening to the front fixed boom (to which the jib boom fastens) would result in having to manually adjust forestay tension. Using the moving boom, much like on a conventional rig, as you move the attachment point forward or aft, you increase/decrease luff wire tension. Also, using a jib boom allows you to install a leech line to control the leech of the jib.
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Old Nov 04, 2012, 07:32 PM
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Without a jib boom you would have no slot between the jib and the main. The sails would not work together as they would not have the correct relative position to each other.
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Old Nov 05, 2012, 03:52 AM
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Thanks D+G, I sort of get the idea now.
Looking at the drawings, I still wonder if the "natural" gap it would form in practice wouldn't be fine anyway. Maybe I'll try it and see.
Finally got time to get on with the fishing line tug project and I've nearly finished a rough prototype plywood hull, see this old thread:

http://www.rcgroups.com/forums/showt...1436244&page=4
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Old Nov 05, 2012, 06:12 AM
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When you are looking at the kind of performance that having a swing rig implies, you do need tight control over sail shape and the slot. Between the leech and luff lines, and the rigid boom there is a set triangle formed, giving full control over sail shape and slot giving you what you "want" and keeping it that way, rather than letting nature give you what you "deserve" with every change in wind speed and direction.
Note that on most rigs with a jib boom, the jib pivot is usually some way back from the forefoot, so that leech and luff lines can act together under tension to maintain sail shape.
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Old Nov 05, 2012, 06:13 AM
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Look at any big boat. The sheeting angle for the jib is never on the center line (unless it has a jib boom).
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Old Nov 05, 2012, 11:12 PM
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What's a clew and what does it do? What is leach tension ? Newbie here trying to learn. Thanks Mike
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Old Nov 06, 2012, 08:43 AM
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Quote:
Originally Posted by Gregg28 View Post
Look at any big boat. The sheeting angle for the jib is never on the center line (unless it has a jib boom).
Almost any full size boat. Full size practice is generally to have a loose foot, so the forward most point of the foresail is on the centerline, thus the sail itself has to be one side or the other. I did see, in a "Classic Boats" several years ago, a full size swing rig, which closely followed model practice.
There will be differences in practice between real and model because the real thing has a crew, models need to work reliably and simply. Thats probably why genoa rigs are rare in working models - tacking is much simpler with a self tacking rig.
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Old Nov 06, 2012, 10:54 AM
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..and on real boats you drop the Genoa and hoist a spinnaker when far enough off the wind. A jib boom gives you a decent compromise between the two sails. Self tacking for upwind work and the boom holds the clew out for down wind performance.
For MG - a triangular sail has three corners and three sides. The clew is the lower outer corner and the leech is the trailing edge. Clew tension/position effects sail draft and leech tension, both of which get set for the wind conditions.

Dave
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Old Nov 06, 2012, 02:47 PM
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So how about tying the jib leech to the mast at regular intervals?
That would control the gap somewhat positively!
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Old Nov 06, 2012, 03:14 PM
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Quote:
Originally Posted by Markx View Post
So how about tying the jib leech to the mast at regular intervals?
That would control the gap somewhat positively!
Only for one point of sail. You would have to bring it ashore to make changes when on a different heading. Sailing upwind, perpendicular to wind, and away from wind all requires a different sail setting (both jib and main). Rather cumbersome to have to find a way to bring it back to shore to allow you to reset the sail - no ???
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Old Nov 06, 2012, 05:22 PM
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No, on a swing rig the jib always stays at the same angle in relation to the mainsail, rather like a slit on a wing.
Tying it at regular intervals would provide rigid control of that gap all the way down the leech.
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Last edited by Markx; Nov 08, 2012 at 04:15 AM.
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Old Nov 06, 2012, 05:27 PM
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Thanks for the info Dave, very helpful.
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