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PostPosted: Mon Dec 14, 2009 3:46 pm 
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Joined: Mon Dec 14, 2009 3:21 pm
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Several people have built a jib for sailing their kayaks successfully, and some are on a second or third version of the jib. After reading about their expiriences , I know what the winter project is for this year, a jib for a Revolution. Reinventing the wheel is not my priority, building and sailing a fast boat has more satisfaction. Has anyone put together a sketch of the jib and the hardware of their most recent jib?


tgoy


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PostPosted: Tue Dec 15, 2009 1:37 pm 
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Joined: Sun Aug 12, 2007 5:17 pm
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Location: Auckland NZ
Tgoy,

I am pleased to hear that someone else is planning to experiment with a jib - it is a useful addition to the sailing perfromance of these boats.

As to the design of the jib, while I use a 'proper' sail - a jib from a Topper sailboat; 1.8sqm I think - which has been cut (one assumes) with a reasonable sail shape, my experience suggests that the most important thing in relation to how the sails perform is to stay the mast.

The reason for this is that with the additional sail area the bend in the standard Hobie mast increases quite a bit. Now even with just the mainsail, mast bend is not good for sail shape and sail performance. But a jib is supported, not by a rigid stick, but by tension in a line (wire/rope) which is much softer than a mast and sail shape of a jib is therefore even more susceptible to lack of tension in the rig.

With the mast unstayed my jib was only really useable on downwind points of sail (from broad reach to full downwind) and then only in very light winds. On a downwind course the apparent windspeed affecting the sails is the true windspeed minus the downwind vector of the boat's speed over the ground i.e the apparent windspeed is less than the true windspeed and the force on the mast bending the mast, reducing luff tension in the jib and affecting sail shape is therefore reduced. Also if sailing due downwind sailshape is of lesser importance - a big balloon of a sail (e.g. a spinnaker) should be quite effective at catching the breeze: a perfect aerodynamic foil shape is of lesser importance than its wind-catching ability.

With the mast unstayed cross/upwind sailing was not really feasible. When sailing upwind the apparent wind over the sails is the true windspeed plus the upwind vector of the boat's speed so the apparent windspeed is more than the actual windspeed. The effect on the force in the sails and on the mast is therefore greater and the impact on sail shape, especially the jib, even more marked. When sailing upwind and across the wind being able to maintain an aerodynamically efficient foil shape in the sails is also more important: when sailing downwind the sails are pushing (or pulling) the boat in the direction the wind wants them and it to go, however when sailing upwind the sails must push the boat in a direction that the wind doesn't want them and it to go, so the sails need to slice through the wind efficiently to lever the boat upwind against the counterforce of the keel. When I tried sailing upwind with main and jib the mast bend increased dramatically, the tension in the foretriangle (the luff of the jib) relaxed as the mast bend increased and the jib fell off to windward and lost all upwind pulling power.

Staying the mast has totally transformed the performance of my jib. I feel that whatever the cut of your jib the same transformation would occur by staying the Hobie mast because the issue (when an unstayed mast is being used to support the foretriangle) is how to maintain whatever shape your particular jib has, through tension in the foretriangle if you cannot create tension in the foretriangle because the mast bends under that tension.

The modification was pretty easy to make and very cheap (a few bits and bobs and bit of stitching on the sail's mast tube but no holes in my boat!). It is a bit fiddly (though still worth the effort IMO) in its current incarnation but I have a few improvements in mind which will make it a whole lot more manageable. With the mast stayed I can adjust the amount of tension in the foretriangle i.e. the forestay to maintain the shape of the jib on all points of sail because the mast is supported at the back. And because the mast is supported it bends a whole lot less thus improving the shape of the mainsail too (which is important: this is the more powerful sail of the two so there is more performance to be lost here through poor sail shape).

Now I can sail with main and jib on all points of sail and can use them in stronger winds without worrying about mast bend (the risk of a ducking remains, of course). I can comfortably get 4.5-5knots (GPS) out of the rig on a reach but am yet to test its true speed potential.


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