What's the difference of cut between a jib with and without self tacking ?

Thanks

The older Nacra jibs were pretty big and would be too big for a self tacker unless you had them cut down so they could clear the mast when tacking.

The newer -self tacking- type jibs are cut much shorter on the foot and are trimmed much closer to the middle of the boat (to the end of the self tacker car track). The newer boats with self tackers have spinnakers so the spinnaker is doing the work going downwind, where as the older, non-spinnaker boats needed more jib for reaching and downwind work.

An original Nacra 6.0 (big jib) will still reach faster than any of the new spinnaker boats if the reach is too high for the spin, due to their much larger jib.

What pete said, but simplified, the older non-self tacking jibs typically overlap slightly with the mainsail. The newer self-tacking jibs are shorter on the foot and don't overlap the mainsail so they can remain sheeted to the front beam and pass through the tack without hitting anything.

Note added

If you don't want to loose the sail area after you shorten the foot and change the balance of the rig. ... you make the sail higher aspect by raising and lowering the hoist and tack respectively.

Yes, is an older non-self tacking jib , but don't overlap the main sail. The foot is slightly rounded off and probably used with barber hauler ...... I must simply verify that pass through the tack without hitting anything ?

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that sail will go on a self tacker ok.

a clew plate added to the sail by a sail maker will allow you to get the jib leach right depending on where you attach the jib shackle to the clew plate.

That depends on what you mean by "cut", but the quick answer is "None, necessarily".

However, the detailed answer is: Because the self tacking jib has to fit inside the triangle defined by the forestay and mast, it is typically smaller than jibs that are not self tackers. In order to max out the available size (which is commonly desired and attempted), it is usual to cut the leech longer and bring it as close to the block(s) on the self tacker track as possible. This means that the sheeting angle is almost directly up the leech when the sheet is pulled tight and it requires having the best trim angle accurately located in the design. On older boats that have ovelapping jibs, like the Hobie 18, the clew is cut higher, the sheeting angle is almost into the middle of the sail, and there is a track allowing for fore and aft adjustment for best trim to account for differences in how the sail was designed or cut and also for close reaching or for different trim in different wind speeds and/or wave conditions. The best you can do with a self tacker is very fine adjustments on the sheet. That is another difference - a 1/4" sheet adjustment on the self tacker is much more significant than the same amount on the old style jibs.

If you are making a sail, then the high loads on the leech of the self tacker requires either a stronger material on the leech panel, a cut that aligns the strength of the material more vertically along the leech, doubling the material on the leech panel, or some other considerations for the higher loads on the leech. It is not uncommon to see leech gutter on these jibs after they have been stressed beyond there design.

I hope that covers what you were wondering about.

I'm sorry, I did not see your photo before my first response. Having looked at what you want to do, I agree with the other responders but already I see that the leech has been stretched as I described in my first post. The change you will make will pull down the leech even more and will stretch the leech more. This will result in the leech "motorboating" which is a self descriptive term arising from the sound it makes when the stretched part starts flapping at high speed. This can be fixed in several ways when it occurs: you can make small "darts" in the sail seams that will take out the stretch; add some cloth up the leech to strengthen it; cut out the stretched part ; or make a new, purpose built sail. The other ting I did not mention before is that many jibs have a leech line sewn in to help prevent this and to adjust depending on wind strength.

I would just go ahead and do what you want, have fun, and then fix or replace the sail when you are ready.