Design considerations for a Jib

posted Dec 15, 2013, 8:45 PM by A Rice   [ updated Dec 15, 2013, 8:45 PM by PacificNorthWest MultihullAssoc ]

As part of last months meeting, Vince DePillis, Commodore, comments on his new sail.


There is nothing quite so engaging and frustrating as ordering a new sail.  At the last meeting, much to my surprise, Jim Kitchen brought along my new jib to display, fresh that day from the land of the long white Cloud, and birth place of Ian Farrier.  



I say engaging because there is nothing as beautiful as the perfect parabola of a fresh crisp sail, on the first day you raise it.  And the anticipation of ordering a new sail-- deciding on fabric and design, what could be more interesting. 


Frustrating because for the average joe like me, there is such a knowledge detriment, and I am never entirely sure that the person on the other end of the transaction is telling me all he knows.  


In ordering my new jib, I started with  a given-- it had to be a laminate sail with high performance fibers.  For a high performance multi hull, Dacron will just not cut it.  And any more, carbon is the default performance fiber.


So then the question is tri-radial, or string.  I think the days of the tri-radial sail are just about done. It is labor intensive to cut out a bunch of triangles and sew them together as a sail.  It is heavier because of all the seams, and it is more likely to be bumpy.   I still have a tri-radial main and it is pretty bullet proof, but with China now pumping out string membranes by the zillions, I know that the next sail will contain no sewn triangles.  


So-- string.  For reasons that are  perhaps a bit arbitrary, I ended up with Doyle's Stratis product.  They cut me a good deal, and they are introducing a new UHMWPE/carbon fabric that sounds promising, and the salesman in New Zealand  participated in several Volvo Campaigns.  Like I say, pretty arbitrary.  North, Calvert, UK, Ballard sails, I'm sure that they could all have made me a beautiful sail.  One of the hardest parts of buying a sail is knowing you will disappoint a small businessman who loves sailing as much as you do, and would be really delighted to have a chance to do a sail for your boat.  


So, having made the choice of sailmaker and technology, you approach the question of design. First question:  full batten, partial batten, or no battens. I knock out the partial battens first.  In my experience the leading end of the batten always causes a creaser a bump in the sail.  Ugly and premature death.  


So then-- What about no battens?  This is a super attractive idea.  Simple, cheap, light, and easy to maintain.  Sure you lose area because of the hollow leech, but why not bring the foot back, and make it a little more genoa like?   But every time I have raised this issue, and talked to people about the compromise inherent in a hollow leech sail, they have told me there is a very significant performance loss.  


At the end of the day, I could not handle the thought of losing the upwind speed, and so I went for a full batten sail.


But how to shape it.  After seeing the square head jibs on the America's Cup boats, I just had to have one of those.  I asked the sailmaker, and he thought it was a good idea.  To to tell the truth, I have very little confidence that this is based on any serious empirical research.  Where are the FEA studies, the pretty multi colored computer simulations of flow around the new sail, etc.?  All the stuff I so avidly consume in the pages of Seahorse magazine?  Hah.  Not for some guy who is buying one sail!  So when you see that square head, keep in mind it is a case of monkey see, monkey do.  


I did have a pretty clear idea about size.  With my previous NORTH 3DL, the sail was just too big - a  gargantuan roach that could not be tacked -- and a super droopy foot that quickly turned into a useless flap that shook the whole sail.  We amputated the extra material and the sail was much the better for it.  So I knew I wanted to be abstemious at the edges.  And the new sail looks perfect from that point of view.  


The thing that really mystifies me is the depth and the luff curve.  I told the sailmaker that the luff curve is highly variable, because of the rotating mast, and that the quality of helming is not particular stellar -- so design me a sail that is forgiving.  He listened, but I have no real idea what he did with that information.  


I have had the sail out once now, and am hopeful that it will prove to be a fast and durable sail.  The square top relation to the main sail is radically different than was my previous sail.  Faster?  Who knows.  


Regards,

Vincent DePillis

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