David, how does your split sheet system look like? It's not apparent from any of the pictures I've seen. Is it based on something in PJR? There are three examples in there but they're all with six sheeting points instead of eight.

Hi Jerry, 

The design I'm going to pursue is more like a gaff jaw on the front of a hinged batten.  The front part of the airfoil is then attached forward of the jaw.   You can see a pic here:

http://www.junkrigassociation.org/page-1401723/41430468

David's wishbones were very nice in that they provided structure to shape the front of the airfoil all the way aft to the hinge.  However, I think their width contributed to their failure.  When the sail begins to fan up it can twist the wishbone, and the wider the wishbone the wider the lever arm for the sail to exert a twisting force on the nose.  Since the nose of the wishbone is prevented from twisting by the mast, eventually something breaks.  In David's case this was confounded by the vertical hinge movement which exacerbated the whole thing.  

When David suggested the arrangement shown in the link I liked the fact that there are very few ways for the sail to impose a twisting movement on the batten.  Further, the design is very similar to the jaw on a gaff rig which is well understood and provides a guide to the design requirements.  This version doesn't have a rigid structure to determine the shape of the cloth once it is past the mast, but most sails don't either. 

I'd have to spend some more time thinking about the design you propose, it does provide a way a making a nice airfoil shape.  The best design will likely have relatively few moving parts.  Every axis of movement provides another way for things to get messed up.  

dennis gibbons wrote:

David,(or anybody else)

Any thoughts abut using Delrin (or other plastic) for the hinge?

It is lighter and stronger than wood but  can also be machined or turned on a lathe if one were to use round battens.

Would it be too flexible ?

Dennis

Of the engineering plastics, I would use solid PVC bar - tough, low cost, and quite strong enough. All the engineering plastics are heavier than wood, but that's not really significant here.

Darren,

Hinge look well engineered, but I cannot fully understand how it works?

Cheers

For comparison the polyurethane I was using has the following traits:

Shore D hardness: 85

Tensile Modululs: 370 000 PSI (2.55 GPa)

Ultimate Tensile: 7800 PSI (54 MPa)

This is the polyurethane resin I was able to get locally.  One of the interesting parts of moulding in polyurethane is there is a great variety and you can select just about whatever traits you like.  For my production run I was considering the below polyurethane:

IE 3075

Shore D hardness: 80

Tensile Modulus: 420 000 PSI (2.90 GPa)

Ultimate Tensile: 10 500 PSI (72 MPa)

I've used tensile modulus for consistency with Dennis, and the fact that for the plastics we're considering the tensile modulus usually close to the flexural modulus.  However,  I think flexural modulus is probably the more important number for hinge consideration.

It looks like the ultimate tensile of Oak is somewhere between 50 and 100 MPa.

Polycarbonate has a shore D hardness around 80.

For both polycarbonate and polyurethane you could use fibre reinforced versions to get even better numbers, but that may not be necessary or worth the extra trouble.