This question of galvanic action was an important one to study whilst I was employed in designing sailing hardware. I had a tank with an immersion heater fitted, and I half-filled it with seawater, heated to 50C or more, dunked test pieces and then suspended them above the surface in the steamy, salty atmosphere: accelerated corrosion tests that would take forever at ambient temperatures. This would very quickly show why putting carbon and aluminium together is a bad idea. Even putting carbon and 302 stainless steel is bad, the steel doesn't last long.

Chris, do I understand that you think the whole of an alloy batten might be advantageously wrapped in carbon? If so, I have to say that this would be bad engineering, as well as being extremely short-lived in the marine environment.

One never combines two materials with differing moduli of elasticity, each carrying a load in the same direction. Carbon, stretching very much less than aluminium, would reach its breaking point first, before the aluminium is carrying any appreciable load at all; then the aluminium is left to carry all the load. The strength of the whole is not the sum of the strengths of each component; you would have the weight of two battens, with the strength of one.

What was being considered was the possibility of using fibre reinforcement circumferentially over a very short length at the ends of aluminium tubular battens, not to add stiffness to the batten, but to add hoop strength against the bursting effect that an internal conical hinge exerts. In this case, the higher strength to weight ratio of carbon is not to the point, as the amount of material to be added is small. A short length of aluminium tubing will do all that is required, if mating sizes can be found. Failing that, glass roving ("string") and epoxy will do the job well enough.  

David - thank you all the information you shared in this thread and for posting the detailed information on the new sail design.  My interest in your sail is related to re-rigging our Freedom 40 which still has the original wooden wishbone booms.  Some thought and effor has gone into this over the years and hopefully I am soon (this winter) ready to make decisions and start construction.  I will start a separate thread then - as seems to be the proper convention :), but some questions here if I may and if this is the right place.

I recall the Fantail planform was carefully designed and you produced a generic, scalable version.  I also was reading the discussion thread on the Rawson 30 and specifically in http://www.junkrigassociation.org/general_forum/4438217#4456954  where your new planform was scaled up as an alternative junk sail design.  In contrast to the Fantail design, which doesn't fit on our two mast boat (to be determined if it will be a ketch or schooner), this new design seems like a possibility.  

Aspect Ratio:  Is the aspect ratio of your planform is critical or somewhat flexible.  In the sketches I've done for our boat, the aspect ratio seem to vary between 2.1 to 2.5, depending on what I am pushing.  

Sail Balance:  You have a certain amount of sail balance that maybe was a result of where your mast fit on the boat and because of the hinged batten design, among other things.  Our mast are rather fixed but seem to accommodate around 10%+/- overlap and to preserve a reasonable CE.  Is your planform and the hinged batten design OK with various overlaps?  I would think the battens would hinge more easily as the overlap decreases.  

Yard angle and Yard length:  Would these have to be adjusted at some point either because of the overlap or aspect ratio?  I believe the aim is for a sling point at 50% or more.  Our mast are at present plenty tall (and not be cut shorter for a while) and would accommodate different halyard positions.

Hinged battens:  Are these critical to the planform or could straight battens with camber induced only by the cut of the sailcloth be an option.  I recognize the cons of a baggy sail and getting it to fill in light winds, and some possible batten stagger issues.  Other issues?  The simplicity of straight battens may out-weight hinged ones.

Hinges:  I am thinking that for larger sails a cone hinge made of PVC may not be strong enough.  Thoughts on this and other possible material choices?  Aluminium may be plenty strong but may be noisy with aluminum battens?  G10 is joyless to machine but just as strong as Al.  A phenolic is weaker but nicer to work with.  Casting hinges out of a urethane gets around some of the machining, but that material isn't that strong, either. 

There are more questions, but I don't want to wear out my welcome.  

Cheers,

Erik

hi david

thanks – i didn't think about this possibility (even if i'm aware of the galvanic corrosion in general…)

…a new lesson learned!