Masts.

I am certainly not an expert on mast construction, but I have observed quite a few failures of masts, aluminum as well as carbon, on Hobies, Nacra’s, Supercats, Darts, Tiapans, A cats etc as well as a lot of iceboat masts. All masts will break if used incorrectly, abused or pushed beyond the design limits. Masts seem to fail for several major reasons. A common way is fitting failure. I.e. a diamond wire becomes unattached or breaks under load and the mast breaks. This has cost me 2 aluminum masts.

The mast actually breaks in compression not in tension. Generally speaking it is a buckling problem. Think of an aluminum beer can empty. It is strong in compression until there is a little crease forming on one side and then the wall collapses and the can is crushed. A mast acts the same way. When the forces of compression or side loading exceed the ability of the mast to resist it, the wall compresses, buckles and breaks. In general the greater the wall thickness of a tube, the more it takes to collapse it. If you take a mast for a catamaran, to get the stiffness required takes a lot less carbon oriented in the vertical direction then Aluminum of the same mast section. The problem is then the walls of the tube are very thin and like the beer can have a big problem with buckling. If you add a lot more carbon (greater wall thickness) to resist buckling it becomes way to stiff. The answer it to use a lot of unidirectional carbon fiber oriented around the mast. (90° to the long axis of the mast) This increases the hoop strength (resistance to buckling) of the mast without increasing stiffness.

The other part of the equation is increasing compression strength by manufacturing method. If you go from hand layup to vacuum bagging you gain quite a bit in compressing strength because the fibers are more compact, (straighter) and there are fewer tiny voids in the laminate that can cause micro buckling which breaks individual fibers, cascading into total failure. Modern building uses fibers that are preimpregnated with the correct epoxy ratio prior to use. These materials are laid out on the tool one layer at a time and smoothed, they are then put in a vacuum bag and placed in an autoclave where they are cured at higher temps ~ 120°C with about 6 atmospheres of pressure on the bag. When this cures the fibers have much less small voids and are straighter then if it was done with conventional vacuum bagging. This gives much better compression strength, as much as 30% compared to vacuum bagging.

Now the more material you put into hoop strength, the heavier it weights. Now the trade off is weight for strength. Each group makes there own decisions as to what is reasonable. A class masts are very light, but will break if they are dropped in the parking lot while rigging, if you run into the mast while trailering or if the boat blows over on the beach. On the water mast failures are fairly infrequent. The only ones I have observed were from pitch poles where the diamond wires and downhaul were cranked on i.e. big compression loads and then big impact as the mast hits the water (think lever arm). I don’t think I want to sail a boat where the mast can’t be broken if pushed hard enough. The question is what failure rate is acceptable?
Cheers,
Eric