I'd like to hear from Ben or some other mast builder as to the designed strength of masts, if there is some 'magic formula' as to how much sail area and or downforce (cunningham, trapping wt. etc.) they use to decide how strong to build the masts.
I doubt if any beach cat mast would break based soley on the lift being generated by the sail itself, no matter what the wind strength.
I think what causes masts to break (other than flipping the boat onto them) is the compression force of trapping, added to the backward bend caused by mainsheet tension, and foreward pull of the forestay and/or spinnker.
Still, I haven't seen too many masts break under sail forces alone, only when the crew lands on them or they roll in the surf.
Timbo, you usually come across as a pretty sensible and switched on guy, but you really need to step away from this thread. Every one of the loads you have defined above is related to the righting moment.
"compression force of trapping" clearly part of righting moment (don't forget same force from side stays).
"mainsheet tension" you apply mainsheet tension to hold the leach of your sail up. Your leach opens up because of the force of the sails, the luff is held closed by the mast (which is held up by the righting moment) and as a result you tension the main to get a similar amount of support. The as such the mainsheet tension is proposional to the available righting moment.
"foreward pull of the forestay" see above plus add in tension as applied to offset mainsheet tension.
"foreward ..... spinnker" drive from kite is clearly proporsional to righting moment.
etc etc etc.
There are many of examples of keel boats adding ballast to their keels in some cases making the boat lighter by taking it out of the bilge making no changes to the rig or sails then breaking the mast in their first serious race after making the change. A great example of this is the 98' yacht Wild Thing, which lengthened its keel and reduced the bulb size shortly before the 2009 Hobart. Having made no rig changes from the way the boat had been rigged for previous years (in fact using old delivery sails) they proceeded to break their mast on the delivery to Sydney for the race.
As stated above the first step in designing a mast is to calculate righting moment as this effects two things the compression on the mast from stays and trap wires and the side force the mast will see from sails as the total side force will only every be equal to the righting moment that stops the boat from heeling over.
See if you can find a copy of "Principles of Yacht Design", Larssone & Eliasson their is a good section on rig design.
As I said above there are practical limits on how thin you can build a mast but as this is a carbon mast the practical limit is about 1/100 of an inch and that definately won't do the job so won't have been an issue in the 18HT rig.
Sail7seas, yes another set of spreaders will help as you are reducing the unsupported lengths and therefor making the mast less likely to buckle, but who wants multiple spreaders on a beach cat?
Timbo, if you're still not convinced rig your boat on the beach, strap it down tight pull on the sails and come back after a week to see what went first.
