we're using a mast section that even the current legal sail shape/design loads up that section that take it to its uppermost limits of strength and durability.....
if its not the case then why are some people adding internal stiffeners....
Because...
About 9 or 10 years ago we had a batch of masts supplied which came out lighter than they had before. Apparently there are tolerances that come in to play when masts are extruded and on that occasion they came out with thin walls and a very thin web. These masts all broke when used with the sloop rig unless they were sleeved. They worked fine for cat rigs (including with spinnaker).
Batches of masts made before and since do not need to be sleeved to make them strong enough.
Other than the lightweight masts, I can't think of an instance of a mast failing. Broken masts are nearly always caused by failure of diamond stays, sidestays, rotation levers, or corrosion.
