Originally Posted by reed1v
Never heard that one so I consulted my Skene's. Its doubtful that mast sizing has much to do with how its stepped. Mostly about loading at height and how stayed. What is below deck has little to do with the strength of the mast and most to do with how well supported it is above deck. Plus there is not much twisting to the foot; which is why deck stepped masts have not much lip to their base plate.
On the contrary, how, or rather, where, a mast is stepped, has a huge influence upon what sized mast section a boat needs. With regards to it's strength & stiffness, AKA section modulus. And a deck stepped mast needs to be 1/3 to 1/2 as much stronger than a keel stepped mast, in the "same" boat.
Brion Toss covers this in his book (from where come the above figures), The Rigger's Apprentice
. And it's detailed to some degre as well, on his website's sub-site, SparTalk SparTalk - Powered by vBulletin
I wrote a post not overly long ago, with a direct link to one of those discussions on SparTalk's forums
, but at the moment can't find it.
Though also, consider this. The Vast majority of racing
boats out there have keel stepped spars, primarily because it allows them to get by with a lighter weight tube. Than if they were to go to a deck stepped spar.
At least on racing
boats which don't have canting keels.
Otherwise, the trend would be to go with deck stepped masts, as such rigs & their step's are generally easier & cheaper to build.
Also, on boats of size, there can be substantial twisting forces to be resolved at the butt of the mast. Albeit, some of these forces are resolved via proper staying.
But a lot of torque, I think, gets created on boats with long booms & big mainsails. Plus the forces exerted on their masts, twisting wise, by spinnaker
poles (with conventional kites that is). As well as boom vangs.
And not to knock Skene's, I'd think that given it's age, some of the recommendations & calculations formulas are a bit dated. Particularly with regards to more modern materials & designs. Coupled with how much of a push there has been to continually lighten boats since the end of WW II, when recreational sailing, & boat designs for same, really started to become popular.