Old as Dirt!
Join Date: Nov 2008
Location: Tampa Bay Area
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Ok, My two cents, My guess is that this is the box section at the front of the mast where the spin halyard sheeve mounts. My second guess is that the crack is a fatique result of cranking the backstay.
The fix, first you must drill stop holes at the base of the crack. If you can't find the base drill the past where you think it is and cut the crack to the stop hole. If you don't, the crack will continue to propagate. After drilling and cutting /cleaning the crack, tig weld it up.
I would then fab plate to pick -up the entire area above and below the box, an inch or so on either side and weld it on. If there is concern about the weld temper, rivot it on.
In order to fix this you need to understand where the stress that caused the crack comes from and either eliminated it, spread it on a larger area, or make the part strong enough to handle it.
Ah...John... You're a little behind the curve on this discussion. See Repairing a Cracked Mast
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."