It's a mechanical engineering issue. A mast (called a column by mechanical
engineers) that's supported only at the ends is less strong in compression
than a column that's supported at two points at one end. The support at the
mast step, for a keel-stepped mast, allows the mast to take more compression
before failing than a deck-stepped mast can. Because the stays and shrouds
take sailing loads almost parallel to the mast, the mast column comes under
significant compression load.

For offshore work, the ultimate compression strength of the mast is
important for situations like full knockdowns and capsizings. The additional
compression strength also comes into play if you lose a stay or a shroud,
and might just give you the additional reserve strength that would keep the
mast from coming down.

Tom Dacon

"Parallax" wrote in message
om...
I have heard ppl say they would only have a keel stepped mast on an
offshore boat. Why? A well built deck stepped mast is as strong as a
keel stepped one and easier to put up or down. If either loses a
stay, it is coming down in a hurry? I fail to see the reason for this
odd preference.