On Thu, 8 Apr 2004 10:19:19 -0700, "Tom Dacon"
wrote:

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.

To that lucid engineering perspective, I would add the following
observations:

1) Dismastings MAY be better with a keel-stepped mast in the sense
that a deck-stepped mast will tend to fail in its entirely, whereas
even if you can salvage ten feet of keel stepped above the deck, you
may be able to rig a jury rig and keep going. Also, if a deck stepped
mast goes, you must IMMEDIATELY cut the shrouds still attached or the
mast will poke a hole in your hull...and this under very likely less
than ideal conditions.

2) Pluses of deck-stepped include no partners to leak water...no mast
boot, no Spartite, and, usually, more room and less obstruction in the
cabin.

3) Keel stepped masts are frequently heavier, but that weight can
translate to the mechanical advantage and lower CG mentioned above.
Also, deck flexing is avoided.

It's a matter of taste and intended use, mostly. I prefer keel stepped
on fiberglass boats, but see no objection to deck-stepped on steel
boats, mainly due to materials used and likely function of the boat. I
suppose the ideal compromise would be an aluminium deck stepped mast
on an aluminum boat! G

For the record, I have a keel stepped mast I am quite happy with.
YMMV.

R.