Your assumptions are incorrect Keel spepped masts NEVER bear on the deck. It
would create a stress concentration that would cause mast failure all on its
own. The only forces on the base of the mast in all cases is down force. All
other forces are born by the standing rig. Please remember that the mast
base must be movable in the fore/aft plane in order to tune the center of
effort. Please also note that some keel stepped masts terminate on support
tables below deck that can be well above the bilge level.. In my case, the
sliding step is 40" above the cabin sole. My mast's static downforce is in
excess of 30 tons. Mast forces on the hull are applied by the rig which
attempt to bend the hull both in the fore/aft plane and across the beam
between the chainplates compressing the deck. On conventional construction
these forces are contained by the deck carlins for the athwort ship forces
and hull stiffness in the fore/ aft direction.
Steve

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...
Again, conventional wisdom says that a keel stepped mast is better
than a cabin stepped mast because the hole in the cabin supports the
mast somewhat.
Considering the loads and torque exerted by a sail on a mast, is this
a significant difference?
I'd think that ease of inspection for corrosion might make more
difference. I'll admit to having no experience with keel stepped
masts but I often look at them out of curiosity. Often I see the keel
step in a really moist area with lots of what looks like corrosion
round the base of the mast. Lack of access here would make me
nervous. By contrast a cabin stepped mast has a step that is easily
accessible.
For mast failures, nearly all I have heard of involve failure of the
standing rigging, common to both types of steps. However, it is far
easier to drop a cabin stepped mast to repair/inspect/replace standing
rigging than on a keel stepped mast. A travel lift can be used to
drop a cabin stepped mast but I'd think you'd need a crane too for a
keel stepped one, kinda complicated. Could this imply that keel
stepped mast rigging is replaced less often; probably not since few
people replace their rigging until it fails.
Due to lack of experience, I cannot really assess this issue.