One very good reason for stepping the mast on deck is that it can be lowered without a crane, another good reason is that a mast stepped above the cabin tends not to leak, I’m sure there a lot of other reasons but the above are why I chose to fit a tabernacle with a single pivot pin.
Welcome to the fantastic world of sailing.
Old Salt Oz.
It's also a question of engineering to take the strain imposed by standing rigging and sail loading.
If you think of it that way, you see that tightening shrouds or stays is like trying to force the mast through the boat. If it is decked stepped then the deck has to have sufficient strength under compression to resist being speared by the mast. Whilst this is possible, on many boats its easier (and it keeps weight lower) to let the deck concentrate on keeping water off the bunks, while the top of the keel resists mast compression.
I agree with "Bergman" but I´m wondering from where the idea that a keel stepped mast should be stronger came? Assuming that the support under deck is properly made a deck stepped one in theory would be stronger - stepping the mast on the keel would lenghten the unstayed section by, at least on bigger yachts, about 2 metres or more.
My keel stepped mast is tensioned to the deck (in the area of the main bulkhead) against the downward pull of the shrouds etc. using bottlescrews. Therefore the "unstayed" section is not longer than a deck stepped mast. The keel stepped mast is in effect semi-suspended between the upward pull from its attachment to the deck and the downward pull from the rigging.
This should provide an inherently stronger structure than a deck stepped mast (assuming nothing else fails). On a deck stepped mast one is relying on the integrity of the bolt that hold the mast to the mast step, on which considerable lateral and twisting forces may be applied.
Incidentally, in my case leaking is not noticeable, although the occasional drop of water does find its way in.
What about lightning?
Does a keel stepped mast provide a better 'path of least resistance' than a deck stepped mast?
Does the mast and rigging provide some kind of 'faraday cage' like a car?
(No I don't know what a faraday cage is really either, but it sound good. I think I'll go down to b&q to see if I can get one.)