Annie, thank you for the efforts on the blog - they make great/interesting reading (no surprise really given your previous record as an author of fascinating books!)
The tabernacle discussion is interesting too. We went for a forward opening tabernacle, but given the layout of things the mast actually steps about a metre below cabin height, with the pivot about 300mm above the roof line (swinging thru a slot in the cabin roof).
The original design from the designer called for building a wooden case dividing the cabin in two with the mast stepped onto the keel and pivoted at cabin roof height (wherry-style) - but this "mast version of a centreboard case" so wrecked the usability of the cabin we started again, and constructed the tabernacle and step in steel (yes, very infra-dig I know). So, the structure is now a piece of box section secured to the "keel", with the open sided box for the lower part of the tabernacle welded to the top, and four steel box-section braces securing this between the deck clamps, port and starboard and a substantial laminated beam forward. This is then bolted (thru the cabin roof line) to the upper part of the box (with the (yet to be finalised) pivot.) The obvious problem with this design is the opportunity for leakage! I think the best current plan is to make a large fabric "boot" that is seized to the mast above the pivot and secured around the edges of the opening in the cabin roof. A bit like a gearstick boot in a car. Time will tell if this works.
We are using an alloy-lamp-post mast and planning for only 16-18sqM of sail so hopefully this will be strong enough!
As a picture is quicker than all these words, these two pages in Befur's Blog might help add some clarity... one two and following pages.
Annie, I still think you are going to beat us into the water with a "fully finished" boat, our current plans are to install the steam plant and try to "see if it floats" before the winter, and then aim to fit the sailing rig in the following season :-/