"How do the sections join together at the bulkheads? That's often a difficult part of building a 'sectional' boat."

The sections join together with stainless bolts, something like that:

http://www.dutyhook.eu/bolt/part-threaded/m10x50-din931-a4.html

The heads of the bolts are embedded in the bulkheads from the middle section (cockpit section). For the embedding part I used epoxy grout (http://www.mapei.com/public/COM/products/149_kerapoxydesign_gb.pdf). The bulkheads from the cockpit section are from 3 sheets of plywood, the third is for the heads of the screws. The other bulkheads are from 2 sheets of plywood. The gaskets are made from an acetoxy silicone sealant (https://www.siroflex.co.uk/en-gb/products/technologies/T4/aqua-silicone_199/) around the bolts with 5 milimeters thickness.

I use a L type pipe wrench (http://www.hanstool.com/product/class/?39.html). I screw with the long arm, it allows to make a complete rotation. Remains a space of about 3-4 milimeters betweens sections and there is no leakage from the bolts.

Thank you for asking! Hope is clear what I did!