Well you might want to look into self bailers fron canoes to drain the cockpit. I suggest looking at surf skis to evaluate the setup of their cockpits. Often, as much space as possible is sealed below the "bucket" that you sit in - this limits the amount of water that is retained when you are hit by a wave. The smaller the amount of water, the faster you can recover and keep paddling.

Consider built in "dry" storage or a hydration system.

There is a new surf ski being built in Sweden (i think) that has a removable deck that will provide coverage in colder weather. http://www.nordickayaks.se/doc/aboutfusion.php 

This looks like what you are interested in, but the details are slim.

Yeah, that's what I want!  both in my surf ski and home built wooden racing sea kayak!

 The deck of a kayak contributes greatly to the stiffness of the hull. Any place the deck is penetrated like the cockpit area creates a weak spot. That's why cockpits are usually reinforced with deck beams and carlins.

Opening up a kayak deck to the extent shown in the picture means what's left is going to require some serious re-engineering to keep the hull from developing a hinge in the middle.

here's another pax 20 with an even more open tailored cockpit during the tws (texas water safari). instead of a deck it has a spray skirt.  

I understand that this idea is going to require some "serious" design changes, but a canoe has no deck at all so .... and I've shown two working examples of this already done.   there is also something called the f16 that is very similar (bottom pic), but none of them have a removable hardtop cockpit deck.  a warm weather boat and a cold water boat, all in one ......

but since I haven't seen these boats in person, or talked with the owners, I figured that going to the builder's forum would be the next best thing. right?  besides, I might not even use a pax 20, it could be the pax 18, or shearwater, or whatever else .... the hull shape is the important thing to select and get right for the intended purpose, that can't change, but the deck can be changed! 

Looking at the picture you first posted and following earlier comments:  I would think that you would want to fill the space behind the seatback, either by bring the permenant back deck closer which would lend strength or at least with something to keep water out.  You might also want to put something like styrafoam in along the sides and under your knees and inbetween your legs which could be carved to almost become a form fitted seat.  Again the idea is to allow as little as possible space for water when a wave comes into the cockpit.  As far as adding strength, a strong internal "sheer clamp" or gunnell would give more stiffness / safety and might also provide a surface you could "clamp" the hard cockpit cover to.  Possibly with several bolts through the cover and sheer clamp with a gasket between.  If you haven't built it yet, make the deck beam at the front and rear of the opening really wide and leave half exposed to let the gasket seal tight at front and back. 

my 2cents worth, maybe 3?

Ed

ed,

I think you might even have a nickel's worth going on here!  I like the idea of gasket surfaces and mounting bases.  and no, I haven't yet built the boat, I want to do as much features engineering as I can before hand.  measure it twice, cut it once kinda thing.  

 the mission for this boat is to do several types of races, and also for some long distance trip making requiring self supported camping.  the texas water safari (tws), colorado 100 (c100), mr360, adk 90, paddling the ICW and the bays of the texas coastline, and exploring the adirondacks in upstate NY.  

I'm not planning on any top side hatches, but rather hatches in the bulkhead, one fore and one aft.  for the tws I'd like to have a spray skirt over the open cockpit for when I finally reach the bay.  for the ICW and saltwater trips I'd have on the hard cockpit cover.

as for building the deck, from what I've seen in the various videos, the deck is made in one piece, after the hull is done.  I was thinking about building the deck, then cutting it into 3 pieces, leaving the cockpit piece off, attaching the bow and stern decks, then (somehow) making the cockpit section to fit.  I can see that I'd might have to have some extra deck structure to strengthen the cockpit cover, and also some bracing in the hull.  this of course might add some weight, but that'll be necessary for this hybrid boat.

I still haven't decided if this will be a pax 20 or not, and I just ordered a catalog to help me go over the different boat options.  shearwater, s and g night heron ...... or whatever else ....