Looking real good Todd!
A while back I looked at many cases for the 20mk3. You can see some of the results in the form of stability curves here.
Sealing the cockpit comings entirely would certainly add some positive buoyancy to the boat in a capsize. The stability curves we calculated assumed the coamings were flooded. Basically you would be increasing the angle of vanishing stability. The water ballast however is by far the largest determinant factor. We think the boat should always be sailed with the water ballast filled unless you are motoring in a dead calm or sailing in drifting conditions. The numbers don't lie and I am confident that the 20-mark 3 WOULD turn turtle without the water ballast in even if the cockpit comings were completely sealed. A small mast had float on the 17 mark 3 and 20 mark 3 can be added to prevent the boat going turtle in all scenarios which would be a good idea if you like to sail the boat without the ballast of if you plan to participate in say the Everglades Challenge for example. I plan to have a mast head float on my CS-20 Mark 3 because I like to sail without the ballast because it's faster! and I don't want to worry about turning turtle when I don't have the ballast in. If you always have the ballast in then no need.
With the water ballast filled the boat should not be able to turtle even if the centerboard is up on the 20 mark 3. (maybe not true for the 17 mk3...see below) This is I think attributed to the taller masts which (once partially underwater) provide the missing righting moment lost from the CB which is good because it remains to be seen if the centerboard could be reached from the water if the boat capsizes to starboard (putting the offset board higher up in the air). I intend to test this extensively with my boat.
We have only been able so far to test capsizing a CS-17 Mk3. Based on these few tests, the 17mk3 was easily righted by one person with the tank full and the board down and even while at the same time scooping the crew into the cockpit. The one test we did with the board UP it looked like she wanted to turtle even with the ballast tank filled. The boat tested did not have sealed cockpit coamings. There were a few extra variables in this test though such as the mizzen being flipped over and could have been scooping water as they tried to right the boat with no CB. Also this test pointed out the possible usefulness of a line tied to the trailing edge of the CB that could be pulled on from the water to re-lower the centerboard.
The 17mk3 is still quite stable without the ballast with an angle of vanishing stability of around 70-80 deg or so. We tested the angle of vanishing stabilty of Grahams 17mk3 here but with no sails at the time and got 85 deg w/o ballast.
And we did a similar test of the first CS-20 Mark 3 with the ballast in only here.
Only because it is pertinent to the above we are also working on a new design...(spilling the beans here) we are currently calling the Core Sound 17/20 SR. (SR stands for self righting). We're pushing the limits of the water ballast in a remake of the original CS-17 and 20. The boat below has almost the same hull shape as the CS-20 mk3 but with added sheer height to gain the maximum righting moment from the cockpit coamings which are now completely sealed. The water ballast tank is a full 550lbs in the CS-20 SR and there is 4 inches of blue closed cell foam blocking between the underside of the cockpit sole and the top of the water in the ballast tank. The purpose of this is to push the center of volume of the water ballast lower in the boat thus reducing the vertical center of gravity. It has the added benefit of being able to fill the tank without topping off with buckets or pumps since the entire tank is below the waterline. It is very hard to get a shallow draft boat to self right and it's impossible to do it without sealing the cockpit coamings. That also makes the boat very stable in the inverted position. To solve that problem. One of the coamings is flooded automatically if the boat turns turtle allowing the boat to be rotated back on her side and then righted. If I had this boat, I would also have a mast head float on it for the same reasons as above. I like to sail w/o the ballast (because it's fast) and don't want to worry about turtling. The 17 and 20 SR will also have an integral outboard well in the stern of the cockpit, a longer forward cockpit seating area for camping under a dodger and we're shrinking the weighted CB down so just the top of the trunk sticks up above the sole.