: My question is: How do I figure the size and placement of the cockpit for the
: double-open configuration.

The usual way to do anything, when you have no data to work from, is called "trial and error". In this case we can avoid a lot of errors by getting a little data ahead of time. Even so, the answers are going to have to be unique to your boat and your circumstances. Maybe after building, and fitting out, a handful of these craft you might cquire enough information to come up with some "average" seat locations -- but I doubt you are going to want to go that far, so let me jsut stick to some ideas for getting this one right.

First, the boat is your test bed. Once the hull is completed you can float it, and load it up with either bags of sand, or get your actual paddlers to sit in it. They move them back and forth, while you look at the boat from the side and see if it is riding level. (you can also put a carpenter's bubble level on the floor of the kayak and they can move a bit and see the results that way.) Once the proper seating locations are defined you just mount the backrests from the coaming at the proper spots, and drop the minicell-foam seat pads onto the floorboards. The velcro "hook" part which you have carefuly attached to the bottom of the foam pads should engage the veritable carpet of velcro plush material that runs down the center of the boat. ( you want the soft stuff on the boat, so if you have to sit there without a foam pad your butt is not irritated by the stiff part of the velcro.)

Before you get to the point of floating the boat there are a few other things you can do. The first is the teeter-totter test. Make a low version of a childs teeter-totter using a well braced, foot-long piece 2x4 or a 2x6 on edge for a fulcrum. On this you set a 6 to 8 foot length of 2x10, or a couple of 2x6's side by side. (yeah, you can use 3 2x4's -- the idea is to get a seat that is wide enough for you friends to fit on.) Invite the friend over for a custom fitting. (let them bring the food) Sit them on the teeter totter in the order they expect to paddle. They will get on and off a few times as you move the board they are seated on back and forth.

Eventually you will find four things of importance: 1) The center of balance of the couple, ( mark this on the edge of your teeter totter with a pencil. 2) The minimum legroom needed for the bow person, 3) the minimum legroom for the bow person, and 4) the distance between their two butts. Assuming the bow person does not want to sit directly on the feet of the stern person, they will naturally sit slightly in front of that person's feet. Once both parties are seated, and the teeter-totter is balanced -- or darn close to balanced -- you can mark where foot rests should go, too. After doing all this get another pencil with a different colored lead, get your friends to shift positions, and remark all the vital measurements. There will be times when they change paddling positions and you should make allowances for this possibility.

After your friends get up from this toy -- and if you give them a few socializing beverages before they try this they will have fun -- you can set the balance board aside and alter use it as a very useful marking gauge. You'll have almost all your information on it.

You cna balance the boat frame to find the approximate center of balance. It is nt the center of bouyancy, but it is only going to be a couple inches away from perfect at most. At this point it is good enough. Lin up the balancing poing from you new marking gauge at this point and you can read off seat locations. Measure back to locations for backrests for both people. your aft end of the cockpit cna go anywhere behind the back seatrest. Go 18 to 24 inches in fron of the bow paddler's backrest for the location of the front of the cockpit.

Try to make the sides of the cockpit straight and parallel and you can make two or three backrests that are the same length, and which are mounted on the coaming. If the coaming is an even width along its length you can re-mount these backrests later at different locations for solo paddling by you, or by a larger or smaller paddler. Once the location fo the backrest is determined, the seat pad just drops into place and the velcro holds it there.

As for yourself, climb on the teeter totter and slide along it until you are balanced, (Did I phrase that correctly?) and mark the balance beam in the appropriate place. Try to avoid getting splinters!
Now yu have a gauge for yourself, whihc is probably close for other solo paddlers.

Hope this helps.

PGJ