Wow! Big boat...

That is one big boat project! With your length boat, and the 90 square feet of sail, you will have a very extreme boat, capable of reaching VERY high speeds. But it will also come under massive stresses... far greater than any soft water boat project of the same scale. Be careful! I would love to see pictures of the project... and if you like, I'll set up your own page at www.icester.net. I have a couple of pages of such "one off" projects.

As for your crossplank, I assume you mean this is a 12 foot one, by "12 ft wide". The traditional way to build one is by using clear sitka spruce. Two planks are lamanated, one on top of the other, in a slight arc. This gives some "spring" so the plank can withstand the large downward forces encountered under sail. Usually the plank is epoxy laminated.

For the runners, I never heard of using spring steel, myself... but am open to correction on this. Most runners are mild steel. This is both hard enough for the ice, and easy to shape and re-sharpen. DN runners are shorter than 36", but perhaps this would work for your boat. The thing to do is start with the dimensions used on a boat of your size and sail area, and not the dimensions used on a DN, which is much smaller. Write someone at one of the sites which has Yankee and Skeeter sailors in their clubs.

The ideal thing is to go to an active sailing area at the height of the season, with a ruler, camera and notebook... and take measurements and ask questions. Of course, that will be too late for the boat you are now building. Where are you, by the way? On the East Coast of North America, Bantam Lake in Fairfield, Connecticut has a large number of boats of all types.

The most important thing in building a crossplank/runner assembly is to assure that the runners are perfectly parallel to one anthor. Bring a tape measure to the lake, even, so that you can loosen the runner brackets and adjust once in awhile. If your runners are not perfectly parallel, they will fight each other as you move, and slow you.