John:

I have most of the materials on hand to start my boat, as well. I
looked through the plans, and cannot find a material list or plywood
thicknesses for the hull. I am running some strength calcs, and
believe 3mm ply with 3/4 x 2" longerons should be more than strong
enough. There is a fairly high torsional load between the mast step
and plank connection, and a diagonal piece of the longeron stock
should carry that load, with either a bulkhead, wood, or metal knee to
get the moment load from the top of the mast backing board to the
diagonals. A bulkhead and center line stiffener seems prudent beneath
the deck. Also, I much prefer scarfs in teh plywood rather than butt
joints and backing plates.

The torsional load comes from the mast step side force which must be
transferred to the plank connection.

It appears you used 4mm ply and smaller longerons and no mid-panel
stiffeners, and no bulkheads. Please confirm. This "feels" a bit
flimsy compared to the ice yachts I am familiar with. As a minimum, a
bulkhead at the aft springboard connection point seems indicated.
This additionally closes a volume and provides a flotation chamber for
weak-ice (or weak-headed moments) with thin ice. Foam hip pads, seat,
and foam fillers at the seat back corners also provide some flotation
in those OH-SH** situations.

Thanks for the reply,

Ken