*From what I think I know about double planking it's usually diagonal both sets of planks.
You are correct.* I have a copy of a film that was produced by Elco during the war about the manufacturing process of their PT boats and the two layers of diagonal hull*planking*(which are slanted in opposite directions from each other) are very obvious.
Runabout manufacturers like Chris Craft, Hacker, and Gar Wood also used double-diagonal planking in their boat hulls, and I've read that they, too, used a layer of canvas between the two layers of planking to ensure watertightness.* But I don't know what the canvas was saturated with when it was applied.
Dolfinite is good stuff--- we use it on our boat to bed things like window frames and such--- but it does dry out if exposed to air.* This is why we were advised by an experienced shipwright to apply a faired bead of Polyseamseal around the joint between a window frame and the cabin side.* The faired bead of Polyseamseal prevents the Dolfinite from drying out.
Since writing the above I looked up some stuff on the internet and it seems that a common material for saturating the canvas between plank layers is "mastic or paint."
-- Edited by Marin on Friday 2nd of October 2009 03:53:03 PM