I built the Arrow-14, using long straight grain, white-ash for all ribs.
I was able to get relatively freshly cut ash from a local mill
(about three weeks after cutting).
The ribs were 5/8" wide x 3/16" thk x 4.0ft long....lightly sanded after band saw cutting, and just prior to soaking.
This ash was easy to bend, structurally strong when epoxied to the stringers (also ash), was light weight...and ash is relatively impervious to moisture in most water/canoe craft.
Rather than steaming the ribs, I soaked the ribs for about 3 minutes in boiling water, using a capped piece of (3 inch dia) 4.5' of PVC pipe, capped on both ends.
The water was supplied by my Mr. Coffee maker. I only soaked two or three ribs at a time: two ribs were soaking, while one was removed with BBQ tongs and worked by hand to the bent radius, rib shape and then C-clamped to the correct stringer (station location). Since the soaked ash drys very quickly, you only have about two minutes to shape the rib by hand before it gets too cold/dry to work any more. I was able to shape all but the last two rib stations, as Platt's plans call for.
Hope this experience helps you...but the "Boiling water soaking" method worked for me. (see my photos in :"moszczak_Arrow_14" of this web site).