i am not the expert in kayak building but have spent my entire life in the wood business. This is my unsolicited two cents, but I can't imagine why you would risk using sub standard material for a project that involves so much work. There seems to be so little wood that goes into a sof why not find a better source. I have a pretty fair size millwork shop and we throw a dump truck load of edgings away a week of all types of species (can't give it away), perhaps there is something like that in your area. As a last resort, our Home Depot sells a KD hem-fir in their framing material that is relatively clear, a few minutes of sorting may yield you a nice 2x4(s) of good length you could rip down to the appropriate size and would have better integrity then knotty pine.