Ok.... Blackmouth season is upon us! A couple questions if I may?

I heard that a good trolling speed in the Sound for salmon is about 2-3 knots... help a rookie...how fast is that? What is the conversion formula to MPH?

Also, when jigging for salmon off the shelf where it drops to literal oblivion (400 + feet!) how deep should i be jigging? or should i be jigging on the shallow end of the shelf?

Thanks in advance for the help!