stern of a vessel
艄 centers on the rear part of a boat or ship: the stern. It is a specific nautical term for the back end of a vessel.
艄 combines 舟 (boat) with 肖, which likely contributes the sound. The character specifically denotes the stern of a vessel.
The left side 舟 is a boat, and the right side 肖 looks like a small figure standing at the back. Picture a tiny captain at the stern of the boat, steering from the rear.
For ソウ, imagine the stern of a boat being sewn (ソウ) together with thick rope: sew -> ソウ, and the stern is where the boat's seams meet.