raft, cut slantwise
槎 centers on wood that has been cut or shaped for a purpose: a raft made of trimmed logs, or the act of cutting wood slantwise.
槎 combines 木 (tree/wood) with 差, which likely contributes the sound and may relate to the idea of cutting or shaping wood unevenly.
A tree 木 is cut slantwise 差 to make a raft: the wood is shaped at an angle so the logs fit together.
For サ, imagine sawing wood to build a raft: the sound of the saw is 'sa', and the wood is cut slantwise.