despatch boat
艅 centers on a type of boat used for dispatching or sending messages, a despatch boat.
艅 combines 舟 (boat) with 余, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a type of boat.
The left side 舟 is a boat, and the right side 余 looks like a person with a message. Picture a small boat carrying a messenger with a despatch.
For ヨ, imagine a despatch boat named 'Yo' that everyone calls out to: Yo! -> ヨ, and the boat speeds off with the message.