bamboo raft
栰 centers on a bamboo raft, a simple floating platform made of tied bamboo poles used for transport on water.
栰 combines 木 (tree, wood) with 伐 (to cut down, fell), suggesting wood that has been cut and assembled into a raft. The exact historical development is uncertain, but the component choice fits the meaning.
A tree 木 is cut down 伐 and the logs are tied together to make a bamboo raft 栰.
For ハツ, imagine a raft made of many hats tied together: hats -> ハツ, and the raft floats on the water.