A hydrofoil is a boat with wing-shaped pieces under it, down in the water. When the boat is still or slow, it sits in the water like any other boat. When it goes fast, those wings push against the water and lift the body of the boat up, so most of it is riding above the surface. Less of the boat is dragging through the water, so it can go very quickly and often more smoothly. People also call the wing itself a hydrofoil; the boat is named after that part. You will hear the word in engineering, ferry travel, and, more and more, in sport — riders now put a small hydrofoil under a board and fly over the water. It is a technical word, not something you say at the dinner table. Though actually, the “foil” here is not kitchen foil. It is the same idea as the wing of a plane, only working in water.

水翼船