Four boats (or hourglass) is a period-2 oscillator, which is composed of four boats in one of its phases. It has two diagonal on-off rotors enclosed by a common stator. While the two rotors are not connected, the stability of the stator still requires them to be in the same phase. It is the most common billiard table osciillator that qualifies under the most strict definition of the term; that the rotor has to be completely surrounded by the stator in all orthogonal directions.
The on phase of this oscillator, the one which led to naming it four boats.
- 16P2.98 at Heinrich Koenig's Game of Life Object Catalogs