Abstract: A novel snatch block particularly suitable for marine use and for movement on a boat deck mounted track, comprising a track slide portion and an upper sheave assembly. The sheave assembly comprises a pair of normally spaced-apart side cheeks; a sheave pin extending between said cheeks; and a sheave rotatably mounted on said sheave pin. Structure is provided for holding the sheave pin at one end to one of the side cheeks, and releasably engaging the sheave pin at the opposite end with the other of the side cheeks to hold the sheave assembly in a normally closed position. Both the side cheeks are pivotally mounted on the slide portion so that with release of one of the side cheeks by the sheave pin, the sheave assembly is capable of expanding to a wider opening than available with conventional snatch blocks.