Lifting Main Mast with Sheerlegs
Sheerlegs are two long spars lashed together in an upside-down V, used as a crane to lift heavy loads. In this case, the sheerlegs are lifting the massive main mast onto the schooner before launching.
This image is from Basil Greenhill and Sam Manning, The Evolution of the Wooden Ship, 1988, p. 170. Used by permission of the artist, Sam Manning.