Abstract: A vang for producing a desired tension in the mainsail of a yacht employs, in association with a multi-sheave block and tackle system conferring mechanical advantage, a valve releasable, variable length, pressurized gas spring strut. Operation against a spring pre-load of the control valve of the strut by application of a force thereon, being a force derived by the block and tackle system from manual exertion on said block and tackle system, enables the gas spring strut to be released from the length at which it is currently held locked, by reason of closure of the control valve, for such extension or contraction as may be required to produce a desired force on the main-boom and a consequent tension in the leech of the mainsail. The length of the gas spring strut remains unaffected by closure of the valve following removal of the force on the block and tackle system.