The Master of a vessel has the Owner's authority to vary charters. This is done by agreeing to or acquiescing without protest, to the Charterer's requirements. It occurs when such requirements are not within the written terms of the agreement (C/P).
It should be noted that any failure to perform any part of any promise contained in a C/P gives the injured party claim in Law for compensation. There are some promises in a C/P which are viewed as being so fundamental to the performance of the agreement as a whole that if they are broken the injured party can refute the whole agreement as well as claiming damages. An example would be in yacht chartering the failure to provide the yacht contracted for by the time specified in the contract (C/P).