Since a time-chartered vessel is under the commercial control of the charterers, what is the master's position with respect to obeying charterers' instructions?

MCA Oral Exam Questions

Although employed as a servant of the owners, the master is always under the time charterers' orders as far as the employment of the vessel and agency matters are concerned. 

He must accordingly prosecute the voyage with "utmost despatch" and give charterers "customary and reasonable assistance" with the crew and ship's equipment. Charterers may only give master orders regarding employment of the vessel, and not regarding navigation, safety, etc. (If the master did obey charterers' order regarding navigation and loss or damage occurred, owners would be liable, with no recourse from charterers. )