If a ship under tow is not in class but is to be insured for her voyage, what steps will the hull underwriters probably take to confirm that she is fit for the tow?

Hull underwriters normally impose a warranty and instruct an approved survey organisation to make a survey of the towing arrangements to verify that the conditions of the warranty are complied with. 

If so, the surveyor will issue an Insurance Survey Certificate confirming that, subject to the attached conditions (e. g. relating to route, weather conditions, towing arrangements, etc. ) the vessel is fit for the tow.