Final answer:
The Lay-up endorsement is purchased when a vessel is not in use and allows shipowners to receive a return of some of the paid premium due to reduced risks. It is not meant for active shipping seasons, repairs, or adverse weather situations.
Step-by-step explanation:
The Lay-up endorsement is a type of insurance provision that shipowners can purchase when their vessel is not going to be in use for a certain period. This endorsement allows them to receive a return of a portion of the paid premium, reflecting the lower risk of an incident when the ship is laid up and not active. The most appropriate time to purchase the Lay-up endorsement is:
(b) When the vessel is not in use
This could be during periods when the vessel is out of service for an extended time, such as the off-season, or when it is awaiting new charters. The purpose of the endorsement is to adjust the insurance cost to the decreased risk. It is not typically related to the duration of the shipping season, when the vessel is under repair, or during adverse weather conditions, contrary to other options suggested in the question. In practice, the terms and conditions of the Lay-up endorsement would be detailed within the insurance policy.