[Zeltino]

If it costs good money for only a few hours, then nobody would pay. Besides, someone else could then come in and steal it. I think the "valet" parking for ships in a public harbour is your best idea. Once the harbour is full, it won't accept any more ships. Or, just set it so that one player can only put a maximum of some amount of ships, and can not put more in. Then, he/she can pull them out whenever they wish, but only them. You could make it so there is no real "harbour", but some script that keeps track of how many boats you put in, etc.