If a person leaves his home for an extended vacation, such as one who goes away for the summer, must he recite a new Beracha on the Mitzva of Mezuza when he returns home?
The obligation of Mezuza applies only to a house in which one resides; if a person owns a home but he does not reside there, he bears no obligation to affix Mezuzot. At first glance, then, when a person leaves his house for a period of thirty days or more and then returns, thereby establishing his residence anew, the obligation of Mezuza takes effect at that point, thus warranting the recitation of a new Beracha.
Rabbi Akiva Eiger (Poland, 1761-1837) discusses this issue in his responsa (#9), where he writes that logic would indeed dictate reciting a new Beracha in such a case. However, he adds, the Birkei Yosef (commentary to the Shulchan Aruch by Rabbi Chaim Yosef David Azulai, Israel-Italy, 1724-1806) writes (in Orach Chaim, 19) that the Beracha over Mezuza was instituted to be recited only at the time when it is initially affixed to the doorpost. Therefore, even though one could argue that, technically speaking, the Mezuza obligation takes effect anew when one returns home after an extended absence, he nevertheless does not recite a new Beracha. This is Rabbi Akiva Eiger's conclusion, and also the position of Chacham Ovadia Yosef (in Halichot Olam, vol. 8, p. 242).
If a person owns a boat with doorways, must he affix Mezuzot to the doorposts?
Chacham Ovadia Yosef discusses this issue in the aforementioned chapter (p. 300), and he concludes that we do not consider the time spent on a boat a permanent residence, and thus it does require Mezuzot. This principle applies as well to other rooms that are not intended for permanent residence, such as elevators, buses, airplanes and jetways leading from airport terminals to planes. In all these situations, even if there are rooms of a size that normally obligates a room in Mezuza, no Mezuza is required, given the temporary nature of the use of these structures.
Summary: One who returns home after an extended absence does not recite a new Beracha over the Mezuzot in his home. One need not affix Mezuzot to the doorposts of boats, elevators, buses, airplanes or other structures that are not used for permanent residence.