Chananyah ben Achi R. Yehoshua says, the entire day they were debating with each other, until my uncle (R. Yehoshua) came and resolved between them and enacted that the Halachah follows R. Gamliel and R. Elazar ben Azaryah regarding a boat, and it follows R. Yehoshua and R. Akiva regarding a Dir or Sahar.

Question: What is the difference between a Sahar and a boat?

Answer #1 (Talmidim): A boat [is different] because its Mechitzos go with it (he is in the Mechitzos in which he was Shoves).

Answer #2 (R. Ze'ira): Since [a boat] is uprooted from these four Amos and put in these four Amos. (Even when he walks, he does not cause himself to leave his four Amos, for the boat would have removed him even had he stood still.).

Question: What is the difference between [these answers]?

Answer #1: If [the boat] was like an Achsadra (it was not surrounded by walls). According to the Talmidim he is forbidden. According to R. Ze'ira he is permitted.

Answer #2: The sea was calm (the boat is not constantly entering another four Amos). According to the Talmidim he is permitted. According to R. Ze'ira he is forbidden.

R. Ze'ira said [above, 27a] that we learn from [this] Beraisa [which distinguishes a boat from a Sahar] that the four Amos that they said are a Techum. (Therefore, we are stringent in a boat only when it goes slowly, and through walking he would uproot himself from his Techum. We explained this like SEFER NIR.)

(Mishnah): Once, the boat did not enter the port until dark (Shabbos night). They asked R. Gamliel if they may get off.

R. Gamliel: You are permitted. I saw that we were within the Techum before dark.

(Gemara): R. Gamliel had a Metzufis with which he could gauge horizontal distance. (E.g. a tube held rigidly at a small angle below horizontal, so he would see what is 2000 Amos away. If he saw land, this shows that they were within the Techum.)

Question: Why [did R. Gamliel say "we were within the Techum] before dark''? Even if it was after dark, is this not R. Gamliel [who permits one who was brought to a Dir or Sahar? A port is no worse!]

Answer: The case is, the port was more than Beis Sa'atayim (Stam, it was not Hukaf l'Dirah), or the walls were not 10 [Tefachim] tall, or there were breaches more than 10 [Amos], or there was Omed opposite Omed and Parutz opposite Parutz. (One may carry in a Dir or Sahar in such a case. They were permitted only because they were within the Techum before dark, they were permitted even if not all of these were fulfilled. We explained this like PNEI MOSHE.)