THE ARGUMENT REGARDING A BOAT
Question: Why didn't R. Zeira answer like Rabah?
Answer: The walls of a boat are merely to keep the water out. (They are not considered walls to make a boat like four Amos.)
Question: Why didn't Rabah answer like R. Zeira?
Answer: All the Tana'im permit a moving boat [for R. Zeira's reason]. They argue only about a stationary boat. (R. Zeira's reason does not apply then. Tosfos - we retract from the two answers given above regarding the difference between Rabah and R. Zeira.)
Support #1 (Rav Nachman bar Yitzchak - Mishnah): Once, they were on a boat that left the Techum on Shabbos. R. Gamliel and R. Elazar traversed the entire boat. R. Yehoshua and R. Akiva did not leave their four Amos, for they wanted to be stringent on themselves.
Granted, if all permit a moving boat, R. Yehoshua and R. Akiva chose to be stringent [lest the boat stop moving and they will not realize];
However, if R. Yehoshua and R. Akiva forbid even a moving boat, they had no choice. [They hold that] it is forbidden to leave their four Amos! (Tosfos - R. Zeira can say that Techumim do not apply above 10 (this is discussed below). They chose to be stringent lest the boat enter water less than 10 deep, even though this is unusual.)
Support #2 (Rav Ashi): The Mishnah discusses a boat after they argued about Dir and Sahar. Presumably, they are similar. Just like the latter are stationary, also a boat!
Question (Rav Acha brei d'Rava): Rav and Shmuel said that the Halachah follows R. Gamliel regarding a boat. This implies that the Tana'im argue about a boat!
Answer (Rav Ashi): This is correct!
(Beraisa - Chananyah): The entire day they argued about the Halachah. At night, my uncle (R. Yehoshua) decided that the Halachah follows R. Gamliel regarding a boat and R. Akiva regarding a Dir and Sahar.
DO TECHUMIM APPLY ABOVE TEN TEFACHIM?
Question (Rav Chananyah): Does the Isur of Techumim apply more than 10 Tefachim above the ground?
We do not ask about [walking on] a pillar 10 tall and four wide [if one end is outside the Techum]. It is [proper to walk on,] like the ground. Surely Techumim applies on top of it!
We ask about a pillar 10 tall and less than four wide;
Version #1: We also ask about flying [using Hash-m's name].
Version #2: We also ask about going in a boat. (It is normally 10 Tefachim above the sea floor. Ga'on Yakov - Rashi's text does not call these different versions, therefore he says that going in a boat is like flying. If so, we need not say that each of the coming answers applies to only one version.)
Answer #1 (R. Hoshaya - Mishnah): Once, they were on a boat...
If Techumim applies above 10, we understand why R. Yehoshua and R. Akiva chose to be stringent. [Perhaps the boat will stop, and R. Zeira's reason will not apply];
However, if Techumim does not apply above 10, there is no reason to be stringent!
Rejection: Rava answered [below] that the case is, the boat was in shallow water (less than 10 above the sea floor). The same answer applies here.
Answer #2 (Mishnah): Once, the boat did not enter the port until Shabbos [...R. Gamliel permitted to get off, for it was within the Techum before dark].
If Techumim applies above 10, we understand this. However, if not, it would be permitted to get off even if it was not in the Techum before dark!
Rejection (Rava): The case is, the boat was in shallow water.
Answer #3: Seven teachings were said Shabbos morning in front of Rav Chisda in Sura, and Shabbos afternoon in front of Rabah in Pumbadisa [which is outside the Techum of Sura. Surely, the same person said them!]
Suggestion: Eliyahu said them. (He flew above 10 from Sura to Pumbadisa.) This shows that Techumim do not apply above 10!
Rejection: Perhaps they were said by a certain Shed [that does not observe Shabbos].
Answer #4 (Beraisa): If one said 'I will be a Nazir on the day that [Mashi'ach] ben David will come', he may drink wine on Shabbos and Yom Tov, but he is forbidden on weekdays.
If Techumim applies above 10, we understand this. (Mashi'ach cannot come on Shabbos or Yom Tov.) However, if it does not apply, also Shabbos and Yom Tov should be forbidden!
Rejection #1: "Hinei Anochi Shole'ach Lachem Es Eliyahu ha'Navi [Lifnei Bo Yom Hash-m" -- Eliyahu will herald the coming of Mashi'ach the day before.] Since Eliyahu did not come yesterday, we are not concerned lest Mashi'ach come today.
Objection: The same reasoning should permit also weekdays!
Rather, Eliyahu will tell the Great Sanhedrin. (Commoners might not know before Mashi'ach comes. Therefore, weekdays are forbidden.)
The same should forbid Shabbos [and Yom Tov! This defends Answer #4. Shabbos is permitted because Techumim applies to Mashi'ach, even above 10.]
Rejection #2: Yisrael are guaranteed [via tradition] that Eliyahu will not come on Erev Shabbos or Yom Tov, lest people go to greet him and not have time to prepare [for Shabbos or Yom Tov. Toras Chayim - the one who gave Answer #4 knew that Eliyahu will inform the Sanhedrin about Mashi'ach in advance ("...Lifnei Bo Yom Hash-m"), and that commoners might not know in advance. He thought that this could be more than one day earlier, therefore he did not anticipate this rejection nor Answer #5.)
Answer #5: The Beraisa shows that Techumim does not apply above 10!
If it applied, Eliyahu could not come on Shabbos, so Sunday would be permitted!
Rejection: The Tana is unsure whether or not it applies. Therefore he is stringent. (If it does not apply, Eliyahu could come on Shabbos, and Mashi'ach could come on Sunday.)
Question: When did he vow?
If he vowed on a weekday, he become a [Safek] Nazir immediately, so also Shabbos is forbidden! (Perhaps Mashi'ach came, but he did not hear about it yet!)
Answer: He vowed on Shabbos or Yom Tov. The day of the vow he is permitted, and afterwards he is forbidden [until Mashi'ach will come and he will finish Nezirus Vadai. Turei Even (Rosh Hashanah 11b) - Tana'im expound that the Ketz (deadline) for the redemption is in Tishrei or Nisan, but if we merit, it can come immediately, in any month.]
(Mishnah): Once, the boat did not enter the port...
(Beraisa): R. Gamliel had a hollow reed - with it he could see [exactly] 2000 Amos on land or at sea. (See Perush Chai diagram, Perek 4 number 10, in the English Charts section, and note 15 in Appendix.)
One can look into a canyon with a reed to see how deep it is;
One can calculate [without a reed] the height of a date tree through measuring his own height, and the lengths of his shadow and the tree's shadow [at the same time. The ratio of height to shadow is the same for him and the tree].
If one wants to prevent vicious animals from inhabiting the shade of a [monument over a] grave [lest they smell the Mes and dig it up], he should insert a stick [as tall as the monument will be] at four hours of the day and see where the shadow leans; he builds a [solid] ramp from the end of the shadow to the monument [so the animal cannot stand in the shade. At four hours it gets hot and animals seek shade. Even though the monument offers shade in the afternoon when the sun is on the other side, the animal already found shade elsewhere. Alternatively, animals do not seek shade then, for it is hot even in the shade.]
Nechemyah brei d'Rav Chanilai was engrossed in his learning and walked outside the Techum. Rav Chisda told Rav Nachman 'your Talmid is in pain.'
Rav Nachman: Have people stand in lines to make Mechitzos, and he may re-enter the Techum [like one who was taken outside the Techum via Nochrim, and put within Mechitzos].