KO'ACH IS PERMITTED REGARDING KARMELIS
Question: What is the source [that Chachamim did not forbid Ko'ach regarding Karmelis]?
Answer (Beraisa): One may not transfer from a ship to the sea or vice-versa;
R. Yehudah says, if the ship is 10 deep (it is Reshus ha'Yachid) but the deck is not 10 above the water, one may transfer from it to the sea but not vice-versa.
Question: Surely, one may not transfer from the sea to it for this is from a Karmelis to a Reshus ha'Yachid - also the other direction should be forbidden!
Answer: We must say that R. Yehudah permits pouring on the side of the ship because it descends to the sea by itself.
This shows that Ko'ach is permitted regarding aKarmelis.
GUD ACHIS MECHITZTA
(Rav Huna): In a small boat of Meishan (the sides slope and meet at the bottom, like a 'V' shape), one may carry only within four Amos;
This is only if it is not four wide within three Tefachim of the bottom [for then its walls are like hanging Mechitzos, which are invalid] - but if it is this wide, one may carry on the whole boat (it is as if the Mechitzos extend to the bottom).
[Even if it is not four wide near the bottom,] if the bottom [until it is four wide] was filled with reeds and thin willow branches, one may carry on the whole boat.
Question (Rav Nachman): We should say Gud Achis Mechitzta (it is as if the wall extends downward, from where it is four wide)!
(Beraisa - R. Yosi b'Rebbi Yehudah): If one inserted a pole in Reshus ha'Rabim with a basket (four by four) on top, and one threw something (from Reshus ha'Rabim) and it landed on the basket, he is liable.
The basket is Reshus ha'Yachid on account of Gud Achis - likewise here we should say Gud Achis!
Answer (Rav Yosef): Rav Yehudah taught that Chachamim [argue with R. Yosi b'Rebbi Yehudah, and] exempt!
Question (Abaye): Surely, you agree with Gud Achis!
(Beraisa): If a pillar in Reshus ha'Yachid is 10 tall and four wide, but at the bottom it is less than four wide, [even] if the narrow part is three Tefachim tall [so Lavud does not apply], if one threw [from Reshus ha'Rabim] and it landed on top of it, he is liable.
This shows that we say Gud Achis!
Regarding the basket Chachamim do not say Gud Achis because goats can stick their heads through [the projected extension of the walls] - this does not apply to the pole (goats are not wont to come close to scratch themselves on it) or the boat!
Question (Rav Acha brei d'Rav Acha): Regarding the boat, fish can break through [the projected extension of the walls]!
Answer (Rav Ashi): We are concerned for penetration of goats, not of fish (for they are covered by the water).
Question: What is the source of this?
Answer - Question (R. Tavla): Does a hanging Mechitzah permit carrying in a ruined house?
Answer (Ravin): Chachamim gave this leniency only in water.
Question: Fish can penetrate!
Answer: This shows that we are not concerned for penetration of fish.
SHIPS TIED TOGETHER
(Mishnah): If ships are tied to each other [one may carry from one to the other].
Objection: This is obvious!
Answer #1 (Rava): The Chidush is, one may even carry from one to a small boat in between them.
Objection (Rav Safra): It says, one may carry from one to the other [implying that there are only two]!
Answer #2 (Rav Safra): It teaches that [even if different people own the ships,] they may make an Eiruv and carry from one to the other:
(Beraisa): If ships are tied to each other, one may make an Eiruv and carry from one to the other;
If they became separate, it is forbidden;
If they were tied to each other again, they are permitted like before, whether they were tied b'Shogeg (he forgot that it is Shabbos or that tying is forbidden), b'Mezid, through Ones, or Misasek (engaged in tying something else).
The same applies to mats spread [for Mechitzos] in Reshus ha'Rabim, one may make an Eiruv and carry from one [resulting Reshus ha'Yachid] to the other;
If the mats were rolled up (they are no longer Mechitzos), one may not carry;
If they were spread again, they are permitted like before, whether they were spread b'Shogeg, b'Mezid, b'Ones, or Misasek (l'Chatchilah, one may not make a Mechitzah on Shabbos to permit something);
Any Mechitzah made on Shabbos, whether Shogeg or Mezid, is considered a Mechitzah.
Question: But Rav Nachman taught that they are Mechitzos only regarding throwing (to obligate one who throws to there from Reshus ha'Rabim, for mid'Oraisa they are Mechitzos), but they do not permit carrying inside!
Answer: He refers only to Mezid (this is a fine. Above, we said 'whether Shogeg, Mezid, Ones...' - really, all of these are Mechitzos for throwing, but Mezid does not permit carrying. Tosfos - the fine is only when the Mechitzah makes Reshus ha'Yachid - if it was already Reshus ha'Yachid, and the Mechitzah is required mid'Rabanan, it is valid even if made b'Mezid.)
(Shmuel): [It is permitted] even if the ships are tied with a drawstring of a cloak.
Question: What is the case?
If the string can hold them together, this is obvious!
If it cannot hold them together, it should be forbidden!
Answer: Really, it can hold them together - Shmuel teaches that another of his teachings does not apply here:
Version #1 (our text) (Mishnah): If a ship was tied with something that can hold it [and one end was in Ohel ha'Mes], it brings Tum'ah to the whole boat.
(Shmuel): This is if it was tied with an iron chain.
Shmuel requires an iron chain only regarding Tum'ah, for it says "Ba'Chalal Cherev" - a sword [or any metal Kli exposed to Tum'as Mes] is like a corpse (it is Avi Avos ha'Tum'ah, it makes an Av ha'Tum'ah) - but regarding Shabbos, anything that holds them together suffices, even a drawstring.
Version #2 (Tosfos) (Mishnah): If a ship was tied with something that can hold it [and Tum'as Mes was underneath it], it brings Tum'ah to [underneath] the whole boat.
(Shmuel): This is if it was tied with an iron chain.
Shmuel requires a solid chain only regarding Tum'ah, so it will not be a moving Ohel, which does not bring Tum'ah - but regarding Shabbos, anything that keeps them together suffices, even a drawstring.