Tosafos 101a BD"H Maskif says that according to his pshat in the nature of the ships (Betzioso deMeishon), ie rafts with spaces between the logs that make up its base, the gemora's suggestion of using gud achis means that the sides of the ship should be considered extended as if they curve round and line the base of the ship thereby covering the gaps in the base.
1) Why is Tosafos not happy to understand the concept of Gud Achis in the conventional sense of extending straight down, ie that the sides should be considered as if they go straight down to the bed of the sea and that the sea bed would be considered as the base?
2) What is the precedent for Tosafos' application of Giud Achis to bend round the corner? Isn't it a machlokes in Eruvin whether we hold of "Koif veGoid"?
When the Ritvo mentions Tosafos' peshat in the Betzioso deMeishon, he brings two ways of understanding the gemora's application of gud achis. Firstly he explains that the sides of the ships are considered as if they go straight down and then he brings besheim r' Hai Ga'on that the sides bend around the base, like Tosafos learns, and on the seconds peshat he says "Ve'enoi mechuvar." I assume that the Ritvo has the same problems with Tosafos' peshat as I mention but I haven't found a yishuv for Tosafos' peshat in any of the regular Rishonim or Acharonim on Shabbos.
I wonder whether you could help.
Yehuda Wittenberg, London, UK
It is possible that Tosfos did not want to say that the sides go straight down to the sea bed because there is a problem of Gedi'im Bokin Bo. The Sfas Emes in fact asks this as a question on Rashi (see his answer).
Tosfos does not necessarily have to have a precedent for his application of Gud Achis, as we see in other Gemaros (most notably Eiruvin 19b) that once Chazal expressed a concept which is based on "Roin," there was often argument regarding how far to extend that principle. Rav Nachman must have understood that being that there is a concept of Kof v'God which is (at least) sometimes said regarding a boat (see Ritva 100b), it definitely could apply here as well, and even in the form where it is mainly Kof, that the sides curve around lining the base of the ship. I'm not exactly sure why the Ritva says that Tosfos' explanation is Aino Mechuvar, as there quite a few questions that the Acharonim raise on Tosfos (i.e. see Sfas Emes).