Bava Metzia 79: Rent-a-Donkey (or a Boat)

If a rented donkey dies midway through the journey, the renter pays a partial fee, and complains about the experience. But what if that donkey can be replaced? Plus, what if a donkey is rented for a man to ride, but a woman rides? She's considered heavier on the donkey... Also, renting a boat that doesn't make the journey, but sinks - the questions whether the rental was specific to that boat or any boat, which makes a difference for liability.