Why do boats or ferries have tyres hung around their sides?

Those tyres are called fenders. In boating, a fender is a bumper used to absorb the kinetic energy of a boat or vessel berthing against a jetty, quay wall or other vessel. Fenders are used to prevent damage to boats, vessels and berthing structures. read more

In boating, a fender is a bumper used to absorb the kinetic energy of a boat or vessel berthing against a jetty, quay wall or other vessel. Fenders are used to prevent damage to boats, vessels and berthing structures. To do this, fenders usually have high energy absorption and low reaction force. read more

Larger boats, work boats, and tug boats (mostly) use them as two way protection. Instead of metal to metal, the tires act as a cushion for boats tying on to, or pushing, other vessels. While smaller boats and recreational boats use "fenders" that hang over the sides to prevent getting scratched up when docking or rafting off each other, larger, heavier work boats often use tires for the same purpose. read more