When a ship strikes another vessel bow-first, the striking ship sometimes survives while the other sinks. Not every time! But most of what we have seen, the rammer stays afloat.
One reason is that ships are divided into watertight compartments designed to limit the spread of flooding. Near the bow is a reinforced watertight partition called the collision bulkhead, which can help prevent water entering the damaged forward section from flooding the rest of the ship. Although the bow may crumple or deform during an impact, it isn’t designed to absorb energy in quite the same way as a modern automobile. Much depends on the speed and angle of the collision, the extent of the damage, and where each ship is struck.
A ship’s bow can telescope or fold back on itself during a collision. That’s a real phenomenon, and it can look remarkably like an automobile’s crumple zone. The difference is between what the bow does during an impact and what it was specifically designed to do.
On a traditional steel ship, the bow contains frames, plating, and structural supports. During a sufficiently violent collision, those structures can buckle, collapse, and fold backward, absorbing some of the impact energy.
The bow structure may collapse or telescope during a collision.
The collision bulkhead, farther back, is intended to stop flooding from spreading into the rest of the vessel.
The watertight compartments help the ship remain afloat if the damaged section floods.
Some examples
Storstad → Empress of Ireland
Rammer survives.
Stockholm → Andrea Doria
Rammer survives.
Florida → Republic
Rammer survives.
And then the wonderful exception:
Pacific → Orpheus
Rammer sinks!