Why can a hollow steel ship float while a solid steel ball sinks?

Answer: The hollow hull displaces water equal to its weight before filling

The engine continually pushes water down to support the ship — An engine is not needed to support an ordinary floating ship. A moored ship remains afloat with its engine off because water-pressure forces balance its weight; propulsion changes its motion, not this basic buoyancy mechanism.

The hollow hull displaces water equal to its weight before filling ✓ — A hollow hull encloses enough volume to displace water equal to the ship's total weight while keeping its opening above water. At rest, buoyancy equals weight, not more than weight. A solid steel ball displaces too little water to balance its weight even when fully submerged.

Water presses upward only on hollow objects — Water also exerts buoyancy on solid objects. Pressure increases with depth, so the net water-pressure force points upward on both a ship and a steel ball; whether that force can balance weight depends on displaced volume.