The Dungeon Master's Guide has the following rule for objects (p.246):
Damage Threshold. Big objects such as castle walls often have extra resilience represented by a damage threshold. An object with a damage threshold has immunity to all damage unless it takes an amount of damage from a single attack or effect equal to or greater than its damage threshold, in which case it takes damage as normal. Any damage that fails to meet or exceed the object's damage threshold is considered superficial and doesn't reduce the object's hit points.
Page 119 of the DMG provides information for the statistics of a keelboat, which has 100 HP, 15 AC, and a damage threshold of 10. A wizard casts a 9th-level Magic Missile at the keelboat, creating 11 darts that do 1d4+1 force damage each. The darts each strike the keelboat simultaneously, and let's say they all do maximum damage for a total of 55 damage.
Now, does the keelboat take 55 damage or no damage at all?