Hull

The hull plating (armour) of a ship or defense unit represents its „health“ (hitpoints).

Damage

When a unit is attacked, the damage to its hull is equal to the difference in the attacker's Attack Power and the defender's Shield Power.¹⁾

While Shield is recharged at the start of every round of combat, Hull is only restored at the end of the battle.

Kills

If the hull is ever below 70% of its initial (i.e. at the start of battle) value, the unit has a chance of being destroyed at the end of the current round of combat.²⁾

The probability of destruction is equal to the fraction of hull that has been lost (ex: 40% chance to explode when only 60% hull remains):

Initial Value

The hull of a unit is determined from its metal and crystal costs to build - this sum is known as „Structural Integrity“ - and the Armour Technology level:

Examples

Example 1: Your Cruiser enters battle and you have researched Armour Technology level 5:

Initial Hull = ((20,000 + 7,000) / 10) * ( 1 + ( 5 / 10 ) ) = 4,050

Example 2: Assume your ship has 1000 Hull and 100 Shield Power and is attacked by a ship with 500 Attack Power:

The attack fully depletes the shield (which starts every round at full power) since 500 is more than 100.
The remaining 400 Attack Power is inflicted on the Hull, bringing it down to 600.
Since your ship has 60% of its original Hull (600/1000), there is a 40% chance of it exploding at the end of the round.
Your ship has a chance to be hit more than once this round if the attacking ship has rapidfire or if the attacking fleet has multiple intact ships.
Your ship still gets to attack in this round, even if it takes more hits and is flagged to be destroyed.

¹⁾

If Attack Power is less than (or equal to) Shield Power, the shield takes damage for the round but the hull is unharmed.

²⁾

This chance is evaluated every time damage is received, meaning a barrage of relatively weak shots can still create a very high chance of explosion.