- This page deals with naval combat mechanics. For information on the recruitment and maintenance of navies see navy. For details about ship types see naval units. For the state of war see warfare.
Naval warfare is the competition of sea-going power between nations at war. While a majority of warfare is land-based, the naval aspect of conflict can be incredibly influential in the spheres of trade, colonisation and conquest.
- 1 Sea battle interface
- 2 Engagement width
- 3 Combat sequence
- 4 Combat statistics
- 5 Morale and strength
- 6 Naval doctrine
- 7 Fleet actions
- 8 Naval missions
- 9 Strategies and tactics
- 10 Footnotes
Sea battle interface
Like land warfare, naval combat occurs when opposing fleets confront each other in the same sea province. Also like land battles, these sea battles can go on for many days, so they should be considered not as a single engagement, but as two fleets maneuvering to gain wind, standing off and skirmishing before the fleets finally collide.
In EU4, there is no player involvement inside combat, but the interface now displays more information, showing among other information which ship is fighting which. The naval combat interface shows the strength of the two fleets engaged in the combat. Combat will begin with a contest of firepower, followed by shock action in which ships attempt to grapple or ram one another. This sequence will alternate until one of the fleets loses morale and is either routed or destroyed. The condition of each ship is expressed as a percentage, and hovering over a ship with the mouse cursor will produce a tooltip which identifies which opposing ship they are currently firing on, as well as their current morale, their strength, and the ship type.
Since the patch 1.16, combat width for naval warfare has been introduced, it has a base of 25, and can be modified by:
- admiral's maneuver +10% per pip
- Coastal sea +10%
Heavy ships occupy 3 combat width, while galleys, transports and light ships each occupy 1 combat width.
Unlike the 'combat width' of land warfare, the 'engagement width' of the two participants are independent, that means one participant could have wider engagement width than the other.
Like land warfare, naval combat occurs when opposing fleets confront each other in the same sea province. Also like land battles, these sea battles can go on for many days, so they should be considered not as a single engagement, but as two fleets maneuvering to gain wind, standing off and skirmishing . A sea battle will last until one side is routed or annihilated. Patch 1.16 also introduced the concept of a naval combat width, and much like its land cousin it is the total number of ships that can engage the enemy at any one time.
Combat is divided into a series of 3-day phases. Phases alternate between Fire and Shock, with the Fire phase happening first.
In combat each ship will try to find a target and make an attack. Targets are selected at random.
At the beginning of each phase, each side rolls a die. The result is used to determine the morale damage and hull damage inflicted by that side during each of the three days of that phase.
The amount of cannons of the attacking vessel affects the hull damage dealt and the hullsize of the defender decides how long it can stand the cannon fire. The Artillery Fire Modifier gives a bonus or penalty to damage dealt depending on if attacker or defender has a technological advantage.
- Attacking unit modifier: The attack modifier from the attacking unit's technology, i.e. "Artillery Fire".
- Attacking unit Combat Ability: Any Combat Ability bonuses the attacking unit has.
Morale is damaged both over time during a battle and also when a friendly ship is sunk. Several ideas decrease the amount of morale lost when a ship is sunk:
|Dip Tech Level||0||3||7||8||12||18||22||25||27||29||31|
|Morale of Navies||+2.0||+2.1||+2.4||+2.6||+3.0||+3.5||+4.0||+4.5||+5.0||+5.5||+6.0|
|Mil Tech Level||0||7||13||16||22||25||32|
The following contributes to the maximum morale of a nation's navy.
|Naval maintenance||20% at minimum maintenance (minimum of 0)|
|100% at maximum maintenance (multiplicative modifier)|
|Navy tradition||+25% at 100 navy tradition|
|Prestige||+10% at 100 Prestige, −10% at −100 prestige|
|Power projection||+10% at 100 power projection|
|Naval reformer advisor||+10%|
|Defender of the Faith||+5%|
|War exhaustion||−2% per 1 war exhaustion|
Certain National Ideas, Idea groups and policies will bump naval morale.
Each point of naval tradition also increases naval morale recovery by +0.10%.
Combat ability is a value that is multiplied with the units' damage dealt (both for casualties and morale), but only for the specified type of unit.
Heavy ship combat ability improvement
Light ship combat ability improvement
Galley combat ability improvement
- Transport combat ability improvement
- The combat ability of transports is not modified by the base game.
- Main article: Naval units
Hull is the defensive stat, and cannons the offensive stat. With the exception of Morale, every combat phase the hull/cannon pips are used against each other to calculate the number of hull damage of each combat phase.
Leaders are rated on a scale of 0 to 6 for each of the following skills:
The leader's skill is added to the dice roll of its respective phases.
The leader's maneuver affects the movement speed of the fleet both on the map. Each point also grants 5% additional trade power when assigned to a fleet protecting trade in a trade node. Finally, it reduces attrition taken by 1% per skill level.
Concluding a battle
A naval battle ends when one side is either:
- Reduced to 0 morale; or
- Flees from combat; or
Upon victory, ships from the defeated fleet may be captured, depending on the difference between the two fleets' maneuver skill. Generally very little to no ships will be captured for leaderless (0 vs. 0 maneuver) battles.
If the defeated fleet still has ships remaining, it will escape to the nearest friendly port to repair and recover its morale. Naval battles, like land battles, may cause war exhaustion as well as affect warscore, naval tradition and prestige. Note that warscore gained from naval battles will generally be insignificant compared to those gained by its land counterparts.
Several ideas increase the chance of capturing ships after a battle:
Morale and strength
- "Morale of navies" redirects here. For bonuses that increase naval morale, see § Max naval morale above.
Morale is an important factor in fighting battles. Each turn of combat a unit will take a Morale hit. Once a fleet's average Morale value has been reduced to zero the fleet will attempt to retreat. Retreat cannot happen until both a fire and a shock phase have completed, so a fleet that has its morale reduced to 0 before that point will be destroyed.
If a fleet loses a battle while having low enough morale, they will be forced to retreat to a port of the nearest sea province. While retreating, it cannot be engaged in combat or be controlled until it reaches the sea province, nor will it repair. The fleet will also move 50% faster, and will recover morale at a normal rate during the retreat.
After every battle is fought a fleet must spend some time not fighting for its morale to recover.
Every month when docked at a home port, a ship recovers a 10% of its maximum morale.
Certain National Ideas, Idea groups and policies will increase naval morale recovery.
Each point of naval tradition also increases naval morale recovery by +0.1%.
Unit strength is an important factor in fighting sea battles. Naval unit strength affects how long it can stand cannon fire before sinking. Each turn of combat a unit will take hull damage reducing its strength.
Galleys gain a bonus 100% strength in inland seas  which are shown on the map below in pale blue.
Fleets that have sustained damage will automatically repair up to their full strength when docked in an owned, allied, or in a province with fleet basing rights. There is a 'repair damaged' button that will detach damaged ships and send them to a nearby port to repair, when they are back up to full strength they will rejoin the fleet.
- Main article: Navy#Ship_Repair
|Available only with the Rule Britannia DLC enabled.|
A country that has a naval force limit of at least 20 may select a naval doctrine. Selecting a naval doctrine, or changing to a different one (which can be done at any time), costs 0.1 ducat per sailor cost of the country's navy (i.e. 5 per light ship or transport, 10 per galley, and 20 per heavy ship).
The doctrines are:
|−15% Naval maintenance modifier|
|+15% Galley combat ability|
|+33% Ship trade power|
|+33% Chance to capture enemy ships|
|+1 Naval combat bonus off owned coasts||Requires British culture group|
|+50% Treasure fleet income||Requires Iberian culture group, but not Portuguese or Brazilian|
||Requires Portuguese or Brazilian culture|
Start patrolling 
The start patrolling button orders a fleet to loop its current movement order: when it reaches the final destination of its current set of movement orders, it will automatically head back to the point where the patrol was started. Patrol routes can include port visits, where the patrol stops until all ships are repaired. While the same function can be fulfilled by light ships assigned to protecting trade nearby, patrols are not limited by ship type, trade power, or trade range.
|Available only with the Mare Nostrum DLC enabled.|
Raiding coasts is a naval ability added in the Mare Nostrum expansion which allows fleets belonging to nations with the raid coasts idea, which in the base game belongs to the Maghrebi culture group ( Morocco, Tlemcen, and Tunis), Habsan and The Knights to raid the coasts of other nations for loot and sailors of a different religion; this idea is also available for custom nations and, with the Golden Century DLC, pirate republics, which can raid the coasts of all neighbouring nations with no attention to their religion. Historically, this ability reflects the raids by Barbary pirates which were active in the 16th to 19th centuries. Raiding can provide a significant early-game boost to the income and sailor pools of nations that can do it.
Raiding a coast is something fleets can do at sea. To raid a coastal province, the following needs to be true:
- the nation…
- …has May raid coasts idea (i.e. countries with Berber national ideas or The Knights)
- …must be able to place a fleet in a sea tile adjacent to the province, but doesn't need to be able to blockade the port
- the target province…
- …must be at most three sea tiles away from a province owned by the nation
- …must have a development lower than or equal to the blockading power of the fleet
- …doesn't have the religion of the raiding nation
- …doesn't belong to an ally or subject of the raiding nation
- …doesn't have the “Raided Coast” modifier
After a province has been raided, its loot bar is emptied, the amount of devastation is increased, and the modifier “Raided Coast” is applied, which lasts for ten years. Each raided province will add a −25 relations penalty with the target country (down to −100), decaying by +1 per year.
The efficiency of raiding is reduced by fleets on pirate hunting patrol.
|per development (raided province)|
|Local sailors per Raided provinces / 4|
Fleets will automatically blockade a coastal sea province under hostile control while they are stationary and have no other orders.
Each point of blockade power will be able to blockade one development point of a province.
Blockades will only take effect if there is enough blockade power to match the full development. If a fleet does not have enough blockade power to blockade all adjacent provinces under hostile control, it will assign its blockade power in precedence of:
- Hostile province under a siege lead by any friendly force
- Hostile province with the lowest development
- Hostile rebel province with the lowest development
The detach blockade button () in the fleet panel will detach a fleet of sufficient size to blockade all enemy ports neighboring the sea province.
Blockade efficiency is determined by:
|+1%||for every point of navy tradition|
Ideas and policies:
Decisions and events:
|+20%||Improved Blockade Tactics||Maritime ideas event: Improved Blockade Tactics||for 10 years.|
|−25%||Blockade Standards Slipping||Maritime ideas event: Slipping Standards
||for 10 years.|
Effects of blockade
A blockade has a number of effects on any neighbouring land provinces occupied by an enemy:
|+20%||Local recruitment time|
|+20%||Local shipbuilding time|
|−50%||Local goods produced modifier|
|−75%||Local trade power|
Blockading a country's ports also applies the following modifier, scaled by proportion of (core, state) development blockaded:
|+0.10||Monthly war exhaustion|
Blockades have the following further effects:
- Siege: −2 penalty to die rolls in non-blockaded coastal provinces, except those owned by primitives. (Provinces without ports, e.g. on the Caspian Sea, don't count.)
- War score: Blockades are worth war score scaling with the proportion of development they represent and local autonomy. Capital and fortified provinces are worth more. Some casus belli have blockades as a war goal.
- Spoils of War: Blockades add ducats to the blockader's treasury (listed under "Spoils of War") in proportion to the province's income.
- Restriction of movement: See § Blocking a strait below.
Blocking a strait
To block a strait the following requirements must be met:
- The blockading alliance or a neutral nation have to control (owns or occupied) at least one side of the adjacent provinces
- The blockading alliance have to control the sea by a naval force (blockade efficiency doesn't matter)
As a consequence, if a war alliance controls both sides of the strait they can march over regardless of blockades.
- A war participant controlling both sides of a strait: The Ottomans can cross the strait at Bosphorus as long as they have control of Constantinople and Kocaeli even when the enemy controls the sea. Same system applies for the second strait between Edirne and Biga.
- A neutral nation controlling a side of a strait: Ulster, controlling Ulaidh, cannot cross the strait across the Irish Sea to Ayrshire in Scotland if Ulster is only at war with England (Scotland is neutral and gives military access to one of the war participants), and England controls the Irish Sea.
- However, if Ulster has called in an ally, say Burgundy, and Burgundy is in a separate war with Scotland and controls Ayrshire, then the first requirement is no longer fulfilled, and the Irish nation can cross the strait despite England having control of the Irish Sea.
Besieging a province of a strait does not fulfil the first requirement; the siege must finish, translating to control.
- Protect Trade
- Hunt Pirates
- Hunt Enemy Fleets
- Blockade Enemy Ports
- Intercept enemy fleets
This mission is available to any fleet that contains light ships. It will add the trade power of the light ships in the fleet to any maritime trade node that the player already have some trade power in, provided that the trade node is within both trade range and supply range.
The trade power of light ships depends on ship model, and increases as diplomatic technology advances. Furthermore, it scales linearly with the naval maintenance slider, reaching full potential at maximum naval maintenance, but suffering a −75% penalty at minimum naval maintenance. AI nations do not suffer a trade power penalty to light ships from low naval maintenance.
|Available only with Wealth of Nations, El Dorado, Mare Nostrum or Golden Century DLC enabled.|
This mission is available to any fleet that contains light ships. The fleet will hoist the jolly rogers and add the trade power of its light ships to a pirate nation in any selected trade node. Unlike the protect trade mission, existing trade power is not needed in a trade node to start a privateer mission.
Light ships on privateer missions receive a +50% bonus to their trade power. Privateers do not inherit any national bonuses or penalties to global trade power. Factors such as overextension or halved trade power when collecting in foreign nodes are notably ignored. The trade power of privateers is instead affected by various privateer efficiency modifiers:
The Enlist Privateers decision increases privateer efficiency by another +25%. It becomes available at 8 Diplomatic technology and requires the nation to have completed Maritime ideas, a ruler with 3 Military, as well as 50% of the force limit.
Each point of naval tradition also increases privateer efficiency by 0.25%. The total trade power from privateers is thus
Privateer fleets can only reach trade nodes in sea zones that are within trade range. A nation's trade range extends from cored provinces of the nation or its subjects, as well as coastal ports of any nation that has granted it fleet basing rights.
Trade range can be increased by diplomatic technology, decisions, ideas, and policies:
The privateer fleet increases the trade power of a pirate nation that acts as a collector in the trade node. This pirate nation will submit 40% of its earnings to the nations that commissioned the privateers contributing to its trade power. The presence of this pirate nation effectively reduces the share of trade value controlled by all non-pirate nations in the node. Income from privateering will be recorded in the economy window as spoils of war.
Any country that would have more than 20% of the trade power in the node without privateers will receive a casus belli against any nation that has sent privateers to that node.
Any country that would have more than 10% of the trade power in the node without privateers will have a −1 opinion modifier per month of any nation that has send privateers to that node, capped at −100.
Sending light ships on privateer missions against rivals is one of the easiest ways to acquire power projection. This can be especially useful to increase the power projection of the player beyond +25 for +1 free leader, or beyond +50 for +1 / +1 / +1 each month.
Activating a privateer mission will force the game to update a nation's power projection on the next game day. This will consequently update, among other things, the trade power in trade nodes and the nation's military force limits. Triggering an update like this can be useful when game stats are temporarily desynchronized due to certain actions, such as sending light ships to protect trade or changing local autonomy levels.)
|Available only with the Mare Nostrum DLC or the El Dorado DLC enabled.|
This mission is available to fleets which contain at least one ship that isn't a transport. Fleets containing heavy ships or light ships may hunt pirates in any non-inland trade node; however, fleets containing only galleys (as well as fleets which are a mixture of galleys and transports) can only hunt pirates in nodes where all nearby sea provinces are inland seas, e.g. in Basra and Alexandria, but not in Hormuz. This does not necessarily mean that fleets will only hunt in inland sea provinces (where they are more effective): for instance, a fleet hunting pirates in Canton will use the Luzon Strait.
The total number of guns of the ships is used to find the efficiency at which they hunt the pirates. The trade power of pirates is reduced by hunt pirate efficiency. 0%, when no fleet is hunting pirates, up to 99%, if the total number of guns from the hunting fleet is greater than or equal twice the total number of guns from pirating fleet.
Note that pirate fleets and fleets hunting pirates do not actually battle or otherwise damage each other, unless the countries that own the fleets are at war with one another.
|Available only with the El Dorado DLC enabled.|
This mission requires an explorer, and a total of 3 light and/or heavy ships. It sends the fleet to explore a region or a coast line, that can be chosen by either selection the region on the UI or clicking on the naval TI with the "explore" tab opened, and the player can attempt to circumnavigate the globe at diplomatic technology 9.
- Main article: Colonization#Discovery
Hunt enemy fleet
|Available only with the Mare Nostrum DLC enabled.|
Blockade enemy ports
|Available only with the Mare Nostrum DLC enabled.|
The fleet will blockade enemy ports and evade enemy fleets that are not significantly weaker. (Can be altered by setting the fleet to be Bold.)
- Main article: Naval warfare#Blockading
Intercept enemy fleets
|Available only with the Mare Nostrum DLC enabled.|
Strategies and tactics
Due to the lack of stacking penalty, optimal strategic deployment in terms of combat results follows decisive battle doctrine strictly - concentrate all ships in one fleet, seek battle if one has superior naval assets, and hide otherwise. Naval admirals can significantly tilt a battle in one's favor, even more so than generals; without an admiral, stack wiping and sometimes even victory becomes unlikely even with seemingly overwhelming advantage.
However, other factors may complicate this. A country may be forced to divide their fleet if they have naval objectives other than destruction or suppression of the enemy navy, such as protecting (multiple) trade nodes, transporting land units by sea, exploring, patrolling for pirates, reconnaissance, blockading and so forth. If the country lacks naval supremacy compared to their enemies, they may be forced to risk losses, support fewer of these objectives at the same time, or leave the sea entirely for the duration of the war.
Bringing an enemy fleet to battle
A country with a naval advantage may wish to bring a reluctant enemy fleet to battle. This can be accomplished in several ways:
- If the enemy fleet is docked at a port, they cannot be attacked directly. However, the fleet may be ejected by occupying (successfully sieging) that province, at which point the leaving fleet will enter into combat with any opposing fleet(s) in the corresponding sea province.
- A small "bait" fleet may be used to lure the enemy into attacking, whereupon the main fleet can reinforce from a nearby sea province or port. This can also be used to lure fleets into or out of inland seas, where galleys have a relative advantage.
- Light ships move faster than heavy ships / transports, which move faster than galleys. A fleet without galleys can easily catch an enemy fleet that contains galleys.
- Putting your ships into port will usually cause the AI navy to be more aggressive and leave port, sometimes even attempting a full blockade, leaving their navy significantly more vulnerable.
- After a defeat, an enemy navy will typically retreat to an adjacent port. Having an army already sieging down this port will give you a significant advantage, as the ships will be forced to leave port with low morale and no repairs, allowing for a stackwipe or a victory in which they lose most of their ships.
- Sometimes a bug will allow the AI remain permanently in a province even if it is sieged down. The only way to fix this is to retreat several sea tiles from the province and then go into port, or in other words give the ai an illusion of safety.
- See in GALLEY_BONUS_INLAND_SEA. :