Adding additional hanger modules gives this US carrier the ability to carry more planes at the cost of increased production cost and slower overall speed

Previously, we examined air power and its role as a force multiplier for your military. This time we are diving into the depths of naval power.

Naval Power in Hearts of Iron 4 is all about power projection. With sea dominance, you can threaten invasions, choke off trade, and/or protect your own lines of supply and communication. 

While in the game, sea power isn’t quite as crucial as its real-life counterpart, it still plays a vital role and cannot be ignored. If playing as Great Britain or another maritime power, it is the most important aspect of the game. 

In this part of the tutorial, we will discover what it takes to build and use a world-beating navy. We will look at the different ship types, the ship designer (assuming you have the Man the Guns DLC), mission explanations, a stat breakdown, as well as an overview of how naval combat works in the game. 

Planning: What do I want my Navy to Do?

He who commands the sea commands everything.

Themistocles, Athenian politician and naval strategist

Naval strategy is different from land and air strategy for one reason; building ships takes a very long time! Most Hearts of Iron 4 games start in 1936. This means you only have a little more than 3 years before the war breaks out if playing on historical. You must make a sober assessment of what your country’s naval requirements and capabilities are and get to work implementing them straight away. To do this most effectively, you need to know how to construct a navy.

Taken from the HOI4 wiki, you can see how long the build times are for capital ships. Planning is crucial!
Taken from the HOI4 wiki, you can see how long the build times are for capital ships. Planning is crucial!

Ship types

* If you own the Man the Guns DLC, the capabilities of all ships is mostly determined by the modules you use on the ship designer.

There are three basic types of ships in HOI4: capital ships, screening ships, and submarines. Let’s look first at capital ships. Capital ships are designed to meet the enemy ships in combat and sink them. 

Super-Heavy Battleship (SHBB) – These are truly monsters of the waves. SHBB’s have serious firepower and armor protection. No other surface ship could hope to compete with them directly; however, they are still vulnerable to torpedoes and planes. The cost to build them is also massive requiring many resources and tying up dockyards for several years. They are also useful for shore bombardment. More on that later.

The Japanese super heavy battleship Yamato is simply a beast on the waves.
The Japanese super-heavy battleship Yamato is simply a beast on the waves.

Battleship (BB) – Battleships are powerful ships in their own right. While not as strong as SHBB’s, they take significantly fewer resources and time to build. Like their bigger brothers, they are powerful but slow and vulnerable to torpedoes and planes. They are also useful for shore bombardment. 

Battlecruiser (BC) – Battlecruisers are the faster, smaller cousins of battleships. They keep most of the firepower of a BB, but trade armor protection for speed. The idea being that ‘they can outrun anything they can’t outshoot and outshoot anything they can’t outrun.’ Their lower armor means they can survive fewer hits, but their higher speed means that they are harder to hit in the first place. 

Heavy Cruiser (CA) – Sometimes referred to as ‘the poor man’s battleship,’ heavy cruisers can still mount heavy batteries that are moderately powerful. They have the lightest armor of any capital ship, which reduces their survivability in combat against other capital ships. However, their speed is quite high which makes them difficult to hit. They are significantly cheaper and faster to build than other capital ships. 

Carrier (CV) – Aircraft carriers in HOI4 are a bit of an enigma. In real life, they absolutely dominated the seas after 1941, but in game, they are effective but not dominant. The strength of a CV depends on the planes it carries and how many of them. We will talk about Carrier Task Force (CTF) composition later in the fleet composition section. 

The ship itself has moderate speed for a capital ship, and light armor. Its build cost and time are less than BB’s but more than CA’s. 

Adding additional hanger modules gives this US carrier the ability to carry more planes at the cost of increased production cost and slower overall speed
Adding additional hanger modules gives this US carrier the ability to carry more planes at the cost of increased production cost and slower overall speed

Now we move on to screening ships of which there are only two types. The primary role of screening ships is to protect capital ships from torpedo attacks and to attempt to penetrate the enemy ‘screen’ to attack their capital ships. 

Light Cruiser (LC) – Light cruisers are a very versatile ship and one of my personal favorites. They have more survivability than their destroyer counterparts but much more attack and armor. They can be equipped with a plane catapult, which makes them excellent scouts for a fleet. While more expensive to build than destroyers, they are still quite cheap relative to capital ships. 

Destroyers (DD) – Destroyers are the smallest, fastest, cheapest direct combat ship that can be built. They have low attack and armor but very high speed which makes them quite difficult to hit. If you load them up with torpedoes, they can actually make for a quite threatening ship that can punch above its weight. They are also critical for combat against submarines with sub detection and sub attack. 

Submarines (SS) – Submarines are not screening ships nor capital ships but rather fall into their own category. They are slow, cheap, and, potentially, lethal. Useful against enemy convoys and unscreened ships, submarines rely on stealth to attack from the depths. If detected, they will quickly be destroyed. However, if they can remain undetected by the enemy, they can do some serious damage with their torpedoes. 

Convoys – Convoys in HOI4 are a generic one-size fits all ship type for logistics ships. In the game, convoys are used to transport resources, supplies, and troops across the seas. They have negligible attack and air defense and can’t be used in combat. The game will automatically pull and return convoy ships to your national stockpile as needed to fulfil your logistics.  


Heavy & Light Attack – The more attack your ships have, the more damage they will do to enemy ships. Heavy attack refers to heavy gun batteries and is better at targeting larger ships. Light attack is for light gun batteries and is better at targeting smaller ships. 

Heavy & Light Piercing – Similar to the piercing concept for tanks, ships use piercing vs. armor values to determine if an attack ‘goes through’ or not. Unlike tanks, the system for naval combat is not binary. If an attack has less piercing than the enemy ship has armor, you will still do damage, just at a reduced scale.

Armor – As referred to above, your ship’s armor rate is how difficult it is for enemy attacks to pierce it and deal full damage. Even if armor is not pierced, damage will still be taken just a reduced percentage. 

Torpedo Attack – This stat is only for ships that have torpedo launchers. Torpedo attack can be devastating because it completely ignores the armor rating of a ship. However, to hit an enemy ship, it must first pass through the torpedo screen. This is why proper screening is so important in fleet composition. 

Depth Charges – The only way for ships to attack and damage enemy submarines is with depth charges. The higher this stat, the more damage you can do to subs. Keep in mind that in order to use depth charges, you must first locate the enemy submarine. You can’t attack what you can’t detect!

Anti-Air – The AA stat represents how much damage your ship can inflict against enemy planes that are attacking it. By destroying planes before they can attack, you are effectively increasing your ship’s defense.

Your ships can only fire at enemy planes if they are being bombed whether by the naval strike or the port strike missions. 

Speed – Speed is critical to your ships’ design. It affects how difficult your ship is to hit. It also determines how effectively your fleet can close with or retreat from an enemy fleet. The faster the ship, the better regardless of type. 

Organization – As in land combat, ships have organization that when exhausted, forces them to retreat from battle. The higher your ship’s organization, the longer it can fight. Organization for different ship types is increased through naval doctrines. Ships with low organization also suffer combat penalties. 

HP – HP is a measure of how much damage your ship can take before being sunk. Once HP reaches 0, your ship sinks. Bigger ships like BB’s naturally have more HP than smaller ones like CL’s or DD’s. 

Reliability – Representing having a secure magazine or lack of fuel leaks, reliability affects your ship’s survival chances in combat. Every time an enemy ship scores a hit against your own it has a chance to score a ‘critical hit.’ Critical hits do much more damage as the name implies and can be devastating. The higher your ship’s reliability, the less likely hits turn into critical hits. 

Deck Size – Only for Carries, deck size is simply the number of planes that can be based on it. The larger the deck size, the higher number of planes that your carrier can use and thus increase its offensive and defensive capabilities. 

Visibility (Surface & Sub) – Visibility is a measure of how easy your ship is to detect. Lower numbers are better here. Remember that enemy fleets and planes (land or carrier based) cannot engage you if they can’t detect you and vice versa. Ships with higher visibility are also easier to hit than ships with low visibility. 

Detection (Surface & Sub) – The opposite of visibility, detection is the ability of your own ship to locate enemy ships and submarines. Only once a fleet or ship is detected, can an engagement begin. Do remember that surface and sub detection are unrelated values meaning that a ship that has a high surface detection stat may not be able to detect submarines. To detect subs, you will need ships that have a high sub detection stat. 

Fuel Usage – Depending on the nation you are playing as, fuel usage might be critically important when designing ships (Hello Japan!) or an afterthought (Hello USA!). Larger ships use more fuel than smaller ones with the largest consuming a significant amount. 

Naval Missions

As with the air force, your navy has specific missions it can conduct. Unlike the air force, most ships can perform most missions. Having said that, there are certain ships better suited for specific missions than others. Knowing how missions function and which ships perform them best will help you effectively operate your navy. 

With all missions, you need to set the rules of engagement for each fleet. ‘Always engage’ all the way down to ‘never engage’ will dictate how aggressive your fleet is and at what strength ratios it is willing to engage at. 

Naval Exercises – Similar to planes, ships can perform naval exercises to gain experience (up to regular). They will only consume 80% of the fuel they would normally during an actual mission. Unlike planes, ships running naval exercises will grant your nation a fairly substantial amount of naval experience which you can use to upgrade designs, speed up research etc. 

On the other hand, running naval exercises increases the chance of naval accidents. Unlike planes, ships are not cheap to build and an accident could tie up some of your dockyards for a while to perform repairs. 

Patrol – This mission has your ships sailing around a sea zone looking for enemy fleets. You will ideally want ships with good surface/sub detection and low fuel usage. If they find an enemy fleet, your other fleets can join battle along with any air wings you may have dedicated to naval strike in the sea zone. 

Strike Force – This mission works by having a fleet sitting in port on stand-by ready to sally out to a specific sea zone and engage if an enemy fleet is detected. This is beneficial for two reasons. First, it keeps your ships from burning precious fuel, as they are mostly just sitting in port. Second, it allows them to project naval dominance score to a sea zone, without actually having to do anything.  

Convoy Raiding – Ships set to convoy raiding will actively hunt down and attempt to intercept enemy convoys. When running this mission, keep in mind that the AI is now intelligent enough to recognize where its convoys are taking losses and switch up their route when possible. As such, you may occasionally need to switch regions making convoy raiding a deadly game of cat and mouse.

Convoy Escort – The opposite of convoy raiding, convoy escort has your ships sailing alongside your convoys attempting to ward off any attacks against them. 

When running convoy escort you need to keep escort efficiency in mind. If the number of sea zones is too large for your limited number of ships, you will suffer escort efficiency penalties. The higher these penalties, the longer it will take your escort forces to assist convoys in battle when they are attacked. You can increase escort efficiency by increasing the number of ships dedicated to convoy escort. 

Minelaying – If your ships have the mine rails or mine tubes modules, they can perform the minelaying mission. In HOI4 mines give you several benefits. In a given region, they increase your naval supremacy score, decrease enemy ship speed, and increase enemy ship accident chance. 

Mines can only be laid if your country is at war. If you have certain techs researched, it is also possible to lay mines using planes. 

Axis forces have laid a considerable number of mines in this sea zone which will give the Royal Navy problems if it wants to operate there without minesweeping first.
Axis forces have laid a considerable number of mines in this sea zone which will give the Royal Navy problems if it wants to operate there without minesweeping first.

Minesweeping – The counter to minelaying is minesweeping which removes enemy mines from a given sea zone Only ships equipped with minesweeping gear can perform this mission. It is also possible to conduct aerial minesweeping if the right techs have been researched. 

Naval Invasion Support – Ships running this mission will escort your troop transport ships all the way to invasion site. Once there, there will be naval bombardment as long as an active battle occurs. If no battle is happening, they will return to port once your troops have landed. 

Shore Bombardment – While not actually a naval mission, shore bombardment is nevertheless a key task that your navy can perform. Any fleet that is stationary next to coastal tiles will perform shore bombardment against enemy forces if a battle is happening there. 

Each point of heavy attack in the fleet will give a -0.1% attack penalty to the enemy while each point of light attack will give a 0.05% penalty. The cap for this is -25%. 


Naval Combat: Where Metal Meets Math?

Naval combat in HOI4 is a complicated matter. There is quite a bit of math involved that is not really in the scope of this guide. Rather, this guide seeks to give you a basic understanding of how the mechanics work. If you wish to see the exact formulas used, here is a link to the wiki.

Spotting & Positioning

As previously stated, before any naval battle can take place, one fleet needs to spot another one. Once a fleet is spotted, then combat begins.

The defending fleet always starts with perfect positioning while the attacking fleet’s positioning depends on their fleet composition and their admiral. Having a high positioning score decreases the amount of screening ships needed to effectively screen your capital ships. 

The storm makes it difficult for the Royal Navy destroyers to spot the Italian submarines. Notice the -20% to detection penalty.


In HOI4, there are four ‘battle lines’ for ships. The screening line, the capital line, the carrier line, and the submarine line. Each line is filled with different types of ships and tries to protect the line behind it. 

Ships in the screening line are called screens and consist of destroyers and light cruisers. They will attempt to protect the capital line from torpedo attack from the enemy screening ships. If the enemy has insufficient screens, they will attempt to attack their capital line with torpedoes. 

Next up is the capital line. Battleships, battlecruisers, and heavy cruisers are the ships that make up this line. They will attempt to use their heavy guns to attack the enemy capital line. They can use their light guns to attack the enemy screening line as well. Finally, they will attempt to protect any carriers your fleet might have from the enemy fleet’s gunfire. 

The last main battle line is the carrier line. Here there are only aircraft carriers. They will attempt to use their carrier air wings from long range to bomb the enemy fleet. If the enemy capital ships or screening ships can manage to get into range, however, it will not be good for your carriers! Protect them with your own capital line and screening line.

Finally, there is the submarine battle line, although perhaps it is better not to think of it as a line. If submarines remain undetected, they can strike anywhere at the enemy battle fleet. Thus, it is important to have sufficient screening for all of your capital ships. 


It is vital to ensure that your valuable capital ships are screened properly. For capital ships a ratio of four screens to one capital is almost always enough to ensure adequate screening. Carriers will need four screening ships and one capital ship to be properly screened.

Ships without adequate screening are much easier to hit by lighter ships and torpedoes. Ships that are screened properly receive combat bonuses. 

Weapons & Targeting

There are four basic weapon types that ships can be equipped with: heavy guns, light guns, torpedoes, and depth charges. They all follow different rules when it comes to which ships they can target. Each shot has a base 10% chance to hit its target. The target’s visibility score also affects how easy it is to be hit with lower visibility ships being more difficult to hit. 

Depth charges – Depth charges are the only weapon type that can target a submarine and even then only when it has been revealed. Only available to equip on destroyers and light cruisers. 

Light guns – Light guns can only target ships in the closest battle line. Mainly equipped on screening ships but can also be added to capitals. 

Heavy guns – These guns can only be equipped on capital ships, but they can target enemy ships in the screening line and the capital line. 

Torpedoes – Torpedoes can do a lot of damage but rely on the enemy having poor screening to target ships in the capital line or the carrier line. 


Damage done depends on the Attack stat of the attacking ship. It also depends on whether the defending ship’s armor was pierced or not. If armor is not pierced, damage will be reduced but not eliminated. 

Critical hits can also be scored, which cause much more damage and have a chance to disable some of the key systems of a ship. Torpedoes have a 20% chance of each hit becoming a critical hit whereas all other attacks have a 10% chance. 


Planes launched from carriers or from land can attack every 8 hours. The amount of planes that can attack is based off the amount of HP in the enemy fleet. However, a minimum of 20 planes are always allowed to attack. 

Before planes can attack enemy ships, they must evade the ship’s AA. Planes with higher agility will have a better chance to avoid being hit. 

The amount of damage planes do is based off their naval attack stat. Their naval targeting stat determines how likely they are to land a hit.

More information about naval bombers can be found in part 3 of the HoI 4 tutorial Air Power here

The famous Japanese Zero. Notice it has the small carrier icon, which means it can be based on aircraft carriers. Planes without this icon cannot be used on aircraft carriers.

Carrier Stacking Penalty

There is a limit to how many carriers you can include in any single fleet before it starts to suffer stacking penalties. Each carrier over four increases the stacking penalty by 20%. The stacking penalty is not a combat penalty but rather a limit to the number of planes that can sortie out. 

However, this only applies to bombers and not fighters; therefore, many players add additional carriers after four but only fill them with fighters.  

Only carrier plane variants can be based on carriers, so be sure to research and build them if you want to include carriers in your fleets. 

Naval Supremacy & Naval Invasions

Indirectly related to naval combat is naval supremacy. Similar to air superiority, it is a tug of war score between your naval presence in a given sea region compared to the enemy. It is determined by the combat potential of all the ships that have assigned missions in a particular sea zone. 

Remember that ships on the ‘strike force’ mission still contribute to this score even though they are not physically present in the region.

Britannia rules the waves! Color marked sea zones make it easy to see where you have naval supremacy. Remember that naval invasions cannot be conducted unless you have the minimum naval supremacy score.

To execute a naval invasion, you will need to have at least 50% naval supremacy in the sea zones along the invasion route up to the target. You will have to use the battle planner (land warfare) to plan the naval invasion and then wait until the plan is ready to execute. It will also require a certain amount of convoy ships depending on how many divisions you have assigned and how long the invasion route is. 

After achieving the required naval supremacy, the British have decided to go for an early D-Day. Notice the planning time. The operation can only be launched after the 49 days of preparation have been completed.

That is naval warfare in Hearts of Iron 4. From planning and construction to tactics and battle, you now have the knowledge required to dominate the high seas. Be sure to check out the final part of the tutorial series, which deals with diplomacy and espionage. (Link to future part 5 here)

Go to the first part of the HoI 4 tutorial: Economy
Second Part: HoI4 tutorial Land Combat
Third Part: HoI4 tutorial Air Power

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