Hearts of Iron 4 (HOI4) – Navy Guide

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Though most players tend to think only about land battles, naval warfare in Hearts of Iron 4 is also extremely important. Knowing how to control the seas is a huge part of the game, as you will otherwise never defeat countries like the United Kingdom or Japan.

However, there are many who have managed to win World War 2 many times and still have no idea how navies work. The main reason why nobody really knows how to use them is because they always get new updates that change how they operate.

Recommended Read: How to Increase World Tension in HOI4

Luckily, things seem to have settled down lately and the strategy to win using them is always the same, no matter what country you choose.

So, in this guide, we will talk about everything related to navies in Hearts of Iron 4: research, naval combat, ship types, how to set up a fleet, doctrines, and how to win every battle that your boats will participate in.

Table of Contents

We can’t talk about the navies in HOI4 before exploring and explaining the Naval Research category. The first thing you will find when opening the research tree will be a lot of hulls.

Hulls are the base of a ship. They are the base that will be used to put guns, engines, and many other important parts on the ships that you will design.

So, what you unlock here is just a big piece of a larger puzzle that won’t work until you put all the necessary upgrades on it.

The main things that evolve with each technological upgrade for a hull are speed, reliability, and range. The only problem is that their production cost will increase.

The upgrades under the Destroyer hulls that follow them with each upgrade are called Depth Charges, and they help in dealing damage to Submarines.

These will usually be very useful additions to convoy escorts that need to defeat the raiding Submarines destroying your whole supply of Convoys.

Under the Cruiser hull tech, you will find a list of Armor upgrades. Generally, avoid researching these unless you really want to make your Battleships extremely sturdy.

Their speed debuff will usually cause problems for the ships and the rewards are just not worth the cost.

A vital upgrade will be the one under Submarines, which are available around 1940, which lowers the visibility of subs. This will be extremely important, so get them as soon as possible if you use subs often.

The Naval support research category exists purely to make your ships better and naval combat to go smoother for your side.

The Armaments sub-category is the guns that you can put on your ships. All of the hulls you researched beforehand are useless unless you get some good batteries to put on them.

Torpedoes are special upgrades that will allow your ships to deal critical damage to Battleships and Carriers.

The Fire Control tech is extremely valuable, and you should always research it when possible since it just gives a flat bonus to all attack stats for all the fleets in your navy.

Transports will also be vital if you plan on doing naval invasions. Even if you think you’re never going to do them, it’s still a better idea to get the tech than to reach the moment when you need to use them and don’t have the necessary research.

The Mine Warfare upgrades are decent. You’ll likely only need the main line ones that increase mine damage since you don’t really want to waste slots on putting minelaying upgrades on your ships.

How Does Naval Combat Work in HOI4?

In HOI4, naval combat isn’t as common as land battles.

The main reason is that you will have to plan for them accordingly for a long time, and a big confrontation between fleets will usually result in almost total destruction of one of the sides.

Otherwise, you will spend most of the time fighting to destroy Convoys or to protect troops doing a naval landing.

When two enemy fleets find each other, they will start engaging. There are 4 lines where the ships can take their positions:

  • Screening Group
  • Battleline
  • Carrier Group
  • Submarine Wolfpack

When battle starts, based on their speed and who spotted who, the ships will quickly move to get in position while the battle starts.

The ships will attack each other for a while until one side realizes that it will lose, so they will attempt to retreat.

During the retreat phase, the losing army will lose most of its navy as the pursuing navy will do its best to leave no ship standing.


The Screening Group, also commonly known as Screens, are made out of Destroyers and Light Cruisers, and their main role is to protect all of the heavy hitters from enemy fire.

Since they’re fast and resistant (with the right design), they go in the front and bear the full force of the enemy fleet.

They usually have enough speed to retreat when they take too much damage, which allows other screens to take their place in the frontlines.

They work very much like a shield for the whole navy since otherwise, the torpedoes used by the enemy fleet would tear through the heavy ships.

There is a stat that tells you exactly how well the screens are doing their job, called Screening Efficiency. The main reason why Screening Efficiency is very important is because it directly lowers the chance that torpedoes will hit your backlines.

A torpedo has a 100% chance of hitting when it fires, which is lowered by the Screening Efficiency. Each Battleline and Cruiser Group ship will need at least 3 screens.

This means that a fleet with 3 Heavy Cruisers and 1 Carrier will need 12 screens to be fully protected from the menace known as torpedoes.

Generally, they are the most important part of any fleet, and since they are easy and quick to produce, most players only concentrate on building them throughout the game. This is usually the strategy for any single-player game since the starting fleet for powerful countries is more than enough.

Usually, the Destroyers exist to take the damage while the Light Cruisers fight the enemy’s screen and try to take it down to get to the weaker backline.

Screens usually use Light Guns, which deal damage to the first non-occupied line in front of them.


The Battleline is directly behind the Screening Group and their main role is to deal damage. They aren’t generally made to withstand attacks, and they are very vulnerable to torpedoes, which is why the Screens are so important.

Part of the Battleline are the Battleships and the Heavy Cruisers. Their main role is to deal the most amount of damage to both the Screens and the enemy Battleline, as they can get the largest amount of guns on top of them.

Battleline ships usually use Heavy Guns, which deal damage to the first two non-occupied lines in front of them. This normally means either the Screens and the Battleline or the Battleline and the Carriers.


The Carrier Group is the furthest away from the battle, with ships such as the Carriers and Convoys. The purpose of this line is to attack the ships without actually getting hit by the opposing navy.

Since Carriers usually hold a lot of planes, their purpose is to survive while the bombers on top of them deal heavy damage to all the ships that are taking part in the battle.

Because they’re the furthest from the battle, they are protected by both the Screens and the Battleline when it comes to torpedoes, which would deal incredible damage if they managed to get through.


The Subs have their own special line, which is under all of the others, that can be attacked using Depth Charges if you first manage to find them.

How to Set Up a Fleet

There is only one correct way to set up your fleet in HOI4, and that is to stack all of your ships into one single, huge task force.

Though the “smarter” way would be to make special fleets that each have its mission, you would generally be a lot better off by just putting all of your ships in a single giant fleet.

Since one battle in HOI4 will instantly decide if you’ve lost all your fleet or not, it’s better to make sure you can always win these kinds of fights.

Otherwise, you will likely get destroyed as a random fleet finds your smaller ones and instantly comes in full force to destroy you.

The only other fleets you might want to set up are convoy raider submarines since they are incredibly annoying and can destroy countries, and minelaying destroyers/subs, just to get an advantage in certain regions where you know you will have to fight at some point.

Otherwise, use a single ship to patrol the waters where you want total supremacy and set all of the deathstack fleet on the Strike Force command. This means that they will instantly join in the fight when someone finds enemy ships in the naval region.

Also, a very important piece of advice, never delete ships! Even if they’re the worst ships in the world, they will be very useful in the huge battle you will have with your main enemy.

Since the main goal of a navy is to win one single game-deciding battle, every ship will count.

In the worst case, you can just refit them to fit your needs better and find a use even for the early hulls pre-WW2.

There are three naval doctrines that you can choose from:

  • Fleet in Being
  • Trade Interdiction
  • Base Strike

The Fleet in Being doctrine focuses on making your ships better in battles. This means that they will get the necessary bonuses to excel in the occasional naval fights that happen in HOI4.

If you don’t have a reason to do something different, always choose this doctrine!

The Trade Interdiction doctrine is useful for harassing enemy countries’ Convoys. If you want to stop their supply of resources and likely ruin their land armies, Trade Interdiction is a really good choice.

However, this should only be chosen if you are a minor country without a real navy.

Never choose Base Strike. It helps you improve Carriers, which aren’t really the ships that win you battles. Just avoid this doctrine at all costs. It’s the equivalent to Mass Assault for ships.

How to Always Win Naval Battles

So, first of all, set up a huge fleet with all of your ships. There’s no other way to do this. Then, make sure to invest in the Fleet in Being doctrine. As specified earlier, this is the only one that will help you win wars.

If you started as a country with a great navy from the start (UK, US, France, Italy, or Japan) then you will only have to produce Destroyers and maybe Light Cruisers the whole game.

This way, you can make sure your huge fleet always has 100% Screen Efficiency and that it is protected in all battles.

Then, set up your fleet in three important naval regions on Strike Force. This will increase your intel in the region and give you naval supremacy, protecting you from any kind of annoying naval invasions.

Once you feel like you have enough intel in the region, you can send one or two destroyers to patrol in the region, trying to find enemy fleets.

This will usually result in them finding someone, moment when your huge fleet will come to destroy anything standing in their way.

Generally, it is best to avoid naval battles since you never know what can happen. The major powers all have huge navies with similar capabilities, and there is no way to predict how the battle will go. It’s basically a 50/50 toss.

Your main goal will always be capitulation, so try to launch naval invasions or just protect yourself from them.

You don’t even need to engage to do this, as the Strike Force command will assure naval supremacy while also generally avoiding battles.

That’s everything you need to know about how navies work in Hearts of Iron 4!

Have any input or suggestions for this guide? Let us know in the comment section below.

Adrian Oprea

Based in London, United Kingdom, Adrian Oprea is a Guides Writer. As a professional single-player RPG player, Adrian has often been stigmatized. He has decided to pour his frustration into writing guides!

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