Hearts of Iron IV Naval Guide: Building a Fleet That Wins

Naval warfare in Hearts of Iron IV has a reputation as a wall of numbers that eats your whole evening. It's not that bad once you know the handful of rules the combat actually runs on. Ships line up in rows, guns can only reach so far, and most of winning is picking the right hull for the right job instead of chasing some perfect design. This runs through the ship roles, the templates worth building, the mission and doctrine setup, invasions, and the submarine problem, with the numbers that matter attached to each.

Ship Types and Their Roles in a Fleet

The rows, and why they exist

Every surface battle lines up the same way. Screens in front, capital ships behind them, carriers and convoys at the back, submarines underneath. What a ship can shoot at depends entirely on where it stands.

Light guns only hit the closest non-empty row. Heavy guns reach the first two rows. Torpedoes can target anything, but the screening line blocks them: a torpedo's chance of reaching the enemy capitals is 100% minus their screening efficiency, and hitting the carrier group behind that takes a second check against their carrier screening. That's the whole game in three sentences. Light attack tears down their screen wall, heavy guns pound their capitals, torpedoes finish things once the wall cracks.

Destroyers

Destroyers are the lightest screen and their strength is numbers. They form the bulk of the screening line, they're the cheapest hull worth building, and they burn very little fuel. They also have the lowest hit profile of any surface ship, so the enemy misses them more than anything else in the water. Convoy escort duty is their second job, and both jobs want quantity over quality.

Cruisers

A light cruiser is a screen. A heavy cruiser is a capital. Same hull, different guns, and the guns decide which row the ship stands in. That's the real light cruiser vs heavy cruiser question, not firepower - every heavy cruiser you add raises your screening bill by three destroyers, while every light cruiser pays part of that bill itself.

Light cruisers are tougher than destroyers and more flexible. Load one with float planes and radar and it becomes your best scout. Load it with light cruiser batteries and it murders other screens. Heavy cruisers carry heavy guns that reach the second row, where the enemy capitals live. They also carry the lightest armor of any capital ship, so they don't take punches well.

Battleships, battlecruisers, super heavies

Battleships are the armored core - expensive, slow to build, able to absorb a beating from the enemy battle line. Battlecruisers share the same hull with lighter armor, so they cost less, sail faster, and are slightly harder to hit, at the price of folding faster when focused. Super heavy battleships sit on a single dedicated hull tech with no later versions, and they stay dangerous the entire game. They're also priced like it.

Carriers

Carriers attack from hour zero of a battle, while your screens have to wait 8 hours to fire and your capitals 6. Their planes sortie every 6 hours after that. That opening window is where carriers earn their cost - a big carrier force can break the other side's organization before their guns ever speak.

The catch is that a carrier is really two production lines. You're building the hull and an air wing to put on it, plus the tech for both. Sortie efficiency starts at only 50%, so half your deck sits idle until doctrines and bonuses push it up. Carriers pay off on top of a working fleet, not instead of one.

Submarines and convoys

Submarines are hidden by default and can only be damaged by depth charges, and only while revealed. Section five covers them properly. Convoys are barely ships at all - 0.25 light gun and 0.1 anti-air between them, so they're more or less defenseless. One merciful quirk: any convoy that survives a battle damaged comes out of it fully repaired, no dockyard time.

Designing Effective Ship Templates

Hit profile, the stat nobody reads

Hit profile decides how often a ship gets hit. The formula is 100 times visibility, divided by half the ship's speed plus 20. A 1936 light cruiser at 15 surface visibility and 32 knots lands around 42.

Weapons have profiles too:

| Weapon | Hit profile | | --- | --- | | Light guns | 45 | | Heavy guns | 70 | | Torpedoes | 100 | | Depth charges | 100 |

A weapon takes accuracy penalties against any ship whose profile is smaller than its own. So light guns struggle against sleek fast hulls, torpedoes struggle against nearly everything that isn't a fat convoy, and big slow ships eat everything. The practical version: roughly 4% more speed works out to about 1% more effective HP, and 1% less visibility is worth about 1.1%. Speed and low visibility are armor you don't pay IC for.

There's also a floor under all of this - hit chance can never drop below 2%, no matter how badly outmatched the shooter is. Every gun in a big fleet lands something. That's a quiet argument for hull count over quality.

The strike destroyer

This is the workhorse. Light attack to shred the enemy screening line, torpedoes for what happens after. The logic is that torpedoes do nothing at all to a fleet screened at 100%, and a great deal to one that's started losing destroyers - so the guns open the door and the torpedoes walk through it.

Fill the slots with the best light battery or dual-purpose battery the tech allows (dual-purpose gives you anti-air from the same slot, which is nice when carrier planes show up), a torpedo launcher mount, the best engine, and fire control or radar if the research is done. Skip the armor plating. Armor on a destroyer costs hulls, and hulls are the point.

The escort destroyer

A different ship for a different job. Sonar, one depth charge mount, a single gun, the cheap engine. Depth charges are the only weapon in the game that can touch a submarine, so if a fleet has no depth charges anywhere, it literally cannot sink a sub.

Keep these out of your strike fleet. Sub detection is the average across every ship in the task force, so mixing sonar-equipped escorts with gun destroyers drags the average down and makes both worse. Two fleets, two designs.

Cruisers: the piercing check

Light cruiser batteries give light attack against the closest row. Heavy cruiser batteries give heavy attack with piercing against the first two rows. Whether that heavy attack is worth anything comes down to piercing versus armor:

| Piercing vs armor | Damage | Crit chance | | --- | --- | --- | | 2x or more | 1x | 2x | | 1x to 2x | 1x | 1x | | Below 0.1x | 0.1x | none |

A heavy cruiser that can't pierce what it's shooting at is a very expensive screen that doesn't screen. Torpedoes skip all of this - they ignore armor entirely, which is a big part of why they end fights.

Spare slots and the treaty

Fire control, radar, and damage control techs are cheap boosts to everything above and worth researching early. One more thing to check before you design anything: if you're playing the US, UK, France, Italy, or Japan, the London Naval Treaty caps production cost at 9,500 for carriers, 5,300 for cruisers, and 10,000 for heavy ships. It lapses on its own once two majors are at war or world tension hits 75%, or you can eat the political power cost and leave (25 for fascists, 150 for everyone else).

Setting Up Convoy Escort and Strike Force Doctrines

What each mission pays you

Missions contribute very different amounts of naval supremacy, and supremacy is what unlocks invasions and holds sea zones:

| Mission | Share of supremacy | | --- | --- | | Strike force | 100% | | Naval invasion support | 100% | | Patrol | 75% | | Convoy escort | 50% | | Convoy raiding | 25% | | Minelaying / minesweeping | 25% | | Exercises / hold | 0% |

Anything set to do-not-engage contributes nothing at all. Air superiority over a region adds up to +100%, making each ship count double, so fighters over the sea zone are often the cheapest supremacy you'll ever buy.

Convoy escort, done right

Escort efficiency is the number that matters here. At 100%, a task force instantly jumps to the defense of any convoy under attack in the fleet's regions. Below it, help arrives late, and late help means dead convoys.

Here's the useful part: escort efficiency is a pure quantity check. The weakest destroyer you can design contributes exactly as much as a battleship. Screening efficiency works the same way. So the right escort fleet is a pile of cheap hulls spread across exactly the regions your convoys actually use - check the routes in the strategic navy map mode (F2) and assign regions to match, rather than blanketing the ocean.

The AI runs its raiders on medium-risk engagement rules, and subs set that way tend to flee from destroyers rather than fight them. Your escorts mostly shoo the wolves away, and in single player that's usually enough.

Strike force, done right

Capitals belong on strike force. They sit in port, burn almost no fuel, and still project 100% supremacy across every assigned region. Your patrols find the enemy; the strike force sorties out to the fight. Capitals on fuel-hungry patrol duty is one of the most common ways players quietly starve their own navy.

Bring the screens with them. 100% screening efficiency needs 3 screens per capital and per carrier, plus half a screen per convoy in the battle, and carriers need 1 capital per carrier plus a quarter per convoy for full carrier screening. Build past those numbers - losses during a fight drop you under 100% mid-battle, and bad positioning (up to -50% at zero) can double the requirement. Holding 100% screening is worth up to +40% hit chance and +20% retreat speed on your capitals, and +10% sortie efficiency on carriers, so the bonus is real money.

Turn on automatic reinforcement and automatic split-off for repairs. A limping battleship sailing home alone is a gift to the other side.

Doctrines after the rework

The old doctrine tree is gone - patch 1.17 replaced it with one mutually exclusive grand doctrine plus four tracks (submarines, screens, capitals, carriers), each holding one active subdoctrine, with mastery milestones that unlock as that ship type actually fights.

| Grand doctrine | Headline bonus | | --- | --- | | Fleet in Being | +10% capital armor, +15 battleship org | | Trade Interdiction | +15% convoy raiding efficiency, +10 sub org | | Base Strike | +40% port strikes, +20 carrier org |

Fleet in Being is the default answer for a gun navy and stays decent even with carriers along. In the screens track, Convoy Escort is the escort pick: +10% escort efficiency off the top, then destroyer sub detection and anti-air milestones as mastery builds. Hunter Killer Groups is the anti-submarine option - +10% fleet coordination, +15% destroyer sub detection, +10% depth charges - but it sits behind a DLC, as do several others in the tree. A doctrine tree with gaps in it isn't a bug; some subdoctrines need DLC or special projects.

Two cheap wins while you're in these menus. Pride of the Fleet gives +5% war support and the ship itself takes critical hits less often and gains experience faster - but if it sinks you eat -10% war support for 30 days, so put it on something durable. And run naval exercises in peacetime: ships gain experience up to Regular at only 60% fuel cost, and that experience is what buys your doctrines.

Naval Invasions Step by Step

1. Research transport tech. The division cap per invasion order starts at 4 and the plan cap starts at 1, and both climb with naval transport tech. The 1940 Landing Craft tech is the big one - it cuts preparation time roughly in half and unlocks floating harbors.
2. Draw the order and read the arrow. Supremacy is checked in every strategic region the arrow passes through, not just the landing zone. A landing that looks safe because you own the beach will refuse to launch over one contested transit region.
3. Let it prepare. Prep runs about seven days per division at base tech, so a ten-division landing is a months-long project. Plan it in peacetime. Note that unlike land fronts, the planning bonus doesn't accumulate for invasions - the timer gates readiness, it doesn't grow strength.
4. Fix your intel. You need over 50% supremacy in each region, and supremacy itself needs at least 40% naval intel efficiency to count. At 10% intel or below, your supremacy is halved. Radar coverage gives up to +20%, and the enemy's own settings leak more: a democracy or free-trade nation hands you 20% for free, a fascist closed economy gives you almost nothing.
5. Count your convoys. Shipping cost is weight-based: 0.1 per support company, 0.5 per infantry-type battalion, 0.8 for lights and mechanized, 1.0 for medium tanks, 1.25 for heavies, 1.5 for superheavies, rounded up. Armor eats transport capacity, and convoys already committed to trade and supply can't carry troops. This is the most common silent failure.
6. Launch the moment it's ready. Supremacy is only checked at the instant of launch - once the boats are away, the invasion proceeds even if you lose the sea right after. Don't sit on a finished plan waiting for conditions to improve; the defender is improving too.
7. Support the beach. Put a fleet on naval invasion support: it escorts the troop convoys, then parks off the coast for shore bombardment. Each point of heavy gun damage is -0.05% to enemy attack and defense, light guns give -0.025% per point, capped at -33%. Close air support in range stacks on top.
8. Take a port within 72 hours. Landing troops fight at -50% attack and breakthrough by default, and while they're still on the transports they get +50% breakthrough but a brutal -80% attack, fading over roughly 48 hours as they get off the boats. Marines, engineers, and amphibious gear soften the penalty. If you haven't taken a port or major city within 72 hours, supply runs out and the landing dies. A floating harbor drops a temporary supply hub right on the beach if you have the DLC and the tech.

If the arrow just won't fill, work the checklist in order: transit region supremacy, convoys, division cap, prep timer, intel. One of those five is always the reason.

Countering Submarine Warfare

The uncomfortable truth about defending

A hidden submarine cannot be targeted at all. That's the whole problem.

A sub that starts a battle on the defending side gets revealed for 16 hours - but here's the catch, ships escorting convoys never trigger that reveal. And passive in-battle detection is close to dead: even a task force averaging 45 sub detection, which is about the ceiling for dedicated anti-submarine destroyers, against an outdated hull with 25 sub visibility, only gets roughly a 0.46% reveal chance per hour. That works out to about six days of continuous battle to find one boat.

Depth charges do literally nothing to a sub that isn't revealed. So convoy escorts protect convoys by being present - their numbers hold escort and screening efficiency up - not by hunting. Expecting your escort destroyers to rack up sub kills is expecting the game to work differently than it does.

Attack the hunters instead

The advantage sits with whoever attacks. A raiding sub that gets spotted and forced into a battle is revealed for those 16 hours. Better yet, every time a sub fires its torpedoes at a task force with any sub detection at all, there's a 3.5% chance it reveals itself, and once revealed it needs 20 hours before it can hide again. Every attack is a roll of the dice against the boat.

So don't chase subs around the ocean. Watch where your convoys are actually getting hit, then push dedicated anti-submarine patrol and strike forces into those specific regions and force the fights. The raiders have to keep shooting to do their job, and shooting is what exposes them.

Building the hunter group

The hulls are the escort destroyer design from earlier: sonar, one depth charge mount, a gun, cheap engine. Keep them in their own task force - sub detection averages across every ship, so a single gun cruiser with no sonar in the group waters down the whole pack. A spotting cruiser with float planes and radar makes a good partner: it patrols, it finds, the depth charge group does the killing. The Hunter Killer Groups subdoctrine (+15% destroyer sub detection, +10% depth charges) is the doctrine fit if you have access to it.

Air power is the best sub killer

Outside of battle, submarines are particularly vulnerable to patrolling aircraft. Naval bombers with decent range over your convoy lanes will spot and sink more subs than your destroyers ever will. There's a quirk where aerial sub detection contributes nothing to the in-battle passive reveal chance, but that only affects fights already underway - planes finding and killing subs on their own works fine.

Two finishing moves

Mine the approaches. Regions hold up to 1,000 mines, and a mined region gives the enemy up to +50% invasion penalty and up to +5% accident chance. The AI barely sweeps, so mines are close to free advantage in single player - and old subs with minelaying tubes on never-engage are great at laying them.

And if you're the one running subs: medium-risk rules only let them attack capitals, high-risk is needed to attack screens, and raiding task forces only cover about 1.5 regions efficiently each, so spread small packs rather than stacking one big flotilla. Snorkels and later torpedo techs cut the reveal chance on every shot.

The cheapest lesson in the whole naval game is probably the escort efficiency one. That bare-bones destroyer you rushed out in 1936 contributes exactly as much to protecting a convoy as a battleship does, so the roach hulls never stop being useful - build them early, build a lot of them, and half of this guide takes care of itself.