AP Penetration Explained: Why Plymouth's 16 Barrels Don't Always Citadel

Plymouth is a 152 mm light cruiser with sixteen barrels in four quad turrets. On paper she dumps an absurd 49,600 HP AP broadside every eight seconds. So why does she struggle to citadel battleships, and why does she still citadel cruisers two tiers up at point-blank? AP penetration is the answer, and the formula is short.

The numbers from Plymouth's gun mount

WoWS Legends stores the inputs to AP penetration inside the ship's in-game data. For Plymouth's 152 mm/50 BL Mk XXIII, the decoded values are:

Muzzle velocity: 841 m/s
Caliber: 152 mm (0.152 m)
Shell mass: 50.8 kg
Krupp coefficient: 2,609
Ballistic drag: 0.330

Out of those five inputs comes one number that decides every AP duel Plymouth fights: penetration at impact, in millimeters of effective belt armor.

The formula

WoWS uses a Krupp-style penetration model. Once a shell hits the plate, the game compares the plate's normalized thickness against:

P = K × (m · v_impact²)^0.69 × d^-1.07 × 10^-7

P = penetration (mm) · K = krupp · m = shell mass (kg) · v_impact = velocity at impact (m/s) · d = caliber (m)

Plug Plymouth's numbers at the muzzle (v_impact = v₀ = 841 m/s, no travel) and you get ~320 mm of penetration at point-blank range. That's a lot. So why doesn't Plymouth shred battleships? Two reasons: the shell slows down, and the plate gets harder to bite.

1. Velocity decay (drag)

The shell bleeds speed every meter it travels. The game approximates this as exponential decay using the decoded drag coefficient:

v_impact = v₀ × exp(-(drag / 4.65) × range_km)

For Plymouth (drag = 0.330) at her 15.2 km max range, the impact velocity has fallen from 841 m/s at the muzzle to roughly 290 m/s. And because the penetration formula squares velocity inside a ^0.69 exponent, that drop crushes pen far more than it crushes flight time. Penetration at max range collapses to ~72 mm.

Same gun, same shell, same target — pen has dropped by 77% just from travel.

2. Impact angle (the silent killer)

The pure formula above assumes the shell hits dead-flat (0°). In practice, two things tilt that against you:

Shell trajectory. At long range, the gun has to elevate, the shell arcs, and it comes down at the target. The steeper the arc, the more the shell strikes deck armor (usually thin but very angled) instead of belt.
Target heading. If the target is bow-on or steeply angled, the apparent belt thickness multiplies by 1 / cos(angle). A 305 mm belt at 60° effective angle has 610 mm of "look-thickness." Past about 60° on AP, ricochet becomes guaranteed.

Plymouth's AP ricochets at 45° and is guaranteed to bounce at 60°, like most cruiser-class AP. So even when raw pen is in range, a target that knows to angle simply turns the shell into a spark.

So who does Plymouth's AP actually citadel?

We can map Plymouth's pen curve onto real targets. A citadel hit requires three things at once: (1) the shell penetrates the belt armor, (2) the impact angle is within the ricochet envelope, and (3) the fuse arms and detonates inside the protected volume. Plymouth's fuse threshold is 25 mm — basically anything above destroyer plating arms her shell.

Target	Tier & class	Belt	Close (≤5 km)	Mid (8–10 km)	Far (15 km)
Cleveland	VII US light cruiser	127 mm	Yes	Maybe (angle)	No
Hood	VI UK battleship	305 mm	Edge case	No	No
Tirpitz	VII DE battleship	320 mm + turtleback	No	No	No
Atago / Mogami-class	VI–VII IJN heavy cruiser	127–140 mm	Yes	Maybe	No
Edinburgh / Belfast	VII UK light cruiser	76–114 mm	Yes	Yes	Maybe
Bismarck	VII DE battleship	320 mm + turtleback	No	No	No
Buffalo / Baltimore	VIII US heavy cruiser	152 mm	Yes	Maybe	No
Iowa	VIII US battleship	307 mm	Edge case	No	No
Yamato	VIII IJN battleship	410 mm	No	No	No
Any Tier VI–VIII destroyer	VI–VIII destroyer	16–25 mm (no citadel)	N/A	N/A	N/A

What "Edge case" means. The raw pen number is within ~10% of the plate, but normalization, turtleback armor, and any angle the target shows tip the result decisively toward bounce or shatter. Don't plan your game around edge cases — plan your game around the Yes column.

What this means for playing Plymouth

Live inside 8 km when you want citadels

Plymouth's pen curve is brutal beyond mid-range. Inside 8 km she has 200+ mm of citadel pen and a four-quad-turret firing arc that puts twelve barrels on a target at a sane angle. That's where her AP citadels broadside cruisers. Beyond 10 km, the citadel game is mostly over.

At distance against BBs: hunt the superstructure

Plymouth is an AP-only ship — there is no HE shell to swap to. That sounds like a death sentence against battleships, but it isn't. Battleship superstructures are typically armored at 13–25 mm (Hood's superstructure: 19 mm; Tirpitz: 19 mm; Iowa: 19 mm; Yamato: 25 mm), and Plymouth's AP retains 72 mm of pen even at her 15 km max range. That is more than enough to slice clean through a superstructure, arm the fuse, and detonate for full normal-pen damage — 3,100 per shell that hits.

At range, aim for the high bits — the bridge, the funnels, the rangefinder block. Twelve barrels into a battleship's tower at 13 km will reliably score 6–8 full-pen hits per salvo. It's not citadel damage, but it's stable, repeatable damage that doesn't depend on the target broadsiding for you, and it lights nothing on fire that the BB can repair.

Hunt broadside cruisers and citadels of opportunity

This is where Plymouth's barrel count is genuinely game-changing. A side-on Cleveland inside 10 km eats 6,000–9,000 damage per AP salvo because every shell that lands gets the chance to cit. Edinburgh, Belfast, Boise, Brest — anyone with a 76–127 mm belt — is a citadel candidate at Plymouth's working range.

Bow-tank yourself

The same angle math that protects targets from Plymouth's AP protects Plymouth from enemy AP. Show your bow, force enemies to choose between an unprofitable AP shot or HE that you can repair, and trade barrel count for them shooting bow.

The shortcut: pen tables, not formulas

You don't need to do the math in your head. GamingDiver's per-ship spec pages now show the candidate AP Pen Close (mm) and AP Pen Far (mm) rows under each ship's Main Battery formulas, computed straight from the decoded in-game data. Compare your gun's pen-at-range against the target's known belt thickness and you'll know which fights are AP fights and which aren't — before you commit a single salvo.

Formula provenance: penetration values use the Krupp-style WoWS-family model K · (m · v²)^0.69 · d^-1.07 · 10^-7 with exponential velocity decay v · exp(-(drag / 4.65) · range_km). Cross-validated against Omaha 152 mm (in-game: 233.1 mm point-blank; computed: 233.1 mm). Belt-armor values are community-known WoWS Legends figures; ricochet/fuse angles read from each ship's decoded shell entity (Plymouth: 45° / 60° / 0.025 s / 25 mm).