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 it comes down to one number that changes every second the shell is in the air.

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 values are:

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

Out of those inputs comes one number that decides every AP duel Plymouth fights: penetration at impact, in millimeters of effective belt armor. And that number is not fixed. It is highest at the muzzle and falls the whole way downrange.

The penetration formula

WoWS uses the canonical community penetration model. Once a shell strikes the plate, the game compares the plate's normalized thickness against:

Pen = 0.5561613 × (Krupp / 2400) × v^1.1 × m^0.55 ÷ D^0.65

Pen = penetration (mm) · Krupp = krupp coefficient · v = impact velocity (m/s) · m = shell mass (kg) · D = caliber (mm)

Three of those four inputs never change: Krupp, mass, and caliber are fixed properties of the shell. Only impact velocity moves, and penetration tracks it through the v^1.1 term. Slow the shell down and penetration falls almost in step. Plug Plymouth's numbers in at the muzzle and you get roughly 330 mm of penetration. By the time the shell has flown 1.5 km it has already dropped to about 293 mm. So the real question is: how fast does the shell slow down?

1. Velocity decay (drag)

The shell bleeds speed every meter it travels, to air resistance. GamingDiver computes this the way the game does: a step-by-step numerical integration of the shell's trajectory under pure quadratic drag (drag force rising with the square of speed) through a standard-atmosphere air-density profile. There is no tidy closed-form shortcut. The trajectory is simulated.

For Plymouth, the impact velocity falls from 841 m/s at the muzzle to roughly 324 m/s at her 15.2 km max range. Feed that into the formula and penetration collapses from ~330 mm to about 115 mm. Same gun, same shell, same target: penetration has dropped by roughly two thirds, purely from travel.

2. Impact angle (the silent killer)

The formula above gives penetration against a plate 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. Plymouth's shells arrive almost flat up close (about 1° at 1.5 km) but plunge at roughly 26° at her max range. The steeper the arc, the more the shell strikes deck armor 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 shows 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 penetration 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 penetration curve onto real targets. A citadel hit requires three things at once: the shell penetrates the belt armor, the impact angle is inside the ricochet envelope, and the fuse arms and detonates inside the protected volume. Plymouth's fuse threshold is 25 mm, so anything above destroyer plating arms her shell. Her working penetration is roughly 222 to 293 mm close in, 153 to 177 mm at mid-range, and about 115 mm at max range.

Target	Tier & class	Belt	Close (≤5 km)	Mid (8–10 km)	Far (15 km)
Cleveland	VII US light cruiser	127 mm	Yes	Yes	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	Yes	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. Plymouth's close-range penetration (~293 mm) lands just under the 305 to 307 mm belts of Hood and Iowa. Normalization, turtleback armor, and any angle the target shows tip the result decisively toward bounce or shatter. Do not plan your game around edge cases. Plan it around the Yes column.

What this means for playing Plymouth

Citadel cruisers, and do it before mid-range slips away

Plymouth keeps real citadel penetration deeper into the map than her reputation suggests. Inside about 6 km she carries 220 mm or more, and even at 8 km she still has roughly 175 mm, enough to citadel every 76 to 152 mm cruiser belt in her bracket. With four quad turrets she can land twelve barrels on a target at a sane angle. A side-on Cleveland, Atago, Edinburgh, or Baltimore inside 10 km eats 6,000 to 9,000 damage per AP salvo, because every shell that lands gets the chance to citadel. Beyond about 12 km the citadel game thins out fast as penetration drops under most cruiser belts.

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 is not. Battleship superstructures are typically armored at 13 to 25 mm (Hood, Tirpitz, and Iowa all sit around 19 mm; Yamato around 25 mm), and Plymouth's AP retains about 115 mm of penetration even at her 15 km max range. That is far more than enough to slice clean through a superstructure, arm the fuse, and detonate for full normal-penetration damage.

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 full-penetration hits per salvo. It is not citadel damage, but it is stable, repeatable damage that does not depend on the target broadsiding for you.

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 do not need to run the model in your head. GamingDiver's per-ship spec pages show AP Pen Close (mm) and AP Pen Far (mm) under each ship's Main Battery formulas, and the Shell Ballistics Chart plots the full penetration curve versus range for any ship. Both are computed from the same model described here, so they always agree. Compare your gun's penetration at range against the target's known belt thickness and you will know which fights are AP fights before you commit a salvo.

Penetration uses the canonical community WoWS model 0.5561613 · Krupp/2400 · v^1.1 · m^0.55 ÷ D^0.65, with impact velocity taken from a numerical trajectory integration (pure quadratic drag applied along the velocity vector through a standard-atmosphere density profile), the model verified near-identical to the in-game ballistics by the jcw780/wows_shell project. Penetration constants are PC-calibrated and not yet verified against in-game World of Warships: Legends penetration values, so treat the curves as close trend estimates. Shell inputs (caliber, krupp, mass, drag, muzzle velocity) and ricochet/fuse angles are read from each ship's in-game data. Belt-armor figures are community-known WoWs Legends values.