THAC0 on the High Seas!

As a Dungeons & Dragons (D&D) player, you already know how thrilling it is to introduce fresh challenges, environments, and twists into your campaigns. One particularly exciting idea is crafting a THAC0 system tailored specifically for a ship-based adventure. Yes, THAC0 — that classic mechanic from earlier editions of D&D — can be reimagined to add flavor and realism to battles on the high seas or spacefaring vessels.

THAC0, or To Hit Armor Class 0, was once a popular way to determine whether an attack lands. The lower your THAC0, the easier it is to hit your target. Imagine, then, how much more dynamic and immersive your game becomes when this mechanic accounts for the unique conditions of life aboard a ship.

Here are some ideas to help you design a ship-themed THAC0 that challenges your players and makes combat feel more authentic:

1. Factor in the Ship’s Movement and Stability

Ships—whether they’re sailing the high seas or cruising through space—are inherently unpredictable. Waves, strong winds, or sudden maneuvers can cause the vessel to sway, tilt, or lurch unexpectedly. How does this affect your combat mechanics?

Dynamic Accuracy: When the ship is rocking and rolling, ranged attacks and spellcasting become harder. For example, if the ship is pitching like a roller coaster, you might impose a penalty to attack rolls—say, +2 or +4—to represent the difficulty of aiming accurately while the deck shifts beneath your feet.
Balancing and Positioning: Characters fighting on a moving platform might have to succeed on a Dexterity check to maintain their footing before attacking. If they fail, they could suffer disadvantage or a penalty to hit.
Stabilization: Conversely, if the ship is stabilized—perhaps by magic or technology—attackers gain bonuses. This dynamic makes the environment feel alive and unpredictable.
- Example:
  Suppose the ship is caught in a storm, causing it to pitch and roll. You might decide that attack rolls for ranged attacks and spells are made with disadvantage if the ship is heavily swaying.
  For instance: During a violent storm, a fighter aiming at a goblin archer on the deck has a normal attack roll of +5. Due to the storm, you call for a Wisdom (Perception) check against a DC 15 to see if they can stabilize their aim. If they fail, they roll with disadvantage, making it much harder to land hits.
- Example:
  When the ship is steady—say, docked or in calm seas—attackers gain a +1 bonus to hit, representing the steadiness that makes aiming easier.

2. Incorporate Weather Conditions

Weather isn’t just a flavor text; it’s a real challenge that can turn the tide of battle. Fog, rain, snow, or storms can obscure vision, dampen spells, or make surfaces treacherous.

Visibility Penalties: In fog or heavy rain, impose disadvantage on ranged attacks or spellcasting that relies on sight. Conversely, in clear weather, attacks are easier, perhaps even granting a bonus.
Environmental Interference: Lightning storms might cause spells to misfire or become harder to control, while strong winds could push projectiles off course.
Temperature Effects: Freezing conditions could cause characters to have disadvantage on attack rolls or require Constitution checks to avoid penalties from cold exposure.
- Example:
  It’s a foggy dawn, and visibility is severely limited. You might state that all ranged attacks are made with disadvantage unless the attacker has a perception or investigation check at DC 15 to spot targets clearly.
  Conversely, in bright sunlight, attacks might gain a +1 bonus because targets are easier to see.
- Example:
  During a lightning storm, spells that require concentration or precise timing could have their attack rolls increased by +2, reflecting the chaotic environment interfering with precision.

3. Leverage Obstacles and Terrain

A ship is a maze of obstacles—rigging, railings, masts, barrels, and other debris—all of which influence combat.

Cover and Line of Sight: Use the environment to provide cover bonuses or impose cover penalties. For example, firing through rigging or behind a barrel might give enemies +2 AC or impose disadvantage on attacks.
Movement Limitations: Slippery decks, narrow walkways, or cramped quarters can restrict movement. Characters might need to succeed on Dexterity saves to avoid slipping or tripping, which could lead to missed attacks or disadvantage.
Strategic Positioning: Encourage players to take advantage of high ground on masts or the ship’s rails, which could grant attack bonuses or advantage.
- Example:
  An archer fires from behind a barrel, which grants the target total cover (+2 AC). Shooting through the rigging might impose disadvantage because the shot is partially obstructed.
- Example:
  A character tries to run across a slippery deck after a rainstorm. You call for a Dexterity saving throw (DC 15). If they succeed, they maintain their footing and attack normally. If they fail, they might have disadvantage on attack rolls or even fall prone, which imposes disadvantage on their next attack.

4. Add Environmental Factors

Beyond terrain, the environment itself can interfere with combat in exciting ways:

Explosions and Collapsing Structures: Fire, collapsing masts, or damage from cannon fire can create hazards that impose additional difficulty on attack rolls or spellcasting.
Fire and Heat: Heat from fires or explosions might cause disadvantage on attacks or force saves against heat exhaustion.
Ice and Slippery Surfaces: An icy deck makes footing uncertain, possibly requiring Dexterity saving throws to avoid falling or losing precision.
Sea or Space Conditions: In space, vacuum exposure or zero gravity could influence attack accuracy and spellcasting, adding a whole new layer of complexity.
- Example:
  An explosion from a cannon or a fire on deck creates a chaotic environment. Characters near the blast might need to make a Dexterity saving throw (DC 16) to avoid being knocked prone or taking damage, which could affect their ability to attack effectively.
- Example:
  An icy deck makes footing uncertain. Characters might need to make a Dexterity saving throw (DC 15) to avoid slipping and losing their attack bonus for the round, or even dropping their weapon.

5. Create a Custom THAC0 System for Your Ship Adventure

To bring all these elements together, you might develop a custom THAC0 that fluctuates depending on the environment:

Base THAC0: Start with your usual character attack bonus.
Environmental Modifiers: Add or subtract based on ship movement, weather, obstacles, and environmental hazards.
Dynamic Adjustments: For example, a character’s THAC0 might be +2 during stormy weather, +1 for slipping on a wet deck, or -3 when firing from a swaying mast.
- Example Calculation:
  During a storm with fog, the fighter’s THAC0 is effectively 15 + 2 (storm) + 4 (fog penalty) = 21.
  To hit an AC 15 target, the player needs to roll a 21 or higher on a d20 + attack bonus. Since the maximum roll is 20, they automatically miss in this scenario—unless they have a special ability or magic that overcomes the environmental effects.
- Another Example:
  On a calm, sunny day, the same fighter’s THAC0 remains at 15. They only need to roll a 10 or higher (since 10 + 5 attack bonus = 15) to hit an AC 15 target. Much easier!

You could even make it more dynamic by having the DM call for perception checks or saving throws to see if characters can compensate for environmental chaos—adding a layer of realism and immersion.

Final Thoughts

Designing a THAC0 for a ship-bound campaign is more than just a technical exercise; it’s a way to make your game more tactile, challenging, and memorable. When you weave in the ship’s movement, weather, obstacles, and environment, combat becomes an unpredictable dance—one that keeps everyone on their toes and deeply engaged.

As a Dungeon Master, your goal is to challenge and immerse your players in worlds they’ll never forget. Creating a tailored THAC0 system for your ship adventures does just that—pushing them to think creatively, adapt quickly, and truly feel like they’re fighting for their lives on the high seas or the void of space.

So, hoist the sails, brace for the storm, and prepare to make your next campaign the most dynamic yet!