RPG Design – d20 Naval Combat

War of Sail

Image by brianjmatis via Flickr

Now that we have the basics of what we want to simulate down, we need a quick d20 system to manage naval combat. This isn’t going to look quick, since I’m going to cover the logic as I go along – but the actual rules part should fit on just a few pages.

The commanders are going to need Profession/Ship Handling and either Knowledge/History (in basic d20, where tactics isn’t a formal study) or Knowledge/Tactics (in d20 modern, where it is). They’ll use the lower of those two skills for most of their combat maneuver rolls.

  • If a commander is doing his or her own steering (using Piloting in the modern setting or Profession/Pilot in 3.0 and 3.5) they’ll be using the lowest bonus of the three – but giving directions to a dedicated pilot while commanding is actually easier than doing it all yourself; as long as the actual pilots skill bonus is at least one-half the bonus the commander is using, there’s no penalty on the commander’s rolls. D20 games often don’t include any form of “leadership” or “command” skill, so we’ll let that go by the wayside – and presume that any high level leader will command the respect and obedience of his or her crew without much difficulty.

The ships themselves get three basic ratings to start with – Speed, Maneuver, and Crew. Those all range from Hopeless (-5), on through Inferior (-2), Average (+0), Superior (+2), and on up to Astounding (+5).

Yes, you can use special powers to enhance your ship and crew. No, you aren’t going to get them past the “Astounding” level; reaching that level implies that they’re already pretty nearly perfect.

Why such a narrow range? It’s enough to be decisive between commanders of relatively equal skill – barring some wildly lucky flukes. On the other hand, these are game rules, and high-level characters are supposed to be able to pull off incredible stunts. Thus a sufficiently skilled commander – say, a fifteenth level ship captain versus some first and second level beginners – can
overcome massive handicaps.

On the other hand, if there are enough opponents to let some of them use “aid another” rolls rather than attacking directly, even a high level commander with a good ship and crew may be in trouble.

So for some common rolls…

  • Keeping the ship afloat in a bad storm, getting it prepared for a long voyage, or making sure that all the repairs are good calls for a Ship Handling + (2x) Ship Crew check. In situations like this, a good crew can often get along without a commander, while a bad one can overwhelm almost any
  • Evading a storm, making a high-speed trip, or outrunning a pursuer will call for a Speed check, made using Piloting + Ship Crew + Ship Speed. If you’re being pursued by another ship, this is an opposed check – and the winner gets to shift things by one range band. Given that a map introduces more complications than we want, the basic ranges here are boarding range, combat range, out of combat range, and lost to sight.
  • Passing through a narrow gap in a reef or trying to steer closely enough to a floating object in bad weather to pick it up, calls for a Maneuver check – Piloting + Ship Crew + Maneuver.

Ship-to-Ship Combat calls for a more complicated check:

  • Roll opposed Maneuver checks, using the lowest of Ship Handling, Tactics, or Piloting* + Ship Crew + Maneuver. The winner may select any option he qualifies for from the following chart. For example, if he or she wins by eight, he or she may select Double Fire, Evasive Maneuver, Victor Fires First, Exchange of Fire – or stall with No Opportunity.

*If the Commander is doing his or her own piloting, use 2x Piloting if the Commander is issuing orders to a dedicated ship pilot.

Result Possible Options Effect
Any No Opportunity Neither ship gets to fire.
0 Exchange of Fire Both ships may both fire from any one arc, determining who shoots slightly quicker at random.
1-3 Victor Fires First Winner fires first during an Exchange of Fire.
4-6 Evasive Maneuver Winner may move out of combat range.
7-9 Double Fire Winner may attack with any one arc, receive fire from any one arc, and fire again with an adjacent arc.
10-12 Close to Board Winner may move in to board or ram at the cost of taking fire from both a primary and a secondary arc.
13-15 Selective Fire Winner may fire from any one arc and choose the arc from which the opponent may fire back.
16-18 Close to Board Winner may move in to board or ram at the cost of taking fire from a primary arc.
19-21 Driven Opponent Winner may force the opponent to choose between entering a hazardous area (if any are available) or receiving fire without a chance to retaliate. Note that, if the opponent is unaware of the hazard, he or she enters it automatically.
22-24 Swift Attack Winner may fire from any one arc. The opponent does not get to fire.
25-27 Evasive Strike Winner may fire from any one arc and move out of combat range without allowing the opponent to fire.
28-30 Double Fire Winner may fire from any two arcs without allowing the opponent a chance to respond.
31-33 Close to Board Winner may move in to board or ram at the cost of taking fire from a secondary arc.

For actual combat, we’ll need another set of ratings – in this case, for how much armament a ship carries on each firing arc. For that, add up the number of major weapons a ship carries in each arc. The rating will vary from Minimal (1-3 weapons), through Modest (5-8 weapons), Military (9-16 weapons), Massive (17-32 weapons), and Capital (33+ major weapons).

  • Attacks rolls are made by the Gunnery Officer, normally using (BAB + Crew Modifier) – albeit at a -4 penalty unless he or she has Weapon Proficiency / Gunnery.
  • If the Gunnery Officer qualifies for additional attacks, and has additional targets, he or she can direct fire into additional arcs – or the ship can deploy additional Gunnery Officers.
  • For damage, use the damage rating of the basic weapon for the arc – usually some sort of cannon (there’s a list of such weapons over here) – and a the following tables plus a d6 roll to determine the number of special damage effects that the attack inflicts.
  • Critical hits do not do extra damage; they indicate that one of the shots that hit from a barrage hit a player character for normal damage.
  • A ships damage reduction is multiplied by the same factor as the damage it takes; the multiplier does represent additional hits after all.
Minimal Modest Military Massive Capital
Damage x1 x2 x3 x4 x5
d6 Roll
1 0 0 0 0 0
2 0 0 1 1 1
3 0 1 1 2 2
4 0 1 2 2 3
5 1 2 2 3 4
6 2 2 3 4 5


Special Damage Effects are determined with d12’s – and come in five levels; level one is annoying, two puts a ship at a notable disadvantage, level three is serious, level four is crippling, and level five will usually put a ship out of the fight – if it doesn’t destroy it outright. It is possible for someone to hit the magazine and blow up your ship in a single round – but it’s rare unless
you’re fighting a fleet.

1) Cargo Damage Levels:
Food stores destroyed (or lost overboard, ruined, contaminated, etcetera. Unusable anyway). You’ll be hungry in days, dead in weeks.
Actual cargo destroyed. This includes things like spare rope and canvas, uniforms, and other gear.
Water stores destroyed. You’ll be thirsty in hours, dead in days.
Ammunition destroyed. Weapons fail in 1d6 rounds due to lack of ammunition.
Ammunition detonation. Weapons fail for lack of ammunition. Roll three more special damage effects thanks to explosion, wild magic, energy surges, or whatever.


2-3) Drive/Power Damage Levels:
Speed -2.
Speed -5.
Speed -10.
Drifting. Drives are irreparable. Vessel must be towed in and rebuilt – or


4) Fire/Radiation Leak/Magical Surge Damage Levels:
Minor. 1 in 6 each turn to upgrade to…
Dangerous. 1 in 6 each turn to upgrade to…
Major. 1 in 6 each turn to upgrade to…
Catastrophic. 1 in 6 each turn to upgrade to…
Ship destroyed and irreparable.


5) Instrumentation Damage Levels: These may be selected at random if the GM wishes.
Communications Systems. Whether it’s flags and signal lamps, running lights, or hyper-radio, you’re not going to be getting information across to anyone for the moment.
Sensors. Whether it’s a sounding line and a crow’s nest or a multiphasic FTL sensor array (whatever that is), you’re down to your own, personal, senses.
Environmental. Whether it’s the hole to measure how much water is in the bilges, the sensors that look for toxins, or the weird little animal that senses hostile intent, you no longer have any way to gather environmental information beyond your own senses.
Library. Whether it’s maps and charts or a mystical intelligence that lives in a crystal ball, you’ll just have to rely on your own brain for awhile.
Navigation. These range from sextants on through complex electronics, but the effect’s the same; there’s nothing left aboard to help you decide where you are.


6-7) Crew/Passenger Damage Levels. Note that a player character may opt to take a crew hit personally for it’s base damage.
Reduce crew quality by one level. If this takes it below “hopeless” the ship cannot be operated.
Reduce crew quality by two levels.
Reduce crew quality by three levels.
Reduce crew quality by four levels.
Crew out of action, ship inoperable.


8) Steering System Damage Levels:
Maneuver -2
Maneuver -5
Maneuver -10
No Maneuver roll.
No Maneuver roll. Field repairs are impossible.


9-10) Environmental System Damage Levels:
Leaks. If not fixed, 1 in 6 each turn to upgrade to…
Patch and Pump. -1 to Effective crew rating, if not fixed, 1 in 6 each turn to upgrade to…
Blocked Off. Various areas are unreachable due to water or vacuum in
the access ways. If not fixed, 1 in 6 each turn to upgrade to…
Progressive Environmental Failures. If not fixed, 1 in 6 each turn to
upgrade to…
Total Environmental Failure. Ship sinks, loses atmosphere, or otherwise
becomes unusable. Field repairs are impossible.


11) Weapons System Damage Levels:
Reduce all weapons ratings by one level.
Weapons failure in one primary arc
Weapons failure in one secondary arc.
Weapons failure in remaining primary arc.
Weapons failure in remaining secondary arc.


12) Wreckage Damage Levels:
Blocked doors and hatches. Various areas are only accessible with great
Blocked access ways. Some areas simply cannot be reached at all, preventing the use of some special functions – the medical area, or the galley, or the mine-laying gear, or some such.
Entrapment. -1 to the effective crew rating until you clear the wreckage and let them out of wherever they’re stuck.
Repairs Blocked by wreckage. No other repair can be attempted until this
problem is repaired.
Ship breaks up, in whole or part. Field repairs are impossible.

The crew – and any player characters who happen to be unoccupied by ship duties or participating in the fight more directly – can attempt some damage control each ship turn. While that’s subject to the special damage notes below, it’s otherwise fairly straightforward:

  • Repairing level one or two damage is DC 25. Level three damage is DC 30, and level four damage is DC 35. Repairing structural damage is automatic, and repairs a number of points equal to the roll made.
  • An unassisted crew gets a bonus of (10 [mutual assistance] + Crew Rating).
  • A player character can try to fix things however he or she can.

Special Damage Notes:

  • Cargo damage cannot be repaired in the field; the vessel must restock from an appropriate source of supply. Player characters, of course, may have special methods, or might jury-rig a weapon or two somehow – but it can’t normally be done properly.
  • Successful fire damage control extinguishes the fire – but the damage is transferred elsewhere at random.
  • Instrumentation damage counts as level two damage for repair purposes and need not be repaired in order.
  • No more than two levels of crew damage may be “repaired” in the course of any one battle.
  • After (2 x Ship Size +2) successful repairs, things start getting harder; there’s only so much you can patch before you have to start doing things right – which requires hours or days, rather than battlefield stopgaps.

The important part about special damage results is that they offer a way to provide challenges for the characters: sure, you won the battle – but if you’re out of water, or canvas, or food, you have a problem. Sails or drive system damaged? Can you jury-rig something before your opponents get a chance to destroy the ship or you drift into a hazard? A raging fire? Time to heroically put it out! Crewmen are injured? How are you at healing?

Structural damage is the boring stuff; splintered decking, holes in non-critical walls, and general breakage. In most cases, the ships should have enough structural hit points to mean that it’s fairly difficult to smash them to bits via raw structural damage. No, that’s not entirely realistic; many a ship has been sunk by having a single torpedo or some such blow a gaping hole in it – but that’s not much fun to play. You can keep the possibility in the game if you like however; simply count a massive torpedo or similar attack as a “Capital”-class threat all by itself and apply all it’s special damage effects to a single random area. That will let a single unexpected torpedo blow up a ship – but keeps it relatively rare.

In any case, it’s usually best to let the player characters use ships with fairly high structural ratings and decent hardness levels. Being blown out of the water by a single barrage is boring – and a route to a total party kill. As a rule, it should take at least nine or ten attacks by a similar vessel to blow a ship out of the water – which means that capital-class ship duels will often be decided by the (average) twenty-five levels of special damage effects, rather than being settled by simply blowing the enemy ship into a scattering of debris.

Thus if your First-Rate Ship of the Line is using forty-four 6d6 Light Artillery Pieces as it’s broadside, then it’s damage is (6d6 x 5) – an average of 105 points. Fortunately, the Hardness of – say – eight for copper-sheathed heavy oak is also multiplied by five, reducing the damage to some 65 points. Ergo, such a ship may reasonably have about 700 hit points.

Hm. Give it some mid-level officers – a roguish captain, a mage and a healer, and a fighter-type to serve as the gunnery officer, and we have a decent match for an ancient dragon. Oddly enough, that also seems pretty reasonable.

For a quick table of ships…

  • Level One (Sailing and Rowing)
    • Weaponry: Light Artillery, 6d6 damage. Includes common cannons, howitzers, anti-tank missiles, magical solar lenses, fireball ballistas, catapults, light missiles, and similar:
    • Hull Materials: Light Woods (Hardness 5), Hardwoods (Hardness 6), Dense Hardwoods (Hardness 7), and optional Metal Cladding (+1 Hardness).
    • Structural Hit Points: 120-160 x1 (minimal ships), x2 (modest ships), x3 (military ships), x4 (massive ships, or x5 (Capital ships).
  • Level Two (Sailing, Steam, and Supplementary Magic)
    • Weaponry: Heavy Artillery, 8d6 damage. Includes large cannon, ship mortar, trebuchet, fireball staves, torpedoes, cruise missiles, and similar.
    • Hull Materials: Iron Plating (Hardness 9), Steel or Dragonhide (Hardness 10), Magical or Psychic Tempering (+1 Hardness).
    • Structural Hit Points: 150-180 x1 (minimal ships), x2 (modest ships), x3 (military ships), x4 (massive ships, or x5 (Capital ships).
  • Level Three (Steam, Primary Engines, and Magical Drives)
    • Weaponry: Ultraheavy Artillery, 10d6 damage. Includes battleship main gun, heavy torpedo, storm cannon, etc.
    • Hull Materials: Alloy Steel (Hardness 12). There are lots of magical and psychic equivalents at this point, but they’re all roughly equivalent.
    • Structural Hit Points: 200-230 x1 (minimal ships), x2 (modest ships), x3 (military ships), x4 (massive ships, or x5 (Capital ships).
  • Level Four (Primary Engines, Nuclear Power, Extraordinary Magics)
    • Weaponry: Superheavy Artillery, 15d6 damage. Includes fusion cannon, antimatter beamer, disintegrator beams.
    • Hull Materials: Duralloy (Hardness 15) with force-field or magical reinforcements that drastically increase the amount of damage the ship can withstand.
    • Structural Hit Points: 330-370 x1 (minimal ships), x2 (modest ships), x3 (military ships), x4 (massive ships, or x5 (Capital ships).

Note that this system – and the special damage effects – gives even a weaker level one ship with a really good commander a (small) chance against a level four ship without one. No, it isn’t really reasonable for a level one sailing frigate to have any real chance against a modern level three destroyer – much less a vehicle out of Star Trek – but this is a game, and it features characters
who can pull off superhuman feats.

There now; that gives us a full range of ships and weapons from launches with a single light black powder cannon on up through massive interstellar battleships, a maneuvering system, a wide range of special problems to harass the player characters with, fires, explosions, a chance of being hit, and the responsibilities of command. That should do it for game purposes.