I am DMing a new campaign and its going to be based mostly on naval battles, and piracy. The issue I am having is that in the DMG (page 119) it sets out the types of ship available. However since they have no offensive stats it seems to me that they should be able to bring siege weapons on board. That being said the most reasonable thing I can imagine is that the restricting factor should be a combination of weight and space available.

Does anyone have any thoughts on how many siege weapons a ship can hold? I was able to find information on siege weapons (DMG page ~250) however it does not mention weight.

The only thing I have reasoned thus far is to use the weight as a hard limit. For example, a Keelboat can carry 0.5 tonnes (1100 lbs). Assuming the crew/passengers is not accounted for in this we could likely carry a few Balista or a Cannon on a single Keelboat.

Looking for people with actual experience running a Naval campaign, or with references backed by RAW (or other official sources).


3 Answers 3


There are some rules for this in Ghosts of Saltmarsh but they stick to non-gunpowder siege engines. Even in previous D&D versions, cannon were often left to third parties (the presence of cannon being oddly contentious amongst fantasy gamers).

I run a pirate-themed Pathfinder campaign (been going since 2009) where I faced this same issue. There's a lot of earlier ed third party rulesets for ships and naval combat, but many of them either:

  1. Completely eschew cannon for ballistae and magic stuff, or
  2. Are super high fantasy so everything's all skyships and gnomish submarines and whatnot, or
  3. Completely avoid statting and just treat ships as plot devices
  4. Toss historicity out and have "frigates" and stuff in an otherwise 1600's kind of world

I wanted something vaguely real-world realistic, though playable. If you're running an actual full time naval campaign, some detail in ship sizes, is that a 12 pounder or 9 pounder cannon, etc. are interesting and important. I'd say the Razor Coast: Fire As She Bears supplement is the best balance of these concerns if you're looking for a book (it's Pathfinder statted but should be easily convertible).

The cannon in the DMG, at 8d10 damage, would be a large usually land-based cannon, I'd pin that at a 32-pounder. Ship cannon will usually be smaller. Scale damage up and down the 3/6/9/12/24/32/48/64 pound range.

The main problem with firing cannon on a ship isn't the raw tonnage to carry the gun, it's the force from the gun ripping up your ship or tipping you over. So there's some hard limitations on what you can do.

So you can handwave as much as you want, but the semi-realistic answer for cannon on the 5e ships in the DMG is:

  1. Galley: Due to their design galleys can't side-mount cannon (it'd both destroy the oars and tip them over). They can mount one or more quite heavy ones lengthwise however, which in calm Mediterranean-like seas is a hardcore stand-off weapon. The "list" galley could probably do a single 24-pounder. Large mature RL galleys, probably double the stats for the one in the DMG, could do a 32-pounder with a couple 12-pounders flanking it. They'd also line the sides with swivel guns (small like one-pound guns on a swivel. Think 3d6 damage with a ball or 2d6 in a cone with shot).

  2. Keelboat: Swivel guns only, no proper cannon.

  3. Longship: No guns, just not designed for it.

  4. Rowboat: No guns, unless you want to swim home.

  5. Sailing ship: This is usually code for "you know, a small caravel like you done seen on the TV," Nina/Pinta/Santa Maria style. Probably a 60 foot long one, given that 100 tonnage value. Guns up on the top deck. Realistically, about 4 12-pounders per side max, maybe a 9 pound chase gun.

  6. Warship: Usually code for a carrack or galleon. This is where you get real gun decks going on (which allow for heavier guns, since they're farther down on the center of gravity). "As many as you want," based on the size of the ship. The Mary Rose, a 700 ton carrack (more than 100 feet long), had a dozen 42-pounders and a small complement of 32 and 18 pounders. Just scale up per 100 tons from the caravel.

I find this in general hits a sweet spot of dangerous but not too dangerous (cannon get a lot of penalties to hit generally). Of course it's part of a carefully balanced set of naval combat rules and such.

Now for other siege engines - ballistae don't have the same recoil problems, they're just kinda big. I'd say you could get about 1/3 more ballistae than you could cannon on a given ship. But same thing with the size class - a small boat might have a heavy crossbow but not really a ballista; maybe one could legitimately fit on a keelboat. Catapults can't really be used if you have sails and rigging, they went out with the galleys.

  • \$\begingroup\$ You don't mention dedicated guns to the brim ships. Frigates like the Constitution. Its nominal armament per side is 15 24 pounders and 10 32 pounders plus 2 24 pounders to the front. And with her 2200 tons displacement/1576 tons weight she is very fast at 13 knots with 'just' 300 feet length. \$\endgroup\$
    – Trish
    Commented Jun 6, 2019 at 17:21
  • 2
    \$\begingroup\$ That's because those are not from the medieval/renaissance era that most D&D settings are. As I mention in my answer, I intended to avoid the problem of "Toss[ing] historicity out and have "frigates" and stuff in an otherwise 1600's kind of world". \$\endgroup\$
    – mxyzplk
    Commented Jun 6, 2019 at 19:12
  • \$\begingroup\$ then something more early? Souvreign of the Sea, 1637, 1522 tons weight, 102 guns on launch, 20 of them 42 pounders. Great Henry, 1514, 1000 tons, 43 heavy canons and 141 swivel guns. Mary Rose, 1511, 500 tons, around 78-96 guns - some of them as large as 4 meters long. \$\endgroup\$
    – Trish
    Commented Jun 6, 2019 at 22:15
  • 3
    \$\begingroup\$ Feel free and contribute your own answer, I am not really in the mood to go add a bunch of stuff to a 2017 answer. I also feel that 100-gun ships aren't a real good fit for the standard adventuring-party model of an RPG so have little interest. \$\endgroup\$
    – mxyzplk
    Commented Jun 7, 2019 at 13:30

The Ghosts of Saltmarsh book offers several template ship types with complements of siege engines

\begin{array}{|c|c|c|} \hline Ship & Length & Ballistas & Mangonels \\ \hline Galley & 130& 4 & 2 \\ \hline Sailing Ship & 100 & 1 & 1 \\ \hline Keelboat & 60 & 1 & 0 \\ \hline \end{array}

The stats for the siege weapons come from chapter 8 of the DMG but additional upgrades for the siege weapons are listed on page 197 of GoSM which produce cannon-like effects. To wit:

The following upgrades can apply to any weapon mounted aboard a ship. A component can gain the benefits of one upgrade, or two upgrades if one of the upgrades is Arcane Artillery.


Explosive Rounds

Drawing on powerful evocation magic, this weapon's attacks are imbued with unstable energy that explodes in a fiery blast. When this weapon hits. it deals an extra 2d6 fire damage.

Using these as a guideline, it is possible to interpolate or extrapolate varying "classes" of ships based on their size and armor.


Likely 2-4 siege weapons for a large ship.

There is a Unearthed Arcana that provides more detailed rules for ships and sailing. For a pirate/naval themed campaign that may serve as a good resource for you in general, but for this specific question a Sailing ship is listed as carrying a Ballista and a Mangonel. A warship of similar size has two of each. Given that this is a ship specifically designed for ship-to-ship combat, it's unlikely that more could be effectively used, though certainly a ship could carry more as cargo, without worrying about firing lines, deck space, crew, etc.

A cannon weighs more than a ballista or even a mangonel most likely, but if you put one on the deck of a ship it would likely take up a similar amount of space, so a reasonable limit would be 4 for this as well. You could theoretically fit many more cannons onto a ship by putting them under the decks (either instead of or in addition to the top-deck cannons), but that would require advancements in cannon and sailing technology likely too advanced for a DnD setting. If cannons like those are common enough to have a dozen or more on every ship, it would stand to reason that they would be used on battlefields as well, making warfare look more like Napoleon than Lionhart.

On the other hand, if you skip cannons and stick to siege weapons, one thing you may have overlooked is archery. It was very common in ancient times for naval warfare to revolve around archers, with each ship essentially serving as a castle for archers to fire from. This could include fire arrows to try and light the other ships on fire, but more often it would simply mean hailing enemy ships with arrows to kill their crew. Then, once weakened, the ship could be boarded and the remaining crew killed or surrendered.

  • \$\begingroup\$ This answer has many assumptions and guesstimations that aren't backed up by either rules or expertise. \$\endgroup\$
    – Akixkisu
    Commented Jun 9, 2019 at 9:42
  • \$\begingroup\$ That UA is the basis for what eventually got adapted into Appendix A of Ghosts of Saltmarsh. \$\endgroup\$
    – V2Blast
    Commented Jun 9, 2019 at 11:42

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