I posted my prior article on firearms for D&D over on reddit, and a poster noted that while he thought this was all neat and stuff, he was much more likely to be interested in naval cannon.

That seems like a great idea, and I found a nice article online to base some initial stats from: Smooth Bore Cannon Ballistics.

The key data from that is the cross-section of cannon shot from 6-lbs to 32-lbs, which obviously includes shot weight, and also velocity. From that, it’s easy to convert to Kinetic Energy and Mass, which means a D&D damage conversion is somewhat trivial.

An a priori note – these are big, heavy projectiles. The critical range is going to be very large. If you get hit with one of these, it’s going to suck.

The Inputs

The primary inputs can be found in the article above. Taking those and just presenting the results, one gets the following table:

Shot Weight (lbs) Velocity (m/s) Dice Crit Range
1860 [3] 6 438 5d6+1 15-20
12 453 5d6+3 15-20
1862 [5] 18 524 6d6-1 15-20
24 524 6d6-1 14-20
32 381 6d6-2 14-20
32 442 6d6-1 14-20
32 518 6d6 14-20


Parting Shot

The table suggests that small cannon such as the 6-lbr will be in the 5d6 range, while cannon that tended to be used on other ships (12-32lb shot) cluster more or less around 6d6. The expanded crit range should take care of getting solidly thwacked by a 6″ steel projectile.
Note that this is based on a system that gave a .50 BMG 2d12, so it’s necessarily flat.
Overall, though, I don’t think it’s crazy-time. If you’re looking at a 6d6 or 8d6 fireball or lightning bolt, the cannon will straddle that well.
Also, the question arises, I’m sure, why the damages aren’t that far away from each other. Wouldn’t more differentiation be better? Well, again: flat scale. But even in GURPS, with the sqrt(KE) damage scale, relying on KE alone provides “only” a factor of 3 in scaling, and larger balls will tend to have slightly lower penetration (energy dispersed over a wider area), so a spread of 2x from the 6-lb to the most energetic 32-lb is all that’s going to be in it.
For simplicity, I might list three ranged of cannon. Small cannon are 5d6 or 3d10 with a crit range of 15-20; medium cannon are 4d8 with a crit range of 15-20, and large cannon are 6d6 with a crit range of 14-20.

3 thoughts on “Naval Cannon for D&D

  1. I do not believe it would be appropriate for smooth bore cannon to have a crit range. Because of the lack of stabilization of the cannon ball a single cannon rarely hits "the broad side of a barn." Cannon are usually used en mass to guarantee hits. Targets are usually Fortifications, ships of the line, whole armies, etc.

    Personally, I think damage should be higher. Are you using a modified Taylor Knock Out Factor "TKO Factor" to determine stopping power, then converting to DnD damage. Seems to me that your damage formula is breaking when scaled up to cannons.

    Disclaimer: I am only familiar with DnD 3.5. I do not know how damage is treated in 5.0. Crit ranges in DnD 3.5 was to differentiate how easy/hard it was to deal extra damage. Scimitar, Kukri, Rapier, Falchion, etc. have higher crit ranges because they are lighter and more balanced than similar weapons. You might be better off increasing the base damage and/or crit multiplier.

    1. I started to reply, but it got big. Look for a full-on post this coming Wednesday. Thanks for your comments – in short, I think there's a case to be made for higher damage, as well as the lower figures. I think that the way I'm using the expanded crit range does take into account he chance of more serious wounds, though. More next week, including an alternate scaling function that does more of what you want at the high end.

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