Tuesday, February 7, 2017

Giants throwing boulders in GURPS

Precis: I play with the math of the GURPS throwing rules for very strong combatants, and come up with a better solution using the slam rules.

While doing some prep work for New Dawn, I once again ran into the weirdness that are the GURPS throwing rules. This also came up in Mecha Against the Giants, but I don't remember doing much about it then.

The basic issue is that the throwing damage rules are scaled for human sized fighters with non-cinematic strength, and the assumption is that if you're throwing something that doesn't weigh very much compared to the amount you can lift, it must not weigh very much at all. Which is maybe acceptable when you're dealing with an average guy throwing a one pound rock at someone a dozen yards away, but falls to pieces when you're dealing with a giant, stronger than an elephant, throwing 100 pound boulders. Or when you deal with that giant's big brother, who is four times as strong, but throws those same boulders three times as far for the same damage.

I like charts and tables, so here's a table going into more detail on how far, and for how much damage, various ST levels can throw things under the standard GURPS rules

ST50 lb100 lb200 lb400 lb800 lb
603d @ 210 yds5d @ 150 yds7d @ 70 yds9d @ 50 yds7d @ 35 yds
904d+2 @ 315 yds4d+2 @ 315 yds4d+2 @ 225 yds7d+1 @ 135 yds10d @ 90 yds
1206d-3 @ 420 yds6d-3 @ 420 yds6d-3 @ 420 yds9d+2 @ 300 yds13d @ 145 yds
1507d-1 @ 525 yds7d-1 @ 525 yds7d-1 @ 525 yds7d-1 @ 525 yds11d+3 @ 300 yds

This table is nicely demonstrative. A ST 120 giant is four times as strong as a ST 60 giant, but the ST 120 giant throws 100 lb and 200 lbs for the same or less damage at the ST 60 giant, even though he can throw them three or six times as far.

The damage amounts are also unsatisfying. A ST 120 giant throws a 50 lb rock farther than a ballista can, but for half the damage, and the same with a 200 lb rock.

Slam Damage to the Rescue?

I was talking about this on the GURPS Discord chat channel last night, and mused that these boulders would do a lot more damage if they slammed into people. A 50 lb boulder has 29 HP, and does 1d of damage in a slam for every 3 yards per second of velocity. Presumably a giant who can throw a rock 210 yards can throw it faster than 10 yards per second.

Actually, if you do the math from the physics, an object thrown in a perfect 45 degree arc for maximum distance travels at the square root of 10 times the distance, in yards per second. If you calculate the slam damage for rocks moving at that speed, you get a damage table that looks like this:

ST50 lb100 lb200 lb400 lb800 lb
6013d+1 @
210 yds
14d+1 @ 150 yds12d+2 @ 70 yds12d+3 @ 50 yds14d @ 35 yds
9016d+1 @
315 yds
20d+2 @ 315 yds22d+1 @ 225 yds22d+2 @ 135 yds22d+1 @ 90 yds
12018d+2 @
420 yds
23d+3 @ 420 yds30d-1 @ 420 yds32d+1 @ 300 yds28d @ 145 yds
15021d @ 525 yds26d+2 @ 525 yds34d @ 525 yds42d+2 @ 525 yds40d+2 @ 300 yds

To me, this is a much more satisfying table. Stronger giants throw big rocks for farther, and much harder, than weaker giants. The optimum rock for damage varies with the strength of the giant, in somewhat reasonable ways.

The only problem is that calculating this damage in play requires performing an ugly square root. That's not necessarily a huge issue for me, because I can write a spreadsheet to do the math, but here's a simple table that pre-calculates it:

Max RangeSpeed
10 yds or less10
30 yds or less15
100 yds or less30
300 yds or less50
1000 yds or less100

Just for grins, let's compare this table with it's breakpoints to the table above:

ST50 lb100 lb200 lb400 lb800 lb
6014d+2 @
210 yds
18d+2 @ 150 yds14d+1 @ 70 yds17d+3 @ 50 yds22d+1 @ 35 yds
9029d @
315 yds
37d @ 315 yds23d+2 @ 225 yds29d+2 @ 135 yds22d+1 @ 90 yds
12029 @
420 yds
37d @ 420 yds47d @ 420 yds29d+2 @ 300 yds37d @ 145 yds
15029d @ 525 yds37d @ 525 yds47d @ 525 yds59d @ 525 yds37d @ 300 yds

Generally, all the damage got a bump, of around 25 to 50%. That's not great, but some things are going to have to be sacrificed for speed of play.

Calculating it In Play

So now I want to see how this works in play, both using the standard rules and these houserules.

The situation is a bunch of delvers are fighting a 18' tall giant with ST 75. He grabs a 270 lb boulder - a rock about 16" in diameter -   and then throws it at a delver.

Standard Rules

The giant has BL (75 * 75 / 5) or 1125. 270 lbs / 1125 is just under 0.25, so his throw distance multiplier is 1.2 for 90 yards of range and thrust damage at -1 per die. If the giant chucks the boulder and hits, the target can expect to take 8d+2 -8 or 8d-6 damage, averaging about 22 points.

House Rules

The giant can throw the boulder 90 yards, so the boulder's speed is 30 yards per second. The boulder has about 50 HP (Campaigns p 558). 50*30 / 100 is 25d, for an average of 87 cr damage on a hit. That delver had better dodge!


Strong giants, or strong superheroes, throwing big rocks can do a lot of damage under these rules, perhaps too much. I don't mind large objects moving fast (30 yards per second is 60 miles per hour) doing a fair bit of damage, but people aren't routinely killed by being hit by a motorcycle at 60 miles per hour, though it's certainly a concern.

I'm not sure what the correct answer is. I like these rules better than I like the standard GURPS rules, but the damage is probably too high. I think that's an artifact of the slam damage rules, especially the extra HP for unliving and homogeonous objects compared to people of the same weight. Reducing the damage by half for unliving objects and to a quarter for homogeneous objects would do a lot to bring things under control while still smoothing out breakpoints. It'd also get rid of a minor weakness, where throwing a living creature (like a knight) does much less damage than throwing a boulder for the same weight.

Actually, what I really recommend is doubling damage for living targets and halving damage for homogeneous objects. If you do that, the ST 75 giant throwing a 270 lb rock does 12d+2 cr, which is a lot, while if he throws a ST 16 knight (weighing 270 lbs with gear), he does 2*16*30/100, or 9d+2, which is still a lot but fairly reasonable.

I think, in play, these rules wouldn't be any worse than the standard GURPS rules and they feel slightly more reasonable to me.

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