So you want to go throwing away a perfectly good weapon?
Why do it?
Range - Sometimes you need to reach out an touch someone you can't otherwise touch. This happens sometimes in a big melee you really need to help a friend and can't reach the enemy threatening him. Sometimes you have a flank or back shot on someone that's across the room - chuck your weapon into him. This is perhaps the best way to throw a weapon - take advantage of someone who turns their back because they feel like they're out of threat range.
Expert Tip: Remember that a successful Fast-Draw is instant, or nearly so. And GURPS Martial Arts says you can attempt one after willingly releasing your weapon - specifically including throwing it! (GURPS Martial Arts, p. 103) So you can throw and then try to quickly ready a weapon to defend yourself. Smart fighters learn Fast-Draw and keep a good backup weapon they can use it with, for this reason - throw, fast draw a replacement, and parry the next attack! A failed Fast-Draw means you don't get it out - so don't fail your roll!
Parry Penalties - most thrown weapons inflict a -1 to Parry on your opponent, -2 if they're small like knives and shuriken (see p. B376). If your opponent really depends on Parry, you can inflict this penalty on him. It's tough, though, because Dodge and Block are unpenalized.
One warning, though - if your opponent's Parry succeeds by 5 or more, he can catch your thrown weapon (p. B373.) So throwing axes at Throwing Art master Shaolin monks is a bad idea, generally.
You Need to Move - you can Move and Attack with only -2 or the Bulk penalty of the weapon (whichever is worse) as a penalty to hit. If you really need to move and really need to launch an attack with better than a max of a 9 to hit, this might be the trick.
You Need to Change Weapons - why drop it if you can throw it effectively? Okay, dropping it is a free action, so it might mean you can get in an attack with the new weapon. But it's worth considering if pitching your throwable weapon into someone is a better way to get rid of the weapon.
So that's when to throw. What should you throw?
Let's look at some of your choices, including those in Basic Set and Low-Tech.
What to throw?
Axes - a solid choice. Axes do swing/cutting damage. They're also excellent melee weapons, so an axe balanced for throwing is a useful way to dice up people in melee and and keeps the constant threat that you can toss it into someone if you need to. Their range isn't bad, either - ST and STx1.5 is okay for a reach-out-and-touch-someone throw in what is otherwise a melee. Hatchets are good if you want a good throwing axe with longer range.
Maces are a more marginal choice - swing/crush is not as good as swing/cutting most of the time. But if it's all you have, you can throw it with as much effectiveness as swinging it.
Iron hungamungas are an intersting choice if you want an axe that'll potentially bend on impact. I can't remember why we didn't do this in the books, but in my own games I give them +1 to hit because of all the pointy/bladed impact surfaces. It makes them crappy melee weapons but better throwing weapon.
Spears - There is a strong "impaling sucks" feeling out there, especially in games with unnatural creatures (who may lack vitals, be homogenous or unliving, etc.) If you think impaling sucks and it's a bad choice, spears are a bad choice. But I generally think they're a pretty good choice. Against living foes, thr+3 impaling is pretty solid damage and opens up vitals as a target. Range is pretty solid (same as axes), and pretty good accuracy. Javelins are less effective in terms of damage but have better range and more accuracy. Either improves if you throw it with a thong - range and Acc both go up.
The cost and weight aren't bad, either, and it's a solid melee choice, too.
Knives - A marginal choice. Knife throwing seems to work well in movies, comics, and abstract combat systems - you throw and hit the throat (which no one seems to armor for some reason) or the eye (surprisingly easy to hit), usually. In GURPS, though, it doesn't do so well. They're fairly pricey ($20 and up), not that small (Bulk starts are -1 and gets worse), and short ranged. On top of that, they do a low amount of thrust/impaling damage. Reasonably or even fairly lightly armored targets will ignore most hits.
As a plus, though, it's a useful melee weapon so you might have it to hand - and GURPS doesn't require a re-ready to switch to a throwing grip from a fighting grip. They are also harder to defend against - they are parried at a -2!
Note that the smallest of hungamungas are thrown with knife throwing skill - so if you must throw knives, consider some small hungamungas and get swinging damage.
Darts - good range, bad damage. A much longer range knife, really. Bulk -2 makes them a good Move and Attack throwing option.
Discs - so-so damage, good range. The chakram is thr+1/cut, which isn't so wonderful, but range is out to STx6 so it's not bad as an ersatz missile weapon substitute.
Sticks - really good range, and very solid damage - swing/crushing and you can throw up to STx10 range with a boomerang.
Shuriken - another marginal choice, like knives, because they are both low-damage and short ranged. Realistically, shuriken aren't in and of themselves lethal. They need to be envenomed to have some real effect. Realistically they are distractions, and seemed to have gotten the most use in ages when concealed weapons and unarmored foes were the norm (such as Tokugawa era and early/pre-Meiji era Japan). If your opponent doesn't suspect a thrown weapon and you wing it at his unarmored limbs, you've got an edge in the upcoming fight.
Their main upsides in GURPS is that they are easy to conceal (Bulk 0) and light (1/10 lb.) - so you can buy and carry a lot of them and hide them fairly easily.
Harpoons - Harpoons historically were thrown into whales, their tether tied off to boats, and the whale couldn't escape. That's pretty much what you get here. It's a heavy spear ($60 and 6 lbs instead of $40 and 4 lbs) with a tether (what length isn't specified, nor is its thickness). Damage is excellent - thr+5 impale! - and if it sticks in, it may get stuck (p. B405 and B411). That's a blessing and a curse. If you want to tether your foe to you, or to something else, the harpoon is a good weapon. If you want your weapon back, or don't care to be tethered to your opponent, it's just a heavy spear.
Mark Langsdorf asked about it in melee - William Knowles pointed out this is covered in Low-Tech, p. 72)
As you can see, generally, the dedicated throwers have better range. The mixed-mode weapons, though, are a safer choice because you're ready to fight with it in hand-to-hand and have the option to throw. If you really need to hit at range, think "missile weapon" not "thrown weapon." But missile weapons are a whole different issue.
Don't forget, next week, Thursday May 9th, is the next GURPS Melee Academy! Details can be found in this post by Doug Cole, or right here:
"Taking advantage of attack/defense timing in GURPS," which can certainly be leveraging Wait in good ways - and bad - but can also be things like two-man fighting where one person soaks up defenses, while the other looks to score hits, or pairing a Reach C, 1 guy with a Reach 1,2 guy. I might even look at fight timing with respect to grappling moves, also including Waits.
Write your thoughts, put it here or in your Blog and link it here, and let's get it on (next week, nine days hence)."
Write blog on the topic, let the other posters know, and join in on the fun!