Hi,

I plan to send my player in a water world. Any idea on how to handle combat in and under water?

Cheers

Probably the simplest thing to do would be to use the 'high gravity' rules from the core rulebook (combined with the Swimming rules from the Skills section, of course).

Well underwater virtually all weapons that aren't designed for it (i.e all SP/Bolt etc) wouldn't work. In terms of melee I would do some things like halve str bonus additions to damage and make all WS tests harder.

Probably make swim checks to make movement actions.

Provided that they can all Swim, unless they are caught in a stream or have an explosion underneath water (which should be loads, loads, LOADS worse than if they are on land), I don't think they should have to do Swim checks.

That said, provided they can all swim, I'd use the rules for High Gravity and take some inspiration from the Vaccuum/fighting in weightlessness rules.

Where do you find thi vacuum / low gravity rules? I found something in the core rule book, and it only says that the AGI bonus is increased and the load you can carry is decreased (or vice versa depending on low or high gravity). I found nothing more...

But what king of weapon? Energy weapon in water? chain? Bolt?

You seem to know about grenade in water... What effects?

Water is much less compressible than air, so any explosion at all is transmitted with much greater force than in air. Grenades wouldn't damage with shrapnel underwater as any fragments wouldn't travel very far at all, but the shockwave would be much more devastating. Have a look at how depth charges used to attack submarines are supposed to work, that sort of info might have a good explanation.

I can't remember where I heard/read it but I believe normal bullets fired into water really only penetrate with any force up to 6-12 inches, to give you some idea of just how useless SP weapons would be underwater. Bolt weapons same principle, the round wouldn't travel far towards the target, though if they exploded might be damaging that way.

Las/plasma/melta weapons would also be less effective, the water would dissipate a lot of the energy that is supposed to be transferred to the target, and might also refract the beam of things like las weapons. If you think that they are basically designed to flash heat the target they would instead be heating all the water between weapon and target.

Flame weapons would be fantastic on the surface if they spray a thick enough mix to sit on the surface and burn, otherwise would be useless (and obviously useless underwater ;-).

Any melee weapons involving power would, I think, need to be modified to be waterproof, otherwise the water would mess with the electrics/motor. Even so I think power weapons would spend too much time constantly heating the water. Chain weapons would probably be the weapon of choice as once you get contact with someone they will still cut them without the wielder having to apply too much force.

primitive weapons are then the good choice for underwater combat.

For the grenades, do you think that keeping the damage but increasing the radius seems logical?

What about harpoons? it is the best effective missile weapon, no? But is it an 'exotic' weapon? or is it in the catégory of the SP weapon?

What about eldar/black eldar weapons? Could it work underwater?

Yeah exactly, knives the ideal as the force as stabbing someone is essentially the same and they aren't unwieldy.

I think increasing the grenade radius would be appropriate. I might be tempted to say that all explosions ignore armour as well as it is the concussive force doing the damage, with all explosives doing impact damage.

I think harpoon as SP/exotic is a judgement call. Given it is basically the only really appropriate underwater weapon I would say it depends on what you want to happen; do you want everyone to grab harpoon guns asap and start attacking each other or do you want the guy with *the* harpoon gun to be a major danger?

IIRC correctly Eldar and Dark Eldar weapons still fire projectiles (razor disks and shards of crystal?) so would suffer the same as human SP weapons. However again if you want the Eldar to be super dangerous could say that the Dark Eldar weapons can fire 'harpoon-like' shards for underater operations...

Where do you find thi vacuum / low gravity rules? I found something in the core rule book, and it only says that the AGI bonus is increased and the load you can carry is decreased (or vice versa depending on low or high gravity). I found nothing more...

P.107 & 216 swimming.

P.210 suffocation & vacuum.

P.213 gravity.

• Basic movement rates are SB*1.5 metres/turn.
• Swim tests are passed automatically outside combat while at or near the surface and in reasonably calm waters.
• During combat or at depth, swim tests are required as part of any move action. GM fiat modifiers apply based on depth and current. 2+DoF makes actors sink at a rate of 1 metre per 20 kilos of weight.
• An actor can hold its breath for a number of minutes equal to its TB, but must test Toughness each minute, or each round if in combat. If the test is failed, the actor gains 1 level of Fatigue. If the actor passes out or similar, it starts drowning and suffers d0 damage at the end of each round, not mitigated by TB or AP.

As you can see, the rules are extremely sparse and kind of sucky, so I'd suggest coming up with something more entertaining and less fiddly yourself.

But what king of weapon? Energy weapon in water? chain? Bolt?

You should probably think of water as a more dense type of air. Most things that work above water also work under water, but at a much lowered speed since friction and displacement are vastly increased.

Explosives and the like are generally water-tight and produces their own oxygen when detonated, so things like SP & Bolt weapons should mostly operate without problems under water (and in space, for that matter).

Hacking, swinging & similar motions will be slow and difficult to land, and do little to no damage. Stabby motions will also be slowed considerably and much harder to land, but will still be able do a fair amount of damage.

SP, Bolt and most Exotic, Launcher and Primitive weapons will have very, very drastically reduced ranges - Dividing ranges by somewhere between 2 and 10 is probably reasonable. Bolt weapons are likely to be the most effective projectile weapons as their projectiles are self-propelled and explode, which means the velocity of the projectiles are much less affected, and the damage they inflict is far less dependent on the velocity of the projectiles.

In addition to being much denser than air, water is also highly reflective. The combination means most beam type weapons - Plasma, Melta, Las & whatnot - will have an effective range of a few metres, beyond which both damage and accuracy will drop like a stone. Plasma and Melta weapons may or may not have a chance of frying the user when fired.

Tiny and near frictionless projectiles like Shuriken and the verious types of shard casters will likely operate under water the same way they do above water, or very close to it. Gravition & Distortion weapons will probably be entirely unaffected.

You seem to know about grenade in water... What effects?

Oh noes, physics!

An explosion displaces a volume of whatever it explodes in. A bomb exploding in the air can create a shockwave of displaced air so powerful it will smash you to bits.

The more dense the medium of the explosion, the more powerful the shockwave is, making explosions far more dangerous under water than above water. If you want a quick and dirty suggestion on how to reflect it in the game, I'd say add d0 to stuff that does Explosive damage and has the Blast quality, for every 30 meters below the surface.

For underwater explosions....yeah. Blast is transmitted much more effectively, fire and shrapnel less so.

I'd probably change X type grenades to I, and as suggested, increase the damage and blast radius. I'd throw in an extra D10 and double the blast, but wouldn't suggest anything too complex in varying it with depth - the cavitation shock wave from a frag grenade will never crack a heavily armoured pressure hull, no matter how deep you go, after all, and simplicity/speed of play is good.

Agree that most 'normal' ranged weapons should either not work or be limited to point blank range. Certainly a bolter can be fired underwater (there is reference to marines fighting under the surface on watery worlds) but beyond that it's your take on which guns fire and which don't.

I'd agree the harpoon is probably exotic. Whilst you're firing a projectile which is solid, the low speed, difficulty in moving fast when aiming, the whole environment, is sufficiently different that you'd want to be trained to use it. It's as least as difficult as swapping from a slug rifle to a las rifle, or a slug rifle to a slug pistol, anyway, and those require new talents.

Melee weapons...the same. Some will work, some won't. It's up to you whether a power fielded blade will (a) short out, (b) boil the water nearby, or © not really be affected by being underwater. I'd suggest either (a) making them ill-advised choices, or (b) making the wielder easy to spot and the weapon hard to handle (maybe port across the overheat trait?)

Swim checks are meant to be called for when doing anything challenging, time-critical, etc. So not for just 'getting around', but if the players intend to make a full move action - and certainly a run action - then maybe. Equally, I might rule that either you can use swim in place of dodge.

Just saw this post and read the comments and ideas...now for my two thrones

1) Swim checks for anything difficult ( dodging, going against the current etc ) otherwise no real need for constant checks

2) Most of your SP class weapons will NOT work at all underwater ( there should be a few exceptions for best quality etd same as modern day..an AK-47 can and WILL fire submerged ( just largely useless if target is farther than 1ft away ) SO unless your putting it to the targets head or other vital areas...dont bother taking it underwater

3) Grenade damage due to shockwave increase is totally correct..I would give the blast radius between x2 and x4 increase

(a) Plasma grenades will absolutely work underwater as will phosphorus -- The type of plasma most likely created by the grenades would be comparable to a plasma torch ( A plasma torch uses an inert gas such as steam. The electrodes vary from copper or tungsten to hafnium or zirconium , along with various other alloys . A strong electric current under high voltage passes between the two electrodes as an electric arc . Pressurized inert gas is ionized passing through the plasma created by the arc. The torch's temperature ranges from 4,000 to 25,000 °F (2,200 to 13,900 °C) ) which is easily enough to work like a charm underwater ( as well as having the effect of flash boiling the water of the surrounding area so id actually add a temporary effect similar to a flashbang as well as total block of line of sight of anything on opposite side of the blast area ( due to the flashboiled water bubbles totally obscuring vision )

4) Las weapons would work but reduced range and damage...HOWEVER..there are specifically designed las weapons listed in the books ( refference landunder weapons ) for use at full normal damage and range ( blue-green coloration instead of the usual reds ) As GM i would give the option for any resourceful PC to get a chance to check and see if there would be a "refit kit for underwater use" for their Las ( most likely a conversion of swapping out the barrel and power assembly to use the different colorations )

I see no reason to not treat Bolt weapons like torpedoes, considering they're tiny rockets when fired in air.

I see no reason to not treat Bolt weapons like torpedoes, considering they're tiny rockets when fired in air.

Those tiny rockets as you put it are mass activated and specifically designed to detonate upon entering a mass greater than air.

Firing bolts in the increased mass and pressure of an underwater environment is tantamount to just pointing the gun at yourself.

Sure he gun will work but your enemes won`t be too worried about it.

So you're saying that a Space Marine can't fight in weird atmosphere because their gun would explose at them? I rather see that the bolt explodes a milisecond after its tip detect something thicker than what it is going through when it is shot. So if it's in water at the beginning, it would detect the resistance, go through the water and then impact on something harder, detect it and then bang.

For standard SP weapons, they wouldn't work in space nor under water, there is no air to burn in their detonation to work. You need special bullets for that.

For standard SP weapons, they wouldn't work in space nor under water, there is no air to burn in their detonation to work. You need special bullets for that.

There doesn't need to be any, as more than sufficient oxygen is released in the reaction. I'm sure you're right about bolter rounds, though.

For standard SP weapons, they wouldn't work in space nor under water, there is no air to burn in their detonation to work. You need special bullets for that.

Lol, projectile firearms DO NOT REQUIRE AIR TO FUNCTION. Oxidizer is already in their propellant.

Google underwater firing.

Yeah, same reason they can fire in space. That said, guns do have ISSUES firing underwater, mainly with water getting into the workings after they fire, especially for a semi-auto gun cycling. IRL Underwater guns often use sealed barrels (with a thin waterproof layer, almost like paper) in an almost "pepperbox" design, firing off barrel one by one and then you remove all the barrels and slot a new set in.

Firing in space is even easier if you have to something to counteract the recoil, in fact your effective range and accuracy far out is increased heavily, with the lack of real gravity exertion as well as the air to slow it down. Granted, trying to hit a far off target traveling fast is going to be a whole new mess of problems (and could end up dropping you down to -60 fast), but for tethered down and pure target shooting, don't see an issue.

What is the deal with void rounds in their many incarnations though....may make more sense as underwater rounds to be honest. Better reliability, but normal rounds can always work in a pinch.

As for bolters, I can see the detonation thing being a possible issue. But there is more than one pattern of bolter shell. For launching, it is no worse off then a bullet. They fire like one, with an initial charge to have the bolt clear the barrel and the backblast range on the wielder, before they ignite, functioning as a mini-torpedo of sorts.

A side note to this is wondering about bolters firing in space, with the 2nd bun meaning the speed would increase even farther than a bullet.

Yeah, same reason they can fire in space. That said, guns do have ISSUES firing underwater, mainly with water getting into the workings after they fire, especially for a semi-auto gun cycling. IRL Underwater guns often use sealed barrels (with a thin waterproof layer, almost like paper) in an almost "pepperbox" design, firing off barrel one by one and then you remove all the barrels and slot a new set in.

Actually, most modern guns don't have problems with that (there are videos of AKs firing underwater, main problem is horribly short effective range of them due the bullet not being optimized for water ballistics so they start tumbling and loosing speed rapidly (few meters).

Dedicated underwater guns are specific due their "bullets" being long darts - here is an ammo for Soviet APS underwater rifle:

So in game terms, limit ALL guns to 5m range max, underwater ammo and dedicated underwater guns extend that to 15-20.