Hi there, just started out as a DW GM and further down the line i was planning on maybe having an anti-tyranid campaign take place on an ocean world, purely because i think that that would open up an interesting new style of warfare for the group as they fight off a bunch of kaiju amphibious 'nids. The problem is that i'm unsure how.

Having them on a warship alot of time would make sense, but for some missions i want to be able to deploy them down beneath the surface in, preferably, some kind of submarine style land raider.

I was wondering, does anyone know of any actual canon seafaring/submersible vessels or have any experience playing a game on an ocean world? And if there aren't any canon boats then what kind of rules would be recommended to just make up some?

I've got tome of blood so i was already thinking of reworking the sea-skimmer rules from that (deathwatch marines on jet-skis, the truly unstoppable force!), but for bigger boats and submersibles i'm somewhat at a loss for those.

Important issue: why would I save this world?

Not to just shoot it in the foot, but an aquatic world isn't one where Humans usually live, and 40k tech doesn't scream Atlantis to me, so I don't imagine people would want to live there. Add in Nids, who have spore-polluted the water and sea life, and this looks like a good place to cyclonic torpedo, to stop the Nid organism from getting stronger. One of the big reasons the Imperium, in my thoughts, isn't even more slash and burn is that various worlds have an intrinsic value greater than can simply be blown up. Certainly, exterminatus ordnance isn't a dime a dozen, but it's a world's resources, hidden secrets, or ever so rarely populace that usually stops the otherwise uncaring Imperium's hand. Make sure that there is something on or about that world that makes it worth keeping around, since aquatic habitation and mining aren't likely.

So, I'm not saying "bad idea", just make sure there's a reason. Space Marines aren't called in JUST because Tyranids are spotted on a world, and especially not one with no people, relics, or value. Bombing them from space, even with a "regular" payload of an average Navy vessel can deal with that, especially when collateral damage isn't an issue. Make sure we have a reason to see the Emperor's finest, and they have to act here.

Well the world in question would have originally been an agri world, with the sea life being harvested as a food source on a massive scale. There would have been cities/spaceports on each of the worlds poles that're built ontop of the ice there and most of the actual harvest would have been done of massive, mobile factory ships. So the worlds intrinsic value would have been as a food source.

Though yeah, i suppose it is rather flimsy.

Perhaps instead of an entire campaign set on an ocean world i'd just have it on a regular planet and throw in an undersea mission sometime.

Yeah i imagine that a land raider should be able to withstand a fair amount of pressure, or at least capable of being modified to be even stronger against that. Maybe by welding shut the from ramp and locking the side ones, and hell maybe even a periscope modification for firing the pintle mounted weapon. And if they're going in a land raider then it's weight pretty much means that it'll be going across the ocean floor, so i imagine that its shape wouldn't matter as much and more than it would when outside of the water. The only problem then is the weapon loadout.

Yeah i thought the same, there should be some kind of submarine tank thingy in 40k, but it's just never been mentioned in cannon yet. Modifying something like a valkyrie or stormraven could be a route, but actually i might, if they want to use an actual sub-rank rather than a land raider, just modify a couple of stormraven stats and call it a sub-tank instead. Like instead of the flyer trait i just call that "Submarine" instead. That might be an interesting idea, thanks for that.

I actually imagine that lascannons, missiles (renamed as torpedoes) and melta weapons should work underwater, though naturally energy weapons would suffer a penalty in distance, but i'm not really sure about bolt and projectile weapons. In real world physics bullets and such pretty much don't work underwater, their thrust is designed to propel them through air rather than water, which is far more dense. Though, as you said, there probably could be some kind of aquatically modified bolt round to compensate.

I actually imagine that lascannons, missiles (renamed as torpedoes) and melta weapons should work underwater, though naturally energy weapons would suffer a penalty in distance, but i'm not really sure about bolt and projectile weapons. In real world physics bullets and such pretty much don't work underwater, their thrust is designed to propel them through air rather than water, which is far more dense. Though, as you said, there probably could be some kind of aquatically modified bolt round to compensate.

Real world bullets do work underwater. In general, the faster the bullet is, the less effective it is when it's fired into or from water. So something like a pistol round will fire just fine and a rifle round won't do much at all. How you translate that into an RPG is up to you.

When I first got into 40K , I saw the Land Raider model and thought "Wow, I wonder what a Sea Raider looks like!" Imagine my disappointment when I later found out that they are called Land Raiders because they are named after a guy named 'Land' and not because they needed to be distinguished from an aquatic counterpart...

I think the only thing current subs have are variants of missile. You may want to give whatever vehicle some kind of missile and modify it for undersea combat. Something like a Frag/Krak missile should be good for most purposes. Add a bunch of launchers instead of guns. Limited quantities etc.

As an added risk of course, you may want to add a few special rules for converting to undersea:

Submersible: This craft is able to move under the water freely. However care must be taken as any impact could weaken the integrity of such vessels causing breaches in the hull. Increase critical damage rolls by +1 this includes critical rolls generated by righteous fury.

My logic for this means that it can nudge a critical 1D10 result into a possible 6, penetrating hit. Basically, possible hull breach. Your choice though.

Does anyone have any idea what depth Space Marine armour can tolerate?

I don't think that has ever come up. I can only guess, but I'd say at least as much as a modern deep sea diving suit, however much that happens to be.

You'll be using the Flying rules a lot, albeit for a different atmosphere.

You'll be using the Flying rules a lot, albeit for a different atmosphere.

Or, if the Killteam is mostly walking along the bottom/inside a flooded complex, the Heavy Gravity rules would probably be a simple simulation of underwater conditions (combined with curtailed weapon ranges).

Adeptus-b: Heavy gravity does sound pretty good, that ought to simulate underwater movements pretty well. Also, i imagine that there would have been a sub-tank that Land discovered, but he called it a Land Submarine, which ended up confusing the mechanicus so much that they never bothered making any.

Calgor Grim: That submersible rule sounds great! I reckon i might get the guys to kinda give all this stuff kindof a test run in the hunting grounds or something, have them hunt an alien squid/whale or something Ahab style.

Though one thing i think still needs to be considered is the effects of depth pressure. I imagine that power armour could take a fair amount of pressure before it starts being compromised, but that would probably have the effect of also making the armour weaker/more vulnerable. So it'd something like for every 25m (or whatever the more realistic number would be) deep they go down they loose an AP on their suit, and if it goes down to 0 then the armour and whatever is inside it is crushed.

According to:

http://en.wikipedia.org/wiki/Deep_diving

http://en.wikipedia.org/wiki/Atmospheric_diving_suit

The latter link looks like it could have plausible mechanics for power armour.

This suggests that specially designed reinforced suit can reach 600m depth before there are problems. 25m per AP lost would only limit you to a 200m dive before the helmet/legs collapse. The link to the suit says that the record was 610m where it could operate for around six hours. I would hate to see our current technology beat power armour. Most dives as well seem to be ok up to around 150m by the looks of the links. Anything after that, throw in some slowly rising penalties depending how deep they go. As a scratch brew rule for coping with high depth:

Depth pressure: Marine armour and physiology allows the marine to endure some underwater pressure with relative ease, however intense depths can cause problems. Should the marine go below 200m depth, every hour they remain underwater or each 100m that the marine descends they must pass a toughness roll (Challenging +0) or gain one level of fatigue. The toughness roll is increased in difficulty for each 100m they descend to a maximum of -60. Intense depths may cause problems for power armour systems as determined by the GM.

At every hours this means even if you are really unlucky you can manage about four hours underwater. With the SM Unnatural toughness cancelling out a level of negative modifiers you can get down to about 900m before hitting that -60 (not counting fatigue penalties).

These are just roughly thrown together though so I haven't had time to read up to rebalance so do feel free to alter or dispute.

First things first: there is ALWAYS a reason for a Deathwatch Kill Team to be anywhere at any time. Anyone who says otherwise has a serious lack of imagination.

Ocean World? OK, why the Hell not? Mechanicus installment/Deathwatch Watch Station/Eldar Ruins/WHATEVER YOU WANT is under the sea and a rescue mission/artifact recovery/smash and grab/sample of new synapse creature needs to take place.

There is always an excuse.

Second things second: You can ALWAYS tweak any existing rule set in your favor. Anyone who says others has a serious lack of imagination and is also an A$$clown. Create a submersible-land raider/water skimmer/whatever you want because you are in the DEATHWATCH and in control of some of the Imperium's most top secret and bada$$ gear.

Seriously, this RPG world is your Oyster.

Second things second: You can ALWAYS tweak any existing rule set in your favor.

...I say "It's caused by the warp" way more often then I'd like to admit...

...I say "It's caused by the warp" way more often then I'd like to admit...

Ah yes, the 40K version of "A Wizard did it..."