This section gives clarifications, new rules and maybe some pointers to using subs and underwater combat. I hope it's useful!
[For notes on swimmers and divers, please see here.]
Some subs have cargo space, and subs are ideally suited to be able to mount top or underbelly weapons, so I see no reason why they shouldn't be able to use cargo space for these weapon mounts - helicopters may mount back and underbody weapons in cargo space. I've done this for a few designs in the Showroom.
Turrets on subs are considered waterproofed for free. This is assumed to come as part of the frame cost.
Subs can be made from metal armour, but in this case the option leaky boat rule should be employed, after all, if damage can seep through the armour, then so can water!
Subs with metal armour have a crush depth modifier of 1,500, 5,000 if the frame is reinforced, or 750 if the sub has a CA frame.
All turrets and EWP's cause drag with a sub (as may do some other accessories as well). A sub takes a 10% top speed reduction for having one or more EWP's, and a further 10% for having one or more turrets. Other accessories will state how much of a drag reduction they give.
All percentages should be totalled, and then applied against the top speed of the sub, remembering though, that a sub will only ever take a maximum of 20 percent drag.
If a sub is streamlined, then calculate the bonuses for that first, and then apply any drag afterwards.
Don't forget that if a sub starts to take in water, the powerplant may short out. If the sub has taken 1/4 of its total spaces in water, then the plant shorts out on a 1-2 on 1d6, if it has 1/2 of its spaces as water, then the plant shorts out on 1-4, and when it is 3/4 or more full of water, the plant automatically shorts out.
However if the plant is surrounded by unbreached component armour, then the plant will not short circuit.
If a craft does not have sonar, then it is limited to seeing things at normal visibility rates of 400-600' modified by depth and lighting.
Searchlights: These increase visibility by 200% as improved lighting does, but only on the side and in the direction that the searchlight is facing and being used in.
Smoke: Smoke, if placed underwater, occupies an area that is 1/2" high, starting from the depth from which it was placed.
Fires can occur in a sub (but the external armour may never catch fire (unless using special chemicals that can burn underwater, which are probably not standard in vehicular weaponry or the sub is on the surface and the location on fire is completely out of the water, such as the top).
However if a sub is taking on water, then this will naturally help in putting out fires.
It is important to realise that the depth that a sub can go down to is calculated by multiplying the lowest armour value on the sub. As such, when a sub takes damage, this safe depth rating is going to change quite rapidly. All changes should be calculated at the end of the phase, after all weapons fire has been resolved, then at the end of the turn all depth damage should be resolved. This is because further weapons fire might take place to reduce the safe depth further and thus increase the risk to the craft.
It's not clear at all, but any explosion underwater will cause the effect of a concussion grenade with a range of 5". This doesn't just apply to torpedoes, it applies to mines to, and other explosives. And note that the 5" is a full sphere centred around the explosion. 3D, not 2D. Some weapons don't cause this - things like lasers and sonic weapons aren't explosive - and neither do personal hand held explosive weapons (like explosive spear bolts), but most others do.
