Why would mechs be used in underwater combat but not on land?

Question

In my world, the planet (which is not Earth, but is suitably Earth-like) has been besieged by deadly monsters for about 1000 years. Their initial attack lead to an apocalyptic scenario which took humanity hundreds of years to fully recover from. Since then, humanity has banded into one large empire/federation and another apocalypse has been averted, but it requires periodic military excursions into the wild to prevent any of these monsters from entering city limits.

The most dangerous variants of these monsters are the ones that dwell in the sea. To combat them, part mechanical part biological mechs were built. They are 15 ft tall and fight alongside small and medium-sized submarines. They are very effective in their role, but are not used in land-based operations. Instead, soldiers either fight with something like the exoskeleton from the movie Elysium (Here: or just have simple ballistic vests that protect them from possible friendly fire.

My question is: why would these mechs be limited to water combat if they have been so effective in fighting monsters?

Answer 1

Have you noticed that sea animals get a lot bigger than terrestrial animals? Buoyancy makes it possible to grow a lot bigger underwater due to the lesser need to support your own weight. For more details, see the "square-cube law," the effects of which are mitigated by being underwater.

That's been mentioned by a couple people already though. The thing I wanted to add is that you don't need to frame the question as "why don't mechs work on land." You can also frame it as "why are there better options on land." Projectiles, for instance, go further faster through air than ocean water, especially deep down where water pressure is high. This is true for energy weapons as well. Maybe terrestrial arsenals have a large and futuristic array of machine/rail/Gatling/laser/etc. guns that perform very well above ground but poorly underwater.

Air support is also a thing for armies. Perhaps individual soldiers can call in bombing raids, missile strikes, and so on, but underwater you've got to carry that stuff with you. So now you need the mech just to hold all of your high explosives.

So, in summary:

1. It's easier to build large structurally sound mechs for underwater use than it is for terrestrial use.

2. Projectile weapons perform well above water, but poorly underwater.

3. Terrestrial armies can call in air support, but undersea fighters have to carry all their firepower with them.

Edit: One other thing just occurred to me. You have to be in a suit deep underwater anyway. Since it's obligatory, it may as well be useful. Hopefully some combination of those reasons will suffice.

Answer 2

Heat

Mechs are inefficient temperature-wise, being submerged in water helps them dissipate heat quickly, which allows them to operate longer than few minutes on land before overheating

Answer 3

There are a few reasons not to use mechs on land, with one of the main ones being that the joints are unable to support the weight of the body if the mech is too large. For details see Plausible Reasons for usage of Combat Mecha.

Luckily, this is helped by buoyancy - mechs don't have to support their full weight in water. However, that just makes a mech a slow submarine with arms, so there needs to be a reason speed isn't important but arms are useful.

Speed could be unimportant because the sea monsters are very fast. They will sneak up on and grapple a large submarine, so torpedoes would be dangerous to the sub itself. The main strategy is for the large mech to grapple and distract the monster, while the small submarines fire harpoons at it. The motion of the limbs makes the mech more attractive to the monsters, so they go for the mech and not the subs.

The arms are used for grappling and cutting - they could have chainsaws. The monsters may also have weak points (at the joints, inside the mouth) that are easier to target at close range.

Answer 4

They are too heavy to go on land.

These mechs have seriously thick armor. The weight of this metal is balanced by gas filled spaces. The underwater mechs are neutrally buoyant. Because of their mass they have inertia but they do not need to constantly fight the acceleration of gravity. They can walk along the bottom of the ocean fine but their servos would not be able to carry them on land.

Answer 5

Underwater mechas are already in use

There are many tasks which need to be performed by workers working underwater; tightening bolts on pipelines for example.

People don't do well underwater, because water is heavy and if you go deep enough the weight will squish you. (Yes, it is more complicated than that; but the point is, unprotected people don't do well underwater.)

Robots have not yet really progressed to the point where one can just tell a robot to go down there and tighten the bolts.

That's why people have developed what is called atmospheric diving suits, which are, to use Wikipedia's words, "one-person articulated submersibles" "which resemble a suit of armour". Some atmospheric diving suits even incorporate motors for power and propulsion. In other words, they are real-life mechas.

Atmospheric diving suits in current use include the Newtsuit, Exosuit, Hardsuit and the WASP, all of which are self-contained hard suits that incorporate propulsion units. The Hardsuit is constructed from cast aluminum (forged aluminum in a version constructed for the US Navy for submarine rescue); the upper hull is made from cast aluminum,[clarification needed] while the bottom dome is machined aluminum. The WASP is of glass-reinforced plastic (GRP) body tube construction. (Wikipedia)

^{Chief Navy Diver Daniel Jackson completes a successful certification dive of the Atmospheric Diving System (ADS) aboard the special mission charter ship M/V Kellie Chouest off the coast of La Jolla, Calif. Photograph by the U.S. federal government, available on Wikimedia. Public domain.}

[Atmospheric diving suits] can be used for very deep dives for long periods without the need for decompression, and eliminate the majority of physiological dangers associated with deep diving. Divers do not even need to be skilled swimmers. (Wikipedia)

Answer 6

Mimicry

The monsters try to avoid military and attack unarmed civilians. Submarines and destroyers are very distinctive; so, the monsters can identify them from far away and just go around them in favor of easier targets on the shore, but the mechs are made to move, sound, and (thanks to organic parts) smell like a monster. So, these mechs go into the water and perform various lures to get the monsters to come to it by doing everything from faking distress to mating dances. By the time the monster gets close enough to realize its mistake, it's already in weapons range.

Once these monsters are up on land, it is a different story. They only come up onto land to hunt, and this means they are looking for food, not companionship... so, they don't take nearly as much coxing to get them to come to you. In fact, making your soldiers on land look as much like a normal person as possible is the best lure you could use to make sure they come towards your guns instead of going around to hit your unguarded population centers.

Answer 7

Something about the biological portions of the mech synergizes with the shape. If the parts are from monsters, then the closer the incorporated part's image is to their original body part on the monster (platonian ideal), then the greater the effect.

For that reason, any kind of mech that resembles a living creature is more effective than the same materials but on a soulless submarine.

Answer 8

Close range combat

The monsters hide in caves in the sea bottom. When they sense the presence of a submarine they hide behind rocks and they prefer to move in narrow gorges between underwater mountain. The only way to engage them is close range combat, obviously they move faster than mechs, but since they chose narrow spaces and the submarines patrol the empty water overhead they can be encircled.

Close range combat 2

Underwater the only available weapons are torpedoes and they are not as fast as missiles. Bullets are useless, speers short ranged. With few alternatives the possibility of close range combat becomes more likely.

Balance

Notwithstanding all the improvements in robotics balance of bipedal machine is still delicate. Especially for a machine that should be capable of sudden changes of speed and direction and should also be able to move sideways while moving the arms for a fight. The water surronding the mech sustains it a little bit, making balance easier.

Answer 9

Another possible reason would be fuel. A giant mech is going to require a metric crap-ton of energy to move around. Perhaps the biological components of the mechs are developed from the monsters themselves, and generate large amounts of electricity from seawater. (I don't know the science, but I'm sure there's something involving high-electrolyte water or trace metals suspended in saltwater that will suffice. Or maybe they filter trace radioactive elements from the water and use them for micro-fusion. Or...)

Thus, a mech can't operate on land, because it can't carry enough seawater around to operate for more than a couple of minutes. But when immersed in the sea, there's no such thing as running out of seawater fuel. Functionally, this is the same as an air-breathing animal needing to stay on the land -- we use oxygen specifically to generate energy, and need a continuous supply of it. The mechs use seawater to generate energy, and need a continuous supply of it.

(This could also lead to a cool story idea where the monsters discover this weakness, and start deliberately throwing/pushing the mechs above the surface of the water to weaken or disable them. Like fighting a human in a river and shoving their face underwater to take away their oxygen supply, only in reverse.)

Answer 10

They're not as good as tanks

Tanks are maneuverable on all sorts of terrain, can be scaled up much better than anything with gangly limbs, have a much more stable shape and are probably much cheaper to manufacture than an equivalent mech, which requires more motors, servos, etc.

Answer 11

The mechs are very sensitive to electro-magnetic interference

These mechs, in order to be able to operate their mechanical-biological interfaces, need some very delicate electronics, which is very sensitive to external radiation (think the electronics of airplane with cellular radiation, but on a 100x scale).
It is not possible to shield this electronic part from external interferences, because this would require to cut the nerves and muscles that are controlled by it.

Underwater, electromagnetic waves have low penetration, so the electronics is protected by the environment it operates in, but in the air the interference (coming from all the other vehicles and field communications) would be too strong

The biological parts are extremely sensitive to sun light

Similar as above: the light of the sun would burn the "skin" of the biological parts degenerating them, so that they would become unusable in a few battles. Because of their high costs, an average operative life of only a few days for these machines would be unbearable for the resources of mankind. Covering these parts with some kind of clothing would surely help, but in battle is easy for the clothing to be torn.
Of course they could be used only by night, but the advantages of having mechs would be cancelled by the disadvantage of having to fight in the night (where the monsters are at advantage because they are used to darkness).

They are too expensive

The human empires have limited resources, so they can afford to build only a few mechs. Since the most dangerous monsters are the ones dwelling the sea, all the mech-building resources are devoted to this effort, and developing a mech that could operate on the surface (where enhanced soldiers are doing decently) would be seen as a waste of precious resources.
Add to the mix that it is not so straightforward to adapt a submarine technology to a land environment, so the development of a land mech would be an expensive and long-term effort, which at the moment would not be feasible.

Answer 12

First, for the biological part

We are starting with (presumably) as our base a subspecies of this monster which has adapted to marine life. Over this short a period that is pretty remarkable and suggests a very fast rate of mutation. Adaptations can include things like:

• streamlined hydrodynamic shape to improve speed underwater. For this reason I suggest the mechs be 15' feet long not tall. This will cause a tradeoff on land. Just look at Crocodiles: they move fast as lightning in the water but are very slow and ineffective predators on land.
• a shift from limbs to fins or flippers and tail flukes
• either develop the capacity to 1- hold breath for a very long time like whales or 2- evolve gills to to get oxygen from the water.

Option 2 would explain why this specific model of bio-mech will not work on land. The living part of it wouldn't be able to breathe. Option 2 would also allow this creature to dive to depths well beyond what a human diver even in a suit can survive.

Now, as for the mechanical part

Submarines are good at hunting other submarines, but marine life is better at detecting other marine predators and prey. Electrosense, the ability to detect minute changes in water pressure which suggest the approach of a fast moving animal, keen eyesight, echolocation.. a submarine just has sonar and has to be careful how and when it uses it.

By the same token, firing a torpedo or anti-ship missile at wildlife is... well it is significant and ludicrous overkill, and a good way to run out of missiles and money. Something armed with weapons more suitable to combating an underwater predator though, and mounted on a bio-mech is a feasible way to fight a war. The submarine's role in this mode of warfare would essentially be to act as a tender / base of operations for teams of these biomechs.

Answer 13

part mechanical part biological mechs were built

Simple, hand wave that the biological part needs to be in the water, or, if it's more of a CNC system than mobility, that it's adapted from the brainstem of a marine animal & operates best in an aquatic environment.

(That said, the heat & buoyancy answers are the ones I'd have offered if they hadn't already!)

Answer 14

Missiles.

In the war between weapon and armor the anti-tank missile has prevailed. Mecha are basically a tanks that trade some armor for better handling of some terrain.

However, there is no underwater equivalent to the infantry anti-tank missile. There are torpedoes but they are a lot bigger and don't move nearly as fast--your mecha have point defense systems that can stop torpedoes. (Yes, there are supercavitating torpedoes, but during the supercavitating phase they are blind. You either use them as unguided weapons or you use the supercavitating capability to get them close to the target and then drop to normal torpedo speeds to actually attack.)

Answer 15

1. High value of gravity of the planet (1.5-1.7 of Earth gravity, for example).

2. Microorganisms (e.g. Ideonella sakaiensis) or other substances in the atmosphere that cause very rapid corrosion of metal (or any specific things in the robots).

3. The land has a lot of karst or other (ice, etc.) caves and caverns (like in the Lem's "Fiasco" in the beginning on the Titan), and swamps, and geysers - as a result, it is a dangerous environment for heavy robots.