Where thrusters are placed and why
Moment arms, and controlling a vehicle that has no contact with anything.
Thrusters on an underwater vehicle are placed as far from its centre of mass, and as far apart from one another, as the hull will allow, because moving the same amount of thrust further from the point the vehicle actually pivots about multiplies the turning effect it produces, letting a given motor do several times more work rotating the vehicle than the identical motor could manage mounted anywhere nearer the middle.
What is actually happening
A car turns by steering wheels that are already gripping the road, and a person turns by pushing a foot against solid ground, but a vehicle suspended in open water has nothing at all to push against except the water itself, which means every single change in its heading, depth, or position has to be generated from thrust alone, with no help from friction, no fixed pivot to rotate around, and no surface underneath doing any of the work for free. Every thruster works by throwing a jet of water one way and being pushed the opposite way in return, and that reaction is the only force the vehicle has available to move, steer, or hold position with, so where each thruster sits on the hull matters just as much as how much thrust it produces. A motor mounted close to the vehicle's own centre of mass mostly just pushes the whole hull sideways or forward, contributing almost nothing to turning it, while the identical motor mounted out toward the bow or stern generates a genuine turning moment around that centre, rotating the vehicle rather than merely shoving it.
The heavy-door comparison
Pushing a heavy door open near its hinge takes real, sustained effort and barely moves it, while the same push applied out at the handle, as far from the hinge as the door allows, swings the whole thing open with comparatively little force, because the turning effect of a push depends not only on how hard it is applied but on how far from the pivot it acts. A thruster bolted near an underwater vehicle's own centre of mass is doing the equivalent of pushing near the hinge, capable of moving the vehicle bodily but nearly useless for rotating it, while a thruster mounted at the extreme bow or stern is doing the equivalent of pushing at the door's outer edge, producing a large turning moment from a comparatively modest amount of thrust. Engineers place thrusters at the hull's extremities for exactly the reason a sensible person pushes a door at the handle rather than beside the hinge, because the same effort simply buys far more rotation from further away. Nobody has to be told this at a door, since pushing near the hinge and feeling almost nothing happen is immediate, physical feedback that the effort is being wasted, and a thruster placed too close to a hull's own centre fails in exactly the same silent, wasteful way, just without anyone standing there to feel it happen.
The number that matters here
Because a turning moment is the thrust multiplied directly by its distance from the pivot, doubling that distance doubles the turning effect for the identical thrust, so moving a thruster from a point close to a hull's centre out to the very end of a hull twice as long roughly doubles the rotation it can produce without demanding a single extra watt from the motor driving it. That relationship runs in both directions, which is exactly why a thruster mounted only halfway out toward the bow rather than right at it is already giving away a real fraction of the turning authority the same motor could otherwise have delivered for free, purely through where it happened to be bolted on. It also means a hull's overall length has a direct, almost mechanical say in how nimbly it can turn, since a longer hull hands its extreme thrusters a longer moment arm for nothing, while a short, stubby hull has to make up the same turning authority with a stronger motor, more of them, or both.
Why this matters in practice
Because rotation and straight-line motion respond so differently to where a thruster sits, most underwater vehicles carry several thrusters split between the extremities rather than one large one near the middle, so that each degree of freedom, moving forward, moving sideways, rising or sinking, rotating in heading, and tilting in pitch, can be commanded largely on its own rather than as an unavoidable side effect of whatever else the vehicle happens to be doing. A pair of vertical thrusters, one near the bow and one near the stern, can lift the whole vehicle straight up when driven together and tip its nose up or down when driven against each other, extracting two entirely different, independently useful behaviours from the same two motors purely through where they were placed relative to the centre of mass. A single thruster anywhere on the hull can only ever do one of those jobs at a time, however powerful it is made. The same reasoning extends sideways as well as vertically, since a pair of horizontal thrusters mounted at the bow and stern, on opposite sides of the hull, can be driven together to slide the whole vehicle sideways or driven against each other to spin it on the spot, again using nothing but where the two motors happen to sit relative to the centre of mass to turn one hardware layout into two distinct, independently useful manoeuvres.
Where this stops being true
The advantage narrows sharply on a genuinely small or compact vehicle, where the whole hull barely exceeds the length a single thruster would need to reach the centre of mass anyway, and spreading thrusters toward extremities that are already close together buys comparatively little turning moment for the packaging cost of finding room for them there. On a hull that small, engineers usually turn instead to thrusters that can swivel or vector their thrust in more than one direction from a single mounting point, trading the moment-arm advantage this article describes for flexibility a longer hull simply does not need in the same way, since a longer hull can already reach the extremities that make separate, fixed thrusters worth fitting in the first place.