Why turning underwater is nothing like turning a car
No friction to push against, and what that does to manoeuvring.
Turning underwater is nothing like turning a car because a car's tyres are gripping solid ground the entire time, using friction against that ground to change direction almost the instant the wheels are steered, while an underwater vehicle has no fixed surface to push against at all and has to generate every part of a turn, starting it, sustaining it, and stopping it again, purely from its own thrust.
The physics of turning with nothing to push against
A car changes direction because its front tyres bite into the road and the road pushes back, an external reaction force delivered at a fixed contact patch that resists the car's own momentum and redirects it almost instantly, which is why releasing the steering wheel lets the car settle back onto a straight line without any further input at all. An underwater vehicle has no equivalent contact patch, no external surface offering a reaction force in return, so a turn has to be generated entirely from thrust acting on the vehicle's own mass, using the thruster placement and moment arms the previous article described, and every part of that turn, from the first degree of rotation to the last, is paid for out of the vehicle's own onboard power rather than borrowed briefly from the road the way a car's turn effectively is. Momentum matters far more as a result, since nothing underwater is quietly cancelling a vehicle's rotation the way road grip cancels a car's the instant the wheel is centred, and a vehicle that has been turning keeps turning, coasting on its own inertia, until an opposing thrust is actively applied to bring the rotation back to zero, a very different situation from a driver simply letting go of the wheel.
The floating-swimmer comparison
Standing on solid ground and turning around is nearly instantaneous, one foot pivots against the floor and the whole body follows it a moment later, because the floor is supplying exactly the kind of fixed reaction point a car's tyres also rely on. Treading water in the middle of a swimming pool removes that floor entirely, and turning around now takes real, deliberate effort, sculling with the hands to slowly bring the body around, and even once the sculling stops the body tends to keep drifting through a little more rotation before it actually settles, since nothing underwater was ever gripping anything to stop it the instant the effort ended. An underwater vehicle mid-turn is doing exactly what a treading swimmer's body is doing, coasting on whatever rotation its thrusters already imparted, and bringing that rotation cleanly to a stop takes a further, deliberate burst of thrust in the opposite direction rather than simply releasing a wheel and trusting the ground to do the rest. Anyone who has tried to stop that drifting rotation in a pool by grabbing the poolside at the last moment already knows the feeling from the other side, the sudden, jarring stop a fixed handhold provides is exactly the kind of stop an underwater vehicle can never borrow from anything external, however close it happens to be to a wall or the seabed.
One figure worth keeping in mind
A car can rotate itself around inside a space not much larger than its own length, since its tyres are actively resisting sideways slip at every instant of the manoeuvre, while a submarine or an uncrewed underwater vehicle commonly needs several times its own body length of open water just to complete a turn of the same kind, purely because nothing is resisting the vehicle's own sideways drift and rotational momentum along the way. That gap is not a sign of poor design, it reflects a genuinely different physical situation, one where the vehicle is managing its own momentum through open fluid rather than being actively steered by a surface fighting to keep it in line, and a designer who tried to shrink a submarine's turning circle down to a car-like fraction of its own length would need thrust and structure completely out of proportion to what the rest of the vehicle actually needs to do its job.
What this changes in practice
Because a turn underwater has to be built entirely from thrust, pilots and control systems alike plan a turn well before it needs to happen rather than reacting to it moment by moment the way a driver adjusts a steering wheel, and a genuinely tight turn is usually assembled from several thrusters working together rather than one, some rotating the hull while others actively cancel the sideways drift that rotation alone would otherwise leave behind. Stopping a turn is treated as its own manoeuvre with its own deliberate thrust command, never assumed to happen on its own the way a car's wheels naturally return to straight, and a control system that forgets this and simply cuts thrust the instant the desired heading is reached will typically watch the vehicle sail straight past it before the coasting rotation finally runs down. This is the practical reason so much of an underwater vehicle's control software is dedicated to anticipating a turn's end rather than merely triggering its start, since starting is the easy half of the manoeuvre and stopping cleanly is the half that actually determines whether the vehicle ends up pointing where it was meant to.
Where this stops being true
The contrast narrows for a small, light vehicle moving slowly, where drag from the surrounding water dissipates whatever little momentum it built up almost as fast as the thrust that created it, so a hovering, lightweight vehicle correcting a small rotation can stop turning almost as soon as thrust is cut, behaving far more like a car easing off the accelerator than like the coasting, momentum-carrying turn described above. It is a large vehicle carrying real speed and real mass through the turn where the absence of friction is felt most sharply, precisely the conditions under which a submarine's turn stops resembling a car's in any useful way at all, and precisely the conditions the rest of this set is really written about.