Dear Samuel,
you got right, basically. But if we left beside mechanical frontiers, like material overloads, which will bend or break rods, bars, if we left aside the cruel forces, which can damage bearings, knocking out cranks, destroy pistons or blowing up cylinders, then every steam engine can be switched from full steam ahead to full steam backwards in simply and eyes blink.
Because: If we do a snapshot of the motion, we can understand why, and why mechanical overloads play the only counterpart:
In our snapshot we consider only one piston, and it's in upwards move! So the steam is below the piston, pushing it up to the cylinder top, and the top valves are open to leave the steam above the piston to the exhaust.
So what happens now, if the gear is switched?
Well, the bottom valves were first closed, lowering by the valve gear from the common march rounds (I guess approximately 40% of the pistons travel steam is let into the cylinder, then followed the cut off and the expansion, so this we in German call the 'filling' , or in english 40% cutt-off) Many triple expansion engines shout not be driven with less than 50% cut off, just to provide enough steam which could be expanded in the LP cylinders, but for our example I just use 40%. So if the gear is switched, the bottom valves were reduced in filling, immediately. From 40% down to 30%, 20%, 10% and last 0%. At 0% percent, to valves won't open, neither bottom nor top valves, regardless which position the piston has. So what happens?
The steam inside the cylinders, because to bottom valves were closed can't go anywhere, because even the steam at-top the piston is cutt-off, with no possibility to exhaust. So this steam at-top is expanded, having little expanded power left, which now works as buffer, so the top-ward travel of the piston will be slowed down by this 'brake' force of the left steam in the cylinder, and the less expansion steam below. At zero, no steam can go anywhere and we have no real power atop or below the piston, so motion is not really possible. Now the gear goes through the zero point, switching the valves. So the top valve, which was exhaust will not connected to the boiler, letting steam enter the cylinder at top, while below the piston the bottom valves are opened and connected to the exhaust. The little more powerful, if so, steam below the piston will rush through the exhaust, and the fresh steam at-top the piston will now press with the boilers pressure against the piston surface, pushing the piston down, direction is changed.
so, basically any reciprocating engine can be switched from one direction immediately to the other.
But now we come to the mechanical frontiers: Titanic has a triple expansion engine. so we have three pistons in motion, three huge cranks in motion and the cranks and weights from the gear also in motion. Each weight in motion means a mass in motion, working like a motion wheel, providing a directional power, even if the engine delivers no power anymore. So, I cannot determine how much this was, because there is an unknown force coming from the propellers, because if you close the valves the propeller will not be turned against the water, so the water will do a force against this momentum, slowing down the turns of the propeller. I can not determine how much this is, but the mechanical momentum must be still high. So Titanics Engine will, if our valve gear is closed still turning in the forward direction, even if the steam provides no power anymore to the piston the same strength as before.
So if we switch Titanics engine from full ahead to full astern, we will ensure maximum boiler pressure from the other direction of the piston against the motion force. Consider, in common steam railway engines this means 40 Tons of power from the opposite direction, and with the left over motion force, we can be sure, this might could be over the mechanical load the cranks, bearings, rods and bars will take. Like hard water hammer, this can also knock out the piston from it's rod or just blast the cylinders or cylinder seals. No good idea.
But: There is a critical round number, where the switch can be done, without going over the mechanical frontiers, also most modern steam engines got safety. so here one can switch from ahead to astern without mechanical overloads, and this magical point right now is the knowledge and 'feeling' of the chief engineer.
so here the safety valves will not blow of, and the bearings won't get damaged, and the bars won't bend or break.
So even Titanic had such a critical rounds number... So many tugboats which shift large ships in harbors had little often only double expansion engines, which turn quickly and fast, but with less motion powers, so these engines were often switched from full ahead to full astern, as needed in those shifting actions, so here the critical rounds were much higher set, as in large engines, like Titanics one.
And I do not know, and be unable to determine how quick titanics engines will get from full ahead below the critical rounds count, to switch the gear to full astern. But, one point: You will be quite faster in direction change than in turbines, because turbines act like large motion wheels, so need to slowed down more carefully than the only up and down going weights of the reciprocating engines, which came much more quickly to the switch point, and in emergency, the chief engineers will act somewhat closer to the mechanical overload point or little more above, and this is even not possible in turbines, because damage of the turbine wheel might result in complete failure of the propulsion plants. In reciprocating engines it might only lead to a damaged bearing or slightly bend rod or a blown-through sealing, thus nothing a crews cannot handle or what will set engine completely out of order. Even a broken rod won't matter, because if two intact rods left, you just dismount the rod, switch off the cylinder and go to harbor at reduced speed with remaining two intact cylinders..
Consider: Even today you can't do with modern diesel engines most times ;o)
Also, many Turbine ships can go faster, because having another boiler plant. Turbines need a continuous steam stream at a special pressure, and if the turbine had to go faster, more steam is required, not more pressure. So large water boilers provide much steam reserve, are slow acting boilers, which cannot be altered quickly to changing steam demands, like turbines have, if they were switched in speed or suddenly stopped. so here commonly fast reacting boilers, as the common water tube boilers of the Samson principle or the single-pass-through principe are used. Those boilers can deliver certain amounts of steam and alter more quickly as smoke tube boilers, like the scotch marine boilers (three pass smoke tube boilers).
so it's not a wise thing, to use water tube boilers with reciprocating engines, as reciprocating engines have a maximum steam demand, all other things is done by alter the pressure, so it better to have a boilers with large reserves, like double drum Samson boilers or common multi pass smoke tube boilers (Stephenson boiler principe).
So turbine propulsion plants require less space, and have a lighter weight, which made them desirable for the military, but: Turbines cannot compete with reciprocating engines in altering rounds and torque force! In shaft power, well, that's another point, but shaft power isn't everything, if we discus about 'racing'. Thats we you don't find tugboats with turbines, because those sea-tractors need torque power, not only speed, and the need the ability to quickly switch round and direction output.
And avoidung u-boats isn't a thing of speed, more a thing of manoeuver-ability, because strong and quick direction changes help to get hit by torpedos, and not the speed. Because torpedos, which in WW II are propelled by four cylinder reciprocating steam engines go 48 kn, much faster than still common war ships go today, so avoidung the hit is the trick, not the run away ;o)
