Why did the British Motor Torpedo Boat go full ahead, not FLANK ahead, to attack...

In this scene from Mckenzie break (1970), Captain Jack Connor reports the surfaced UBoat to the MTB, for the MTB to strike the U boat. It's blindingly obvious that the MTB has to hurry, to stop the UBoat from boarding escaped Nazis who escaped from a prisoner of war camp, and from diving away.

In actuality, the MTB does show up too late, as most prisoners do embark on the UBoat and the UBoat does dive away!

But why did motor torpedo boat (MTB) just go FULL ahead, not FLANK? If the MTB sped up to FLANK, then it could have gotten on the scene faster?

After watching the clip - the crew of the MTB appear to be British, ordering British commands. Given what I have said below - they do actually order the equivalent of the American term of flank speed.

The edit of the wrong EOT (engine order telegraph - they show an American one instead of a British one) means that someone uninformed matched the word 'full' to 'full' instead of using a clip of what the British word means on an American equivalent device.

tl;dr:

Bit of an issue here, and not a direct answer to the question:

The MTB is a British Motor Torpedo Boat of the Royal Navy - the bridge-to-engine room engine order telegraph / throttle repeater - whereby commands are sent from the bridge to the engine room - seen here is an American device.

The USN only use these commands, the British do not.

The British would have in fact ordered full ahead.

But that in of itself would be incorrect too, as full ahead is the same as flank speed in USN terms - ie. emergency use only - the load generated on the engines and the rate at which the fuel is consumed at this setting is unsustainable and for short periods only, as in emergencies when something is going very wrong (the only other time would be at sea trials after construction).

So, whilst full ahead could have been ordered, like flank, it would have been short lived and they would have spent most of the time at half ahead at a certain amount of revolutions.

• Slow is the minimum speed for still being able to steer, the revolutions being set to achieve around 5 knots.
• Half ahead would actually be standard speeds at sea, everything inbetween slow to full, and set additionally via the revolution telegraph.
• Full ahead is for emergency use only and maximizes both engine power and revolutions.

(Much of this has changed with modern technology whereby you have control of speed via blade pitch instead, as an example - so you might feel the speed change but not the engine as its throttle could be unchanged)

(and again the Merchant Navy has different settings as well)

The differences really did cause an issue for those unaware: as a US pilot might order full ahead thinking the vessel would now engage at full speed, only to be thrown backwards to the bemusement of the British crew knowing that the pilot had accidentally ordered the equivalent of flank speed instead.

Incidentally, the submarine shown at the end is a GUPPY-modified post-war USN ww2 fleet submarine, further muddying the (Turkish) waters (where the submarine sequence was filmed - given that the US transferred numerous GUPPY modified submarines to Turkey post-war, this is probably a Turkish Navy submarine).

https://en.wikipedia.org/wiki/Flank_speed

Allegedly an engine order telegraph from a period RN MTB:

https://www.clipper-maritime-antiques.com/ships-engine-telegraph/antique-ships-bridge-telegraph-from-a-royal-navy-motor-torpedo-boat