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  1. Does Revolutions XX00 mean the RPM of the propeller shaft?

  2. Why do the crew talk about propeller shaft RPM? What are they communicating?

In The Last Ship that features the USN, I never heard this metric. Sea Patrol (2007) portrays the Australian Navy. s02e03 Episode Script | SS

Port 10, steer 315, revolutions 1800.

Alright, that gives us All under control, sir. Revolutions 2000.

[At 43:55/45:00] Up ahead, both engines. Revolutions 1800.

Sea Patrol (2007) s02e13 Episode Script | SS

Take us round the headland.
Half ahead both engines.
Revolutions 1000.

Bring us up to full steam.
Half ahead both engines.
Revolutions 2000.

I've got it, Nav.
Half ahead both engines.
Revolutions 2000.

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As Paulie_D has stated, Revolutions is short for Revolutions-Per-Minute. Your ship example is not as uncommon as you think. Many industries put more emphasis on RPMs to measure power output than they do on the actual work done. RPMs is a much more consistent way of measuring power output when the power is being supplied to an array of variable gears or working against a fluid and/or dynamic medium like air or water.

An example of this is the car racing culture. Your average daily driver puts the speedometer as the main performance instrument. On the other hand, a race car will have the Tachometer as the main and usually largest and most central performance instrument. Aircraft base most of their performance calculations on RPMs and manifold pressure. Cyclist will focus their attention on cadence (RPMs), using that to convert their energy output to watts.

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Does Revolutions XX00 mean the RPM of the propeller shaft?

Yes, it does

Why do the crew talk about propeller shaft RPM? What are they communicating?

Because they know from experience what revolutions will produce the correct amount of power will produce a specific speed in the water.

It no use to the engine room to tell them "Make speed 25 knots" because it will mean nothing to them.

Tell them to set the engine to specific power setting they can monitor is simpler and more precise.

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  • The Hunt for Red October uses similar terminology for a Russian vessel, blended a little differently. After the MHD propulsion goes down, Ramius orders "Make revolutions for 20 knots and engage the propellers." Presumably somebody else will pick the shaft RPM to give the desired speed; the captain doesn't have to bother with the math. – T.J.L. Mar 4 at 13:56
  • "It no use to the engine room to tell them "Make speed 25 knots" because it will mean nothing to them." Why will this mean nothing to the engine room? The engine room can pick the RPM that will yield a speed of 25 kts. – Accounting Mar 4 at 21:19
  • @Greek-Area51Proposal - It’s more about the fact that 25 knots is arbitrary to a ship. Your ship’s speed is relative to a reference. That reference can be the water, a land feature, or a GPS coordinate. It can also be very subjective to the operator of the vessel. On the other hand, RPMs are a fixed reference of performance. The engine room would be more concerned with measuring performance through RPMs, engine pressures, engine heat, torque measurements, horse power/wattage measurements etc. than they would relative speed. – Dean F. Mar 5 at 18:41

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