I’m watching an old war movie involving a submarine during WWII (Operation Petticoat) and the captain needs to get out of a situation in a hurry and barks out the order ahead 2/3rds! Really? Why not ahead 15 knots? Why is it better to use fractions instead of actual speed?
Ahead 2/3rds isn’t a speed. It’s how much you open the throttles. It may be 15 knots in calm seas, but it may only be 10 knots in rough seas. The guy operating the engine isn’t navigating. He doesn’t need to know the actual boat speed.
I assume it’s somewhat related to one of these, no?
To expand on engineer_comp_geek, the throttle setting isn’t directly related to the speed. Rough seas, currents, and wind all contribute to the final speed a ship will have. The speed over-ground and speed through-the-water are different, and a throttle generally isn’t calibrated for speed.
Ships with variable-pitch screws will also generally run the shaft at a fixed RPM and adjust speed with pitch. I was on a ship where the “throttle” setting was specified in feet of pitch.
And multi-screw ships can run one screw slower than the other, or even use one backing and one ahead, for increased manuvering ability. What speed in knots would you specify when one screw was backing at 2/3 and the other was ahead 1/3? 
I’m pretty sure that the Cp’n said pretty much exactly that when the sub first left the dock, along with ‘rudder commands’, fwiw. Or that is what I seem to remember watching it myself last night 
I am not even sure if that sub was a twin-screw, though…if not, never mind.
The submarine was (probably) supposed to be a Gato class. (The movie subs were Balao and Trench class – SS-285 Balao for the pink sub, SS-393 Queenfish (Trench class) for the opening and closing scenes, and SS-311 Archerfish for the grey sub.) The Gato class had four Diesel engines powering four electric motors driving two screws.
Any respectable modern captain would ask for “ahead 110 percent”
But the Sea Tiger didn’t have 4 engines. They stripped #3 & #4 to rebuild #1 and #2. 
True true, but in their defense it was much easier to reprogram the computers then reengineer the ship
That is called an EOT. Engine Order Telegraph.
Ship speeds are never ordered. To dificult to order because of changing conditions and on vessels with twin screws it becomes impossable.
Merchant EOT
The EOT you have linked is probably for a merchant ship. Stop, dead slow, slow, Half ahead, and full. Also the same in astern. Also Stand By Engines and Finished with Engines. For emergency ahead or astern ring it against the stops twice.
Navy EOT
Stop, 1/3 ahead. 1/2 ahead, Full Ahead, and Ahead Flank. Also the Navy EOT will also have RPM settings.
Speed changes are the number of turns of the shaft.
When I was in the Navy, this was phrased as “All ahead Bendix”.
(Bendix being the manufacturer of one EOT, on which their name appeared in small letters just past “Flank”).
What does it mean when a sub captain says “make revolutions for 10 knots.” Obviously, revolutions are the propeller or propeller shaft, but what does the order mean? Does he say that to the navigator who calculates everything to make the speed 10 knots and then passes the specific orders to the people at the controls?
The OOD should know how many turns it takes to make 10 knotts. He will tell the man at the EOT how many turns he wants the main motors to make.
He’s telling the OOD/conning officer to do what it takes to make 10 knots. He may ring up ahead 1/3 which is (for example) 100 RPM and 8 knots in calm conditions, and adjust RPM to make the actual speed to average 10 knots. There can be some trial and error to find the right RPM.
For example, “All ahead one-third, make 110 turns for 10 knots.” The “10 knots” in this case isn’t a command, it’s just for logging purposes. After 30 minutes, the OOD/conning officer looks at the actual speed made, and may say “All ahead one-third, make 112 turns for 10 knots.”
Thats funny. Thanks for sharing that.
In his book War Fish, George Grider tells of a running torpedo and gun battle the USS Wahoo took part in during WW2. With batteries depleted, the Wahoo was on the surface, pursued by a Japanese destroyer. The captain called down to the engine room for maximum speed. The engine room replied that he had it. Then Grider (third officer) mentioned to them that there was a destroyer astern closing fast.
That got a sudden 3 knot increase in speed.
Grider also mentions that the speeds available were Ahead 1/3, Ahead 1/2, Ahead 2/3, Ahead Full, Ahead Flank and Ahead Pearl. The last one was the speed they hit when they were out of torpedoes and heading home.
I haven’t seen that movie in years, but I do recall clearly that the sub was a US WWII era “fleet boat” (Gato class or something similar), which are a particular hobby of mine. Yes, they are twin-screw.
I got nothing, except to note that “Twin Screw Boat” sounds like the title of unusually good porn.
HBO was going to have a series with that name.
It was an “adult” reboot of the Love Boat starring the Olsen twins.