It would all depend. If the engine room and boiler room were cold iron with shore steam I would guess 6 to 8 hours.
If one or two boilers are on line to power supply steam for the hotel generators the time could be cut to 30 to 60 minutes until the ship could be moved at reduced power and speed.
While lighting off the cold boilers the main engines could be lined up. But the engines would need to be warmed up before getting under way. The steering gear would also have to be started. And a lot of little things. On the ships that were turbo electric warming the engines may not be necessary.
I did some googling and apparently the normal time to get a cold engine up to full steam was twenty-four hours. But that was using all the normal procedures. Apparently in an emergency it could be done in as little as an hour. But doing it that fast risked causing some serious damage to the engines.
One battleship, the Nevada, was able to get underway during the attack. It happened to have two of its boilers running when the attack began so it was more ready to move than it normally would have been in harbor. Even with this advantage, it took the Nevada forty minutes before it could move.
To answer your other question, the best thing the American Navy could have done if it had a little advance warning was deploy torpedo nets.
But that assumes the Navy would have realized this was needed. The ships were anchored in pretty shallow water - about forty-five feet. And WWII torpedoes that were dropped from a plan dived deep into the water - about seventy-five feet - before coming back up. So the Americans though a aerial torpedo attack was impossible in Pearl Harbor, which is why the torpedo nets hadn’t been deployed.
Among the other surprises it inflicted that day, Japan used specially designed torpedoes that worked in shallow water.
So it’s possible that even if the Naval commanders had gotten a warning and had enough time to deploy torpedo nets, they might not have realized the need to do so.
From my very limited understanding torpedo nets were a pain in the ass to deploy. I have no idea how long they took to put up but little advance notice is probably not enough time if “pain in the ass” to do so is part of the equation.
My sense is it was something done in particular circumstances when you didn’t think the ship would be moving for a while. Not something you would do in a safe harbor (which they considered Pearl Harbor to be) and not something you would have time to deploy when you heard planes were coming to shoot you (the planes would arrive long before you managed to deploy the nets).
Indeed you can see booms on WWI era warships (those slanted things on the hull) to deploy torpedo nets but they were not seen on WWII era ships. One can only assume the navy didn’t think they were worth the trouble.
FWIW: A ‘Scotch’ boiler has no pumps: the water circulates using thermal circulation. That means that if you just light up the furnace, it will burn out: the local water will turn to steam, and the fire will burn out the pipes. It takes several days to start from cold.
If it’s not a Scotch Boiler, the water is circulated by pumps. Steam pumps or electric pumps, or some electric pumps for startup and steam or thermal pumping once it’s hot.
Pumped boilers don’t take as long to start as Scotch Boilers. and If you have some boilers already going, or can take electric power from shore, or have a separate smaller power source to run the pumps, you can start much faster.
"Modern’ warships of the era were able to start much faster than typical civilian ships of the same size, partly because of design and partly because they were fully staffed and cared less about fuel costs. If they were in port and the boilers were not fully staffed, it would have taken longer to bring them up to steam.
Of course they still can’t accelerate worth a dam: if you put on full power on a stationary ship, the propeller just spins. I believe that warships had better acceleration than civilian ships (again, they had better design and cared less about fuel costs), but I don’t know how fast those big warships would have been able to pull away from dock even with a full head of steam.
What would the benefit be of being out at sea? A battleship can’t maneuver fast enough to “dodge” much of anything. Now, getting the crews awake and manning the AA guns, that would certainly help, but is there any reason they couldn’t do that while still sitting in harbor?
My dad was in WWII and (so he told me) in the first convoy into Pearl after the attack.
He told me that had the ships made a run for the sea they could have been sunk in the narrow entrance/exit channel which would have rendered the whole harbor useless for a long time (it is not a deep channel…a big ship sunk there would block the channel).
IIRC there was a ship making for the open ocean which got attacked and, realizing sinking in the channel would be bad, instead intentionally ran aground to keep the channel clear.
So, getting a ship that is slow to come alive through the channel and out to sea was probably not a good idea in this case. They’d have to make that dash before the planes arrived to make the effort worthwhile. Once the planes were there best to just man the AA guns and do the best you can.
To light off a scotch boiler you do just light the fire off. You do not put in the biggest burners or light off all burners. If you heat a boiler too fast tubes and other thing will come loose. Tubes are thin walled compaired to tube and crown sheets.
Marine water tube boilers have no pumps on them to circulate the water it is all natural.
The pumps that a boiler need supply the boiler with water not circulate it.
A scotch boiler takes longer to get up to pressure because it has more water in it and less heating surface area.
Since a ship at sea is moving and under air attack, doing its best to avoid being hit. With early WW2 sights it was difficult enough to reliably hit a city, the Germans sometimes were not sure what the target was after a raid. Now imagine how difficult it was to hit something which was small, maneovering and shooting back.
The single biggest thing the US could have done wish a bit of advance notice would be to get a whole bunch of the US aircraft in the air.
Several classes of aircraft were obsolete, and none of the fighters were matches for the Zero, but the P-40s, for example, had good results on the historic attack.
The US fighters would have been devastating against the bombers, especially since the Japanese did not have fighter coverage for their bombers during the attack. As was seen by the US at Midway, sending “low and slow” torpedo bombers against fighter flying CAP was suicidal.
First, the Japanese would have to find them, and say had the battleships departed early the night before they would not have been able to locate them. The Japanese did not have scouting planes looking to the south, east or west of Hawaii.
Had the battleships simply gone out into the ocean close to the coast and been detected, the US would not have had as many BB sunk and damaged. In the first wave, the Japanese had 49 Kate bombers with custom designed amour piercing bombs. The Arizona was sunk by these bombs. Attempting to hit moving, maneuverable ships at sea with level bombers was quite difficult.
There were 40 torpedo bombers, but they would not have been able to attack as many BB, and if the US had fighters flying CAP, then it would have been much more difficult to sunk the US ships.
In addition, in a harbor, a ship’s AA were limited because of the danger of hitting ships next to it.
OTOH, the IJN had about 20 submarines operating in the waters around Hawaii and there would have been a danger from that.
Actually not. The British had sunk one Italian battleship and damaged two others in their attack on Taranto. What is most often quoted is that up until 10 Dec 1941, when Prince of Wales and the cruiser Repulse were caught without air support, no battleship at sea had ever been sunk from the air alone.
Actually not. Alan Zimm addresses this in his book, Attack on Pearl Harbor: Strategy, Combat, Myths, Deceptions. The channel was wide enough that even had a battleship been sunk in it, it would not have blocked the channel.
It was actually a tremendous benefit to US strategy that the BBs were sunk at their berths at Pearl. If they had gotten out to sea, naval strategy would still have been built around them, and that way lies madness in a world of aircraft carriers. Plus the fact that by sinking in shallow water, inside a naval base, the battleships became rather easier to salvage and rebuild. By then, of course, they had been relegated to shore bombardment duties, which is really all they were good for anymore.
That may be true if the ship sunk perfectly parallel to the direction of the channel but if it swerved for whatever reason and sunk at some angle it would block a lot more of the channel (not to mention if it tipped over when it sank…they were tall too). The BBs were as long as that channel is wide (or near enough).
Not a risk worth taking. I do not think any of the entrance is more than 50 feet deep so it would not take a lot to block it.
With hindsight many have noted that the Japanese not attacking the dry docks and oil facilities was a strategic mistake. Had those been badly damaged Pearl would have been out of action for a lot longer and unable to do the near miraculous repairs on USS Yorktown which enabled it to participate in the Battle of Midway (estimates were the Yorktown would require months of repair after the Battle of the Coral Sea…they did it in three days).