How long did it take a typical capital warship to ramp up to flank speed?

Question

For a typical capital warship (including aircraft carriers) of any navy circa 1920s-1960s I am looking for a rough power ramp up time for the power plant to go from min load to max load.

That is to say, if you are cruising at a dead slow speed making turns for only a few knots, with all boilers lit and cut in on the line, how long would it take the ship to respond to an engine order for flank speed?

I'm not looking for the actual time it takes the ship to accelerate to flank speed, only the amount of time for the power plant to ramp up to where it is producing maximum output shaft horsepower.

Anyone have any information on that?

Answer 1

TL;DR: that is difficult to say. Mainly due to technological limitations and fluid dynamics, and as such there is a correlation between output of the engines (or reactors) and the current speed of the vessel. In other words: time to ramp the power plant to max output is THE time ship needs to accelerate to flank/emergency speed. For USS Iowa it will be about 45 min to emergency speed (assuming 15 kt comfy maneuvering speed), about 120 min during peace time for flank (from same 15kt). The difference between flank speed and emergency speed is in the time to get there, as the latter is wartime.

Full answer: In fact, you should ask about time to flank speed, as this would be your only measure of maximum output shaft horsepower (on all shafts). The power delivered to the shafts is just enough to propel ship at desired speed, and a bit more if ship is accelerating, and the power output is increased in increments as long as the target speed is not achieved. And this is where we start getting into trouble with said physics and fluid dynamics.

For example, HMS Canada could cruise at 14 knots using less than 18% of nominal power, yet reaching 22 knots required her full (nominal) power of over 38k shp (303 RPM). And during trials she exceeded 24 knots, at 52.6k shp (and propeller shaft RPM exceeded 335). See "British Battleships of World War One: New Revised Edition" for more details.

So you see, the problem at some point is the water... Because at those speeds water is... well, hard would be one way of putting it. Another is: when one goes that fast it's not a lubricant anymore...

Another problem is cavitation, which was recognized well after 1960s, and which reduces the efficiency of powertrain.

And last but not least... With the exception of nuclear-powered ships designed from keel as such, ships' designs are optimized for cruising speed (which is not the max speed), and those designs are not efficient at high speeds. There are ways to alleviate that, but obviously older designs would be deficient in that regard too, sometimes greatly. This has to do with fuel consumption, which is not a concern for nuclear-powered vessels. So while it's not in your question, I would recommend to do any comparison on era-by-era basis. To illustrate the difference of the hull optimization: USS Iowa was rated for power output of 158MW. That's slightly more than 80% of the Nimitz Class CVs 194MW. Both are rated at 30+ kts max speed, so it shows the problems, limitations, general approach and specific solutions used: battleship with a full load displacement of a bit more than half of the Nimitz Class CV's (58kt vs 102kt) has that much power and is just a smidge faster than a CV. The only real difference is that Iowa is rated for 10000 nautical miles of range, Nimitz's range is 25 years and fuel for all his aircraft.

So. By now you should see the problem with your question...

Typically battleships of that size (25-30k tons displacement) required about 50-60 min to reach maximum speed. They could reach maximum power only during max-speed runs, as power produced by the engines must be dumped somewhere, and you literally can't run ship's propeller shaft from start to 225-335 RPM immediately. If that would happen, the propeller shaft would overheat and warp in seconds, most probably, and the propeller itself would break off (big ship = big propeller, which is HEAVY), quite probably before the warping.

USS Iowa, when on her trials, reached contracted 32.5 knots, and that required about 212k shp. And she was running light for that (so no full bunkers, no full ammo load, no full crew), as well as it was a shallow water run.

And one must not forget that USS Iowa was built with designed overload of 20%, so we don't really know how fast really she could go. Same thing would apply to most WWII-era ships.

According to an article I found USS Iowa requires superheat condition on her boilers if the required speed is above 22 kts. Going from "no superheat" to "full superheat" requires 30 min. Only then she can hit her top speed of 27 kts. If we assume the 205 RPM of the propeller shaft to be at her max speed - that is the 32 kts - then to reach that speed, during trials, required increasing the shaft RPM by 10 RPM every 10 min. So it gives you simple calculation that it requires 220 mins to reach flank speed from rest. From Class trials in 1985 (i think, when reactivated) we know that it takes about 25 shaft RPms to add 4kt, but that same 4kt require doubling the engine output (and these go at about 4200 RPM at 202 shaft RPMs), which cannot happen instantaneously.

However, from the description of the "Close the barn doors" stopping maneuver (done once by USS Wisconsin), it is necessary to go all back full from all ahead full, and this happens within 700 feet (so in less than a minute), mclearly requiring the dump of all power to shafts in under one minute, basically immediately (with obvious all stop somewhere in the middle).

Bottom lining the answer: USS Iowa will hit it's not disclosed emergency speed (though fastest of the class reached 35+ kts) in about 45 mins: 30 min to reach superheated status on boilers and max speed of 22 kts, then about 15 min to ramp it up all the way up to emergency speed, 2 min or so for every 10 RPM from 130 to 225.

Answer 2

A major limiting factor on rate of acceleration of an oil fired steam ship is how quickly the boilers can be brought to full pressure.

Large ships, such as battleships and aircraft carriers, have multiple boilers, and at cruising speed will only be operating some of them, as running all boilers at reduced heat is very inefficient. To achieve full power, all boilers have to be operating at their maximum pressure, and that means bringing cold boilers on line. This takes some time, as the boiler must be heated evenly and slowly, lest parts of it expand too quickly, which could result in a rupture, and a horrible death for anyone in the boiler room if that happens.

Here is a simple description for bringing a water tube boiler online. Fire for five minutes, wait 15 minutes, repeat until steam comes out of the bleed line instead of air (this usually takes eight or nine cycles), then slowly step it up to full pressure by firing for 30 minutes, and then waiting ten minutes. The whole process can take three to four hours, though it can be rushed a bit if the alternative is sinking, albeit while placing the machinery room crew at some risk of being boiled alive if a steam line ruptures.

If we assume the warship is operating under wartime conditions, and the crew has kept the inactive boilers at least partly heated in anticipation of a change of speed, all boilers could be brought online in around two to three hours.

So, to begin with, the battleship captain must allow a minimum of two to three hours just to get enough steam pressure on all boilers to even contemplate high speed. And then there is the matter of accelerating a mass of 30,000+ tons in water, after all boilers have achieved operating pressure.

This isn't as much of an issue with nuclear powered warships, as the heat is applied to the water in the boiler internally with a heating loop, not externally by first heating the boiler. The water itself acts as a thermal buffer for the boiler, so they can be heated more quickly without as much concern for uneven heating and thermal shock.

The US navy has abandoned oil fired steam plants entirely, replaced with either nuclear plants or gas turbines. The gas turbines are much lower maintenance, and can be brought to full power in a matter of seconds.