Feedwater is distilled water. A steam ship must have enough distilling plant capacity to feed the boilers and provide potable water for the crew. The boilers require much more water than the crew. The storage tanks are segregated and if the water in one of the feed storage tanks becomes slightly contaminated, that water can be transferred to the potable water tanks; the boilers need water that is more "pure" than the crew needs.
Before lighting off the plant in port you can fill or "top off" the feedwater tanks from shore supply, usually tanker trucks; but some shore stations had feedwater piped to the piers and you could hook up by hose connections. Once underway you were on your own and dependent on the distilling plant. Distilling seawater is very simple, it just takes heat. Even a small ship uses thousand of gallons of feedwater a day; the two distillers in the KIRK produced 12000gallons a day and both were running most of the time. If one went down for some reason, the crew would be put on "water hours" (strict rationing) until it was fixed.
Once the plant is on line and ready to go to sea, the pressures and temperatures in the steam system are always the same, based on the boiler steam drum and superheater "set points" and various regulators to supply auxiliary steam and "Hotel" steam. Changes in speed (or even electrical load) are adjusted for by increasing or decreasing the boiler firing rate to increase or decrease the volume/flow rate of steam being supplied to the main engines and turbogenerators...the boiler(s) should always operate at the same pressure and temperature...the "set points". Burning more fuel or less fuel does not change the pressure or temperature in the steam system, it changes the amount of water converted into steam.
Back in WWII that was not the case in some ships. The steam pressure was maintained at a constant value, but the temperature could be adjusted coming out of the superheater...cutting burners in or out.
The capability to respond rapidly to speed increase demands is dependent on the number of boilers on line; or, in those ships with variable superheater outlet temperatures, by cutting burners in or out (we still had a few of those around in the 60's and 70's for guys like me to get trained on). Modern steamships were built with a fewer number of larger boilers and combustion control systems that made it possible to cruise economically on one or two boilers, but respond quickly to high speed demands without having to bring another boiler on line right away. "Full power" was the only time you really need all boilers on line and that very seldom happens without plenty of advance notice.
In WWII...economical cruising was not generally a big concern...you had the number of boilers on line that you would need to respond to any expected speed demand. Ships in a Task Group or Task Force refueled alongside an oiler every three or four days to keep at least 85% fuel onboard; we continued doing that in peacetime. Even nuclear aircraft carriers did that...keeping aviation fuel "topped off" every few days.
Hope this helps some more...
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