That would matter in a swell from ahead. Watching the speed with a short averaging period it can be seen (and felt) that a dip into a larger swell will knock the speed down significantly. Speed will recover afterwards only slowly in the smaller sets.
Same thing with the old anti-piracy zig-zag, that really bleeds the speed off fast if done too aggressively.
I am not sure of the correct steam term, but is there a measurement for steam “storage”? Obviously heat in = steam out in the long term, but if there was extra steam just waiting you could possibly get full torque right that second and then turn up the fuel to catch up before the pressure dropped??
I was aboard an ancient steam tug once with a 300 HP piston engine. I asked someone how they managed to tug things with only 300 HP, and they said the instant full torque allowed the tug to do a lot with only 300 HP.
The operative measures are the pressure, weight and rate of steam delivery, and its temperature. The boilers on a typical typical steam tanker of around 125,000 tons would each deliver 100,000 pounds per hour of steam at 900 degrees F and 900 psig. This was enough to power a 30,000 hp turbine and turbogenerators. Just for trivia, it took almost as much steam to drive the cargo pumps for offload.
The majority of the stored energy will be in the temperature of the system, most of it liquid water. There will also be some energy in the form of pressurized steam, but I suspect that will be trivial in the big picture. If the burners flame out, the turbine will keep running for a while, and I’m curious if we’re talking seconds or minutes before you get a noticable power drop.
Thanks, those figures help, but I need to know how much water is in the hot side to calculate the amount of stored energy.
It doesn’t work quite like that. Steam is produced at the same rate it is removed or pressure will quickly rise or fall. Heat input has to match steam production or pressure will rise or fall. There is massive “inertia” in a large steam plant and things don’t happen instantly. You can tweak a setting now and its effect may not be seen for another ten minutes or more.
If the boiler is running at max load and the throttle is instantly closed, steam flow virtually ceases and drum pressure rises rapidly until the safeties open. The furnace and tubes are too hot to cool without water flow and even though fuel is reduced very quickly it takes time to cool the system to reduce pressure, same for powering up, it takes time to go up or down and maintain equilibrium.
When an automated steam plant is in bridge control an overly enthusiastic mate can wreak havoc on the plant by rapidly opening the throttle. Pressue drops, water level “swells” the feed pumps suffer a panic attack and might trip on overspeed, the DC heater level drops and auxiliary steam pressure falls. Not knowing why the mate took that action (it could have been to avoid a collision) the engineer can only call the bridge and tell them they will lose the plant very soon unless they do something different.
Great explanation! I was thinking more like a 1/2 HP air compressor running a 1 HP air tool briefly from the stored air in the tank.
The stored energy in hot water makes sense to me. My friends are into steam locomotives and told me of an accident near here caused by mineral deposits. The sight tube got a bunch of crap in it and got hard to read, the engineer thought he had enough water, he was wrong, part of the boiler not designed to be dry was dry, it cracked, and all the water flashed to steam instantly and launched parts of the locomotive all over the place 
Then you need to look up the boiler specs. From an operational point of view, if there is less water than the low-low trip or more than the high-high trip the boiler is either melting or flooding the system with carryover. Neither condition is good.
The amount of “stored energy” depends on the rate of steam extraction and the volume of the steam drum. The only steam storage systems I have ever heard of are those of little locomotives used in factories where flames were not acceptable, like explosives plants. The concept doesn’t really apply to marine steam.
Excess fuel going into the engine at low rpm and not getting burned vs the engine controls not injecting excess fuel in the first place is a different scenario in that one is wasting more fuel but from the standpoint of the end goal, increasing rpm, it’s a similar situation.
True. This is also why airplanes with variable pitch propellers get off the ground quicker than fixed pitch, with fixed pitch you get “stuck” at less than 100% power for awhile until speed builds up and then RPM picks up. You get full RPM right away from the variable pitch prop. (you don’t directly control pitch, you set a target RPM for the prop governor)
The scale didn’t cause the Gettysburg disaster, it was incompetence and lack of maintenance. No one knew how to use test cocks to backup a dodgy gauge glass.
As Steamer explained there is no steam storage. Steam systems are like diesels in that they are heat machines. They require heat to be moving constantly or they shut down for various reasons. Fuel and air are supplied based on demand for the work to be done in both cases. A boiler also requires water for the steam to form which makes things go round and round. If you change the flow of fuel or air to a diesel its output decreases or increases. Same with steam but with the added need for water. Steam is a symphony with many more parts needed to make it work. Engines on the other hand are limited only by their design parameters. Low speed engines are not very forgiving but they are much simpler than steam plants, less expensive to maintain and don’t require the engineering personnel to have the experience a steam plant does. Steam is fading into oblivion for those reasons.
That has always been my way of describing a marine steam plant. It is what makes the job of conductor so fascinating, satisfying, and maddening at the same time. Jeez, I miss steam!
Going back to the analogy of a underpowered car climbing a hill in high gear. On a vessel, reducing the pitch on a CPP must be the equivalent of down-shifting the car.
You’re moving on a slippery road now.
In a car, down-shifting allows the engine to increase rpm to deliver more horsepower. The car will still slow down if rpm is not increased. The reason to downshift is because the engine could not deliver enough power at the reduced rpm of a higher gear and the car slowed down due to that lack of power.
Your ship with a CPP wheel offers much the same versatility but is equally constrained to limits of load and rpm. If you maintain rpm but reduce pitch your boat will slow down, reduce it enough it stops. The fuel burn reduces because you produce less power at low pitch than at the same rpm and higher pitch. If you don’t want to slow the boat, you need to speed up the engine when low pitch is selected. The combinator is programmed to control the best combination of rpm, pitch, and power without breaking something but if you manually control those elements you risk wasting fuel or overloading or not operating efficiently. In a car that would be like always running in low gear or high gear, you either get nowhere while making a lot of noise or you overload the engine most of the time.
If a car is kept in the same gear the relationship between vehicle speed and RPM is fixed. Changing gears in a car allows that relationship to change.
With a fixed prop ship the relationship between vessel speed and RPM is also fixed… Changing the pitch of the CPP likewise allows that relationship to change.
Kinda, with one important caveat: In a car, selecting a lower gear increases the torque to the drive wheels, also increasing the likelihood of wheel spin. Some vessels have enough power to shake the screw loose at high pitch / slow speed. I guess that doesn’t apply to big ships, but it certainly applies to a lot of the old mini coasters that have been re-powered a bunch of times, and now have a high speed diesel three or four times as powerful as the original engine, still running through the same old CPP. In such cases, you want slightly finer pitch and a bit of revs on in heavy weather, to avoid losing thrust and gaining all manner of noise when she digs her nose in.
Yes that make sense.
I was thinking about this:
With a low-speed fixed prop the automation reduces the rpm to avoid the torque-rich situation. The automated CPP vessel must reduce the pitch in that situation.
That depends on the combinator and its programming, it could be operated in power mode, rpm mode or shift into load mode when limits are reached.
It very much does that in an airplane, but it is somewhat transparent to the pilot.
Back to steam, is there some computerized controls that could be grafted onto a turbine that would adjust prop pitch, number of nozzles, steam pressure, etc, etc, to the optimum configuration?
No steam plant I have ever sailed in has ever had a CPP prop. Steam reciprocating engines deliver torque and the old minesweepers that I sailed in were supplied with “passage propellers” of a coarse pitch ( a bit like driving a tractor in a gear that gave 40kph on a road). Unfortunately these bronze propellers could not be found at the end of WWII. Perhaps Australia wasn’t the safest place to store them.
The difference between the operation of a steam turbine plant on a warship and that of the tankers that I sailed in was the difference between night and day.
A frigate had a small amount of feed water in the system and the watch keeper stood between the two boiler fronts monitoring water levels with engine telegraph and engine rev orders in front of him. Overhead were manual overrides to the feed water controller and throttles for the steam turbine forced draft fans. The burners were increased or withdrawn by hand signals to the ratings on each boiler front.
The response was immediate, a frigate could accelerate from stopped in the water to 20 knots in not much more than a ship’s length.
The older tankers I sailed in including T2’s were altogether different. At full away the setup was more like a power station and we normally gave an hours notice before manoeuvring. The last steam ship I sailed in was UMS where even the soot blowing was automated. We didn’t do much at all in heavy weather reducing speed to prevent damage rather than load. The Japanese automation used to switch to a high speed program area when manoeuvring but at 400,000 tonnes high speed only existed as an electron.
