Steam vs Diesel RPM in (Heavy) Weather

Torque vs horsepower was discussed here with regards to IC vs electric cars. What about the performance of a steam ship in heavy weather when the RPM of the main engine falls off due to wind and seas?

Surely you just get the stokers to shovel faster?


It’s often the case that in heavy weather it would be prudent to reduce rpm in heavy weather to avoid damage. The diesel ships I sailed on the governor gain setting could be adjusted to reduce the risk of over-speed.

I was wondering about the response of steam vs diesel as the weather begins to worsen.

Or just in general as the load on the engine increases.

I’ve only been fair-weather sailing on a steam vessel, and that one without electronic control. But perhaps I would worry more about the boiler. It seems like the less top=up time you have and the more agitated the steam drum is, the less effective the steam generation might be.

In a modern and well maintained automated steam plant you could say the bark is worse than the bite.

The sounds of the turbines and reduction gear winding up and down in response to throttle and load are truly incredible and a little scary the first time it is experienced. The boiler automation manages to prevent excursions beyond high and low water levels but the feed pump governors, fans, and fuel pumps get a good workout.

As far as comparing to a diesel plant, even if the inertia of the entire system from screw to heat input were the same, the governor on a diesel instantly reduces heat input while the resistance to turning (compression) remains essentially constant so the deceleration rate is greater and the ramp back to speed setpoint is longer than on a steam turbine plant which opens the throttle very quickly in comparison.

Mind you all the above is a very broad description, there is a heck of a lot going on in a steam plant in response to large load changes over short periods and the system may be working on the ragged edge in really bad weather. The steam engineer is not comfortable in those conditions.


I was wondering more about a higher load in general rather than changes in load from heavy pitching.

How a steam plant responds really depends on how the plant is set up once up to speed after maneuvering. The way I always did things is to run the plant at a set 1st stage pressure on the HP turbine. This usually delivered the desired shaft rpms (ship’s speed) but it would vary depending on weather and seas. The advantage would be that the plant would run at a steady load with only minor variation. If the automation was kept “on the wheel”, meaning a set rpm the plant load could be all over the place.

Not sure if this answers the question you were asking.

It’s kind of hard to “overload” a steam plant if that is what you mean. It is after all an external combustion engine. The worst you can do is turn daylight to dark with smoke for a while.

RPM has everything to do with load (or vice versa) with a fixed pitch screw, load changes with ship motion and rpm changes with load until the governor controls the change by adding or removing heat.


Thinking through what you are asking here and as a deckie here is how I understand the steam plant:

There are multiple components to the steam engine, but generally we think of a diesel as one unit. Steam ships had boilers to generate steam and turbines to make the prop turn.

If load increases on the steam ship due inclination from rough waves, the turbine will be placed under greater load and may lose RPMs, which in turn could increase boiler pressure. Fuel burn will stay the same (although there is greater slip and less speed made good).

If the load decreases the prop can race. Can’t remember where I read it, but the old steam ships of WW2 had trick wheels to speed up or slow down (not sure what). The story made it sound pretty intense to be in a steam engine room during a storm spinning trick wheels, just trying to protect from the prop racing.

I would expect turbines to take longer than diesels to spool up under variable loads, and that would make the timing of mechanical adjustments to prevent prop racing a bit trickier.
(crawling under the desk in case engineers start throwing rocks)

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If you hit a strong headwind the turbine merely slows down while continuing to consume the same amount of steam in my experience. One class of steam ship that I sailed on had a torque indicator and alarm for the shaft. We were instructed to throttle in or reduce the number of steam nozzles in use in extreme cases. It was sometimes fun to ring the bridge and ask “Who is doing their steering ticket at the moment?”and to be met with surprise that I knew that an inexperienced helmsman was steering instead of the autopilot. The larger than usual rudder movements caused the shaft revs to fluctuate.


In the case you described, when the load increases the rpm drops. The governor opens the throttle to allow increased steam flow to recover the rpm drop back to setpoint. Greater steam flow means the drum pressure will drop until the fuel flow increases enough to boil the feed water delivered to replace that which left the drum with the greater steam flow. In the meantime, the fuel pressure has to rise to add heat to the furnace and the fan speed has to increase to feed oxygen to the increased fuel flow.

Old manual controlled systems relied on the engineer to open the throttle to provide a certain pressure in the steam chest for a given steaming condition. There was a row of nozzle valves that could be individually selected open or closed to configure the outlet area of the steam chest at full throttle and there was an actuating beam operated by the governor that sequentially opened the nozzles to adjust speed.

I believe most of the horror stories surrounding prop racing steam ships was from the recip days when they had to quickly close the throttle or close a butterfly valve downstream of the throttle to avoid engine damage.

In my experience in heavy seas, the turbine would accelerate and decelerate quite rapidly under changing loads, as would steam flow. In very heavy seas the rate of change of speed and the noise level of steam flowing was something to behold.

You cannot go from dead slow to full ahead on a slow speed diesel without ramping up at a rate that matches the amount of fuel injected to the rpm and air flow or really bad things will happen.


You can absolutely over torque the shaft on a steam plant. Its not like it would break right away, but if you consistently operate outside of the design envelope you’re asking for a fatigue failure. I think the shaft over torque alarm is even displayed on the bridge.

Probably a poor choice of the word overload on my part.

My intent was to point out that increasing heat input rapidly in a steam plant will make smoke before it breaks something as the rpm will generally increase rapidly enough to avoid over torque.


On a motor plant the fuel gets stirred up and turns to liquid shit that plugs the filters & purifiers for a week afterwards and complains for constant attention. On a steam plant you never notice, you just burn it.


The poor junior cleaning fuel strainers every few hours notices it.


But you can over speed it…

The ship I was on had overspeed trips on the propulsion turbines. Yes, they can overspeed and have witnessed it once.

The title of this thread should have left out the word “Heavy”. I was wondering about how the two types of plants would perform as the load increased.

To increase the power output (load) on a steam plant you have to make more steam. To make more steam you have to add heat (fuel) water, and air.

Unless you want to make a bunch of smoke to hide the convoy from U-boats you follow the old steamboat saying of “fuel follows air on the way up and air follows fuel on the way down.” That means you need to increase the air supply before adding more fuel to increase power and decrease fuel before decreasing air when reducing power. The air supply is via independently powered and controlled forced draft fans and fuel is controlled by pressure regulators. There is a lot of wriggle room for combustion control and rate.

A diesel uses turbochargers to supply combustion air and those units rely on exhaust flow and temperature to deliver air. That means the supply of air is dependent on the amount of fuel burned and rpm. Unless the turbos are a very modern style of electrically boosted system, it requires a carefully controlled ramp up to increase power output without damaging stuff. There is little wriggle room.