Steering gear

[Georges Guay]

«Speed increases helm response. Less rudder angle is needed for same result.»

Test No.1; A vessel is dead in the water and ready to manoeuvre. The weather is flat calm, no current. You put the rudder hard to Port and order Full Ahead. You do a complete 360° turn.

Test No.2; The same vessel is back dead in the water in the same conditions. You put the rudder hard to Port and order Dead Slow Ahead. You do a complete 360° turn.

What will be the difference between the two turning circles other than the time it will take to execute the turn?


«Steering engines on larger ships were massive, being powerful enough to be prime movers in lesser craft.»

Do you really believe that the steam steering gear would be as efficient to turn a rudder while the ship is alongside than when she’s proceeding at full sea speed, taking due note that a force up to 423 tons against a stern inertia of a 50,000 tons displacement vessel will be generated?


«Titanic's rudder was wide near the waterline where the wake is well aerated and creates less resistance to turning the rudder.»

Would a well aerated rudder, which will result in a lesser resistance to turn, will produce the same lift as one in a homogenous water flow?

 

[David G. Brown]

Georges --

You can do whatever you want with the helm if the ship is dead in absolutely still water and it will not cause any effect. So, you could not do a 360 under any circumstances. Without adding power the time to complete the maneuver is therefore infinite. But, ships do not start moving just because the shaft is rotating. The coupling between propeller and water is fluid so there can be some lost revolutions with no perceptible movement of the hull. And, slamming into full forward would break the laminar flow over the propeller thus precluding any transfer of power until it was restored. So, the second scenario you postulate should actually produce better results than the first.

(Aside -- can anyone imagine what it would have been like to do a "hole shot" in Titanic?"

I think your first discussion would make more sense to imagine the difference between a ship making bare steerage way versus the same vessel operating at normal speed. Given equal amounts of rudder, faster is faster.

As to your second point, well you're right that in absolutely still water the steering engine would have less work to do and so should in theory be able to act faster. However, the engines I've seen are not designed for operation in still water. They are designed to maneuver a the ship's normal operating speed. A steering engine for the Olympic class ships had to have enough "oomph" to maneuver at 22 to 23 knots. That would have been the design criteria and not dead slow harbor speeds.

Your third point is curious. Obviously, the rudder works best in a homogeneous water flow. That was exactly my point. The design of Titanic's rudder assumed this fact. It had more area where the water flow was less effective and less area in the more effective deep homogeneous water. Good design I would say. Perhaps not perfect design, but the concepts involved were just beginning to be understood.

Titanic's rudder was flat as a plate of rolled steel. It had no foil shape. So, in and of itself Titanics rudder could not have generated lift in the sense of this discussion. It could only create flat plate resistance to the flow of water. This would have caused some steering force, but would not have been efficient. The lift you speak about is created by changing the shape the hull presents to the flowing slipstreams. With right rudder, the water on the starboard side flows a shorter distance than on the port even though everything has to get back together at the back edge of the rudder. Conversely, the flow on the port side is faster. Several principles are in play, Bernoulli being the best known, but the result is a low pressure on the port quarter and high on the starboard. This pressure differential is the same as "lift" on an airplane wing except in this case it rotates a ship's hull around the pivot point.

But, fluid dynamics aside, the steering engine would have been designed to handle the largest anticipated normal load. For Titanic, that was maneuvering at 22+ knots. So, there should have been no problem with steering at the ship's expected normal speed. And, therefore, there should have been no particular problem cutting didoes around an iceberg as long as there was sufficient sea room.

-- David G. Brown

 

[Samuel Halpern]

The Hunting Gear or Follow-up Gear?

Hunting gear is the term I read about. Thanks.

Not really sure where this discussion is going. As far as the time it takes to swing Titanic's rudder from one side to the other at full speed, as far as I know there is no data available to actually tell us. We can only make assumptions. That said, there is the separate question about how long it would take to turn the wheel full over from one side to the other. The wheel worked the telemotor master unit. Telemotors had two master pistons that were interconnected by hydraulic lines to two slave pistons in the slave unit located near the steering engine. The piston pairs operated by one pulling and one pushing when force was applied to the wheel allowing positive control in either direction. The slave cylinders in this system were fitted with springs that would return the pistons to their centered position if the wheel were to be released. Turning the wheel over to one side would increase the back pressure felt because of this, and the wheel would spin back to center if released. But even if one could get the wheel fully over (4 full turns) in say 7 or 8 seconds, the steering engine would immediately try to follow but would slow down as it back pressure from the rudder increased as it got nearer to the stop. The rate of rudder swing was probably not constant but may have looked something like the attached.

It should also be noted that the a rudder of type fitted on these vessels, the stall point occurs at about 45°. That means the increase in rudder force stats to flatten out as the rudder gets closer to that point. The flattening starts at about 28-30°. I believe that is the reason why a maximum of 35° deflection was later adopted as a standard.

Anyway, in developing a turning model there are a number of parameters that can be adjusted so that the turning characteristics match closely to the real world results. What's most important is that that the we get a model that can reproduce what we were told, such as the tactical diameter of the turn and the time it took for the vessel to veer off so many degrees from straight ahead while going at full speed. Also in the dynamics you need to reproduce the amount of speed reduction that occurs due to increased hydrodynamic drag as the drift angle approaches its steady state value. And that is what I did when I developed my model based on the analytical work of Professor F. A. Papoulias of the Department of Mechanical Engineering at the US Naval Postgraduate School in Monterey, CA.

 

[Georges Guay]

Thanks Samuel. The diagram shows it all. For a Hard Over, it would take for the actual rudder around twice the time to meet the telemotor rudder angle. If it took 8 seconds to turn the wheel to 40°, the rudder would reach 40° in 16 seconds. That would meet Lloyds’ 1922 requirements spirit.


For the remainder...

«Speed increases helm response. Less rudder angle is needed for same result.»

Sea Trials, Ship Simulators or Ship Model have shown that the Tactical Diameter does not change if the turn is done at any «constant» speed. The only parameter that will change is the period of time to execute the turn. Saying that «Speed increases helm response and Less rudder angle is needed for same result» is not possible.

The Transfer Diameter and Advance are virtually the same whether achieved at Full or Half Ahead.

Compare to a longer, deeper, heavier and faster cargo vessel, Titanic had a larger Transfer Diameter and Advance. The rudder hydrodynamics knowledge was just not there. At harbour Speed with the central turbine stopped, she must have been very sluggish at the wheel.

Titanic was built under Transverse Framing which did not give her much longitudinal strength as would be needed a vessel of her length and trade. She was a kind of hybrid between a Tall Ship and a Steam Ship. The steel quality was debatable. So why then would she suddenly be equipped with the state of the art steam steering gear apparatus, where a shout to the quartermaster would result in a 40° actual rudder angle in 7 to 8 seconds at full sea speed?

A fractal is a mathematical set hat exhibits a repeating pattern displayed at every scale. It is also known as expanding symmetry or evolving symmetry. Fractals can also be nearly the same at different levels. Fractals also include the idea of a detailed pattern that repeats itself. The tree is within its leaves!

 

[Samuel Halpern]

I did one more thing with the curve of rudder angle Vs time shown in red above. A rudder similar to that on Titanic other older vessels of the time tends to stall at about 45°. That means beyond 45° the pressure on the rudder starts to decrease just like an airfoil. I was able to then map the two curves to show how rudder pressure would vary Vs time with the rudder deflected to a maximum of 40° in 16 seconds. See attached.

What this shows is that 85-90% of 40° rudder pressure would be developed in about 10 seconds.
The last 10° of rudder travel produces relatively little greater gain over rudder pressure at 30°. I can easily see why a maximum of 35° rudder travel was later developed by the standards.

 

[Georges Guay]

It is a complex world. The maximum pressure (F1) will develop in the first stage, as the drag (D1) but unlike the lift (L1). As soon as the lift develops (L2), the pressure force (F2) (evolutive component) will move forward and the drag (D2) will diminished, until equilibrium or when the maximum rate of turn at constant speed is attained.

At Full Ahead, if you turn a rudder hard over at once, the heading will barely change. The vessel develops a centripetal list, afterward a centrifugal one, both barely perceivable and then, she really starts to turn, quite constantly, until her maximum rate of turn related to her turning circle is attained. The whole process will take more than 16 seconds so when you really need it, as in the case of a Hard-A-Starboard at Full Sea Speed, it looks like an eternity! No fun there…

 

[Georges Guay]

 

[John E. Smith]

So the steering gear was under the docking bridge and aft of the 3rd class rooms. I found this while looking at the plans.
Are the things in front of the docking bridge, connected to the steering gear room or not?

 

[Tim Aldrich]

So the steering gear was under the docking bridge and aft of the 3rd class rooms. I found this while looking at the plans.
Are the things in front of the docking bridge, connected to the steering gear room or not?

Are you referring to the capstans?

 

[A. Gabriel]

I have heard it said that in the extreme emergency that the ship's wheels failed, the steering quadrant could be worked manually by means of lines and the aforementioned capstans?

 

[John E. Smith]

No. Not the capstans. The objects in question I circled in red. I’m just wondering, if these objects are related to the steering gear, or related to something else

 

[Ioannis Georgiou]

They are the skylights (for light & air) over the steering gear.

 

[Jim Currie]

These were steel access hatches (You can see the hinges). They were on top of the two raised coamings with the three forward facing portholes.
The hatches were situated directly above the port and starboard Steering Engines. The potholes afforded natural light in the steering flat. The hatches provided means for repair and replacement of the steering engines. only one was in service at any one time.

 

[John E. Smith]

Ok. That helps

 

[David G. Brown]

On D deck in the steering flat there are two capstans shown which were intended as emergency steering. These would work through tackles P&S which are also depicted running from the capstans to the steering quadrant. In theory, these tackles could also be man-hauled, but that would be a daunting process. Most steering engines had a "trick wheel" (origin unk.) that would allow using the regular steering system should there be a breakdown in the telemotor or its hydraulic pipes. Also, the wheel on the docking bridge would have had a means of connecting to the steering engine, possibly through a rod and crank arrangement. All very standard stuff in the early 20th century. There was some fear of telemotor, main steering engine, and/or quadrant gear failure.

-- David G. Brown

(PS to Mike Standart -- do you recall that odd tandem wheel setup on the ship in Toledo?)