Steering gear

  • Thread starter Colin W. Montgomery
  • Start date
Mar 22, 2003
5,131
614
273
Chicago, IL, USA
Hi Cal: Good to hearing from you again. I read your post with great interest and went back to my own notes on this subject. I agree with the count of engine crank shaft to secondary shaft having 19 teeth to 55 teeth, and the secondary shaft to pinion input gear having 18 teeth to 87 teeth. These give the ratios of 2.89 and 4.83 respectively. However, pinion to quadrant count must be handled a little carefully. You are correct in stating 31 teeth on half the quadrant. However, this is much more than a 40 degree turn of the quadrant. If you actually measure a 40 degree turn of the quadrant you will see that the quadrant has to turn almost 19 teeth of travel, not the 31 that are there. This gives a ratio 1.36 revolutions of the pinion to swing the rudder 40 degrees to its stop. Thus, 1.36x4.83x2.89 = 19 revolutions of the crankshaft for 40 degrees rudder swing from amidship. The leverage ratio is computed by noting that if the quadrant would have been a full circle, it would have contained just about 168 teeth. Thus 168/14 = 12 is the leverage ratio for pinion to quadrant. This multiplied by 4.83 and that product multiplied by 2.89 gives a total leverage of 168 to 1 from the steering engine to the rudder.

If still believe it would take about 7 to 8 seconds to swing over 40 degrees. As far as the turning capability of the ship, the famous 37 seconds was a measurement from the time the order was received by the helmsman to the time the Olympic's head veered off 2 points to port. This was at a speed of 21.5 knots with both engines going ahead. The response of the ship, heading Vs. time was probably very close to the following plot adapted from http://web.nps.navy.mil/~me/tsse/TS4001/lectures/11.pdf.

93546.gif
 

Georges G.

Member
Feb 26, 2017
510
69
38
60
«One thing to realize is that the time to move the rudder over when running full ahead is going to be greater than the time to return amidships. At some point the engine will begin to feel the force of the sea against the rudder and slow down. As the rudder angle increases, so does the force of the sea against the rudder, so it's the power of the engine that limits the speed

«Another question is how long would it take the helmsman to turn the wheel hard over? Working from a diagram of a Brown's telemotor sending unit, I estimate that it took about 6 revolutions of the wheel to turn it hard over. Unlike some modern ships, the helmsman was working against the large return spring on the receiving telemotor, so he couldn't just give the wheel a spin.»

«For Titanic running at 22.5 knots, and a full rudder deflection angle of 40 degrees, the force calculates out to 423 tons. Since the underwater area of the rudder works out to be about 402 sq. ft., this force would create a pressure of about 1.05 tons per square foot on the rudder plate.»


Samuel, taking into account the above statements, do you still believe that from a hard over order at 22½ knots, it would take about 7 to 8 seconds to swing the rudder over to 40 degrees?
 

Georges G.

Member
Feb 26, 2017
510
69
38
60
I read on the thread that Titanic was equipped with an electrically Rudder Indicator which relayed to the Bridge the real rudder position! On top of the wheel telemotor stand, there was a mechanical pointer that showed the wheel position. Then why install a duplicated electric rudder indicator, if was not to show a possible difference between the wheel position against the rudder position? Was the wheel position attained faster than the rudder position? Was there a Lag Time between the wheel and the rudder?

On a modern electro-hydraulic steering gear system, the wheelsman will turn the wheel over with his little finger in 2 seconds and confirm the mechanical wheel pointer over. Once done, he will look at the rudder indicator. When the indicator pointer shows the rudder over, generally in 14 seconds, he will confirm the rudder over. From the wheel mechanical pointer position to the rudder indicator pointer position, we can say that the lag time is 12 seconds.

What about Titanic?
 
Dec 4, 2000
3,213
463
213
Georges -- you are right that the duplicate indicators were to show the difference between the position of the telemotor and the actual tiller quadrant attached to the rudder. The telemotor wheel on the bridge only opened a steam valve actuating the steering engine which did the actual work of moving the rudder. For some seconds the bridge wheel and indicator would have been well ahead of the rudder. That difference would have reduced to zero once the desired rudder angle was achieved.

The lag time does have some benefit. Ship is not steered only by the resistance of the rudder. Think of the rudder as a trim tab and the hull as a side view of a wing. Tabs on airplanes help generate extra lift. The same lift is created in the water. High pressure helps swing the stern away from the desired direction. This, naturally, swings the bow toward the desired heading. Too rapid an application of the rudder will cause a "stall" condition which actually increases the response time of the ship as it turns to that new heading.

Sam can probably desribe this is spectacular scientific terms. Bottom line, though, is steering is not a slap-dash affair.

-- David G. Brown
 
A

Aaron_2016

Guest
How many seconds roughly would have passed between the moment Murdoch first ordered hard a-starboard to the moment Moody informing him that the helm was hard over? When Olliver reached the bridge he got there in time to see Murdoch closing the watertight doors and see the berg passing aft of the bridge. Yet he was not aware of any helm order before the collision. Was this order a work of fiction?


.
 
Mar 22, 2003
5,131
614
273
Chicago, IL, USA
Was the wheel position attained faster than the rudder position? Was there a Lag Time between the wheel and the rudder?
The wheel could be turned faster that the rudder as the rudder tried to catch up. It was a standard design at the time for it to take 8 turns of the wheel to shift the rudder from one side to the other. The rudder of course could not get ahead of the wheel because if it did the steering engine would stop and reverse to keep it aligned. They had a name for that type of tracking which I now forget.
The 1922 edition of Lloyds Rules had the 1st requirement that a rudder be capable of going from hard over one side to hard over the other side in less than 30 seconds while the vessel was going full speed ahead. The American Bureau of Shipping adopted the same rule in 1939 and in 1951 the American Bureau adapted 35° to either side as a standard. It was also recognized in tests that there was little to gain in actually putting the rudder beyond 28° as far as responsiveness. My guess is that at angles beyond about 30° the rudder is getting closer to its stall point.

How many seconds roughly would have passed between the moment Murdoch first ordered hard a-starboard to the moment Moody informing him that the helm was hard over?
My guess, simply a guess, is that Moody would confirm when the helm indicator showed the helm was hard over.
 
Last edited:

Georges G.

Member
Feb 26, 2017
510
69
38
60
«They had a name for that type of tracking which I now forget.»

The Hunting Gear or Follow-up Gear?

«The 1922 edition of Lloyds Rules had the 1st requirement that a rudder be capable of going from hard over one side to hard over the other side in less than 30 seconds while the vessel was going full speed ahead.»

Why did Lloyds implemented such regulation? Did they realise that after a few near misses or accident reports, rudders on these ever growing ships were turning too slowly.

«One thing to realize is that the time to move the rudder over when running full ahead is going to be greater than the time to return amidships. At some point the engine will begin to feel the force of the sea against the rudder and slow down. As the rudder angle increases, so does the force of the sea against the rudder, so it's the power of the engine that limits the speed.»

At 22½ knots, turning from amidships a of 400 ft² rudder at 40 degrees within 7 to 8 seconds, which would generate a force up to 423 tons against a stern inertia of a 50,000 tons displacement vessel, is for the least, impressive! Can you imagine what looked like the cavitations on the rudder turning side! The steering gear was design for hard over orders at manoeuvring speed but positively not for exceptional emergency orders at full sea speed. 7 to 8 seconds at harbour speed would’ve been more than excellent. But at full sea speed, I suspect that Lloyds regulation was not attained?
 
Dec 4, 2000
3,213
463
213
A couple of things which have to be put into the mix...

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

Titanic's rudder was wide near the waterline where the wake is well aerated and creates less resistance to turning the rudder. It was narrowest down deep where the opposite would be true.

Steering engines on larger ships were massive, being powerful enough to be prime movers in lesser craft. They drove a gear mated to a semi-circular rack on the outside of the steering quadrant. The power was available to make rapid changes in helm angle.

Photograph evidence of the ship's "S" shaped wake indicates Titanic was nimble enough.

-- David G. Brown
 

Georges G.

Member
Feb 26, 2017
510
69
38
60
«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?
 
Dec 4, 2000
3,213
463
213
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
 
Mar 22, 2003
5,131
614
273
Chicago, IL, USA
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.
rudder.gif
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 G.

Member
Feb 26, 2017
510
69
38
60
rudder10.gif

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.

turnin10.jpg

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!
 
Mar 22, 2003
5,131
614
273
Chicago, IL, USA
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.
rudder.gif

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.
 
  • Like
Reactions: Georges G.

Georges G.

Member
Feb 26, 2017
510
69
38
60
rudder10.png

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…
 

Jane Smith

Member
Aug 16, 2018
71
0
6
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?

65E47896-8598-4DDD-91C2-FDCC8E172596.png
 

A. Gabriel

Member
Jun 13, 2018
139
30
38
Philippines
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?