David Allison

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George, I apologise for taking so long to respond, however this is not an easy topic. The following is for your consideration.

I spoke to another pilot regarding hard over rudder manoeuvres who said there are two different viewpoints among ship handlers. I find it rather perplexing in this year 2020, two different viewpoints exist. It’s really about safety and collision avoidance techniques. It would seem to me, only one viewpoint is valid.

1) The first viewpoint. Spin the wheel in the minimum amount of time, to its limit of 35°.

2) The second viewpoint. Spin the wheel and pause at 20° rudder deflection, then continuing to its limit of 35°.

The pilot I spoke with subscribes to the seconds technique. I am curious about the technique you would use.

Regarding stopping distance, naval architect Edward Wilding said this at the British Inquiry regarding the Olympic.

“The trials that I have were made again off Belfast Lough. Both engines were running at about 60 revolutions, corresponding to a speed of about 18 knots. The helm was left amidships and both engines were reversed. The way was off the ship in about three minutes and 15 seconds from the order to reverse engines being given, and the distance run was just over 3,000 feet. I might mention in that connection that, so far as we on the bridge could see, the engines were not reversed as quickly as we had seen them, and the distance is probably a little on the large side; but that is what we actually observed, and it would be very difficult to put an estimated correction on it.”

The sketch (to scale) represents Wilding comments. Just prior to initiating the emergency stop manoeuvre at t = 0, the right winged propeller as viewed from behind, is rotating clockwise at about 60 revolutions per minute. Just prior to the ship stopping, the right winged propeller is rotating approximately 60 revolutions per minute counter clockwise. At some point along this timeline, the reciprocating engines stop reciprocating, which is coincident with the winged propellers stopping rotation.

Olympic 18 knots .JPG


Now Titanic was going about 22 knots, not 18 knots.

You are a lamp trimmer on the night of the disaster. It’s your first time working in the reciprocating engine room. You hear the engine telegraph ring and feel a slight shock. Two seconds later you notice the reciprocating engines have stopped reciprocating, followed by the engines starting to go slow astern. (The right winged propeller slowly starts rotating counter clockwise)

Would you hazard a guess as to the speed of the Titanic at this moment in time?

All of the skippers you spoke with over the years who considered Captain Smith was proceeding too fast, might consider this.

If you slow Olympic(1912) from 18 knots to run at 9 knots, the emergency stopping distance of 1/2 nautical mile will double to one nautical mile. The reason for this is the power available for stopping at 9 knots is only 1/8 of the power available at 18 knots. Travelling at 9 knots is a bad idea regarding stopping distance.

As for the inertial stopping distance from 9 knots, considering the angular momentum and the inertia of the turbine rotor, using a time halved constant of 10 minutes is quite appropriate. The inertial stopping distance at 9 knots will then be just over 2.2 nautical miles. (About 16 ships lengths)

All of the steam ships of 1912 faced the same problem. They didn’t have the luxury of going into reverse and increasing their power to shorten their stopping distance. Consequently they all maintained close to maximum speed in the vicinity of ice, provided they had adequate visibility. It is the right and prudent thing to do as operating at maximum speed gives them the shortest stopping distance.

As a ship increases it speed, the increase of total resistance is not linear. For displacement vessels it follows the curve in this sketch. The hump in the curve is not an accident and can easily be explained.

total resistance.JPG


I think the final chapters of the Titanic disaster, yet to be written, will be most interesting to read. It’s almost 120 years of people thinking Captain Smith was overconfident and proceeding too fast, when in fact he was being most prudent regarding the ship's speed.

In the emergency stopping sketch there is a problem when the ship has stopped at t = 195 seconds. The turbine which automatically shuts down at the start of the manoeuvre, does not have a brake. You can use the right hand rule which tells you the direction of the angular momentum vector throughout the emergency stop manoeuvre and then accordingly judge its affects. The rotor and center propeller is left to windmill. The right winged propellers at the moment just prior to stopping is rotating at 60 revolutions counter clockwise. The left winged propeller at the moment just prior to stopping is rotating at 60 revolutions per minute clockwise. This action forces the turbine to rotate counter clockwise.

Now if the engineer is not on his toes, the ship will develop significant sternway after t = 195 seconds. The engineer must have a plan to keep the ship dead in the water and bleed off this angular momentum. I would suggest 10 minutes at slow ahead, followed by 5 minutes slow astern should do the trick nicely.
 

Georges Guay

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Feb 26, 2017
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Hi David,

It is much easier to reduce the wheel than it is to increase it, especially when you realize that it should have been put over earlier. When she turn too fast, it is easier to reduce the rate of turn than too increase it in extremis. Facing a peril, it is much better to turn faster than slower. Pilots who lack self confidence increase the wheel gradually whereas seasoned shiphandlers set the wheel over first then reduce it. If there would be a risk for a rudder to stall, naval architects would have corrected the problem right away a long time ago. Etc. For the engines settings, it is the exact opposite; better two small kick ahead on the engine than a big full astern!

If it takes 3½ ship’s length to crash stop from 18 knots, it will not take half that distance to crash stop from 9 knots but rather something like a ship’s length, since kinetic energy = mv²/2. The propellers thrust going astern is only 40% the power ahead. So don’t worry about the steam pressure.

When you have no other alternatives, the proper maneuver to steam an ice class vessel across an icefield is to enter at minimum speed for steering then once safely in, to increase full; certainly not entering at full speed then sink slowly! Smith had two choices; reducing or avoiding. He choose to go to bed. I would tend to reduce since steaming full in the vicinity of a not so well outlined icefield in the middle of a pitch dark night was not a promise to not meet another berg at full speed!

I would say that since 108 years, every enthusiast have tried to save the liner. As far as I know, nobody ever succeeded …
 
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Alex Clark

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Is the forced rotation of the centre propeller caused by the port wing propeller directly or the windmilling action? Pardon my ignorance of what must be the basics. I’ve only ever commanded a single screw vessel .
 

Alex Clark

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The statement ‘this action forces the turbine to rotate counter clockwise’. Is that a reference to the windmilling? I’d assumed so, but the wording of the previous line makes it sound a little like the port propeller’s continuing rotation is having an effect on the centre propeller. I’ve probably misread that, but wanted to clarify.
 

David Allison

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Oct 15, 2015
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If it takes 3½ ship’s length to crash stop from 18 knots, it will not take half that distance to crash stop from 9 knots but rather something like a ship’s length, since kinetic energy = mv²/2. The propellers thrust going astern is only 40% the power ahead. So don’t worry about the steam pressure.

George,

Yes, it’s coming up to 110 years on the foundering of Titanic in 2022, not 120 years. My mistake. It’s not unusual for me to make errors however I try to avoid important ones. If I could address your comments, one at a time.

The Titanic/Olympic similar to a destroyer, had underwater hull profiles built for speed. Typically the block coefficient of this profile is approximately 2/3 with Titanic/Olympic’s block coefficient about 0.68. Such ships have a final turning circle diameters of 4 times their length. This gives Titanic a final turning circle diameter of 4 x 850 = 3400 feet where 850 feet is the length between perpendiculars. (Turning circle shown as 32 rays of length 1700 feet – 32 points x 11.25° = 360°)

TurningCircle.JPG


For a destroyer to increase its speed from 15 knots to 30 knots it typically takes 8 times as much power! The increase in power is proportional to the cube of the ship’s speed. The reason for the rapid increase in power is predominately due to the resistance of ship generated waves. Whatever power is required to push the ship through the water at 15 knots, it then requires 8 times this power, to push the ship through the water at 30 knots.

A destroyer when proceeding at 15 knots has a huge amount of reserve of power available to decrease its stopping distance, should an emergency arise. If the destroyer was steam powered and proceeding at 15 knots, it has none of this reserve power available to stop. The power it’s developing to push the ship through the water at 15 knots, is the maximum power available to stop the ship from 15 knots. There is no time to bring additional boilers online.

Consequently, it can be shown if you make the decision to run a steam powered destroyer at 15 knots, its stopping distance will be double its stopping distance at 30 knots. If you are concerned about stopping distance, you need to operate the steam powered destroyer at the maximum speed possible. For steam powered ships, the harder you push the ship, the shorter it’s stopping distance.

The Titanic was steam powered so a decision to run at 9 knots compared with 18 knots, doubles its stopping distance from what it is at 18 knots. For Titanic we have the data for 18 knots which Edward Wilding provided at the British inquiry. This analysis is in deep seawater and importantly, in flat calm conditions. The only waves are ship generated waves.

This is the associated physics.

Comparing Titanic’s straight line stopping distance at a speed of 18 knots with its stopping distance travelling in a straight line at 9 knots.

The ship at 18 knots has four times the kinetic energy, twice the momentum and twice the velocity, when compared with operating the at ship at 9 knots. (The subscripts below are the ships speed in knots)

aaa.JPG


Momentum is a linear function. Kinetic energy varies with the square of the ship’s speed. Power varies with the cube of the ships speed.

Power (P) measures the rate at which someone or something (in this case Titanic’s engines) do work and therefore is the work done, divided by the time to do the work. A good video explaining this can be found here. It can be viewed as negative work as the work done on the ship is destroying kinetic energy in bringing the ship to a complete stop.

Comparing the time required to bring the Titanic to a complete stop from 18 knots with 9 knots.

bbb.JPG


Power must be increased by a factor of 8, to increase Titanic’s speed from 9 to 18 knots. We then compare the ratios of Power for each speed.

ccc.JPG


For the steam powered Titanic, it takes twice the time to stop the ship from a speed of 9 knots, compared with 18 knots. (2 x t18) (2 x 195 seconds = 390 seconds)

With a speed of 9 knots, it takes twice the distance to stop the ship from a speed of 9 knots, compared with 18 knots.

ddd.JPG


The bottom line George, Captain Smith was anything but overconfident in deciding to proceed at 22 knots. The captains you spoke with who criticized Captain Smith for indiscriminately speeding through an ice field, have not thought this through. Almost 110 years of people accusing Captain Smith of recklessly speeding through a known ice field are unfounded. He was proceeding at a prudent speed. All steam powered ships in 1912 did not slow down in the vicinity of ice, for this reason. Slowing down not only increased their stopping distance, it also increased their radius of turn. (To be explained) Logically they then travelled at close to their maximum speed, provided they had adequate visibility. In the vicinity of ice if visibility deteriorated they either proceeded dead slow or stopped.

The need to blame someone for the Titanic disaster extends to this day. You see and hear it all the time. Some even accuse Captain Smith of criminal negligence. They go to great lengths to discredit him.

George, I would like to know if you understand this analysis. It is not one ship length, but rather 6000/850 ≈ 7 ship lengths. I think you are considering the Titanic had reserve power at 9 knots which it would not.
 

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Mar 22, 2003
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For the steam powered Titanic, it takes twice the time to stop the ship from a speed of 9 knots, compared with 18 knots. (2 x t18) (2 x 195 seconds = 390 seconds)
The only thing that these equations show is that for the power at a speed of 18 knots to be 8 times the power at 9 knots requires that 4 times the energy must be delivered in half the time. It is what is needed to maintain a speed of 18 knots compared to 9 knots because the resistance is 4 times greater.
With a speed of 9 knots, it takes twice the distance to stop the ship from a speed of 9 knots, compared with 18 knots.
??? This does not follow.
 
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David Allison

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Sam, You are missing the point. All of the stopping distances in your graph, are at the maximum power available. So for example from initial speed of 10 knots, they then use maximum power available to stop, about 10 ship lengths. They go into reverse and increase their power to stop. How long and far would it take the vessel to stop from 10 knots if they went into reverse and were not allowed to increase their power to stop. This is basics of the analysis. This was the situation for steam powered vessels of 1912 when they elected to run at lower speeds. The maximum power available to stop, is the power needed to push the ship through the water at 10 knots. They didn't have the luxury of increasing their power to stop. .
 
Mar 22, 2003
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David,
You said, "With a speed of 9 knots, it takes twice the distance to stop the ship from a speed of 9 knots, compared with 18 knots." Why is the distance doubled? I understand that the available power is perhaps 1/8, but I don't see how you can say that the distance will double. I believe its a little more complicated than that.
 

David Allison

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The only thing that these equations show is that for the power at a speed of 18 knots to be 8 times the power at 9 knots requires that 4 times the energy must be delivered in half the time. It is what is needed to maintain a speed of 18 knots compared to 9 knots because the resistance is 4 times greater.
This in flat calm conditions with the rudder held steady. Resistance is directly related to the horsepower required to propel Titanic. The resistance is 8 times greater at 18 knots compared with 9 knots. You are suggesting kinetic energy is directly proportional to resistance which it is not.

It then does follow with a speed of 9 knots, it takes twice the distance to stop the ship from a speed of 9 knots, compared with 18 knots.
 
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Resistance is directly related to the horsepower required to propel Titanic.
Sorry David, but that is not correct. What is correct is that the power required at twice the speed is approximately 8 times that of the given speed. (These are for speeds that are below where serious wave making becomes a factor, which for vessel the length of Titanic, can easily be ignored). In other words, power needed goes up as the cube of the speed. To double the speed requires 8 times the power.

Look at it this way, to maintain a speed of say 9 knots requires a certain thrust (force F) from the ship's propellers. That force is need to balance the resistive force that is acting on the hull going through the water at 9 knots. The thrust and resistive forces are equal and opposite, otherwise the ship would either accelerate or decelerate instead of maintaining a constant speed. Now the energy being imparted to the vessel by the ship's propellers that is needed to balance the resistance acting on the hull is nothing more than force (propeller thrust) times distance travelled. The amount of power being supplied is nothing more than the rate that that energy is being imparted, or simply thrust (F) times distance travelled divided by time travelled (the ship's velocity V). Thus power P is given by P = FV.

The resistance of the hull increases approximately in proportion to the square of the ship's speed, therefore, the power needed, which is force times speed, increases as the cube of the speed.
 
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Georges Guay

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It then does follow with a speed of 9 knots, it takes twice the distance to stop the ship from a speed of 9 knots, compared with 18 knots.

Proceeding at 9 knots along with the company of a fully engine crew standing by, a brand new twin screw Titanic would be crashed stop near her own length! You would have to hold on the railing to not break your nose …
 
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I am apologizing for continually harping on this question on the "4 on and 4 off watch schedule" because it seems almost bizarre to this 21st Century "landlubber". I am just wondering if this was something that was in common practice in the Merchant Marine at that time and how long that practice was in use ?
 

David Allison

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" Resistance is directly related to the horsepower required to propel a ship" [Titanic]
Sorry David, but that is not correct.
Sam, That statement comes directly from the US Naval Academy document found here. Below is their example and model testing of a Navy-YP.

"Model testing is carried out over the expected speed range of the ship with resistance data collected at each testing speed. Effective horsepower is then calculated and plotted as shown in Figure 7.4. It will be observed from the figure that the doubling of speed of the Navy YP from 7 to 14 knots increases the power by a factor of 10! Speed and power are not linearly related."


NavyYP1.JPG


Now compare a diesel powered Navy-YP with the same Navy-YP powered by steam. Both engines producing the same maximum 500 EHP.

At 14 knots they both have the same stopping distance using 500 EHP.

Now reduce their speeds to 7 knots. Both use 10% of their max power to maintain 7 knots, 50 EHP.

Now the diesel powered Navy-YP can go into reverse and increase its power to 500 EHP to be stopped from a speed of 7 knots. Its stopping distance is then considerably less, compared with its stopping distance from 14 knots.

Now the steam powered Navy YP cannot immediately increase its power to stop. It has only 50 EHP available to stop from a speed of 7 knots. Its stopping distance is considerably more compared with its stopping distance from 14 knots.

So if you’re out and about in your steam powered Navy-YP on some dark calm night, and worried about visibility and bumping into an immovable object, it’s a very dumb idea to run your Navy-YP at 7 knots.

This is very basic and easy to understand and is the fundamental reason why ship captains of the day, did not slow down in the vicinity of ice. In doing so, this increased their stopping distance considerably. They may not have been able to express why they operated their ships this way, (British Inquiry) however it was in their mariner DNA to do this.

Sam, I’m sure you are aware of the sea trial test data for Titanic/Olympic from Wilding. The faster their speed, the shorter their stopping distance for speeds 18, 20 and 22 knots. This seemed to puzzle Wilding particularly at 22 knots. Again, the basic reason for the shorter stopping distance as speed increase is the phenomena discussed above.

Anyone interested in reading about Titanic and for those who criticize Captain Smith for proceeding too fast, this criticism is totally unfounded. The perfect storm for Titanic was flat calm conditions. If Titanic wasn’t a new ship which they were breaking in, I’m sure Captain Smith would have increased her speed to the maximum possible in the vicinity of ice. It is the right thing to do and may have saved the day.

Titanic falls into a category the same as destroyers which have a final turning circle diameter of 4L, where L is the length between perpendiculars.

From the Naval Academy

“As shown in previous sections, the power required to propel a ship through the water is the product of total hull resistance and ship speed, and so engine power increases even more rapidly than resistance. Often, ship power is roughly proportional to the cube of the speed, so doubling (2x) the speed of a destroyer from 15 knots to 30 knots will require 2^3 =8 times as much power!”

The hull of destroyers and Titanic are more streamlined compared with a Navy YP and so rather than taking 10 times the power to double its speed from 7 to 14 knots, it only requires 8 times the power to double their speed and for the destroyer from 15 to 30 knots.

Steam ship handling 101
– In flat calm conditions in the vicinity of ice, operate your ship at the maximum speed possible to give the shortest stopping distance.
 

David Allison

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David, your question seems trivial, and the answer is obvious.
Hi Doug,
Yes I believe it is trivial. You may be familiar with the following information from Edward Wilding at the British inquiry. It is a gift to mariners as a result of the tragedy and should interest any curious ship handler.

Testimony of Edward Wilding, recalled

Does that complete the information?
- No, there is a little more information that I think the Court wishes to have. Since the accident, we have tried the "Olympic" to see how long it took her to turn two points, which was referred to in some of the early evidence. She was running at about 74 revolutions, that corresponds to about 21 1/2 knots, and from the time the order was given to put the helm hard over till the vessel had turned two points was 37 seconds. (2 points = 22.5 degrees)

This should puzzle experienced ship handlers in terms of the angular velocity. In degrees per second this is (22.5 degrees / 37 seconds = 0.61 degrees/second) Experienced ship handlers will immediately recognize the Titanic is capable of higher angular velocities at a speed of 22.5 knots. At minimum in the order of 1.25 degrees/second. The problem is if you put the rudder hard over, the rate of turn is very small. Count out 37 seconds for 2 points and it will emphasize just how small this rate is. You need to turn quickly to avoid an obstacle ahead and putting the rudder hard over limits you to 0.61 degrees/second. If you go gently on the rudder you will be able to attain a higher angular velocity. (rate of turn) This is for Titanic/Olympic in flat calm conditions and deep seawater. I haven’t looked at balance rudders on modern ships, but I suspect it is the same. As I mentioned to George there are two schools of thought among experienced ship handlers today. Only one is correct and it is the one where ship handlers pause at 20 degrees rudder displacement, before continuing to put the rudder hard over. One can’t really blame First Officer William Murdoch if by modern day standards and practice at least some ship handlers believe the right thing to do is put the rudder hard over to avoid an obstacle ahead.

Ship handling 102
-Avoid hard over turns
 

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