Optimal speed to avoid collision

[Steven Christian]

I am not saying for one moment that there was any similarity between the situations of Endurance and Titanic but didn't Ernest Shakleton comment something about reducng a ship's speed to about 4 knots when there was ice around?

That sounds about right. I know on one of their better days they made around 120 miles. That was after being warned by whalers that there was heavy ice ahead on their way to Antarctica. 120 miles/24 hours = 5 mph which is close to 4 knots. So it sounds like he practiced what he preached cause the Endurance was capable of around 10 or so knots. But he could have been following Frank Worsley's advice who in my opinion was the unsung hero of that expedition. Or at least he deserved a lot more credit in saving everybody's life with his navigating skills. Tough guys on that voyage. Cheers.

 

[Arun Vajpey]

As I said (and as the man himself admitted) the situations were not comparable. Still, Sir Ernest Shackleton was a man who kew about travelling through ice and IMO what he felt was the optimal speed under those circumstances is relevant irrespective of the maximum speed the ship was capable of doing.

Following is an excerpt from Shackleton's testimony at the British Inquiry int the Titanic disaster. I have copied and pasted what I conidered to be his general views that could apply to the Titanic disaster but the entire testimony makes very interesting reading.

25044. And supposing you were passing through a zone where you had ice reported to you, would you take precautions as to the look-out? Supposing you only had men in the crow's-nest, would you take any other precautions?
- I would take the ordinary precaution of slowing down, whether I was in a ship equipped for ice or any other, compatible with keeping steerage way for the size of the ship.

25045. You would slow down?
- I would slow down, yes.

And supposing you were going 21 to 22 knots, I suppose that would be the better reason for slowing down?
- You have no right to go at that speed in an ice zone.

25073. Just one question, Sir Ernest: Do you frequently find a haze in close proximity to an iceberg?
- Generally when the temperatures are different - the temperature of the water and the temperature of the air.

25084. The pace you speak of, four knots, was when you were in among the ice, turning and twisting, as you have described it?
- Yes, when we were in the ice region. I would not like to compare in any way the North Atlantic, with its comparatively few bergs, with the south, but if I were going 20 knots, I would want to get down to the steerage way just the same as when I am going six knots I want to get down to four knots.

25085. But you do not compare the state of things which you found, as you were approaching the south Pole, where you had to turn and twist among the icebergs and masses of ice, with what prevails in the North Atlantic?
- No, I do not compare it. The point I look at is, when you get a very fast speed, you must slow down, even as we in narrow waters had to slow down in our little ship.

25086. Slow down to four knots?
- We did.

25087. What do you suggest a liner should slow down to?
- I am not qualified to give an opinion, but I should suggest a liner should slow down sufficiently to give her steering way, which is, of course, More than the full speed of my own smaller ship.

25088. What do you estimate would give a vessel like the "Titanic" steering way?
- I am not qualified to say. I do not know enough of the turning movement of ships over 10,000 tons; I should say 10 knots.

25089. (The Commissioner.) That would be half-speed, practically?
- Yes, My Lord.

25090. (Sir Robert Finlay - To the witness.) Is your suggestion that all liners in the Atlantic should slow down to 10 knots as soon as they know that they may come across an iceberg?
- As soon as they know they are in an absolute ice locality, which they can tell now because of the wireless.

 

[Incony]

At only 10 knots, and assuming the turning circle remains the same, stop or hard astern changes the time from sight of the iceberg to maybe @ two minutes.. (10 knots is nearly 17 feet a second) i.e 500 yards covered in 93 seconds without slowing down = it just means it takes longer to hit the berg, it does not mean one can turn faster.. one cannot suppose that the berg would be seen sooner at 10 knots.. ? so the distance from sighting to the Titanic doesnt change... just the time taken to cover the distance does.. ok.. one could work out how far the Titanic would go, before hard astern at 10 knots would affect the speed... but i bet it needs more than 500 yards.. and two minutes - and the Titanic would still hit the berg. The force of the impact changes, but not its possible effects, because the contact time increases at a slower speed.. so its possible the damage could be even more severe. As an engineer, i know that dealing with loads weighing 250 tons that are moving at only 1 foot a second... if it hits something like your hand - its still going to flatten it. - at 17 feet a second it just happens faster..

so... the kinetic energy of contact, ( a partial inelastic collision ) is lost by contact both in the berg and the Titanic hull, it a bit like looking at cars colliding in slow motion or an explosion in slow motion... as opposed normal time.. so going slower means there is more opportunity for contact.. and the longer the contact the greater the opportunity for energy to be transmitted? so it might have meant more holes or bigger holes.. or both.. because the berg would not break up as much.. neither would the Titanic or the Berg be pushed apart faster..?

 

[Arun Vajpey]

At only 10 knots, the distance from sighting to the Titanic doesnt change... just the time taken to cover the distance does.

True. Also true that that the lower speed the Titanic's turning characteristics would be different; I would like an experienced seamn like Jim to tell us how different.

But the fact remains that Fleet would have seen the iceberg at the same distance as he did and rung then bells at the same moment that he actually did. It follows that Murdoch would then scan the Ocean ahead as he did and when he spotted the iceberg himself, he would have instinctively worked out that he had a timeframe X seconds before the ship's bow reached the iceberg. Likewise, in the real life scenario, Murdoch would have worked out a time factor Y seconds before the two objects met.

Since 10 knots would have been less than half of the Titanic's actual speed at the time, X seconds would be about twice as much as Y even allowing for human estimation errors. If Murdoh had given the same order at the same time, the Titanic would have been twice the distance away from the berg as it really was that night if it was travelling at only 10 knots. Even allowing for the slower turning reponses at 10 knots, the ship would have started its turn while much further away from the iceberg. Considering that even at 21.5 knots Murdoch almost pulled it off, IMO there is a very good chance that he might have actually done so at 10 knots.

The only difference could be is that if the Titanic had been travelling at only 10 knots and the crew responses from the moment of first sightng had remained exactly the same till the hard-a-port helm order, Murdoch then might not have stopped the engines. In the real event that did not make any difference anyway.

 

[Jim Currie]

At only 10 knots, and assuming the turning circle remains the same, stop or hard astern changes the time from sight of the iceberg to maybe @ two minutes.. (10 knots is nearly 17 feet a second) i.e 500 yards covered in 93 seconds without slowing down = it just means it takes longer to hit the berg, it does not mean one can turn faster.. one cannot suppose that the berg would be seen sooner at 10 knots.. ? so the distance from sighting to the Titanic doesnt change... just the time taken to cover the distance does.. ok.. one could work out how far the Titanic would go, before hard astern at 10 knots would affect the speed... but i bet it needs more than 500 yards.. and two minutes - and the Titanic would still hit the berg. The force of the impact changes, but not its possible effects, because the contact time increases at a slower speed.. so its possible the damage could be even more severe. As an engineer, i know that dealing with loads weighing 250 tons that are moving at only 1 foot a second... if it hits something like your hand - its still going to flatten it. - at 17 feet a second it just happens faster..

so... the kinetic energy of contact, ( a partial inelastic collision ) is lost by contact both in the berg and the Titanic hull, it a bit like looking at cars colliding in slow motion or an explosion in slow motion... as opposed normal time.. so going slower means there is more opportunity for contact.. and the longer the contact the greater the opportunity for energy to be transmitted? so it might have meant more holes or bigger holes.. or both.. because the berg would not break up as much.. neither would the Titanic or the Berg be pushed apart faster..?

First of all, we must understand how a ship behaves when a hard-over emergency turn is ordered.
In real life, using a steam steering gear of Titanic vintage, there would have been a small lag between helm movement and the change of direction. i.e. unlike a car steering system, the bow would not start to move left the moment QM Hichens started to apply left rudder i.e. there would be a momentary delay.
When the bow did start to move left, it would start to do so almost imperceptibly, but quickly gain swing speed. However, the moment a heading direction changes, the ship begins to slow down - fairly quickly if it is a hard-over turn, and that is without touching the engines.
I believe the answer to Williams' question might be found in the evidence of Captain Rostron if you ignore his waffle about Carpathia's speed.
Just before Carpathia arrived beside Boxhall that morning, Carpathia was making about 14.5 knots. she had to swerve suddenly to avoid an ice berg . That berg was North East of where Titanic sank because North East is the direction taken by Boxhall when rowing away from the sinking ship. If I remember correctly, there was only one iceberg close to Boxhall at that time and it was SE - between him and the approaching(or stopped) Carpathia.
So there's a simple answer for you , William - 14.5 knots and with lots of lookouts.

 

[Arun Vajpey]

However, the moment a heading direction changes, the ship begins to slow down - fairly quickly if it is a hard-over turn, and that is without touching the engines.

Is that an actual slowing down of the ship itself or a reduction of rate of "forward" movement ie in the original line of travel?

 

[Incony]

i understand the turning circle was no different at 11 knots or 22 knots for the Titanic. and i perceive that all that changes is the time to reach the berg... not the distance... ie the sighting of the berg , wither you are going at 10 knots or 22 knots stays the same.. what changes is how long it takes to reach the sighting point which remains the same.. and after that how long you have before you cover the same 500 yards.. Qoute from the web "TITANIC’S SEA TRIALS Performed in Belfast Lough on Tuesday April 2, 1912, over a period of 12 hours. The ship averaged 18 kts for a 2 hour run, with bursts up to 21 kts. Her Turning Circle was determined to be 3,850 feet with a forward motion of 2,100 feet. Emergency stop from 20 kts took 850 yards (unloaded). Emergency stop from 22.5 kts would require a little over 1,000 yards (half-mile). During her 570 nautical miles (nm) run to Southampton, she briefly reached 23.5 kts (but, was not loaded) By comparison, OLYMPIC reached 22.75 kts at 78 rpm of her outboard screws. " So at 10 knots i suppose its possible to stop in a little over two minutes with the full load that it had.. but in consequence still, the turning circle has not changed... Titanic would still have hit the berg, and more to the point stayed in contact with it much longer.. i dont like like only ifs or wise after the event supposition, 500 yards was not enough to avoid collision at the speed it was going. nor would it have been at 10 knots, it might have stopped a 5 knots safely, but that's a bit like saying all the folks would have been saved if there were enough lifeboats... it wasn't doing 5 knots, it wasn't doing 10 knots, it was doing at least 20 knots, and didn't have enough lifeboats. and the distance was about 500 yards. No one knows the shape of the berg underwater... itself a consequence, since two thirds of the 70ft? high berg is underwater.. pretty much the visible part was half the length of the Titanic. The Carpathia had a lot less mass than the Titanic, so Momentum and advance and allusion are different - and at 14.5 knots one would need to know the same things, the distance from the sighting to the berg, and the Carpathia turning circle... obviously only knowing speed doesnt tell you the result, even though for the Carpathia a change of course after sighting was a result... which tells you two more things, for the Carpathia there was sufficient distance and angle, to turn away from the berg... i.e was the Carpathia berg dead ahead, to port or starboard?, an angle of only 1 degree makes a huge difference, as it would have even for the Titanic.. whose characterises were totally different to the Carpathia.

 

[Incony]

folks - do the maths. speed is not of consequence given the distance..

 

[Jim Currie]

i understand the turning circle was no different at 11 knots or 22 knots for the Titanic. and i perceive that all that changes is the time to reach the berg... not the distance... ie the sighting of the berg , wither you are going at 10 knots or 22 knots stays the same.. what changes is how long it takes to reach the sighting point which remains the same.. and after that how long you have before you cover the same 500 yards.. Qoute from the web "TITANIC’S SEA TRIALS Performed in Belfast Lough on Tuesday April 2, 1912, over a period of 12 hours. The ship averaged 18 kts for a 2 hour run, with bursts up to 21 kts. Her Turning Circle was determined to be 3,850 feet with a forward motion of 2,100 feet. Emergency stop from 20 kts took 850 yards (unloaded). Emergency stop from 22.5 kts would require a little over 1,000 yards (half-mile). During her 570 nautical miles (nm) run to Southampton, she briefly reached 23.5 kts (but, was not loaded) By comparison, OLYMPIC reached 22.75 kts at 78 rpm of her outboard screws. " So at 10 knots i suppose its possible to stop in a little over two minutes with the full load that it had.. but in consequence still, the turning circle has not changed... Titanic would still have hit the berg, and more to the point stayed in contact with it much longer.. i dont like like only ifs or wise after the event supposition, 500 yards was not enough to avoid collision at the speed it was going. nor would it have been at 10 knots, it might have stopped a 5 knots safely, but that's a bit like saying all the folks would have been saved if there were enough lifeboats... it wasn't doing 5 knots, it wasn't doing 10 knots, it was doing at least 20 knots, and didn't have enough lifeboats. and the distance was about 500 yards. No one knows the shape of the berg underwater... itself a consequence, since two thirds of the 70ft? high berg is underwater.. pretty much the visible part was half the length of the Titanic. The Carpathia had a lot less mass than the Titanic, so Momentum and advance and allusion are different - and at 14.5 knots one would need to know the same things, the distance from the sighting to the berg, and the Carpathia turning circle... obviously only knowing speed doesnt tell you the result, even though for the Carpathia a change of course after sighting was a result... which tells you two more things, for the Carpathia there was sufficient distance and angle, to turn away from the berg... i.e was the Carpathia berg dead ahead, to port or starboard?, an angle of only 1 degree makes a huge difference, as it would have even for the Titanic.. whose characterises were totally different to the Carpathia.

A few corrections to your post.
1:: The size of a turning circle is the function of drift angle. The value of the drift angle varies from vessel to vessel and for the same vessel under differing values of speed and helm angle. Thus, the size of Titanic's turning circle would increase as the speed dropped. It is the same for any normal ship.
2: Speed trials in the Irish Sea or any where around the UK coast, except in enclosed waterways, are notoriously inaccurate due to the extremely strong tidal streams.
3. If we accept the available evidence, then the iceberg was a mere 144 feet ahead of Titanic when she began to turn left.... not 500 yards.
4. Consider the berg as a single, stationery, immoveable point and the ship as a "cross" with first point of contact on the ship as a single point at the end of the arm of the cross, moving at 38 feet/second. Then apply the laws of mechanics and physics (as I have done for my new book). I think you might get a surprise.

 

[Arun Vajpey]

o at 10 knots i suppose its possible to stop in a little over two minutes with the full load that it had.. but in consequence still, the turning circle has not changed... Titanic would still have hit the berg, and more to the point stayed in contact with it much longer.

I am sorry, but I don't agree to that. At 10 knots, the turning circle would have started when the Titanic was much futher away from the iceberg. I think (but stand to be corrected) that's what Jim means above by "drift angle". If one consideres the berg as an (almost) stationaty object, the Titanic would have performed a wider arc around it by starting its turn much further away and would almost certainly have avoided impact.

 

[Samuel Halpern]

At 10 knots, the turning circle would have started when the Titanic was much futher away from the iceberg.

The turning circle starts when the rudder is put over Arun. It has nothing to do with speed. Any deep hulled vessel will decrease in speed during a turn because of increased hydrodynamic drag. For all practical purposes, if Titanic going at half the speed she really was, and if the helm order came when at the same distance to the berg as it actually did, she would have hit at a lower speed than she actually did, and the kinetic energy taken off would have been far less than it really was. Even if she stayed in contact with the berg for a longer period of time, the impacts would have been less severe, and fewer rivets breaking away.

 

[Samuel Halpern]

Drift angle, Arun, is the difference between the ship's heading and the direction she is moving at given point in time during a turn. It's sort of analogous to motor vehicle in a turning skid.

 

[Jim Currie]

All great academic stuff, lads.
However, if the berg avoided by Carpathia was the same one that did for Titanic, then a lower speed helped, but the real secret was vigilance, otherwise, we would be discussing two disasters.

 

[Samuel Halpern]

folks - do the maths. speed is not of consequence given the distance..

That video is totally misleading. Sure, momentum is mass times velocity. Sure, the mass is very large. But the mass doesn't change. If you cut the velocity in half, the momentum of the vessel will be half as much. The kinetic energy needed to change the momentum is a function of the square of the velocity. A vessel going at half the speed will have 1/4 the kinetic energy compared to it going at full speed. Speed does matter.

 

[Arun Vajpey]

Titanic going at half the speed she really was, and if the helm order came when at the same distance to the berg as it actually did

That is the part that I fail to understand Sam. IF the Titanic was going at half the speed that it really was, the berg would have been spotted by Fleet when the ship was at the same distance as the actual event. No arguing about that. But from that point on, several events happened before Murdoch actually gave the first helm order.

• Fleet took at least a few seconds to acertain that he was indeed seeing something ahead before ringing the bells. Let us say 5 seconds, although I personally think it was a wee bit longer than that.
• He rang 3 bells at an interval of a second between rings......2 seconds. This alerted Murdoch on the bridge.
• Murdoch immediately started scanning the sea ahead & soon spotted the iceberg.......5 seconds to do so.
• Having seen the berg, Murdoch had to assess its position and distance to make a decision about his order. I think you have explained this yourself in another thread Sam, but I cannot recall your opinion on time factor involved. I think you said something like 7 seconds.
• Murdoch then gave the hard-a-starboard helm order.

(At this point I'm ignoring Fleet's phone call to the bridge, Moody's response etc because that is probably irrelevant as far as Murdoch was concerned. He would have started scanning as soon as he heard the bells and very probably had already spotted the berg the time Moody yelled out Fleet's phone message.)

It therefore took some 19 to 20 seconds at least from the moment Fleet spotted the iceberg and Murdoch gave the helm order. In that time, the Titanic had travelled about 750 feet towards the berg (assuming a speed of 21.5 knots or approx 38 feet per second) on the actual night. But at only 10 knots, the ship would have covered only about 350 feet before the helm order.

In other words, if the Titanic had been travelling at only 10 knots instead of 21.5 knots that she actually was, Murdoch's first helm order would have come when the ship's bow was 400 feet further away from the icerberg than it actually was that night. Considerng that Murdoch's attempts came very close to succeeding in avoiding the collision, what could have been the consequence of the hypothetical scenario of a lower speed?