Optimal speed to avoid collision


Arun Vajpey

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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

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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.
 

Jim Currie

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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.
 
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Arun Vajpey

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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.
 
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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.
 
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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

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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?
 
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Incony

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Arun Vajpey, even if the Titanic was going only 10 knots, and so covered the same distance in a longer time, the turning circle hasnt changed... it takes longer because the speed is slower... so even if the longer time means things happen in relation - faster, like the commands on the bridge and the action in the engine room and the rudder turning - the result doesnt change, it just takes longer.. because the distance from the berg to the titanic doesnt change, just the time to cover the distance... and if the distance isnt large enough and the turning circle is the same, it doesnt matter how fast you can make the rudder turn because even if your going slower... 500 yards want enough distance... ? the titanic needed 800 plus yards to turn sufficiently ( 2 degrees? ) ?
 
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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.
Do you want to know what would have happened is the ship started her turn away from about 400 feet further back than what she actually did? She would not have contacted the berg in her bow. She would have, however, initially made contact with the iceberg in the vicinity opposite the fourth funnel, near the juncture between the reciprocating and turbine engine rooms, and contact would probably have extended further aft of that point to include the dynamo room. (see below.) The ship would have been lost if the damage to the hull caused flooding in those 3 compartments.
1631214945594.png

The problem that most people have in conceptualizing the accident is that a ship turns like a motor car when going ahead. That is just not the way things are. It is more like motor car being run in reverse. Her stern swings out away from the direction of the turn. The danger zone is along the side of the vessel all away to the stern.
 
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Mar 22, 2003
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Just to add to the scenario above, where the turn starts about 400 feet further back from where it did. If it is realized early enough that the ship would strike the berg near her quarter if no further action taken, the OOW could order the helm shifted hard over to port in an effort to swing her stern away from striking. Timing is critical, but if pulled off successfully, the berg might have been sidestepped completely.

Lots of ifs.
1631295523516.png
 
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Arun Vajpey

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Just to add to the scenario above, where the turn starts about 400 feet further back from where it did. If it is realized early enough that the ship would strike the berg near her quarter if no further action taken, the OOW could order the helm shifted hard over to port in an effort to swing her stern away from striking. Timing is critical, but if pulled off successfully, the berg might have been sidestepped completely.

Lots of ifs.
Thanks for that Sam. I know that this is not an easy question to answer but given that Murdoch, probably the most competent officer on board was on the bridge, do you think that he could have timed the second helm (hard-a-port) order well enough that night to miss the berg going by your "if" scenario above at 10 knots?

I am thinking about your conjecture in another thread (to which I agree) that on the actual night, Murdoch quickly realized that an impact with the iceberg was inevitable and sought to minimize the damage. The one and only thing that I believe differently is that rather than assume that inevitability even before he gave the first hard-a-starboard order, I think that it was perhaps 6 or 7 seconds later when he realized that the berg was closing too fast and the ship was not turning enough.

Ironically, the reason I think that way is because my mind keeps going back to your serial diagrams in the article Encounter in the Night. Even if your thinking about distances have slightly changed since you published that article, I think the "blossom effect" of the closing icerberg is demonstrated extremely well. Adding my own medical knowledge to that, human depth perception is relatively mediocre and reduces at night. In other words and using arbitrary figures only, any man in Murdoh's position and with good vision might have not been able to make an accurate judgement of size-speed-distance when that berg at 1800 feet from the bow but be able to do so with a fair degree of accuracy at 1300 feet. (The distances mentioned are just examples to explain what I am saying).

Correct me if I am wrong but the way I read your earlier posts and opinons expressed elsewhere on this subject compared with the one above in this thread, Murdoch had no chance whatever of avoiding an impact with the iceberg that night with the Titanic going at 21.5 knots. But if the ship had been going at 10 knots instead, there was a chance of avoiding impact altogether provided Murdoch gave his helm orders at the right time, even allowing for a bit of luck to be involved.
 
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Jim Currie

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Just to add to the scenario above, where the turn starts about 400 feet further back from where it did. If it is realized early enough that the ship would strike the berg near her quarter if no further action taken, the OOW could order the helm shifted hard over to port in an effort to swing her stern away from striking. Timing is critical, but if pulled off successfully, the berg might have been sidestepped completely.

Lots of ifs.
View attachment 77697
Only if the 2nd helm order is given immediately after the fist and the engines are left severely alone ;)
 

Stephen Carey

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Slowing down in ice, fog, restricted seaways and weather is subjective, and not all ships are the same of course.
On a 21knot container ship we slowed down to around 15knots in ice at night and in fog, depending on the conditions and didn't maintain full speed. That's quite a reduction.
On the same run and conditions in a 12knot iron ore carrier, we slowed down to 8-10knots!
The container ship had an ice-strengthened bow so in clear ice conditions in daylight we just barrelled through it at 21knots. On the ore carrier we tried that at 12knots and put a hole in the bow from a bergy bit...
In other words it's not that simple, and often you don't know what's out there until you meet it.
 
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