Question about speed measurementsindicators

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Sean Munger

Guest
Maybe this is a very basic question, maybe not (please forgive if it is! bear with me).

How was actual speed measured on Titanic's bridge (and presumably on other ships of the day?) I recall reading something about a device called the "Svenska Log" which was a gizmo utilizing a pipe protruding through the bottom of the ship's hull, but I thought this was an invention first used on ships of a later day, and assume Titanic would not have had one.

With what instrument (or calculation), then, would officers on the bridge utilize in order to take a reading of Titanic's precise speed at a given moment?

Just curious. Thanks.
 
Aug 10, 2002
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Sean:
According to testimony by Quartermaster Rowe, they used a taffrail log. It is like a propeller on a line that is towed astern of the ship. As it is towed through the water the propeller twists turning the line which turns dials on a meter. You read and record the readings each hour. The pitch of the propeller and gear train in the meter are such that it records nautical miles traveled through the water. It varies from observed miles traveled by slip. The observed distance is the distance measured between two geographic positions, it is the true distance traveled. mIn Titanic's case when out in open ocean it would have been the distance between
two celestial fixes. I hope this answers your question.
Regards,
Charlie Weeks
 

Dave Gittins

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Apr 11, 2001
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To add to Captain Weeks' remarks, they couldn't get a speed reading at a given moment in Titanic's day. They could only work out an average, given a time and a distance run.

Later logs did give speed as well as distance. In quite recent years yachts were using a hybrid log that had a towed impeller that sent electrical impulses to an instrument on the stern.
 
Mar 22, 2003
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To add to this, they were also working on developing a slip table which, among other things, was to give the speed of the ship as a function of the revolutions carried on the reciprocating engines. At the end of each watch the junior officer would call up the engine room and record the average revolutions per minute that were being carried into the ship's log. In addition, the distance run through the water taken from the taffrail log reading on the after bridge on the poop deck was called in every two hours by the quartermaster stationed there.
 
May 31, 2006
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I am interested in how Titanic determined its position during the Atlantic crossing. Did the navigator have to rely on celestial navigation only? What aids other than dead reckoning were available to him if there was cloud cover? Having to rely on "shooting the stars" as a USN ships navigator in the early 60's in the the Pacific and the South China Sea, I know how frustrating "celestial only" positioning can be- and I had LORAN to rely on- except when it didn"t work! Is there a thread on the Titanic site that deals with Navigation? I would love to find it! AJG
 
Mar 22, 2003
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Hi Alan: There is no thread that I'm aware of specifically dealing with how the ship was navigated. There are threads dealing with a few specific related issues such as: Time Zone Practices and Sea Time, Time of collision, Titanic Position, Delay? Wot Delay?? (The "Corner"), Distance from the corner to the CQD location, Navigational Refutation of Titanic's CQD Position. All aree under Collision / Sinking Theories topic.

At least you had LORAN when it worked. Unfortunately, they only had sextants, chronometers, almanacs, pencils, paper, tables and charts. If you are interested in the subject and more, I would highly recommend Dave Gittin's e-book, "Titanic: Monument and Warning." Appendix 1 specifically deals with navigation. It comes in a CD-rom. See: http://users.senet.com.au/~gittins/Book.html. You will also find Dave's style of writing a joy to read.
 

Dave Gittins

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Apr 11, 2001
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Sam, flattery will get you anywhere!

Alan, it really too much to go into here. I'll just say that in 1912 British navigators used methods learned for the Board of Trade examinations. These used spherical trigonometry and logarithms, rather than the short method tables that you no doubt know. Given a visible horizon and a celestial body, they could get latitude and longitude at most hours of the day and into the night.

The beauty of the old methods is that they gave latitude and longitude without the use of plotting charts. In theory, the methods were precise, but given the limitations of the vernier sextants of the time the navigators were probably kidding themselves a bit. There's an example in the Titanic story of a navigator working out longitude to a quarter of a minute.
 
Mar 22, 2003
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Flattery works sometimes, depending who is being flattered.
happy.gif


Yes it is interesting how precise numbers could be, but sometimes the results were not accurate as we have seen.
 
Dec 2, 2000
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>>Yes it is interesting how precise numbers could be, but sometimes the results were not accurate as we have seen.<<

Indeed, and if they weren't always on the money, they did at least manage to get into the bank, and all without using any fancy aids such as the Global Positioning System. I would hope that contempoary navigators could do as well using a sextant, a few tables, and good old fashioned skull sweat in crunching the numbers.
 

Jim Currie

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Apr 16, 2008
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OK Guys! However I did have an old Admiralty vernier sextant which I used all my sea-going days alongside my pals who used modern vernier ones. I knew my errors and could find where we were as well a they could - even better.. My star telescopes were much better than their single wide lenze version.

Sailorjim
 
May 3, 2005
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It seems a bit curious that ships such as Titanic had no instrument giving instantaneous readings of speed, such as a speedometer. Automobiles and other vehicles of the time had such devices.

Were the old methods of calculations of speed so ingrained that no one even thought of designing such a device ?

My apologies to sailorjim and others, but I was never a sailor even though I served four years in the USN. LOL.
 
Dec 2, 2000
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>>Were the old methods of calculations of speed so ingrained that no one even thought of designing such a device ?<<

I would be amazed if somebody didn't think of trying to build such a device. The problem is that the best you could have done at the time would have been something which would have measured the flow of water through something like a tube, or turns of a prop on a logline. It couldn't possibly have accounted for factors such as current, slip, drift, wind, wave and so on. Terra firma doesn't move, but water does even when you're sitting still.

In other words, they would have been obliged to verify their information with navigation sights to make sure it was an accurate reflection of reality.
 
May 3, 2005
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>>would be amazed if somebody didn't think of trying to build such a device. The problem is that the best you could have done at the time would have been something which would have measured the flow of water through something like a tube, or turns of a prop on a logline. It couldn't possibly have accounted for factors such as current, slip, drift, wind, wave and so on. Terra firma doesn't move, but water does even when you're sitting still.<<

So many factors are involved that only some kind of computer taking inputs on all the data...from water, wind, etc....would be able to give a true instantaneous speed indicator. Are there any such devices on modern ships nowadays ?

Bicycle speedometers worked on a gear off one of the wheels and more or less just converted RPM's to MPH's, so we get back to the reckoning of ship's speed by RPM's. Bicycle wheels ran on solid ground...As you said the ground doesn't move but water is constantly in some kind of movement all the time and from all directions.

Incidentally.....In reference to Titanic's speed and the temperature just before the collision, the wind chill factor in the crow's nest would have been something close to 10 Degrees Fahrenheit. It seems as if that would have some effects on the lookouts ability to see things.
 
Jan 29, 2001
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Was the Titanic fitted with a Taffrail Log at it's stern which measured the ship's speed thru the water? I recall a letter written on-board Lusitania in which the writer was given a bit of a fright having noticed the log activating in the water surround thinking it was a torpedo!

Michael Cundiff
NV, USA
 
Dec 4, 2000
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Titanic was equipped with a taffrail (or "patent") log. It was consulted regularly throughout the navigator's "day's work." As with all such devices, it measured distance through the water.

Distance through the water should not be confused with distance "made good." The water in which Titanic (or any ship) steamed was always moving due to currents. The speed of this movement sometimes increased while at other times decreased the speed made good from the distance measured by the patent log.

Speed made good could only be determined by measuring the distance between "fixes," or accurate positions of the ship at specific times. Out of sight of land, these fixes were obtained by "shooting" the sun, sometimes the moon, and certain stars with a sextant.

Because speed made good could only be obtained between fixes, it was always computed at the end of the measured run. Surprisingly, modern GPS receivers do the same thing. The difference is that a 1912 navigator measured distance made good over hours and GPS does it over fractions of seconds.

Patent logs were noted for suffering sometimes large rates of error. A new line, for instance might register "low" until it was stretched and worked tight. Navigators of 1912 were advised that a 4% plus/minus error was to be expected from trailed logs. It was considered more accurate to gauge speed off of the number of revolutions ("turns") being made by the engines. This explains why the officers put so much emphasis on the never-completed slip table.

-- David G. Brown
 

Jim Currie

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Apr 16, 2008
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Actually the log line did not stretch David.

Because it was continually in torsion, it could not be made from normal 3 or 4 strand RH rope. It was a special, braided cotton line which did not twist or stretch. However, it was very prone to kinking therefore it had to be streamed (payed-out in a special way. Instead of just paying out the impeller first and letting it pull the rest of the log line until it was fully out behind the ship, the impeller (fly) was kept on deck until the last moment. The line itself was payed out in a bight until it was all out with one end attached to the taffrail register and the other to the impeller (fly. Then the fly was thrown over board and it fell away astern until the line was fully stretched out to it's full length. Then it would start turning. here's a wee sketch:
207837.jpg


In fact, they were unjustly cautious about these things since they worked on more or less the same principle as the engine revs. The biggest problems were caused by being fouled by seaweed of floating debris. In fact, the design did not change very much between the time of Titanic and the time of the old QE2.

Jim