Speed measurements/indicators


Jim Currie

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Jim, do you know the nautical adage; «When the pilot points to the leading lights … the cadet looks at the finger»? :)

My example was regarding a simple way I would adopt to make a cadet understanding the difference between the distance run through the water from the distance run over the ground of a vessel proceeding in a non-static liquid. You can change the current from a ¼ of a knot to Gulf Stream 6 knots, the final outcome will all proportions guarded be the same. But I understand that you could’ve forgotten the basics since your maritime academic degree, if you ever held any, was conferred since ages. Serving ashore as an insurance clerk would certainly not help either.

Jim, when you sailed back and forth through the Pentland Firth, the Messina Strait's, the Strait of Corryvreckan or the Niagara Falls, did you compare the patent log with your DGPS WAAS? Have you kept any «evidences»? How can you understand those distance run differences if you’re not even able to make the distinction between an Ocean Current from a Tidal Current?

Being almost always on the defensive, even about what might seem like the most insignificant things, bring some people to enjoy arguing just for the sake of arguing. ;)
“That's the beauty of argument, if you argue correctly, you're never wrong.” ;) ;)
;)
 
Mar 22, 2003
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Well, the corollary to that is, "if you argue incorrectly, you're never right.” ;)
Unless engine rpms were reduced down to about 71 during the run from noon to the corner as I posted in #28 above, the distance traveled through the water over a given time period, which is what the log shows, should have been about the same as that over previous time periods of the same duration with the ship carrying the same revolutions. And the ship was making better than 22 knots through the water since noontime Saturday carrying an average of 75 rpms.
 

Doug Criner

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I don't remember there being such an underwater pit-log, but it is possible. My recollection is hazy, but I don't remember there being a separate speed-through-water indicator on the bridge, but it is possible that there was a pit-log that provided an input to the dead-reckoning tracer. Speed was ordered from the bridge by the engine-room telegraphs. The telegraph pedestal had three knobs to signal the ordered rpm of our two engines. The quartermaster would log any ordered changes of speed or course, along with the time. Speed orders were not ever given in knots.

The three rpm knobs on the bridge were understood to mean to apply to both engines. When maneuvering, with the possibility of each engine being ordered for different speeds, then the engine-order telegraphs alone were used - the three rpm knobs were set to the ship's hull number (537).

90% of our time was steaming in formation, along with other ships, escorting the carrier. Our assigned position was a distance and bearing from the carrier. We continually checked our pelorus bearing from the carrier and our range from the carrier. If we were getting ahead or behind our assigned position, the order would be given to the lee helmsman, for example, "drop two turns." The radar repeater on the bridge was not very precise, so we had to call down to the combat information center to get our range to the carrier.
 

Jim Currie

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Well, the corollary to that is, "if you argue incorrectly, you're never right.” ;)
Unless engine rpms were reduced down to about 71 during the run from noon to the corner as I posted in #28 above, the distance traveled through the water over a given time period, which is what the log shows, should have been about the same as that over previous time periods of the same duration with the ship carrying the same revolutions. And the ship was making better than 22 knots through the water since noontime Saturday carrying an average of 75 rpms.
Or perhaps if you keep repeating the same old nonsense often enough it'll become fact?
You wrote "the log shows, should have been about the same as that over previous time periods of the same duration with the ship carrying the same revolutions. "
You have obviously never used a Patent Log Sam., otherwise you would not have put your name to that nonsense. I know you have heard of set and drift, so perhaps you had a memory block?
We are waffling on about Patent Logs and things nautical. This is disrespectful to those members who find it hard to keep up. Perhaps if I explain in terms they can relate to, we can sort this out? Consider the following. I'm sure everyone can relate to this. First let me point out, that the rotator of a Walker Patent Log, performed in exactly the same way as does the turbine in the water- flow meter of a domestic water system, In the Walker Log, 9000 revolutions of the rotator = 1 nautical mile.

Like the rotator on the Log, the Turbine flow-meter (better described as an axial turbine) translates the mechanical action of the turbine rotating in the liquid flow around an axis into a user-readable rate of flow.

The turbine wheel is set in the path of a fluid stream. The flowing fluid impinges on the turbine blades, imparting a force to the blade surface and setting the rotor in motion. When a steady rotation speed has been reached, the speed is proportional to fluid velocity.Knowing this, apply it to the following hypothetical situation


A Turbine flow meter is installed. It measures tons of sea water passing through it per hour. 9000 revolutions of the turbine is equal to one tonne of flow.
At Noon, the Register shows 000.00 Tons and the Revolution Counter also indicates zero.
At 5-50 pm, the Revolution Counter shows 1131.3.K. However, the tonnage counter has failed.

Question 1: How many tonnes passed through the turbine during the period Noon to 6 pm?

Question 2: If there had been an error in the Revolution Counter and it was reading 11.7 K too low ( i.e more water than indicated had flowing through the turbine), what was the true value of delivery by 6 pm that evening?
 
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Mar 22, 2003
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What is so hard to understand? The log measures the distance travelled over the body of water the ship is sailing on. The body of water is the frame of reference. If the ship makes 22 knots through the water carrying 75 revolutions, it cannot make 21 knots through the water unless the revolutions are reduced proportionately, or there is a very strong head wind that is strong enough to slow the steamer down by 1 knot (not the case in question here.)
 

Jim Currie

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No Sam, to be correct, the body of water through which a ship sails, is not a frame but a cuboid which had dimensions equal to the beam as well as the air and water drafts of the vessel.
As you point out, a strong head wind (acting on that part of the vessel above the water) will effect its forward speed and consequently the distance in one direction it will travel. However, by the same token, a head current or any current forward of the beam,s will effect the forward distance traveled.
At least 2 independent and very well qualified witnesses testified to the expectations regarding a current in that area which would have slowed any ship down had they encountered it. At least one source of evidence demonstrates that such a head current existed in the area in question:
Captain Moore of the Mount Temple thought that his ship would pick up half a knot from the eastward setting North Atlantic Current.
Likewise, Titanic's 4th Officer Boxhall also expected such a current and even off set it with a guestimate of reduced propeller slip due to the flat calm which gave him his 22 knot figure he used to calculate the Distress position. He was obviously using a ship speed of 21.5 knots as derived form the Olympic Slip Tables since the ones for Titanic had yet to be completed and he did not use the Patent Log. Had he been using the ones you concocted, he would have used a speed greater than 22 knots to work his distress position.
Then there is the calculated Noon position of the SS Mesaba which put her to the north and east of her expected (DR) Noon position.
I sailed to New York twice a month for over a Year Sam.,I do not remember a single incidence when we did not get a head current of some sort in that area. Explain how it got "switched off" for the passage of the Titanic.
 

Jim Currie

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The distance run by Patent Log Divided by time is how Lowe arrived at his speed. Over the ground or water would not enter his mind. He would simply assume that Titanic had traveled 125.7 miles since Noon. If she had not altered her course before 6 pm, he would have run on at the same speed and course for another 2 hours to obtain the 8 pm DR. However the coursew was altered before 6 pm so he had to find out a position for where she turned/ The use of the term "At The Corner" is misleading
However, if there had not been any impediment to the forward passage of the ship. and she had made 22 knots as she did up until Noon, then the 6 pm reading would have been 132 miles and Lowe would have used 22 knots to calculate a 5-50 pm turning DR and subsequently, the 8 pm DR
 
Mar 22, 2003
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Or the real log reading put down was something like 135.7 miles since noon and he read the handwritten 3 as a 2. 135.7 miles divided by 6 hrs is a speed of 22.6 knots through the water, which over a 2 hour period gives 45.2 miles by log, totally consistent with Hichens' 45 miles by log reading taken at 10pm, and consistent with the ship carrying 75-76 rpms since noontime Saturday. But I highly doubt you would be inclined to consider that as a possibility.
 

Jim Currie

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Sam. Navigators in 1912 and right up to and after 1960, worked in exactly the same way/ They used logarithms to the base 10. When Low came on the bridge at 6 pm, the first thing he would do after the handover, would be to find out the speed. We know he got that from the Patent Log because he said so in so many words. His work would be as follows:

Patent Log at 6 pm = 125 .7 Log 2.09934
Run time 6 hours ....................Log 0.77815.
Log 1.32119 = Av. Speed 20. 95 knots.
In fact , Lowe and the others would have used 4 figure logs. I use 5 because I'm used to them and besides that-s how my tables are tabulated/

The logarithm for 135.7 is in fact 2. 2937... nothing like the logarithm for 125.7. Lowe would have spotted that in a New York Minute.

In fact, both Boxhall and Pitman, (Lowe's boss) were using what seems like Olympic Slip Tables and were thinking in terms of 21.5 knots = 75 rpm.. Pitman and Lightoller said as much and you can be sure that's why Boxhall used 22 knots for his distres position. Since Boxhall had worked the 7-30 pm sights and knew the Noon position, you can also be sure that he did not get a 22 + knots average speed from Noon until 7-30 pm sights. He most certainly would have provided that information for Smith before 10 pm that evening. By the way, this is what ships ahead look like from the port wing of the bridge.
1580586719293.png

That was taken in the Suez Canal. As you can see, the ship right ahead is in the direct line of sight and would be dead ahead to the helmsman and to anyone on the starboard side. No problem with parallax on a ship with a beam such as the one you see above.
 
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