Speed measurements/indicators


So? It all depends how long you were in that particular current. If the the average current was setting 070°T at 0.5K for several hours earlier, then the course and speed over the ground would be something like 266°T at 22.1K during that period of time. No big deal. :)
 

Doug Criner

Member
Time out. This discussion reminds me of something from my ancient naval experience in an old destroyer.

There was something called a dead-reckoning tracer. It kept a graphical plot of own-ship's track along with target's track - very handy when maneuvering violently back and forth, attacking a submarine (not for plotting a steady course over time), and deciding when to turn hard left or right, and when to drop depth charges. So, where did the DRT get its dead-reckoning inputs? For ship's course, from the gyro compass. I can't remember where the speed input came from - maybe just from the engine rpms? Or from the engine-order telegraph? But, of course, ocean current was not important, nor was travel over ground! We were all about trying, or more like playing like trying, to kill someone.
 
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So? It all depends how long you were in that particular current. If the the average current was setting 070°T at 0.5K for several hours earlier, then the course and speed over the ground would be something like 266°T at 22.1K during that period of time. No big deal. :)

No big deal for sure … one has just to «cherry pick» one of the 32 compass rose points as a set, a unique force vector as a drift and an estimated period of time, to stretch a vessel’s position a on a chart where it best suit his own presumptions. And not only that, as set and drift is also subjective to lateral wind leverage and hydraulic forces acting upon a vessel.

Questions:

The lateral windage centroid of a tramp ship is located at the rear of her center of flotation. The vessel is located in the center of an ocean gyre where the current is zilch. She is steered by hand through a fresh breeze on the Port beam. Being steady for hours, in which direction the vessel may well set; up wind, downwind or no effect?

A tramp vessel is propelled by a left hand propeller. She is steered steady by hand for hours in flat calm weather. In which direction the vessel will tend to set; toward the Port side of the route, the Starboard side or no effect?

How can we determine by eyesight alone, no glasses needed in that case, if a vessel is drifting and in which general direction?

So, if someone attempt to deduce from the parallelogram forces law where a vessel will land from a magnetic steering compass and a patent log considerably influence by wind, current, time, lateral wind leverage, propeller transverse trust, the ability of her wheelsman, the accuracy or her quarter master to properly record the patent log register vs. time, the age of her master… he will soon find out that these influences can knock out all common rules into a meaningless cocked hat vectors structure!
 

Jim Currie

Member
If a vessel steers 266°T at 22.6K by patent log and there is a current setting to 196°T at 1.3K, the course and speed over the ground would be something like 263°T at 23.1K. :)
As you know, the Patent Log was not a speedometer, Georges... it simply measured the apparent distance that the device had been towed through the water. In theory, it is a little turbine. much like the turbine in a hydro plant. The device in question rotated 9000 times per mile of distance it traveled through the water i.e. the number of rotation were directly related to the speed of the water flowing past it. In theory, a head current would increase the pressure on the rotator blades and the rotator would turn faster. thus indicating a greater distance over the ground or water and consequently an exaggerated speed.
However, as a ship handler, you will know that the patent log normally is located in the neutral zone within the wake pattern. Consequently, the log error was normally very small. You might be interested to know that HMS Cumberland which was scrapped in 1959 (The year I got married), recorded an error of less than 5 miles over a voyage of 3000 miles.
 
Jim, you’re in command of a brand new passenger liner, on a great circle passage from Bermuda to the English Channel Light Vessel. The distance is 2,800 nautical miles. Your cruising sea trial speed is registered as 22 knots, in dead calm weather and no current whatsoever. The general average Gulf Stream current along the route is estimated at 2 knots by Pilot Book and the Weather Routeing agency forecasts light winds all the way.

Once on Full Away of Sea Passage, the perfectly rigged patent log starts reading, the Doppler Log Water Tracking push button is depressed and the GPS Distance Run Over the Ground is activated, so that all instruments starts reading and recording the distances run through the water and over the ground at the exact same time.

At End of Sea Passage, all instruments arrived at the same time or after 117 hours and are compared. Since everything went according to the voyage plan, the difference in distance between the patent log and the Doppler log water tracking is nada zero zilch! But compared to the GPS, it seems that something went wrong. The recorded distance over the ground is effectively 2,800nm but the distance run through the water from the patent log and the Doppler log is only 2,567nm or a difference of minus 233nm against the GPS.

On your way back, in the exact same conditions and route, the GPS still shows 2,800nm. The difference between the patent log and the Doppler log water tracking distances are still nada zero zilch. But now, it took 140 hours to cross and the distances recorded by both water logs are 3,080nm or plus 280nm against the GPS.

Eastbound Log (2,567nm) + GPS Difference (233nm) = 2,800nm
Westbound Log (3,080nm) - GPS Difference (280nm) = 2,800nm

The least we can say is that these results are absolutely astonishing, aren’t-they!!! How can that be possible?

Now Jim, do the same exercise only westbound but in that existing chaotic current system, while crashing into a few gales low along the run and under an overcast sky all the way through and tell me how you are going to manage to reach Saint George’s Cut by magnetic compass and by patent log alone. Please be advised that there are many hundreds of shipwrecks all around Bermuda Island … might is well you provide the lookout with a good set of glasses! :)
 

Jim Currie

Member
Georges, that was a saga worthy of Jules Verne or Captain Maryatt. However, you are not comparing like for like. I suggest you start with Difference Between GPS and DGPS and take it from there.
Incidentally your example tracks are way of "base".

You waffled -on about ocean currents around The Cape. You haven't lived, pal until you have taken loaded bulker or worse still, a light one through the Pentland Firth or the Messina Straits. Or for a real thrill... try the Corryvrecken.

As for the idea that the Labrador Current at the Titanic wreck site? I suggest you consult oceancurrents.rsmas.miami.edu/atlantic/slope-jet... If you do, you will discover that unless that particular current was turned-off during the month of April, 1912, there was no way that the Labrador Current could have flowed over warmer water in a southerly direction in the vicinity of 42 North, 50 West . I suggest you play the animation.
 
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. ;)
 
So, where did the DRT get its dead-reckoning inputs? For ship's course, from the gyro compass. I can't remember where the speed input came from
I would assume Doug that the destroyer had what was called an underwater pitometer log system. It works very much like an airplane's airspeed indicator by measuring the difference between dynamic and static pressure heads using a rodmeter which projects through the vessels hull in the forward part of the ship. They used that system on submarines as well to plot their DR course through the water even when submerged.
 

Jim Currie

Member
It seems he came up with that by dividing the distance the ship was from the corner at noon by 6 hours. Nothing to do with log readings unless they reduced rpms from 75 down to about 71 for six hours.
Your selectivity regarding the evidence of 5th Officer Lowe is astonishing, Sam. You pounce on his "15825. Did you look for any lights at this time at all.
Answer ...........-I glanced over in that direction casually and I saw a steamer there." and turned that "casual glance" into a swinging bow.
Yet you ignore the most obvious pointers as to how the man navigated between 6 pm and 8 pm. I remind you:
Senator SMITH.
In order to ascertain the ship's position accurately at 8 p. m. you must know her speed at 8 p. m., must you not?
Mr. LOWE.
If you take the average speed from 12 to 6 - that is giving her a run of six hours - she will not jump up in two hours, from 12 to 6 o'clock, from that average speed. You have six hours in there to take a mean on.
Senator SMITH. :But you had means, had you not, of ascertaining definitely how fast the ship was going?
Mr. LOWE: In what way, sir? We have the log -
Senator SMITH. (interposing) Between 6 and 8 o'clock.
Mr. LOWE.
We have the log.

I should think that it would be obvious - even to Long John Silver with both eye sockets covered, that Lowe was using Patent Log readings and dividing them by the time interval between them to obtain a speed to use in the calculation of Dead Reckoning positions.

It would be equally obvious to LJS that since Lowe did not arrive on the bridge until 10 minutes after Titanic was assumed to have been at The Corner, he could not calculate an 8 pm DR before he had calculated a 5-50 pm DR. The work would be in two stages. (It would be recorded on the "Day's Work" sheet.).
Stage 1: Course and distance from Noon to 5-50 pm using the known course and an estimate of the speed derived from the 6 pm Patent Log reading divided by 6. This would give him a DR for 5-50 pm
Stage 2: Course and distance from 5-50 pm to 8 pm using the same speed derived at 6 pm (because as Lowe thought "she will not jump up in two hours".
The foregoing is the reason why Lowe was able to the following answer:
Her speed from noon until we turned the corner was just a fraction under 21 knots.

Sam. you cannot ignore the foregoing or waffle around it with suggestions of Patent Log inaccuracies. The Bottom line is that Lowe got that speed from a reading of the Patent Log .The man would have been very particular about speeds, given that his main task that evening was the construction of Slip Tables. You are very much aware of that since, in the past,you have written extensively about the subject.
I can well understand your reluctance to accept this part of Lowe's evidence. After all, you have also written extensively about the question of time change, and if you accept the evidence of Lowe, concerning speed, you will have to go back to the drawing board.
 

Jim Currie

Member
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.” ;) ;)
;)
 
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

Member
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

Member
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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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.)
 
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