OK Navigators, what are these for?

[Cadence]

Originally Posted by Cadence

Maybe I misunderstand? We are speaking of a vessel that went into service in 1934 The photo may be of her restored to her days of glory?

I may be all wet? I had no idea Sperry came out with a gyrocompass as early as 1914.

[KJThomas]

Originally Posted by Snowpetrel

The 264 is obviously the course, we used to have them on the older container ships, but the + and - numbers I am not so sure about. Maybe allowance for set and drift? Or deviation, but if it was deviation I'd expect it to be East or West. She didn't have a gyro, so its not gyro error high or low.

Another thought is it might be the difference between the steering compass and the standard compass.

It is gyro error. The gyro repeater is visible to the left of the steering compass. Obviously, she did have a gyro! At least in later years. Obviously not when launched.

[Uricanejack]

Originally Posted by Don C L

So then back to the arrow... could it be that the gyro error would fluctuate + to - ? (I can't imagine why..) and they would need a way to show the helmsman which to look at? (still curious about that spinning arrow) Well for that matter what DOES cause gyro error?

I wouldn’t say fluctuate. Fluctuate implies randomness and not steady

I would say Gyro error varies. I think varies is probably the correct terminology but the old grey matter isn’t what it once was. I ither forgot or never knew.

Again my old brain has forgotten which was which, the Dalek or Armour Brown. They both used the same basic principle in the opposite way.
Known as damping.

A the axis of free spinning gyro points to a constant point in space. Or “star” which is not a hell of a lot of use to a terrestrial navigator. Spaceman yes. Sailors not so much.

The gyros star will appear to rise and set every day just like the sun. This movement is known as the precession of the gyro. It takes 24 hrs or 23hr 56 min or 4 minutes per degree.

The earth itself is just a big gyro. It’s axis point in one direction. In space.

The precession can be broken into two components tilt or altitude. And azimuth.
Both will complete a cycle every 24 hrs.

A free spinning gyro must have the freedom to spin.
The freedom to tilt or rise or fall on a vertical axis.
The freedom to rotate on a horizonontal axis

The spinning is left alone or it just won’t work

If you apply a force to the horizontal or vertical axis of it will move 90 degrees in the direction of rotation from the force applied.

By applying a constant force to one axis the gyro can be damped in tilt or azimuth

This is done by offsetting the spinning wheel.

Now instead of 24 hrs to complete a precession it will only take an hour or two depending on how much force.

This gyro is no use even on a space ship.

So mercury is used on the other axis. It’s no constant but as the gyro tries to rise the mercury’s flows to the end of the axis an adds weight to the end of the axis.

So you apply another force on the other axis of the gyro. At the same tim. 90 deg from the first one.

The gyro will now instead of appearing to go in a circle. It will gradually spiral until it reaches a balance point slightly up or down in altitude or tilt. And slightly of from North.

It takes 3 or 4 hours for a gyro to settle close enough to north to be used as a gyro compass.

It will be almost constant but will if I rember correctly. The gyro still continue to spiral in very tiny wee almost imperceptible small rotations round north. So the gyro error will very slowly change.

So gyros almost always have a very small almost but not quite constant error.

The ships direction will have an effect as will latitude and speed.

Traditionaly the compass error was taken at least once per watch. The gyro was compared to the compas.every time the error was taken to determin the gyro error.

Traditional practice the error was taken using the magnetic compas. Mathematical it can be taken from a gyro repeater.

The logic behind the tradition. The magnetic compas. Was and still is the ships compass.
So the OOW would figure the compas error out. Using the magnetic compas.
Calculating the compass error. Apply variation to determin deviation.
At the same time compare to the gyro.

The gyro repeaters were mechanically and electrically connected to the main gyro. Then adjusted to read the same as the gyro.
Since they were adjusted separately each repeater might have a slightly difrent error in its setting.
This is not the gyro error but a separate reapeter error.

The gyro was compared to the compas every alteration of course.

The gyro error would occasionally be zero or + or - about 1 maybe 2 degree.
By keeping a record it would be fairly constant on a particular heading but may be within the + - 1 or 2 east or west or north or south headings.
It would also vary with latitude and speed.
A correction could be applied for both. Speed and latitude

Modern gyros have difrent methods of doing pretty much the same thing. To make a gyro seak north. With a little bit greater presision.

[Uricanejack]

Originally Posted by Uricanejack

I wouldn’t say fluctuate. Fluctuate implies randomness and not steady

I would say Gyro error varies. I think varies is probably the correct terminology but the old grey matter isn’t what it once was. I ither forgot or never knew.

Again my old brain has forgotten which was which, the Dalek or Armour Brown. They both used the same basic principle in the opposite way.
Known as damping.

A the axis of free spinning gyro points to a constant point in space. Or “star” which is not a hell of a lot of use to a terrestrial navigator. Spaceman yes. Sailors not so much.

The gyros star will appear to rise and set every day just like the sun. This movement is known as the precession of the gyro. It takes 24 hrs or 23hr 56 min or 4 minutes per degree.

The earth itself is just a big gyro. It’s axis point in one direction. In space.

The precession can be broken into two components tilt or altitude. And azimuth.
Both will complete a cycle every 24 hrs.

A free spinning gyro must have the freedom to spin.
The freedom to tilt or rise or fall on a vertical axis.
The freedom to rotate on a horizonontal axis

The spinning is left alone or it just won’t work

If you apply a force to the horizontal or vertical axis of it will move 90 degrees in the direction of rotation from the force applied.

By applying a constant force to one axis the gyro can be damped in tilt or azimuth

This is done by offsetting the spinning wheel.

Now instead of 24 hrs to complete a precession it will only take an hour or two depending on how much force.

This gyro is no use even on a space ship.

So mercury is used on the other axis. It’s no constant but as the gyro tries to rise the mercury’s flows to the end of the axis an adds weight to the end of the axis.

So you apply another force on the other axis of the gyro. At the same tim. 90 deg from the first one.

The gyro will now instead of appearing to go in a circle. It will gradually spiral until it reaches a balance point slightly up or down in altitude or tilt. And slightly of from North.

It takes 3 or 4 hours for a gyro to settle close enough to north to be used as a gyro compass.

It will be almost constant but will if I rember correctly. The gyro still continue to spiral in very tiny wee almost imperceptible small rotations round north. So the gyro error will very slowly change.

So gyros almost always have a very small almost but not quite constant error.

The ships direction will have an effect as will latitude and speed.

Traditionaly the compass error was taken at least once per watch. The gyro was compared to the compas.every time the error was taken to determin the gyro error.

Traditional practice the error was taken using the magnetic compas. Mathematical it can be taken from a gyro repeater.

The logic behind the tradition. The magnetic compas. Was and still is the ships compass.
So the OOW would figure the compas error out. Using the magnetic compas.
Calculating the compass error. Apply variation to determin deviation.
At the same time compare to the gyro.

The gyro repeaters were mechanically and electrically connected to the main gyro. Then adjusted to read the same as the gyro.
Since they were adjusted separately each repeater might have a slightly difrent error in its setting.
This is not the gyro error but a separate reapeter error.

The gyro was compared to the compas every alteration of course.

The gyro error would occasionally be zero or + or - about 1 maybe 2 degree.
By keeping a record it would be fairly constant on a particular heading but may be within the + - 1 or 2 east or west or north or south headings.
It would also vary with latitude and speed.
A correction could be applied for both. Speed and latitude

Modern gyros have difrent methods of doing pretty much the same thing. To make a gyro seak north. With a little bit greater presision.

Some of the above is crap. My memory is faulty. Probably should check stuff before I post stuff I think I remembered

Precession is not what I posted.

Precession is the change of azimut when a force is applied.

When a constant force is applied the gyro will oscillat ither side of the meridian.

Damping reducises the oscillating until it settles.

It still spirals and settles near north.

[El Pinguino]

Golly, someones grey matter is in better shape than mine...
I had to go and find this.. https://www.britannica.com/technology/gyrocompass

Meanwhile some may try and riddle this out.... it was very clever how Mr Sperry fitted 360 degrees into those repeaters... as you can see 10* takes up about 40 actual degrees....

How did he do it?

I used to know but have forgotten but it was very very clever...

[GrowleyMonster]

Originally Posted by El Pinguino

Golly, someones grey matter is in better shape than mine...
I had to go and find this.. https://www.britannica.com/technology/gyrocompass

Meanwhile some may try and riddle this out.... it was very clever how Mr Sperry fitted 360 degrees into those repeaters... as you can see 10* takes up about 40 actual degrees....

How did he do it?

I used to know but have forgotten but it was very very clever...

The compass card in the repeater is not a flat disk. Only the part you see is made flat for viewing. Yes it is very clever and very nice for the man steering by it. Some other repeaters have an inner and an outer ring.

[SeanPatrick]

A fairly good explanation of precession:

https://www.youtube.com/watch?v=XHGKIzCcVa0

[El Pinguino]

Originally Posted by GrowleyMonster

The compass card in the repeater is not a flat disk. Only the part you see is made flat for viewing. Yes it is very clever and very nice for the man steering by it. Some other repeaters have an inner and an outer ring.

Close but no banana...

There are two flat discs....rotating at slightly different rates.... between the - lets say - 260 and 270 and hidden from view are other numbers 320 , 020, 070, whatever....

As the inner disc rotates more slowly it exposes a different set of numbers...

[Lodesman]

Originally Posted by Don C L

So then back to the arrow... could it be that the gyro error would fluctuate + to - ? (I can't imagine why..) and they would need a way to show the helmsman which to look at? (still curious about that spinning arrow) Well for that matter what DOES cause gyro error?

Assuming it is gyro error, there are obviously two slots - the one on the left indicates a negative (-) error, with positive(+) on the right. The arrow is not necessarily meant to point left or right; perhaps it should be pointing fore or aft?

Would the QM possibly have more than one gyro, and the arrow indicates which is in use (port and stbd, or fore and aft)?
Warships out of that era were typically fitted with 2, and there would be a barrel switch somewhere on the bridge to switch between them.

[georgiah]

True Course
Mag variation
Mag heading
ships deviation
Per Standard Magnetic (Ships Compass course)

[El Pinguino]

At last... I think we have a definitive answer..... from the Old Pharts site I frequent

'Folks,

Have been following this thread with interest?
As someone who spent 5 years as a navigating officer on Cunard cargo vessels from 1967 to 1971 have been pondering the question posed by Cisco.
Perhaps the plus/minus refers to "twist"?
When I first joined Cunard it was the time that the Queens were being disposed of as well as many navigators. Some transfered to the cargo fleet. I was advised by these experienced gentlemen that to counteract the gulf-stream they applied a "twist" allowance of up to +/- 2 degrees to N. Atlantic GC courses. Strangely it worked on the many crossings I made during those years as navigator and our landfalls were generally spot-on.
Hope you all have a prosperous and healthy New Year. Cheers, Chris'

[El Pinguino]

We also have an answer as to that queer looking radar..

'The radar was a British Thomson Houston. It had the logo BTH on the chassis. Similar radars were fitted on Sylvania & Caronia.
The 2nd R/O did the maintenance

https://en.wikipedia.org/wiki/British_Thomson-Houston

BTH morphed into AEI and then into Marconi'

[El Pinguino]

And also ... regarding 'twist'...

QM and QE would both have been doing 700 mile 'days runs'.... thats a long time between 'sights' ( even when you divvy it up between morning stars/noon/evening stars and don't allow for overcast conditions...) so they would have been running on DR for considerable distances.

'What about LORAN ?' I hear you ask....
Even after the war and the invention of LORAN I doubt they had it.... I never came across it on any British ship I sailed on... with one exception and she had been built for a Norwegian owner.


Another handy little factoid for you .... one of the largest British companies - Blue Star Line - owned by Vesteys the Butchers - only started fitting radar to their ships in the late 1960s.... a very conservative animal was the British shipowner...

This is where I sought enlightenment https://www.shipsnostalgia.com/showthread.php?t=289523

Seems someone had the answer at #2 but then the thread wanders a bit..... until several days ago.

[Teknishn]

I'm coming in here late to the party, but I was thinking back to the days when I was teaching DR in the USN. I'm wondering if the +/- isn't for Set and Drift? Just a thought....

[El Pinguino]

Originally Posted by Teknishn

I'm coming in here late to the party, but I was thinking back to the days when I was teaching DR in the USN. I'm wondering if the +/- isn't for Set and Drift? Just a thought....

Well yes, that's pretty much what ex Cunarder 'Callpor' said above.

With 100 years experience of running across the North Atlantic the Cunard navigators called it 'twist'.... applied to port of track when approaching New York, to starboard when crossing the stream outwards.

I am assuming they had a morning arrival in New York so they would have had a ( nominal) 12 hour run at 30 knots since stars the evening before.