Help please on wartime navigation

Trevor Bell
21st April 2009, 20:17
Hi all

I am new to the forums and hoping that some of you may be able to help in some research I am doing about the sinking of RMS Rangitane in 1940. I am no mariner and need help on some ocean navigation issues. As I understand it, navigation in wartime (radio silence etc) was by dead reckoning - estimating actual track travelled by knowing course, average speed, current, wind direction etc until a fix can be made and then changing course to correct any deviation. Taking the 6000 nm from New Zealand to Panama, in WW2 British ships were routed by Naval Control through a number of waypoints about 1000 nm apart to avoid enemy raiders. Assuming that no fix can be taken over 24 hours, how far adrift would a ship typically be? Was it realistic to be able to take regular fixes at night and could fixes always be taken in the daytime? It's pretty obvious I don't know much about ocean navigation so any help would be much appreciated.

Trevor Bell

non descript
21st April 2009, 20:27
Firstly a warm welcome to you. Secondly thank you for an interesting first post and I am confident that some of our navigators will be able to offer you a proper answer.
Good to have you on board

21st April 2009, 21:15
Welcome Trevor:
Navigation across the Pacific would be a "piece of cake" to any ship in ww2 which had trained deck officers. The routine would normally be as follows when out of sight of land -
1. At dawn the chief officer who would normally do the 4 to 8 watch would take star sights ( moon and planets if available) and get a pretty good fix.
2. When the sun had risen to a reasonable height the 3rd mate who did the 8to 12 would take a sun sight when it had reached a reasonable altitude and the second mate also would take a sight as he was the navigating officer ( he did the 12 to 4 watch.)
This would provide one position line only. A calculation would be made to ascertain when the sun was on the meridian and at this time several of the officers (and often the master) would observe the maximum altitude of the sun and this would give them a definite latitude. The earlier positon line would be advanced to the noon sight, crossed with the latitude and give a good fix.
Later on in the afternoon, the second mate would take another sun sight providing a single position line and at dusk the chief officer and possibly others would take stellar sights again and hopefully get a good fix ( the afternoon position line would be run on to this fix as another confirmation.
This was the basic daily routine.
If the weather was bad and sights were not taken , a dead reckoning position would be estimated, taking account of the information published about ocean currents etc.
Of course these interesting and satisfying routines have, I believe, fallen into disuse with the advent of GPS and I think that the demise of skills like astronav, morse light signals and semaphore are a great shame making us a lot of screen watchers and calculator nerds !
Separation lanes also have made life at sea boring. In my days at sea we would pass close by ships passing by and call them up on the Aldis lamp and chat. I met many old training ship colleagues in that way. Nowadays, ships passing are miles away, it seems.
Mind you, you must make allowance for the fact that I am a right Meldrew in my old age (82)

Trevor Bell
22nd April 2009, 15:54
Thanks for the reply which begs more questions! First, if two competent navigators side by side took a fix using the same method, how much difference in nautical miles might there be between the two calculated positions? Second, if there was complete cloud cover and the ship was travelling on an old fashioned compass bearing with a helmsman at the wheel (remember my interest is in WW2), what would the drift typically be after 24 hours - 1 nm, 5 nm.... 20nm... more? I am trying to find out how a ship can be over 50 nm off course after one day.

22nd April 2009, 16:21
Two competent navigators shouldnt differ by more than a couple of miles, but it is an art as well as a science and we all have our off days.
As to the 24 hour error. A DR position is calculated using just course (which is pretty accurate) and distance (which depends on the accuracy of the log or known revolutions). Once you allow for set & drift (effect of tide/current) and leeway (effect of wind) it is called an Estimated Position and thats where the art comes in. You just have to use the best information you can get from publications for set & drift and guess the leeway from your experience of that ship. (Normally you would adjust your course to counteract leeway). 50 miles would seem to be a bit excessive.
I'm not old enough to know for sure but I dont think "radio silence" would prevent the use of passive radio aids and I'm pretty sure Loran was used in WW2.

22nd April 2009, 17:00
Greetings Trevor and welcome to SN. Bon voyage.

Trevor Bell
22nd April 2009, 17:58
Thanks Sidsal and Lancastrian - this is getting interesting! Thanks for the info on Loran. According to Wikipedia, Loran was developed from GEE which itself was not introduced until March 1942 for aircraft positioning over Germany with a maximum range of 400 miles - I am looking at an incident in November 1940 in the middle of the South Pacific so I guess that there was no passive positioning system in use at that time. As for DR, assuming 15 knots over 24 hours, a one degree error in course would only be a 6 nm error - a long way from the 50 mile error I am researching. Does anybody know what the currents are like in the South Pacific!

22nd April 2009, 18:34
"Does anybody know what the currents are like in the South Pacific!"
You have to pay for that! -

22nd April 2009, 19:02
This is the US National Geospatial and Atmospheric Agency Website and has a ,pdf file for the Pilot Charts of the South Pacific.

They are free and high resolution so you can enlarge the area of interest.

22nd April 2009, 23:42
Needless to say, the electronic navigation aids commonly in use today (GPS, Satnav, Loran, Decca, etc.) were unavailable during WW-II. However, all licensed deck officers were required to be capable of celestial navigation. It was possible to ascertain a ship's position during daylight by getting a celestial "running fix" of the sun. That was done by taking a series of several sextant observations of the sun over a period of time, usually one hour apart. Each of the lines of position obtained thereby were then run up along the ship's dead reckoned course and speed. The ship's position was then plotting at the intersection of the lines of position.

Celestial observations by sextant were not possible at night. That was because, although the moon, stars and planets were visible, the horizon was not. Consequently, nocturnal celestial fixes could only be obtained at morning and evening twilight, when both the stars and the horizon were visible.

The other principal use of celestial navigation was for ascertaining compass error. That was done by taking an "azimuth" or "amplitude" of the sun or other celestial body. Essentially, the officer took a compass bearing of a celestial body, noting the time and ship's position as closely as possible. By means of mathematical tables he then computed what the bearing SHOULD BE at that particular time and position. Whatever the difference was between the computed bearing and the compass bearing told him how much error there was in his compass. Deck officers generally tried to work out at least one azimuth or amplitude during the course of each four-hour watch, if possible.

23rd April 2009, 04:38
They were certainly some good navigators given the handicaps they had. Like Lighthouses being switched off during the war and fake ones set up to lure any enemy shipping ashore.
Them old Navigators could find the harbour entrance in pitch black or thick fog with little or no lights to guide them.

23rd April 2009, 08:35
This is the US National Geospatial and Atmospheric Agency Website and has a ,pdf file for the Pilot Charts of the South Pacific.

They are free and high resolution so you can enlarge the area of interest.

How kind of the US Government to make these available. If only the UK Government would be so helpful in putting its information (and photos) in the public domain!

For Klaatu83. How ever many times you shoot the sun, its still only a running fix of dubious accuracy. The only ones worth having were star sights.

Trevor Bell
23rd April 2009, 10:01
Thanks again for the advice. I now have an estimate of current throwing a ship off by about 10 nm in 24 hours, of compass error of 1 degree causing 6 nm error in 24 hours so what about leeway - would this be significant on a 16000 ton ship? Even if all these errors are cumulative (maybe they cancel each other out?), I am still struggling to account for a 50 nm error in 24 hours. Any further thoughts?

Pilot mac
23rd April 2009, 10:20
Smart arses will also catch Venus on the meridian in the afternoon!


23rd April 2009, 14:09
Trevor, was the vessel at the time under VNCS or NCS as different Rules apply in regards to the routing of a Merchant Vessels in wartime, she may have had orders when at sea to avoid a MERZONE.

Trevor Bell
23rd April 2009, 15:05
I am not familiar with VNCS or NCS unless they refer to naval control service. I know that the vessel had been routed by the New Zealand naval control service which was a military department which compulsorarily routed all naval and British registered merchant ships across the Pacific to Panama. Each vessel was given a set of secret sealed waymarks (which changed regularly) through which they had to pass to avoid using the normal quickest route thereby avoiding German raiders. The vessel was over 50 miles adrift from the route specified by the naval control service.

23rd April 2009, 15:12
"what about leeway - would this be significant on a 16000 ton ship?"
Depends on strength and angle of wind to track. The Pilot Chart will give an idea of the prevailing winds, but not necessarily what happened on any one day. 3 degrees would not be unusual if not allowed for.

23rd April 2009, 20:13
Beg to differ a little with Klaatu83. I have been known to take a moon sight at night when the horizon below was clear. Also we did not take hourly sun sights as the small angles between position lines would tend to largish errors. The morning sights of the sun would be run off to the noon meridian ( or ex-meridian) sights which would give an exact latitude which, crossed with the transferred earlier position line , would give a reasonably ccurate noon position. Star/planet/moon sights at dawn and dusk could provide very accurate poistions and I venture to suggest that four or five position lines crossing in a very small"cocked hat" was a very satisfying experience .
The ships I sailed in during ww2 had magnetic compasses only and quadrantal compas cards - the 0 to 360 cards came later. Amplitudes/azimuths were taken when posssible twice in a watch, and whenever an alteration of course was made, in order to acsertain deviation ( variation being obtained from the chart). The deviation of the compass was noted religiously in the log.

23rd April 2009, 20:17
PS Never came across Loran or Decca during the war but at the very end of the war one ship I was on had radar fitted. An armed guard was placed on it in port. The screen showed just a grassy green fuzzy line and the aerial was manually turned by the operator. An echo would show up as a peak off the grassy fuzz. The radar was housed in a thing that looked like a small stubby lighthouse on the monkey island.

Trevor Bell
23rd April 2009, 23:34
Not surprisingly, this is all getting very technical! Sidsal - you referred to "a reasonably accurate noon position" and "a very small cocked hat" - what sort of accuracy are we talking about and how big (in miles) a cocked hat?

24th April 2009, 17:00
Trevor: I would say an accurate noon sight would be within half a mile and a good cocked hat would be similar.. Often with star sights one would get four good crosses and one way out caused perhaps by taking the wrong star or making a stupid mistake in the maths !

Trevor Bell
24th April 2009, 18:12
Thanks again to all the respondants - there is a wealth of knowledge out there! I have another technical question. Let's say that you have to sail from A to B which are 1000 miles apart. As I understand it (again bearing in mind this is under WW2 conditions) there are two options, sailing great circle which is the shortest route or by Mercator which would be constant bearing. I have tried to find the maths which would tell me what the maximum deviation would be between these two options. Is there a simple way of calculating this?

24th April 2009, 18:42
Not that I know of, other than plotting both routes and measuring it with dividers. The sailings are all explained here -
What exactly are you trying to discover from all this research?

24th April 2009, 19:31
Trevor, are you saying that the vessel was out of position because of a navigational error/weather and if so, how do you know that was the reason?, it may be because the Master has recieved a Merzone warning either by encrypted broadcast (all ships) or special encrypted message (his ship) to prevent his ship "standing into danger"and as he was under full Naval Control (NCS) his route is determined by the Naval H/Q under a "special cover" document which covers everything he needs to know. Sightings were taken each day @1200Z, Lat & long, great circle or rhumb line track and speed. The problem you may have is that you may not be "privy" to any passage ammendment message (PASSAM) to determine as to whether he was out of position due to weather/danger or navigational error and you most probably may never know.
The "special cover" document was very much the same Post War in the event of a "problem" occuring and all Senior M.N. Officer's were made aware of it when they attended a course when going for their Tickets. Just as a matter of interest I would be obliged if those who know would P.M. me, no prize I am afraid.

24th April 2009, 19:56
Working out great circle courses and distances was ( and should still be) entirely within the capability of a competent navigator , as was plane or a Mercator calculation. This was without any calculator. Purely a matter of trigonometry. There was ( and is I should think) also Composite Great Circle sailing. That is - when a Great Circle course could mean that a vessel entered high latitudes with possible ice problems, a great circle route would be set to a particular latitude, then a due east or west course would be sailed and then the last leg would be a great circle course to the destination.
All Certificate of Competency exams would have a question involving this.
I remember that a favourite example in the navigation books like Nicholls Guides would be a courses from San Fransisco to Yokohama.

Trevor Bell
24th April 2009, 20:09
Thanks for the link Lancastrian - I will have to brush up my trigonometry! The purpose of all the questions is all about RMS Rangitane which was sunk by German raiders in 1940 in the south Pacific. At an official inquiry many people including Rangitane's captain insisted that the raiders knew where to find her because of leakage of information, collusion etc etc. This included accusations that the Germans had details of the NCS waypoint coordinates. But this doesn't make sense because Rangitane was over 50 miles far to the north of her track to the first waypoint. If the Germans knew the waypoints, how was it that they were 50 miles too far north at exactly the same time? So I am trying to establish why Rangitane was off course. To answer Chadburn's point - at the official inquiry the NCS gave evidence (as did the captain) that there was no last minute re-routing. I have my own theory.... but I need more technical evidence first! What I have discovered from all the learned wisdom in this forum is that it is highly unlikely that Rangitane was off course because of navigational error or wind and current. Any further thoughts would be welcome!

24th April 2009, 22:50
OK Trevor , I have now found your site and even read that very long Enquiry Report.
Going back to great circles, it was the normal practice to alter course only once a day, so as she was one day out from NZ, if you calculate the rhumb line course and the initial great circle course you will have the number of degrees difference which simple trig will convert to miles between the two tracks.

Trevor Bell
26th April 2009, 17:00
I think that this thread on wartime navigation has run its course (yes, the pun was intended!) Many thanks to all contributors - I have a very much better understanding of the issues and have dusted off my slide rule to do some serious calculations. However, keep watching because I shall be starting a new thread about wartime merchant navy codes. Again, thanks to all.

26th April 2009, 17:14
Best of luck Trevor. A master I sailed with used to refer to my slide rule as my "guess-stick" !
Re poss thread on MN Codes I have tried to get a copy of MERSIGS which all ships had giving the flag signals for convoy manouvres etc and the various zig-zags. I have been unsuccessful !

26th April 2009, 18:08
Best of luck Trevor. A master I sailed with used to refer to my slide rule as my "guess-stick" !
Re poss thread on MN Codes I have tried to get a copy of MERSIGS which all ships had giving the flag signals for convoy manouvres etc and the various zig-zags. I have been unsuccessful !

Well if you do find a copy, dont put it on the site as its probably still classified!

27th April 2009, 20:13
You could be right that MERSIGS may be still classified.

27th April 2009, 23:05
First of all thanks to Sidsal for reminding me how we operated in pre GPS days. I was a navigating officer (2nd Mate) in that era and proud of my art.
I can certainly remember the days when we congregated on the bridge at "Noon" to take a Latitude and the Old Man would say his sextant reading and we would have to accept that as gospel!! Ah well, we got there eventually. Wonderful sad they have passed.!!!
Dave 437

27th April 2009, 23:16
Best of luck Trevor. A master I sailed with used to refer to my slide rule as my "guess-stick" !
Sid - He was a bit harsh on slide rules, my father used them all the time as an airline pilot in the 50's and 60's and I still possess his aviation circular version. He preferred the linear version because of the reduced accuracy of the inner scales on the circular. I remember him talking about using a special version when he was on pathfinders in WWII. When I went to sea in the late 60's calculators were taking over and I remember spending a vast sum on a Texas Intruments scientific calculator, which consigned the slide rule to the scrap heap.

Trevor Bell
29th April 2009, 20:03
Just an afterthought - when using a sextant, is it realistic to claim a one minute of arc repeatability? i.e. if 10 people made the same sighting with the same sextant, would they all get exactly the same angle to within one minute of arc?

29th April 2009, 20:26
Dave: See you were in Clan Line. Fellow Conway cadet AJR Tyrrell went in Clan Line and was master, I believe, then joined P&O and became Personnel Director.
Beer Sailor: I have an aircraft circular slide rule - it's called a computer but isn't !! I used to fly years ago and it was used to allow for drift etc.
Trevor. Probably they would get pretty near the one minute difference but it's a long time since I used a sextant although I still have one. My original one was a wartime "utility" version and I sold it when I was hard up. I bought a Japanese Tamaya sextant when I circumnavigated under sail in the 80's.