NavList:

Trevor Bell

A few questions and a few comments follow:

I strongly suspect that researching  official records will greatly quench your thirst in forming the conclusion that German raiders were at the ready …meaning at Rangitane’s ETA on 27 Nov,1940. ( and thus knew intended position).

I am researching the sinking of the RMS Rangitane in WW2 300 miles off New Zealand on her way to the UK via Panama. I have established that she was about 50nm off a whole circle course after about 45 hours having sailed 420nm of the 1500nm to her first pre-arranged waypoint. How did you establish this  50 NM error?  Which side of the intended track ?I am trying to establish whether being off course (what was her intended course (great circle or plane sailing)and how do you know ?) could have been the result of normal navigation error. One could say yes but then again could be due to poor helmsman ship, and/or possibly several factors. (I believe nothing to this effect was determined by the subsequent enquiry?)

I know that the weather at the time was not good - overcast skies and rain for much of the time. I have learnt a little bit about old fashioned (pre-GPS) navigation and would like to test out some conclusions on you learned guys.

1. I have been advised that under normal wartime conditions of radio silence ( not sure where this fits in as chronometers were used for keeping “accurate” time.)and out of sight of land an accuracy of about 2 arc minutes or 2 nm was routinely achievable with celestial navigation on a large ship. Depended on navigator , his assistant –recording time from the deck watch-But with constant cloud cover, I assume that a position could only be estimated from bearing( read compass course) , log, knowledge of currents and leeway extremely unlikely in ocean currents , etc.
2. Compass error could be as much as one degree - about 7nm in 420nm from poor steering and calibration. Without becoming too long winded let us just say his positional error (for this factor only ) could be + -  4 NM yielding a circle of position EP (estimated position) whose radius is 4NM.
3. Errors from a poorly calibrated log could be as much as 5% - 23nm after 45 hours. Do you know that the vessel carried a taffrail log or more importantly was it in use ? ( Taffrail logs were prone to shark attacks, fouling and the like. Depending on the sea conditions engine rpm was as good a measure as say a log in determining speed made good (distance with time factor)
4. Errors from a mid ocean 1kt current could be as much as 42nm but I understand that this would have been automatically compensated for by the navigator. Ocean currents are neither constant in direction or speed (set and drift) . I would suggest you see a PILOT CHART for South Pacific –November (year does not matter)  I could look this up for you but since you are going to play with so many variables you might as well jot your thoughts down on this chart.
5. I do not know how much windage would have affected a 16,000 ton ship. As noted above I respectfully suggest you study the pilot chart first , (unless you happen to have the ships log) . Unless the vessel was caught in a very strong blow , I would not spend too much time working on this aspect. (The  calculation is a simple one for your guesstimates  .)

If all these factors were cumulative then I assume that 50nm deviation after 45 hours might have been possible, yet many people have told me that it is highly unlikely.( Listen to your friends, you might win some lotto but you ,at best, are only able to calc. the odds at winning) But I need to explore one further possibility but my maths isn't up to it:
I understand that there are two fundamental ways to navigate between two points - whole circle (called a GC or Great circle)and Mercator and I think I understand the relative benefits. But how do I calculate the difference in track after 45 hours using the two different methods? Can anybody point me to some software which might help? If your math  is wanting I suggest you consider “acquiring”  say an old (meaning used) hand held calculator with built in navigational routines . My favourite is the HP 41CX (Hewlett=Packard) with the navigational pack. It will do a great job for your tasks at hand. In the alternative there is   website devoted to HP calculators that includes different modules..one being the nav. pac. (You see the 41CX on the monitor and use your mouse to get going…you are also able to download the manual.  )  If the study of nav. interests you, perhaps you may want to invest in a nav program. Aside from my 41CX pgr I use Coastal Explorer (more than enough for you I suspect and finally Star Pilot. but this is rally for use in celestial nav.

Hope I have not been too critical , but my intention is simply to give you some food for thought. No doubt others , far brighter than I , will  provide you with , hopefully. more useful information. Sorry to conclude by saying I have written this is haste, which may encourage others to help  out.

Best of luck.

IFH

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