General Discussions  
This is the place to discuss general issues related to the U-boat war or the war at sea in WWII. 
Re: Accuracy of Coordinates
Posted by: Ken Dunn ()
Date: October 11, 2020 01:00PM

Hi Urs,

Thanks much for the information on the bubble sextant. I have never used a sextant myself & knew there was more to it but not the details.

A friend had sent me some information on the Gyro sextant that used a different technique to come up with an artificial horizon. Here is some of the text that went with it:

"To use the gyro sextant a supply of compressed air is required, or the gyro may be run up by using a motor car type foot pump. After running up, the instrument must be allowed five minutes to settle and great care is necessary in handling to keep the instrument level and to avoid jerky movement. The sun or star is then observed in much the same manner as with a normal sextant, the lines in the field of view obtained from the rotating gyro being used as horizon. Rough direct reading of the altitude is possible and the averaging mechanism is intended to give the average altitude over a period of one, two or three minutes, the time of the observations being taken in the mid-point of the interval. It is possible to make six or eight observations before the gyro runs down to such an extent as to cause noticeable flicker and erratic behavior.

The reading of the scales is not self-evident and some practice is required before the instrument can be used with confidence. The averaging mechanism indicates 3 degrees when set but the graduations are such that 2½ degrees appears to be set. The figures indicated by the averaging mechanism are added to the indication on the drum and on the main index which reads tens of degrees. In this respect the Hughes Bubble sextant is very much superior in that the reading is direct and self-evident."

Before your post I didn't know what the bubble sextant was even though it is mentioned in the above description. Now I do. (:

The gyro sextant doesn't seem practical but perhaps the bubble sextant was.

That said I have never seen a photo of anyone on a U-boat using a sextant other than the normal one so I am guessing these are late war devices. Any idea when they were deployed on U-boats?


Ken Dunn

Options: ReplyQuote

Subject Written By Posted
Accuracy of Coordinates Sergio 10/08/2020 03:37PM
Re: Accuracy of Coordinates Michael Lowrey 10/09/2020 12:27AM
Re: Accuracy of Coordinates Mark McShane 10/09/2020 05:12AM
Re: Accuracy of Coordinates Ken Dunn 10/09/2020 06:25PM
Re: Accuracy of Coordinates Urs Heßling 10/11/2020 12:01PM
Re: Accuracy of Coordinates Ken Dunn 10/11/2020 01:00PM
Re: Accuracy of Coordinates Urs Heßling 10/11/2020 02:09PM
Re: Accuracy of Coordinates Sergio 10/12/2020 09:45AM
Re: Accuracy of Coordinates Ken Dunn 10/12/2020 12:32PM
Re: Accuracy of Coordinates Sergio 10/12/2020 01:51PM

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