NavList:

Lars you wrote concering the Hughes Aircraft Marine Sextant: What was the intended use in RAF for this small sextant? 

In the early days it was common to use a marine sextant for air navigation.  It was used in the R34 airship, by Alcock & Brown, by Coutinho & Cabral, and in the Graf Zeppelin airship.  The only problems were: it might be night time, the horizon might be obscured by cloud or mountains, or you might have difficulty knowing your exact height to calculate dip, because in mid-ocean you might not know the surface pressure to apply to your pressure altimeter.  The advantage of a marine sextant as opposed to a bubble sextant was that you didn’t have to worry about acceleration errors.  Anecdotally, Coutinho only used the famous bubble attachment for his marine sextant when the natural horizon wasn’t available.  I read somewhere, but I can’t remember where, that on the approach to St Paul’s Rocks, he did have a natural horizon, so he didn’t need use his bubble attachment.  Also anecdotally, the crew of the R34 airship calculated dip by checking their height over the sea by using their marine sextant to measure the angle subtended by their shadow, but that takes a bit of getting your mind around deciding if it’s a 2D or 3D problem.   

Maritime aircraft usually flew at low altitude, and in some parts of the world in perfect sunlight, so it made sense to have a marine sextant available.  It might also have been used while fleet shadowing to measure ‘distance off’ to keep out of range from a capital ship of known dimensions (e.g. at 3nm a 300’long ship would subtend approximately one degree, 300/6080x3, 1 in 60);  and for a flying boat, when on the sea to check position, or horizontally when taking soundings close to land. DaveP