In my opinion the short story is characterised by a Chinese-box narration.
First of all we find a third-person narration at the beginning that presents us, on the action level, a scene in which another character, the man, becomes in turn a narrator because he tells a tale to the woman, and, in this way, she becomes his narratee.
The role of the man as a narrator is emphasised by the words of the woman "You used to tell – your – your simple and – and professional – tales very well at one time. Or well enough to interest me. You had a – sort of art – in the days – the days before the war".
Moreover this statement probably contains another hint that made me reflect and think about the modernist writing.
We know, in fact, that before the war the tales of the man were "simple and professional", in other words, the opposite of the one he tells the woman now, during the war. As a matter of fact, he can be considered a non-reliable narrator-character because his story is full of gaps, ambiguity, uncertainty about what is real and what is not, that are typical features of the modernist writing.
So his shift from the "simple and professional tales", before the war, to the sort of tale he tells now can be a reference to how the war, with its horror and distrust, contributed to shape a new way of writing based on a new conception of reality.
It is also interesting to notice that there's another "tale" in this text that contributes to make its structure even more complex, because it's a story within a story within a story. This is, of course, the one that the Northman tells the Commanding Officer.
It is, in fact, striking that Conrad decides to use the word "tale" also in this case, as if he wanted to highlight the intricacy of his short story's structure. In addition, the tale of the Northman doesn't seem linear at all, or at least it is not perceived like that by the Commanding Officer.
So in conclusion, there's a third-person narration that presents us a frame story in which the characters become in turn narrators and narratees, we are the real readers whose thoughts and interpretations Conrad couldn't predict and the ideal reader is the one who can possibly understand all the subtleties and references meant by Conrad in this text.