Submarines and surface ships have devices to confuse systems that count the blades (sometimes called 'blade reduction' or 'blade rate suppresson'), on subs you will see them gain lines as they close- a skilled operator in DW can even tell you its a sub just by seeing this compaired to the range of a contact.
A sub with Five blades will have Five lines visable on DEMON, the shaft line will also be the same line as the 1st blade line.
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