Interesting usage of the word that is new to me. The passage in which the term occurs comes from the Dionysiaca, an epic poem by the late Hellenic poet Nonnus who wrote in Egypt in the 5th century AD.
Bull, you are astray out of your country; Nereus is no bulldrover, Proteus no plowman, Glaucos no gardener; no marshground, no meadows in the billows; on the barren sea there’s no tillage, but sailors cut the ship-harbouring water with a steering-oar, and do not split with iron; Earthshaker’s hinds do not sow in the furrows, but the sea’s plant is seaweed, sea’s sowing is water, the sailor is the farmer, the only furrow is the ship’s grain and wake, the hooker is the plow.
The note on grain reads: If a line be drawn along the ship’s course, the part ahead is called the grain, the part astern is the wake. I found further elucidation at the Society for Nautical Research.
The Commonwealth ‘Fighting Instructions’ of 1653 include a command to form a line of battle in the Admiral’s ‘wake and grain’. In the 1654 ‘Fighting Instructions’ it was changed to ‘wake or grain’. Further instances of the use of the word ‘wake’ cast light on its use at that time. ‘Grain’ as a useful word in this context has fallen out of use. ‘Grain’ may have been used to mean the same as the modern use of the word ‘wake’, i.e. in line astern, or it may have meant the opposite, i.e. in line ahead. It may even have been used to mean in line either ahead or astern, a meaning that ‘wake’ might have had at the time. Either way, the meaning of the instruction was clear; that ships were to take up their stations in line, either astern or ahead of the admiral as appropriate.
So it may have meant this, it may have meant that or it may have meant something else. That’s all clear then!