Achieving 6.0 nautical miles easily before 1700hrs BST ensures that he should cover the distance comfortably before the tide turns again at 1848hrs (UT) or 1948hrs BST, perhaps 1950hrs local time.
So, in this example, the tide turns from north-east to south-west at 1330hrs BST. There will, of course, be a period of slack water but the tidal diamonds sometimes do not show this clearly.