High water at Liverpool that morning was 11:18, at a height of 32'10" and was the highest tide that year. "Laver's Almanac" quotes high water at Menai Bridge as 28 minutes before high water Liverpool. What is termed 'slack' in the Swellies is actually a brief period of uneasy equilibrium between two opposing flood streams which typically occurs 1 hour 42 minutes before local high water, or at 09:08 on the morning of the move. Owing to the strength of the southwest-going ebb, which runs at 8 knots during a , there is a confused complex tidal flow among the numerous rocks and islets generating many powerful eddies. On a big tide in particular it is vital for an outbound vessel to be through the Swellies before the tide turns against her. It is therefore the local practice to start an outward transit with the last of the northeast-going flood 20 minutes before the 'slack', or at 08:48 on the morning of the move. Captain Hewitt had worked with the Caernarvon harbour master, Captain Rees Thomas, in preparing his plan, and had several times made a passage of the Swellies in the "Conway"s motor boat, checking his timings at . He also consulted "Conway"s old log book for the timings of the previous transit, and had planned to arrive at the bridge at 09:20.