Note 15.

Due allowance has to be made for these time variations in 'The Slack', such that the draft of an outward dead tow cannot be more than would inhibit her from passing under Britannia Bridge perhaps as much as an hour and a half before local HW (L'pool -2.40). The tidal range ^{(see Fig.9)} of such a middle height tide would be midway between the 14.5 ft neaps and 19.0 ft springs ranges quoted by the Chart = 16.75 ft.So an hour and a half before HW the depth over Cheese Rock would be 15.25 + sin[90 ~ ( ⁹⁰/₃₃₇ x 180)] x ^16.75/₂ = 20ft 10ins.^{(see page 11)} After allowing Emrys' 'absolute minimum' 2 ft underkeel clearance, 20' 10" on a middle height tide clearly indicates that the channel isn't really suitable for any outward tow deeper than 19 feet. Only by progressively increasing the risk factor by being limited to the bigger tides with their brief period of 'Slack' and more severe and turbulent streams could any tow perhaps up to 20' 6" be passed through, until we come to the extreme example of Conway at 22 ft draft, which at the 1¹/₂ hrs before local HW is shown by Fig 8 to have had less than her own draft over the rock even on the very highest tide of the year.