6.

3. Navigation Overview.

At around 22 feet draft Conway remains the deepest ship ever to have made the Swellies transit, considered only possible on the highest tides of the year (a fact which we will challenge shortly), and consequently when the streams run at their hardest. (The Admiralty chart states the rate at Ordinary Springs to be 8 knots.) The tow would only have been possible around the period of 'The Slack', which as we have seen is at it's shortest duration on these highest of tides. Thus a primary factor in the loss of the ship was the perceived need to make the transit on the highest possible, and therefor the most dangerous tides of all.
Fig. 6 Southwestern end of the Swellies at low water on a 10M Liverpool tide, photographed from Britannia Bridge on the line of the southern transit markers through Cribbin Gutter (far right). To give scale to the photo, the nearest point of Cribbin Rock (left of the Gutter) is about 700 feet from the camera. The surface width between Cribbin Rock and the Caernarvonshire shore (extreme right) is about 150 feet. The wind locally was southerly force 5, but might have been stronger than this outside at sea, so the water level shown here might be more generous than usual. When 'Conway' passed through here outward at L.pool - 2.00 there would be about 19 feet more water here, which may be judged against the buildings on Gorad Goch.

It was generally assumed that since the ship had gone through the one way she could equally as well go back the other, but the two transits are completely different. Inbound with the tide behind him Captain Goddard had been 18 minutes between the bridges (the Sub-Committee report states 13 ?) so it would be reasonable to allow 20 minutes for the outward tow in similar circumstances. Since this is longer than what on a big tide can be expected to be a short period of 'Slack', part of the tow had necessarily to be made either with or against the stream. For a deep dead tow in a narrow channel neither is in the least desirable, but we've discussed the sudden onset of 'The Ebb' and how it rapidly gathers strength on a big tide, which for the prudent mariner would decidedly rule this option out, leaving him with the only other alternative of having to make part of the tow (in either direction) with the tide behind him.

It's therefor the local practice to make the inbound transit with 'The Slack', which is straightforward enough. It's self-evident when the stream is going 'Slack', and off you go. Then, if 'The Slack' proves to be short lived at least the tide will be turning with you. But through fear of meeting 'The Ebb' in the Swellies on a big tide it's the local advice to make the outward transit not with 'The Slack' but with the last of the dying east-going stream. Dr. Sherwin : "The zone of the slack travels westwards, so when it goes slack at Britannia Bridge it will already have been slack several precious minutes at the Suspension Bridge, which adds meaning to starting the outward transit before it slacks at Britannia Bridge. This also means the slack lasts longer for the inbound vessel which carries the slack with her, than it does for the outbound ship which goes against it." Notice too that the RWYC (Caernarvon) advises yachtsmen to aim to be passing Menai Bridge at the commencement of 'The Slack' both when inbound and when outbound.(Menai StraitsTides at a Glance) So given Capt. Goddard's "eighteen minutes between the bridges", This would mean Conway starting the transit twenty minutes before 'The Slack' when there would be twenty minutes less water than she had when inbound. The least depth in Swelly Channel is accepted as the depth over Cheese Rock, which lies about mid-way between Britannia Bridge and Cribbin Gutter, or at the end of the inbound transit (giving further time for the water to rise), and therefor at the beginning of the outbound one, which is another twenty minutes disadvantage in water depth. In that forty minutes an equinoctial spring tide at Britannia Bridge would rise nearly three feet. (see Fig. 8) So the outbound

EGo to page :- 1; 2; 3; 4; 5; 7; 8; 9;F

10; 11; 12; 13; 14; 15;

List of Contents.

LOSS HOME PAGE