Seymour Narrows — BC, Canada

Seymour Narrows is the narrowest part of Discovery Passage, the main shipping channel north from the Strait of Georgia. It is nearly 2 miles long and at least 0.4 mile wide, with high, steep shores. (PAC 202)

How the pass behaves

Tidal streams in Seymour Narrows reach 16 knots. The flood sets south and the ebb sets north. When either stream is running at strength the eddies and swirls are extremely heavy, and a strong wind against them turns the races very dangerous to small vessels. (PAC 202)

The strongest stream is near Maud Island on both the flood and the ebb. On the flood, rips and upwellings build along the west shore from about North Bluff, meet the rips from the east side south of Maud Island, and leave a band of maximum turbulence that reaches beyond Race Point. Violent rips extend about 0.4 mile south of Maud Island, and the Sailing Directions advise avoiding the west side of the narrows on the flood. On the ebb the water is smooth as far north as North Bluff; from there to Puget Bluff the channel is all whirls and eddies, fading out abreast Separation Head. (PAC 202)

What remains of Ripple Rock lies slightly west of mid-channel, about 0.25 mile west of Maud Island, with 13.7 m over its shallower head. Upwellings over it come about every five seconds and are swept downstream as eddies and whirlpools. (PAC 202) The rock's two peaks once lay close under the surface; they were removed on 5 April 1958 in one of the largest planned non-nuclear explosions on record. (Wikipedia)

How slack works here

Seymour Narrows is a reference current station: the Canadian Hydrographic Service tabulates its slack water and maximum flood and ebb daily in the Canadian Tide and Current Tables, Volume 6. Slack can last as much as 12 or 15 minutes, but on a large tide the change is considerably quicker, and severe local weather can change it considerably. (PAC 202)

Greene Point Rapids, further north in Cordero Channel, are referenced on Seymour Narrows in the same tables, so the turn here also sets the timing there. (PAC 201)

Planning a transit

The Sailing Directions advise low-powered vessels, vessels with tows, small craft under about 20 m, and anyone without local knowledge to go through only at or near slack, keeping mid-channel. Fatal accidents have happened in small vessels that tried it at full strength, and small vessels have capsized here even near slack in reasonable weather — so trim and stability matter even on a slack-water plan. A vessel met head-on while you are stemming the stream may not be in full control; give it as much room as possible. (PAC 202)

To wait for the turn, Duncan and Menzies Bays to the south, and Plumper, Deepwater and Elk Bays to the north, all have anchorage. (PAC 202)

Large ships plan for slack too. Cruise ships gather in the Seymour Narrows area at slack water from April to October, and the Pacific Pilotage Authority asks all vessels to monitor the Vessel Traffic Services channel. (Pacific Pilotage Authority)

Nearby

South of the narrows, Discovery Passage runs past Campbell River to Cape Mudge, where the streams are also troublesome. North of them, the passage runs about 12 miles to Johnstone Strait with streams that do not exceed 3 knots. (PAC 202) The more protected inside route between the Strait of Georgia and Johnstone Strait runs through Calm and Cordero Channels instead, by way of Gillard Passage and Dent Rapids. (PAC 201)