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The reason that slack water in our passes doesn't match the high/low tide times is because the water is held up by these "bottleneck" passes during the tidal change.
Lets start with a situation of extended high slack tide.. so that the water levels actually evened out on either side of the pass (not going to really happen, but bear with me). As the tide drops, say, in Georgia Strait, the restriction of the passage holds up the water in, say, Burrard Inlet. The differential in water level leads to the current flow out the pass. As the tide drop continues, the level in Burrard inlet lags behind, and remains higher than the strait, and so ebb flow continues. When the Strait hits low slack tide, the level in Burrard inlet is still higher, and so the flow is still out of the inlet. The current in the pass will stop when the rising tide in the strait recovers enough to temporarily match the level in the inlet, after which the tide gets ahead of the level in the inlet and the flow reverses. This puts the zero current times out of synch with the high and low tide times.
This phenomenon is really easy to see in a place with a lagoon, like Squirrel Cove. It's on a smaller scale but illustrates the point. It's also dramatic to see this at Skookumchuk Narrows near Egmont. The water level difference is as much as 6-8 feet at full flood! (with 15knot current!!) I can send you some pics of that if you like - PM me. In places like Gabriola Pass, the slack current is extremely shortlived.
As far as timing your passage, the tide and current tables are indispensable. Also, for passes not specifically covered, most cruising guides will often give comparison data with areas that are covered. And - also very important, remember that the government tide/current books are in PST, so in the summer you need to add the hour.
I hope all this made some sense!
Last edited by Faster; 06-15-2006 at 08:44 PM.