Need help, please with tide/current issue

[FrankLanger]

I would appreciate clarification about tide/current to avoid running into trouble in a narrow, shallow passage that I need to navigate in the Pacific Northwest. The tide changes the water level by about 10 - 12 feet twice per day, and the current can run through that passage at up to 9 knots.

Intuitively, I would have thought that current would be slack/slowest at highest tide level and again at lowest tide level, as that's when the ebb/flow changeover would occur. But the tide/current tables actually show the least current about 2 hours after the high/low tide levels.

Can someone help me understand why the time lag between tide and current? I need to time my passage through the narrow section to minimize current, but with the tide high enough to avoid grounding in the shallows.

Any help would be much appreciated.

Frank.

[Faster]

Hi Frank

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!

[FrankLanger]

Thanks for your very clear and sensible answer. I wouldn't have thought that the geographic barriers you mentioned would hold the water up that much--would have expected it to simply flow faster through those areas to keep up with the tide. But that's the only explanation that makes any sense. So I'll make sure to consult both the tide and current tables, and also to keep my wits about me when I navigate those passages.
Thanks again!
Frank.

[Faster]

Happy to have helped.

Which particular passage were you thinking of in your original post? There are not many around here where depth is an issue...

[FrankLanger]

Hi, I was referring to Dodd Narrows, which is narrow, and a bit shallow at the edges.
Thanks again!
Frank.

[Faster]

One more thing, Frank. Contrary to what you would intuitively assume, all the passes to the Gulf Islands flow out into Georgia Strait during flood tides, and back into the islands during ebbs. The bigger push is up through Sydney Island and Haro Strait, up the island chain and out the passes into Georgia Strait. So Dodd, Porlier, Active all flood out of the islands and ebb back into them. The current books indicate the (true) compass direction of the currents as well as the timing.
Dodd narrows can be particularily troublesome for those transiting outside of slack times. Large eddies have literally swallowed small boats, thrown 30' logs around and it's risky to go through early or late. It's also one of the busiest places around, traffic wise, at slack times of course.

[FrankLanger]

Thanks for your reply. I have seen diagrams which confirm your statement that the tide comes in through the Juan de Fuca strait, proceeds up through the Gulf Islands and leaves through the passes to the Strait of Georgia.

I have been through Dodd Narrows once when it was flowing faster than I had hoped, as we were about 90 min. after slack--we made it o.k., but at a little faster speed than I would have liked in that narrow channel, and dodged the odd swirling eddy. Even in our 9,000 lb. 30 foot boat, the resulting instability was noticeable--that's why I want to make sure I understand this issue, to ensure I time it well.

Sailing in the Gulf Islands/Strait of Georgia is lovely, but it would have helped if "they" had made the various channels and passes a little wider and straighter, without the ubiquitous underwater rock formations!

Thanks again for your replies.

Frank.

[Faster]

Yeah. "they" made things nasty here and there.... but "we" did blow up Ripple Rock and took care of that one!!

Cheers and good sailing.
Ron

[nolatom]

Sometimes the same thing happens because the tide range on opposite sides of the canal or restriction are different. I'm thinking of the Cape Cod Canal here, which has an 8-9 foot tidal range on the Mass. Bay side and only about 3-4 feet on the Buzzards Bay side. So the slack water in the Canal is at about half-tide rather than at high or low. And at about 3 knots current, you want to make sure you're transiting in a following current, or it'll take half the day to make the 4 miles (all numbers from memory here, it's been a while).

[sailingdog]

Looks about right to me... I prefer going around the Cape rather than motoring in the canal..