Wednesday, July 8, 2009

Tide and Timing


We are getting ready for the homeward push. Once again, we are anchored behind Center Island in Lopez Sound. Now here's the timing bit.

We will need an ebb tide to give us a ride out of Lopez Sound and down into the Strait of Juan de Fuca. But an ebb tide will prevent us from entering Admiralty Inlet, so we want a flood tide for the trip on the South side of the Strait. In other words, we want to see low slack (tide at its low point) at about the center of the Strait. Paying attention to the tidal current now, instead of the tide, we need to be at Smith Island (at about the center of the Strait) at about 12:00. Other than considering the tremendous inertia of the water streams, I cannot explain how the tidal current and the tide can be out of sync. But they frequently are.

In the past, we have gone behind (to the East of) Smith Island, and had difficulties. Looking at the various current tables last nite, we discovered that apparently behind Smith Island, a gyre forms. I guess this is not surprising, when you consider that the water coming down the Strait has to divide and part go North thru Rosario Strait, and part go South thru Admiralty Inlet. Smith Island is apparently the knife that splits the stream, and behind it, all is chaos. So this time, we will go to the West of Smith Island.

Paying attention to the tidal current is important for us - we make 5 knots or so in still water under engine power, and maybe 7-8 knots under the most favorable conditions for sailing. At the entrance to Admiralty Inlet, we have seen tidal currents in the 5 kt range. Yes, that means that at full throttle, we are essentially stationary in the flow, burning diesel and going nowhere, if we are going against the tide. Going with the tide, we could make 10 knots.

So, figuring out boat speed, and factoring in the loss of a knot or so due to headwind, and adding in the expected help from the ebb tide, we need to leave here at 09:30. Ish.

Timing is everything.

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