Words by Les Rohlf Photos by Kel Rohlf
One of the new-to-us considerations for our cruise through the Inside Passage in British Columbia and Alaska is the high tidal extremes. The difference between high and low tides is much greater than what we’ve experienced during travels in Florida. In Juneau, for instance, during last week’s new moon, the difference between high and low tides was over 18 feet.
As all that water moves into (floods) and out of (ebbs) the inside passage fjords and channels on a twice-daily basis, strong currents can be created, especially where the flow of water is constricted by a narrower channel.
Narrows can see currents of over 10 mph. Furthermore, underwater obstacles create waves and whirlpools as they interrupt the flow of this fast-moving water. Swirling, fast currents make boat handling a chore at best and very dangerous at worst.
Internet Rabbit-hole: Google “Ripple Rock Explosion” for an account of how one such obstacle was removed.
Planning considerations for trips like this include:
Transiting narrows and rapids as close to slack water as possible – slack is the very short time between when one tidal cycle ends and the next begins. “High slack” occurs just as the flood tide finishes and the ebb tide starts. “Low slack” describes the point at which the ebb tide is finishing and the flood tide begins. We arrived a Seymour Narrows well ahead of low slack at a time when the current through the narrows would have provided an eight knot “push”, which would have been dicey for our smaller boat. We waited for the current to slow down a little before slack (about 3 knots of current) to ensure we completed the mile-long narrows before encountering the “headwind” of the incoming flood.
Moving in the same direction as the tidal flow – you can get a little boost of speed and use less fuel by moving in the direction of the tidal currents during that part of the day. Correspondingly, moving against the strongest time of tidal flow can decrease speed and/or increase fuel usage.
Winds blowing against tidal flow create waves, which can hinder passage in certain areas. When checking weather forecasts and planning for a specific day’s travel, the times of flood and ebb tides, along with wind direction and intensity, will determine whether safe transit of an area is practical.
Lots of planning considerations that aren’t needed with running down the muddy Mississippi.
Whatsoever the Lord pleased, that did He in heaven and on earth, in the seas and in all deep places. (Psalm 135:6 KJV)