Let’s make an example of an early June passage from Anacortes, Washington, to Juneau, Alaska, via the Inside Passage. The idiosyncrasies of all the tides and tidal currents for a passage such as this would fill a book (or several). In this case we’ll focus only on the departure and arrival.
At a speed of advance of 12 knots, the voyage will take us about three days. We depart just before sunrise on June 3 at 5:00 a.m. local time.
We begin our voyage at Cap Sante Marina in Anacortes, where the controlling depth of the marina and entrance channel is 10 feet. Our vessel drafts 5 feet. Our predicted tide is 7.6 feet, added to 10 feet for 17.6 feet of water. Minus 5 feet for our draft means 12.6 feet of water below our keel. That’s more than plenty. However, had we departed later in the morning, we would have had a tide 2 feet lower than the chart datum. So, 10 feet for the channel depth minus 2 feet equals 8 feet, minus an additional 5 feet for our draft, which would leave us with only 3 feet of bottom clearance. We see from the chart that the channel was last dredged in 2013, and the charted controlling depth is based on that. What is the chance that the channel has silted up by 3 feet in the past six years? I don’t know, and I don’t especially want to find out.
Our arrival to Juneau on the morning of June 6 presents an additional challenge relating to tides: passing under the Juneau-Douglas Bridge, just prior to our destination of Harris Harbor.
From our chart, we see that the vertical clearance of the bridge is 51 feet, based on Mean High Water (MHW). We expect to reach the bridge at 4:00 a.m., just after the high tide of 17.8 feet. Bridge clearance is based on MHW, and we see from the chart-insert table that MHW for Juneau is 15.3 feet, so the water is actually 2.5 feet higher than this, meaning 2.5 feet less bridge clearance. This gives an actual bridge clearance of 48.5 feet. Let’s be a little conservative and round this down to 48 feet. If we’re worried about a difference of six inches, we don’t have enough clearance.
For our purposes, let’s assume that we have an “air draft” of 24 feet. We subtract this from 48 feet to give 24 feet of bridge clearance. Any lower tide than high will yield even more air draft; simply add or subtract the difference between MHW and the computed tide to the total air draft. In this example, even the worst-case tidal state is plenty.
The better we intrinsically understand tides and tidal currents in the areas we navigate, the better we can anticipate which charted hazards are actually hazardous to us, and which are merely interesting. If you’re not certain, it’s usually best to go around a rock, rather than over it. Good watch!