It’s not the case that slack current persists for 2.5 – 3 hours. Slack current generally persists for a much shorter time than that, perhaps only a few minutes in some places. Rather, it’s the case that slack current lags behind high or low tide by a few hours. So if there is a low tide at noon, that doesn’t mean there will be slack current at noon. Instead, slack current will come later, perhaps around 1:00 or 2:00.
The relationship between currents (flood, ebb, and slack) and tides (high, low) is something that is best learned visually. I recommend going to deepzoom.com. Click on a yellow square tide station, so you can see a graph of the tide. Then press play so you can watch the arrows that show the current.
Here’s an example, comparing a current station in Rosario Strait to a tide station in nearby Strawberry Bay.
First, let’s go to 10:08 in the morning, right around low tide, and see what the currents are doing at that time. Uh-oh, currents are doing 4.2 knots! Currents are VERY FAST at low tide:
Now, let’s go forward a few hours to 12:03 PM. Ahh, that’s better! 0.1 knots, that’s a nice, slack current, safe and easy. But that good slack current doesn’t happen at low tide. It happens almost two hours AFTER low tide:
But the delay between high or low tide and slack current is not always two hours. It varies tremendously, from a few minutes up to many hours. It’s a complicated relationship. Your best bet is to consult current tables, not tide tables. If you use tide tables to travel at “slack tide,” you’ll very rarely encounter “slack current.” You’ll more often encounter fast currents, sometimes very fast.
For this reason, I don’t like to say “slack tide.” A better term for a high or low tide is the “stand of the tide,” but almost no one uses this term.