When the Ocean is a River: Ocean and Tidal Current Basics

Ocean and Tidal CurrentsAfter our second time around the islet, we went and got the anchor out.  It was the first time I’d ever heard a racing skipper call for the anchor in the middle of a race…

Believe it or not, anchoring in the middle of a sailboat race is not unheard of in the Pacific Northwest.  With significant currents and fickle summer winds, not going backwards can be a significant competitive advantage!

In our case, we were trying to cross a small river of current just at the mouth of Cadboro Bay outside Victoria, British Columbia.  It was only about 200′ wide and was running at about 5 knots.  Unfortunately, there was also about 5 knots of wind in exactly the same direction.

It’s a curious fact of the physics of sailing that what makes a boat go is actually the difference in speed between wind and water.  If current and speed are matched, the boat will bob helplessly along like a cork in a stream and the crew become mere passengers, hostage to the whims of the tide.

Out of ten boats, 8 got to the stream before the wind died down and made it across.  One other was swept with us down and into an eddy behind the islet.  We both tried again to no avail and the other boat gave up and retired from the field.

We were a bit more stubborn and readied the anchor to wait for more wind or less current when someone called out, “Gust on!” and we dove into the stream again.  This time we made it across.

We were an hour behind the rest of the fleet and resigned to an inevitable DFL finish (Dead F’ing Last) when we rounded the next island and spotted the fleet far in the distance in altogether the wrong direction.

The faster boats had made it out into the Straight only to find no wind at all and be swept miles past the turning mark.  Suddenly we were in first place in possibly the most absurd race in sailing history!

Why You Need to Understand Currents

It may sound too bizarre to be true, but that race really happened.  The effect of currents on sailboats can be profound and currents can be far more than just inconvenient.

Many sailboats have been damaged or lost over the years when ignorant or imprudent skippers have tangled with currents they shouldn’t have. If you sail in an area with strong currents, you need to understand both their effect and how to predict them.

There are two types of currents:  Ocean Currents and Tidal Currents.

Ocean Currents

Ocean currents are large, steady movements of water in the oceans over thousands of miles.  The most famous example is the Gulf Stream which is about 60-100 miles wide and runs at up to 5 knots for thousands of miles, drastically effecting the climate of the North American East Coast and Europe.

Ocean currents are caused by differences in water temperature and salinity.  They are very steady and predictable (though there can be large varying back-eddies in the faster currents such as the Gulf Stream) and any ocean going sailor should take them into account when route planning and navigating.

This mesmerizing animation shows the currents of the world at any given time.  You can spin the globe, zoom in and out, and even explore what they looked like on different days in the past.

Wind Against Current

I’ve had the not so pleasant opportunity to see what even a light North wind can do when it opposes the Gulf Stream.  You haven’t seen a boat slam until you’ve seen a flat bottomed Open 60 downwind racing sled trying to beat into steep waves.  We actually had to slow down to keep the boat together.

When wind and current are going in opposite directions, the waves become much steeper.  The size of the wave isn’t what makes it dangerous or unpleasant, it’s the steepness.  Current in the same direction as wind will tend to smooth the seas making for fantastic, fast sailing.

If you’re planning to sail in or across a current, pay very close attention to wind direction in the forecast.

Tidal Currents

Technically, these are called Tidal Streams, but at least in North America, Tidal Currents is the commonly used term.

When the tide is coming in (Flooding) or heading out (Ebbing), the water is flowing from somewhere to somewhere else, creating a current.

In large open coasts, the effect is minimal.  Throw in some islands or especially large basins with small openings and you can get some intense rapids worthy of whitewater kayakers.  In fact, whitewater kayaks are common on some of British Columbia’s tidal rapids.

This is a video I shot last year at Skookumchuck Narrows which can run at 15 knots and throws up huge standing waves frequented by kayakers (sadly, none were playing in the rapids when we were there):

Luckily, tides go in and out about every six hours and in between the water holds still long enough to sneak by the rapids.  This is called slack water, slack tide, or just slack.

Note that slack water does NOT in general coincide with high or low tide!

The flow of the tides through island chains and into and out of large sounds is more complicated than that.  Use your current tables for currents and tide tables for tides!

In passes with tidal rapids there will be a virtual parade of boats through the pass in the half hour before and after slack.  You’ll only get a couple slack waters in daylight hours, so you sometimes need to plan your whole day around the current tables.

If you are going to go through a pass at a time other than slack keep these things in mind:

  • Eddies can really throw you around.  Make sure you’re going fast enough to have good steerageway and steer manually through the pass.
  • A fast current can throw up some impressive standing waves, even in deep water.  Carefully study the chart before you enter so you understand where the rocks are, if any.  You may not notice the difference between rocks and standing waves.
  • White water is white because it’s full of air.  Boats don’t float well in air.  If you get into actual whitewater, the boat will sit lower, and the rudder and propeller won’t bite as well.  You’ll have the uncomfortable combination of water on the decks and poor control over your boat.  Stay out of the white water!

Work With the Currents, not Against Them

Of course, most tidal currents are running at just a knot or two and choke points with rapids are a rarity.  If you time your travels to work with the currents instead of against them, you’ll have fast, pleasant days.

There’s nothing slower than trying to work both upwind and up-current on a light wind day.  Most people give up and start the engine.  If you plan ahead, you can get to where you want to go without the “iron genny” by planning your trip around the tides.

Wind Against Current (Again)

Just like with Ocean Currents, if the wind is going against the current, the resulting waves can be steep and nasty.  This can be accentuated by shallow water.

The big difference, though, is that Ocean Currents are hundreds of miles wide, while you can get incredibly localized effects with Tidal Currents.  If you watch the waves on the water, you can sometimes choose the smoother water or even identify and hitch a ride on a back-eddy that’s heading your way!

You don’t need to fear currents, but you do need to understand them, respect them, and work with them instead of against them whenever possible.

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8 thoughts on “When the Ocean is a River: Ocean and Tidal Current Basics

  1. Thanks so much for the informative post about working with the tides and currents rather than against them! Eddies–especially important, yes. We spent a good portion of our travels in BC and AK working with the eddies. Traveling up an eddy while the main current is against you is a good river trick that works well in some ocean settings indeed. Fair winds!

    1. Great to hear from you.

      I’ve had some great experiences in kayaks working up an eddy on the fringes of a tidal stream or river. It’s great fun when you can do the same in a 20,000 pound sailboat.

    1. Fascinating wasn’t the word we were using onboard at the time…

      It is certainly a recognized PNW racing tactic, though. Some boats broke out the anchors near the start of this year’s Fastnet drift race, too!

  2. Great video footage!
    We have got some crazy currents here in New Zealand but I have never seen anything like that. We attempted to go against a current in full ebb in Tory Channel (at the top of the South Island of New Zealand) – would have been running about 7kts in the centre. We picked our way around the edge by the rocks and nearly got sucked down a big eddy that felt like the plughole to nowhere! Scary stuff!

    We also had a race once where there was absolutely no wind. We had no steerage and were going as fast backwards as the other yachts were who were pointing in the right direction!

    Fun times!

    1. My wife was on a RIB in a different rapid and they hit one of those scary bathtub drain whirlpools. They almost lost somebody overboard when they hit it. You want to stay away from those!

  3. Great post and introductory anecdote to set it all up.

    We’ve only been sailing a year, about the same amount of time we’ve owned our boat, but have already crossed The Gulf Stream once, used the Eddies along the northern coast of Cuba to head west, crossed the Yucatan Current (same as the Gulf Stream), and spent the last year around the Yucatan Peninsula where our boat lived. You get to learn a lot about that current when it’s not even a nautical mile offshore. You basically hit it a couple minutes after heading out of the marina. We’ve bucked against it, had our bow fully submerge into waves and burst out of the troughs as well as had easy afternoons cruising along it in fair winds.

    I actually mention it a little bit in our latest post on Love & Rum about traveling down to Guatemala, but not quite so detailed like your post here.

    Just started following you on Facebook, definitely looking forward to more of these in-depth type posts!

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