Making allowance for tides

I did my first ocean swims last year and was surprised at the number of beginner and expert swimmers who didn’t seem to understand how the tides worked on the swims they were undertaking. As a professional mariner and scuba diver I have a close relationship with tidal flows and their effects. If you have a basic understanding of how they work, you can use them to make life much easier. Get it wrong and you could find yourself in a hard slog at the end, instead of getting a little bit of help when you need it.

While tides may seem a little confusing they do follow well defined patterns and are fairly predictable. The tide is simply a 3-4m high wave that circles around the globe. As the wave approaches the tide comes in and flows to all the river mouths, harbours and coastlines. As the wave peaks we have high tide, and once the wave has passed all this water flows back out and we have low tide. Look at waves coming up a small creek. It takes a little over six hours for the tide to come in and a little over six hours for it to go back out again. There is of course a lot more to it than this if you are navigating a ship across the ocean, but it’s all you need to know for swimming in the harbour.

So how does all this affect you? Just think of the harbour as a very large river, which it basically is. The Waitemata Harbour is just an extension of a river that goes all the way to Riverhead and beyond. On the incoming tide the water will flow past Rangitoto, around North Head and run up under the Harbour Bridge to beyond Riverhead. On the outgoing tide it simply flows in the opposite direction – back out to Rangitoto and beyond. If you plan ahead a little, you can easily use this same tidal flow to give you a bit of assistance at the end of the Harbour Crossing event when you need it most. Being downstream of your target can turn an easy swim into a hard slog at the end when you are tired.

When you are in the water, pick a moored yacht, harbour marker, or something similar in the water that is ahead of you and line it up with a landmark in the background. A tree, building, or any feature that you can easily see will do. This will give you a reference point on the water to swim straight at your target and show if the current is taking you off course. Keep an eye on your two reference points. If the mark in front starts moving to the left of the one behind, you need to swim more to the left to counter the current more. If it goes to the right, swim more to the right. You can stay upstream on this course if you like, to give you some room, but don’t drop downstream of that line or you will pay the price later. See the diagram below. At the start you will think you are going to swim miles off course but stick with it, and you will be surprised how far the current will carry you.


As you get further across you will change the direction you’re heading, and slowly start aiming towards your target more as the current changes. Pay particular attention if the tide is going to change during your swim, and the current starts going in the opposite direction. This will keep you upstream of your target, so as you approach the finish you will actually have the tide helping you instead of going against you. The difference will be significant and you will have an easier run home instead of slogging it out. Use your two landmarks, they don’t lie and will tell you if you are on target, or are being taken downstream by the tidal flow.

Like anything with a bit of practice it becomes pretty easy. If you train at the beach, practice it a bit. Don’t over complicate it to start with, just pick a line and swim it. Even if it’s just up and down the beach a few times, just getting used to keeping your two marks lined up and moving the front one left and right with your course.

So to sum it up, here’s the main points:

  • You are basically swimming across a very large river where the flow goes up and down it.
  • Pick a marker on the water (mooring, boat, course marker or harbour marker) and line it up with a feature in the background.
  • If the front marker you have starts going left, swim more to the left. If it goes right, swim more to the right to keep them in line.
  • Make small adjustments to your course, you don’t need to swim in big zig zags to keep your marks lined up.
  • Trust your marks, and not your “internal” compass. You will swear you are swimming miles off course to start with, but your landmarks won’t lie.
  • Stay upstream a bit of your line. It is much easier to come back downstream if you need to, than it is to get back upstream.
  • If you are upstream your swim will end easier, downstream and you will finish slogging it out and swim further.

You may even improve your times as you will end up swimming the shortest possible distance. I hope this helps a bit and makes your swims a little bit easier!

Good luck. Blue skies and smooth seas.

~Gordon Campbell