Understanding Current

Book Extract

A great number of venues you will race at will have current caused either by tide, river flow or the wind. This can be very confusing for sailors, especially if they normally sail inland.

The first thing to do is to research the venue. Is it likely to have current? Look on the internet to find tide tables. This will give you the high and low water times and the rise and fall of the water. Consider any river flows by looking at Google Maps. Or is it a big expanse of water that might have wind-driven current?

Is the current likely to be the same across the whole of the course or is it likely to vary? If it is going to be different across the course, you may be able to use the current to your advantage or there might be only one way to go that will pay.

Current Uniform Across Course
Let’s assume the current is going to be uniform across the whole course for the race. This means that, as long as you stay inside the laylines upwind and sail a straight-line course between the marks downwind, you can concentrate on your normal wind and boat tactics – tacking on shifts, sailing into gusts and avoiding dirty air.

This sounds easy but many sailors get this wrong and overstand marks, sail big loops down reaches and on the wrong gybe down the run.

Let’s think about the main factors to consider depending which way the current is flowing relative to the course / wind.

Wind Against Current
Beware being over the start line. The current is pushing you over. Get a good transit and don’t get on the line too early.

The beat will feel shorter and you will need to tack before the usual laylines as the current is pushing you up towards the windward mark.

The waves will be steeper and closer together than normal. Be prepared to modify your technique to cope with this.

You can shoot the windward mark since the current is pushing you up, so a late tack in can be a good tactical move.

The run will feel longer as you sail into the current and the fleet will be very close and bunch up as it rounds the windward mark. The big priority is to try to keep your air clear.

Make your turn late on the leeward marks since you will have to be careful not to get swept into them by the current

Try not to get pushed high on the reach by the current else you will end up running into the mark with the tide against you. Not very fast! Use a front or back transit to sail the direct line. If there is no transit, look at both marks to make a judgement as to whether you are still sailing the direct route.

With wind against current, the beat will feel shorter and you need to tack earlier for the laylines

Wind With Current
The current is pushing you back from the start line. Get a good transit and don’t get too far away from the line. You will have to keep sailing to stay on the line. There is often a big line sag in the middle of the line. The beat will feel longer, and you will need to overstand the normal laylines as the current is pushing you down.

Beware of getting too close to the windward mark. The current is pushing you onto it. Never try to shoot the windward mark.

The run will feel shorter as you sail with the current. This will mean that the fleet will spread out as it rounds the windward mark. Try to stay in the middle of the run else the current will sweep you past the mark and you will end up reaching to get to it.

Make your turn early on the leeward marks since you will have to be careful not to get swept past them by the current

Try not to get pushed low on the reach by the current or else you will end up beating into the mark with the tide against you. Not very fast! Use a front or back transit to sail the direct line. If there is no transit, look at both marks to make a judgement as to whether you are still sailing the direct route.

With wind with current, the beat will feel longer and you need to tack for the laylines later

Current Across The Wind
The current will be pushing you towards one end of the start line. Beware of starting at the end the current is pushing to. It is very easy to get there too early, then you will end up in a raft on the pin or the committee boat.

The beat will probably have a long and a short tack. Watch your laylines – one you will have to tack earlier on, the other you will need to overstand.

Be aware of the current flow on the windward mark – it will either be pushing you hard onto it or sweeping you past it.

The run will have the tide across it, so it is likely to have a favoured gybe for the whole run. Try to get a transit on the leeward mark and sail a straight line to the mark.

If the leeward mark is a gate think about which mark is better for the current. Will you get caught in a leebow from another boat when you round the mark or will the boat in front get swept wide and open up a lane for you.

The reach will either have the current behind you or you will be sailing into it. If sailing into it, this will close the fleet up and make the reach feel a lot longer. The opposite is true if the current is behind you.

With wind across current, if the windward mark is laid directly upwind, one tack will be longer

Current Varying Across The Course
The current can vary in strength or direction across the course. If this is the case, the import factor is to ensure you sail in the least adverse current or the strongest advantageous current.

How can you predict where the current is strongest or where the direction might change?

Venturi Effect
This is the effect where the flow of water increases as it is squeezed or compressed through a gap such as between a shoreline and an island.

If the current is squeezed into a gap, it will get faster

It could be that it is just compressed trying to get around a headland. Remember this will also cause a change in direction of the flow. In extreme circumstances a headland can cause the flow to sweep around it and then swirl into a reversed flow (a back eddy).

The current can be compressed going around a headland – which will also change its direction and possibly cause eddies

Shallow Water
Current flows slower in shallow water because of the friction with the sea or riverbed. So, if current is flowing along the shoreline, it is likely to be slower inshore and faster out to sea.

The current flows slower in shallow water

However, if a current is passing over a shallow bank it will speed up because of the Venturi effect mentioned above. So be careful, not all shallow water is slower moving!

The Venturi effect can happen over a shallow bank, increasing the speed of the current

Bends Around Land
Current will try to take the easiest route around shorelines or islands. This will bend the current direction and might mean the current is different at the top of the course to the bottom.

Once you have predicted how the current will be flowing on the course it is important that you check exactly what it is doing on the course.

A good visual clue as to changes in current strength or direction are tide lines. Where different current flows meet you will probably see either a disturbance in the wave pattern, a difference in water colour or maybe a line of seaweed or rubbish floating in the water.

A change in the water surface can show a change in the current

To tell the direction and strength of the current look at any fixed objects such as a mark, lobster pot or the anchored committee boat. Look at how the water is flowing around it.

You can see the direction and strength of the current against a buoy

To be more accurate use a sponge or an almost full water bottle and throw it into the water next to the object. How far does it drift in one minute and in what direction? Don’t forget to retrieve your bottle or sponge!

This is where your coach can also be useful. They should have a tide stick so they can get readings around the course and feed them back to you before the start. This will help you make the best strategic and tactical decisions about the current.

 

© Not to be reproduced without written permission from Fernhurst Books Limited.

Optimist Racing is written by top coaches Phil Slater and Steve Irish, who have coached from a regional level to Britain’s sailing stars, including Ben Ainslie and Hannah Mills (who writes the foreword to the book).