The Current Changes Everything — Racing on a River

Learning to Race — From First Start to Better Results

You can be sailing faster than another boat and still watch it move ahead.

Your sails are drawing. The boat feels lively. There is a reassuring ripple along the hull. Yet a nearby rival, apparently moving more slowly through the water, is making better progress towards the next mark.

Before blaming your sail trim, look at the riverbank.

On a river, the water beneath you is moving too. The race takes place between marks anchored to the riverbed, so success depends on how effectively you move past those fixed positions.

For those of us sailing at Upper Thames Sailing Club, this makes current part of the racecourse itself. Wind, stream, banks, trees and other boats all have to fit into the same decision.

Aboard Champagne, our A-Rater, that is a particularly absorbing challenge. Her tall sail plan makes the wind impossible to ignore. The quieter influence beneath the hull deserves just as much attention.

Two Speeds, Two Different Stories

Start by separating two ideas:

• Speed through the water: how fast the hull moves relative to the surrounding water.
• Speed over the ground: how fast the boat moves relative to the riverbed and banks.

A GPS normally reports the second. A conventional water-speed log measures the first, subject to its accuracy and calibration. The RYA makes this same distinction in its navigation terminology. rya-enavigation.org

The underlying relationship is:

Velocity over the ground = velocity through the water + current velocity

Velocity includes direction. When the boat and stream are aligned, simple addition or subtraction works. When they cross at an angle, you must combine the directions too.

Imagine two boats travelling directly upstream. These are illustrative figures, not measurements from the Thames:

Boat	Speed through water	Opposing current	Upstream speed over ground
A, in stronger stream	2.0 knots	0.8 knots	1.2 knots
B, in weaker stream	1.8 knots	0.3 knots	1.5 knots

Boat A is sailing faster through the water. Boat B is moving upstream faster.

Over five minutes, that 0.3-knot advantage amounts to roughly 46 metres.

Boat B has found a better combination of boat speed and current.

One important distinction: upstream and upwind are different directions. Depending on the wind, the upstream leg might be a beat, a reach or a run.

Weak Current Can Have a Strong Effect

A gentle stream is easy to dismiss, particularly when there are no obvious swirls around the marks.

But small speeds accumulate over time:

Distance carried by current = current speed x time

At 0.1 metres per second, the water travels 30 metres in five minutes.

That does not mean every boat loses 30 metres to its rivals. If two boats experience exactly the same current for the same time, both receive the same displacement. The competitive difference comes from different current, different routes, different travel times and different progress towards fixed marks.

Weak current also becomes proportionally more important as the wind fades.

If your upstream component of speed through the water falls to match the opposing stream, you stop making upstream progress. The sails may still be drawing and the rudder may still respond, yet the bank barely moves.

A boat can be sailing quite normally while going nowhere along the course.

Read the River as Carefully as the Wind

Current is rarely identical across the whole width of a river.

Friction from the banks and bed influences the flow. Vegetation and channel shape also affect its distribution; research on natural channels illustrates how bank conditions can change velocity patterns. U.S. Geological Survey

A useful starting expectation is weaker flow near some margins and stronger flow in parts of the main channel. Treat that as a question to investigate. Bends, shallows, structures and local eddies can complicate the picture.

Useful clues include:

• Naturally floating leaves passing a fixed bank feature.
• Water streaming past an anchored buoy.
• Swirls or changes in surface texture near structures.
• Your own progress against the bank.
• Repeated gains and losses by boats using different lanes.

Ripples also reveal wind, so a darker patch of water is not automatically stronger current. A drifting leaf can be affected by wind too. Look for agreement between several clues.

For me, one of the attractions of Thames sailing is that the river rewards this sort of attention. The sail tells one story; the bank tells another. Good decisions come from reading both.

Upstream: The Bank Can Help — Until the Wind Disappears

When sailing against the stream, a lane of weaker current can be extremely valuable.

However, the bank may also bring trees, buildings, disturbed wind, shallows and very little room to tack.

Imagine making 1.6 knots upstream through the water in the main channel, against 0.6 knots of current. Your net upstream progress is 1.0 knot.

Move towards the bank and reduce the opposing stream to 0.2 knots. If you retain 1.4 knots of upstream movement through the water, your progress improves to 1.2 knots.

But if shelter reduces that movement to 0.8 knots, you make only 0.6 knots over the ground.

The weaker stream no longer compensates for the lost wind.

On a beat, use the upstream component of your boat’s movement, rather than simply subtracting current from total boat speed.

The practical question is:

Does the reduction in current outweigh the loss of useful sailing speed?

Aboard Champagne, I want to keep that question beside the temptation to follow a promising strip of quieter water. A bank lane needs enough wind, depth and room to remain useful.

Downstream: Helpful Flow Must Repay the Detour

When travelling downstream, stronger favourable current can improve progress. That often makes the main stream attractive.

But reaching it may require a detour, additional manoeuvres or sailing into another boat’s wind shadow.

The quickest route depends on total time:

Leg time = distance travelled / average speed over the ground

For a simplified direct downstream leg:

• A 300-metre route at 1.5 metres per second takes 200 seconds.
• A 360-metre route at 1.7 metres per second takes about 212 seconds.

The second route is faster over the ground but still takes longer.

These figures assume constant average speeds. Real river racing adds shifts, gusts, traffic and acceleration, but the principle remains useful.

Helpful current has to repay the cost of reaching it.

This connects directly to our earlier discussion of fleet positioning. A route towards one side needs a reason, and that reason must remain valid as conditions change.

Your Bow Does Not Show Your Track

Current can carry the boat sideways while it sails forwards.

Suppose you sail directly across a river at 1.0 metre per second relative to the water. The crossing is 20 metres, so it takes 20 seconds.

If the river flows downstream at 0.3 metres per second, you also move 6 metres downstream during the crossing.

Your bow can point at a gap while your actual track passes well below it.

To arrive opposite your starting point, you need an upstream component to your heading. The required angle depends on your speed and the stream.

The racing habit is simple:

Check where the boat is actually going, not just where it is pointing.

Compare an approaching mark with a fixed feature beyond it. Watch whether that alignment changes as you close. One glance is rarely enough.

Mark Approaches: Pointing at It Is Not Making It

Current makes the final approach to an anchored mark especially revealing.

When the stream carries you away from the mark

You may need extra up-current allowance. If your speed falls during a tack or in disturbed air, that allowance can disappear.

A mark which looked comfortably reachable can become an emergency extra tack.

When the stream carries you towards the mark

The final distance can close faster than expected. Prepare the crew and sails early enough to round under control.

When current crosses the approach

Aim sufficiently up-current to produce the required track, while allowing for the rounding side and other boats.

The correct allowance depends on the actual geometry. There is no universal instruction to aim upstream by a fixed number of boat lengths.

This also changes laylines. A still-water layline is an unreliable guide when current materially changes the boat’s track. Changes in stream or boat speed can change the approach again.

A useful call aboard Champagne would be:

“Are we making the mark over the ground?”

That question invites the crew to check the track, rather than agree that the bow looks promising.

Tacking Decisions: Protect Momentum and Choose the Lane

A tack costs speed. Against current, the resulting reduction in upstream progress can be particularly painful.

There are two decisions to make:

1. Is this tack worth the speed and distance it will cost?
2. Will it put us in a better combination of wind, current and clear air?

Short tacks near a bank may keep you in weaker stream, but repeated manoeuvres can consume the benefit. One longer tack may preserve speed while taking you into stronger adverse flow.

Neither approach wins automatically.

For Champagne, the useful evidence is how much speed we retain and how quickly we accelerate again in the conditions of the day.

There is an important piece of physics here: in uniform current, every nearby boat is being carried by the stream. The current does not suddenly push only the boat that tacks.

The tacking boat loses relative distance primarily because its sailing velocity changes during the manoeuvre. A slower passage also leaves it exposed to adverse stream for longer before reaching the fixed mark.

For practical purposes, watch how much progress the manoeuvre actually costs against the bank, then judge whether the new lane repays it.

Current Can Change the Wind You Sail In

Airflow over the sails depends on the boat’s movement relative to the air.

A moving water mass changes the wind experienced in a frame travelling with that water. The boat’s own motion then contributes to its apparent wind.

That is why crossing into a different stream can alter the wind angle or strength experienced aboard, even without a change in the wind relative to the bank.

This deserves care: it does not mean simply putting current on one side of the bow creates a free lift.

In uniform current, an otherwise symmetrical boat has no magical advantage merely because the stream is described as being on its “lee bow”. Changes in current, real wind shifts, boat performance and the course geometry are what need examining.

For a developing racer, the practical response is to notice the change, adjust trim and reassess the track.

The Start Line Moves Past You Even While You Wait

A start line is fixed to its marks. Your waiting position is in moving water.

Stream carrying you towards the line can erase a carefully judged gap. Stream carrying you away can make an apparently well-timed acceleration leave you late.

A useful pre-start check is to sail the intended approach and watch how the line closes in the actual stream.

This brings us back to time-on-distance: judge the approach relative to the line, with enough room and time to accelerate.

A Simple River-Reading Exercise

Before racing, in suitable conditions and clear water:

1. Identify the stream direction using fixed objects and several surface clues.
2. Sail comparable short stretches in two lanes, as traffic and room allow.
3. Compare progress against the bank, wind quality and freedom to manoeuvre.
4. Approach a mark and observe the difference between bow direction and track.
5. Record one observation and one question for next time.

If using GPS, remember it measures movement over the ground; GPS alone cannot separate current from boat speed through the water. Use any device in accordance with the event’s equipment rules.

Check the Environment Agency’s Thames stream warnings and the club’s decisions before sailing. Strong stream is a reason to reassess whether to go afloat, not merely an extra tactical challenge. GOV.UK

Conclusion: Race the Water as Well as the Wind

River racing asks us to hold two pictures in mind.

One shows the boat moving through the water: sail trim, balance, speed and handling.

The other shows that moving water carrying the boat past banks, anchored marks and the start line.

At Upper Thames, that combination is part of what makes the racing so absorbing. A short stretch of river can contain several different opportunities, and the best one can change with the wind.

With Champagne, my aim is to keep asking better questions:

Where is the useful wind? What is the stream doing here? Are we actually making the mark? Will the next tack improve our position?

The river rewards those questions.

The next time another boat edges ahead, look beyond its sails. The answer may be underneath it.

Follow Champagne and the Learning to Race series at pmrsailing.uk.

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