Kayak Tidal Current Effect Calculator

When paddling in a tidal current or river flow, your actual speed over the ground (SOG) depends on whether the current is with you, against you, or at an angle.

Simple inline current (same direction as travel): SOG = paddling speed + current speed (following current) SOG = paddling speed − current speed (against current)

Worked examples:

• Paddle speed: 4 knots, following current: 1.5 knots → SOG = 5.5 knots
• Paddle speed: 4 knots, opposing current: 1.5 knots → SOG = 2.5 knots
• Against a 4-knot spring tide: SOG = 0, so you are holding station or going backwards

Time implications: At 5.5 knots, 10 nautical miles takes 1 hour 49 min. At 2.5 knots, the same trip takes 4 hours. Timing your paddle to use a favorable tide can halve your effort.

Typical current speeds:

• Calm estuary: 0.5-1 knot
• Normal tidal channel: 1-2 knots
• Spring tide in a constriction: 3-5 knots
• Fast river: 2-8 knots depending on gradient

Crosscurrent compensation: If the current runs perpendicular to your heading, you must angle into it. The required crab angle is:

crab angle = arcsin(current speed / paddle speed)

For a 4-knot paddle speed and a 1.5-knot crosscurrent, that is arcsin(0.375) = 22.0°.

It is arcsin, not arctan, and this page used to get it wrong. The two look interchangeable and are not. What you need is the heading whose sideways component of paddling exactly cancels the current, so paddle × sin(angle) = current, which inverts to arcsin. Arctan answers the different question of what angle the two speeds make when laid out at right angles, and it comes out low: 20.6° instead of 22.0° in the example above. The tell that arcsin is the right one is that your speed over ground comes out as √(paddle² − current²), which is paddle × cos(arcsin(current/paddle)). If a page quotes that speed formula and an arctan angle in the same breath, as this one did, the two do not describe the same triangle.

The ferry angle calculator works the same geometry from the river-crossing side, and the two now agree.

The rule of twelfths, and why timing beats effort

Tidal streams do not run at one speed for six hours and then reverse. They build and fade roughly as a sine wave, and the old sailor’s approximation splits the flood or ebb into six hourly parts:

Half the water moves in the middle two hours. Put the other way: a channel running 4 knots at peak is running about 1.3 knots in the first hour and the last. If a crossing is impossible at peak flow, it is often trivial ninety minutes either side of slack water, and no amount of fitness substitutes for reading the tide table first.

The practical rule most sea kayak coaches teach: plan crossings for the slack, and if you must go on a running tide, go with it and accept the landing it gives you.