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.

How fast is it actually running right now?

Tidal streams do not hold one speed for six hours and then reverse. They build from slack, peak at mid-tide, and fade again, roughly as a sine wave. Two different rules of thumb describe that, they answer different questions, and mixing them up is the reason people misjudge a crossing.

The rule of twelfths is about tidal HEIGHT, how much the water rises each hour: 1/12 in the first hour, then 2, 3, 3, 2, 1. It tells you when the slipway is covered. It is not a stream-speed rule, and reading it as one makes the first hour look calmer than it is.

The 50/90 rule is about stream SPEED, and it is the one you want. Measured from slack water:

Read that table the other way and it is the number that matters most on the water: an hour either side of peak the stream is still at 90% of maximum. Not a third, not half. You have to give it two full hours to fall to half, and the useful quiet is in the last hour before slack.

So the practical rule most sea kayak coaches teach holds up: plan crossings for the slack, and if you must go on a running tide, go with it and accept the landing it gives you. Ninety minutes after peak is not a plan.

The tidal current speed calculator works this out for any point in the cycle and shows the sine value alongside the 50/90 approximation, which parts company with it by about 3 percentage points in the second hour.