Sailing

Navigation compass#

Sun, Moon and wind bearings on a compass rose.

North up keeps true north at the top, as on a chart. The outer scale is marked every 5° with numbers every 30°. Each body has a coloured mark on the scale at its exact bearing.

Compass mode turns the card with the phone’s live true heading. The fixed triangle at the top marks the way ahead; the centre shows heading in degrees and its compass point. Hold the phone flat with the top edge of the screen pointing ahead. North up keeps the chart fixed for planning and shows wind speed in the centre. Your mode is saved.

Heading is live at the phone; Sun, Moon and wind markers still describe the selected planning place and time. Compass mode needs a valid true-north reading and location access. While a reading is unavailable the card stays north up, the centre shows no heading, and the status explains why. Low accuracy is labelled; move away from metal or magnets and recalibrate the phone.

The Sun and the Moon sit at their true bearing for the chosen time: the Sun further out, the Moon nearer the centre, so they never cover each other. Drawn solid when above the horizon, hollow and dashed when below it.

The green arrow is the wind at the chosen time. It comes in from the rim on the side the wind blows from, with vane feathers at its tail, and points the way the wind blows. Its speed and gusts in your selected unit are in the list below; North up also shows speed in the centre.

The list under the rose gives each bearing in degrees with its compass point, the altitude (height above the horizon), the Moon's rise and set, and the wind with gusts.

Wind#

Wind and gusts in your chosen speed units.

Six forecast hours from the chosen time: mean wind speed, the direction it blows from, and gusts, all in your selected speed unit. It comes from the best forecast source for your location, which forecasts over land and the coast; conditions offshore can differ.

Tides#

Predicted tide heights and turning points.

The modelled water level from a little before the chosen time through the next four high and low waters, with each turning point's time and height. Dashed vertical lines are midnights; day names are along the bottom.

Heights are the model's water level in metres and may use a different reference level from the chart datum in official tables, so compare the rise and fall rather than the numbers; times are approximate. Always check official tide tables before passages.

Sea state#

Wave height, period, direction and current.

Four hours from the chosen time: significant wave height and the direction the waves come from, the surface current's speed in your selected unit and the direction it flows toward, and the water temperature. From MET Norway's ocean model, which covers north-western European waters; elsewhere the section says so.

Sun and nautical twilight#

Daylight and nautical twilight at sea.

The bar runs from midnight to midnight and is coloured by the Sun's height: daylight, then civil, nautical and astronomical twilight, then night. The white rule is the chosen time.

Nautical twilight (Sun 6° to 12° below the horizon) is outlined in the accent colour: the sight windows, when the sea horizon is still visible and the brightest stars and planets are out, so a sextant can measure their altitude.

Below the bar: nautical dawn and dusk (the Sun at 12° below), and sunrise and sunset with their true bearings, useful for a compass check.

Celestial sight practice#

Practise correcting a measured altitude.

Choose the Sun’s centre, a navigation star or one of the five naked-eye planets. Your selected planning location is the assumed position. The observation time is shown in UTC, with calculated geometric altitude Hc and true azimuth Zn.

Enter a corrected observed centre altitude Ho as whole degrees and decimal arcminutes. Apply the appropriate almanac corrections to your sextant reading first: index error, dip, refraction, limb and parallax as applicable. The app does not apply these corrections. The Moon is deliberately excluded from the sight calculator.

The intercept is (Ho − Hc) × 60 nautical miles. Positive means toward the body, negative away. The sketch plots a position line perpendicular to Zn at that offset, with a labelled local scale. This is one line, not a position fix. Changing the body, time or location clears your observation. You can share the worksheet explicitly.

These approximate on-device positions support learning and sight planning, rather than replacing a current nautical almanac or certified navigation equipment. Verify observations and use independent navigation methods.

Star and planet finder#

Navigation stars and planets on a bearing chart.

The north-up chart uses compass bearings: east is right, unlike a mirrored overhead star atlas. The outer circle is the geometric horizon; the rings mark altitude 30° and 60° and the centre is zenith. Nineteen bright stars and Mercury, Venus, Mars, Jupiter and Saturn are calculated for your location and selected time.

Stars are circles and planets are diamonds. Tap a mark or a listed object for its altitude and true bearing; Use for sight practice opens its sight worksheet. The list filters by minimum altitude and object type, with a Show all option. Dim chart marks remain above the horizon but below your threshold.

Star positions use Hipparcos I/239 catalogue coordinates at epoch J1991.25, proper motion and precession to the chosen date. Catalogue star magnitudes help rank brightness; lower numbers mean brighter. Planets are ranked by altitude without invented magnitude values. Atmospheric refraction, cloud, glare and obstructions are not modelled, so above-horizon positions alone do not guarantee visibility.

Current and tidal-stream passage#

Course to steer, passage time and speed over ground.

Surface forecast shows up to 24 hours of hourly current speeds and flow-toward directions from the existing ocean provider. Missing samples are gaps, and a chosen time outside coverage gives no current. The strongest available hourly sample is labelled; it is not an interpolated peak or a slack-water prediction.

For tide-specific planning, choose Manual tidal stream and enter set (true direction flowing toward) and drift (in your selected speed unit) from an official local atlas, prediction or observation for the chosen time. Manual values are saved on the phone and must be checked for each new plan. High and low water do not determine local slack water; surface-model currents are not necessarily purely tidal.

Enter speed through water, desired ground track and distance. Passage distance is how far you need to travel over the ground, in nautical miles, along that track; with the speed over ground it gives the passage time and arrival. The vector triangle shows one hour of motion: the amber arrow is the boat through the water (on the course to steer), the green arrow the current added to it, and the white dashed arrow the result over the ground. With no current the green arrow has no length and the amber and white arrows coincide. The course to steer cancels the cross-current, then the along-track component determines speed over ground and passage duration. The app reports when a track cannot be held or an opposing current prevents forward progress.

This assumes one constant current and boat speed along the passage. It does not model changing streams, leeway, hazards or coastal routing. Course is true; apply local magnetic variation and compass deviation separately.