Actual Currents - Tidal Current Predictions

Getting Around

The Settings Panel

The panel on the right side of the screen; on a phone, tap the ☰ button in the bottom-right corner. It holds every app control: Time Control, Visualization Mode, Basemap Styles, Tools, Markers, and Display & Units. On desktop, the arrow tab on the panel edge hides and shows it. On a phone, swipe down or tap the map to close it.

Location Search

The search box in the top-left corner, also available on the start screen. Type a town, harbor, or landmark, then tap a result. The map flies there and data loads on its own.

Live Data Readout

The readout in the top-left corner, under the search box. It reports conditions at the small crosshair in the center of the map: position, current speed and direction, and water depth. The values track whichever layer is active: wind speed when Winds is on, the roughness score when Rough Water Index is on, water temperature when Sea Surface Temp is on. Just pan the map; no tapping needed.

Date Display & Calendar

The date and time at the top of the map is the moment the map is drawn for. Tap it to open a calendar and jump to any date. Viewing days other than today requires an Unlimited subscription.

Map Buttons (phone & tablet)

The button stack on the right edge of the map. Top to bottom: zoom in, zoom slider, and zoom out; a compass that appears whenever the map is rotated, tap it to snap back to north-up; a locate button that appears when your position is off screen, tap it to fly to your GPS spot; a bookmark that opens your saved places; and an ⓘ that opens this guide. On desktop, the ⓘ at the bottom of the settings panel opens this guide. The separate small ⓘ beside the gold Unlimited badge in the lower-left corner shows map and data credits.

Map Layers

Pick one in Visualization Mode in the settings panel. Exactly one layer is on at a time; tap a tile to switch. The app always starts on Currents. The Heatmap Opacity slider below the tiles sets how strongly the active layer is painted.

Currents

Animated particles ride the predicted tidal current, and color shows speed on the legend at the bottom. Zoom in until data loads; a hint appears if you are too far out, and fully zoomed out you see ocean-scale currents instead. Scrub the time bar to watch the flow build, turn, and go slack. A repeating NO DATA stamp means that water is outside the model, which covers US coastal waters.

Winds

Forecast wind drawn as drifting streaks with its own 0–40 kt legend; the map brightens so the colors read. Pills under the tiles switch the forecast source: HRRR is US-only and sharpest, out to about 2 days; NAM is US-only, out to about 3.5 days; ECMWF is worldwide, out to about 10 days. Use ECMWF outside the US.

Rough Water Index (RWI)

Scores the water 0 to 10 where wind blowing against current steepens the sea, the classic recipe for short, steep chop. The readout translates the number: Calm, Moderate, Rough, Very Rough, Dangerous. It has its own forecast-source pills and covers US coastal waters.

Sea Surface Temp

One satellite temperature image per day, useful for finding fronts, warm eddies, and cold pockets. The time buttons switch to −1d/+1d for stepping between days, and the app jumps to the newest image on its own; a message appears if a date has no image. Leaving this layer returns the clock to now.

Basemaps

The map background, chosen under Basemap Styles. One is always on, and your data layers stay on top when you switch. Starts on Standard each launch.

Standard

A clean chart-style map drawn in dark, light, or night colors to match the app appearance.

Satellite

Aerial imagery.

Nautical

Official NOAA electronic charts: depth areas, contours, soundings, buoys, beacons, and hazards in standard chart symbology. The closer you zoom, the more chart detail appears. Soundings are shown in feet or meters to match your units choice. US waters only; the symbols are drawn for reference and are not tappable.

Bathymetry

Shaded seafloor relief, worldwide. Read channels, banks, and drop-offs at a glance.

Tools

All in the Tools section of the settings panel.

Virtual Currents Gauge

The predicted current at any exact spot. 1. Tap the tool; it highlights and the cursor becomes a crosshair. 2. Tap a point on the water: a marker drops and a chart opens with 24 hours of speed and direction there; the red dashed line is the selected time and follows the time bar. 3. Tap somewhere else to move the gauge; there is one at a time. 4. Tap the tool again to turn it off, or close the chart with its ×.

Virtual Tide Gauge

The predicted water level at any exact spot. Tap the tool to arm it, then tap a point on the water: a marker drops and a chart opens with that location’s 24-hour tide curve plus the water depth at the selected time, useful where there is no official station. Tap somewhere else to move the gauge, and tap the tool again to turn it off.

NOAA Tides

Shows every official NOAA tide station as a marker. Tap a station for its published tide predictions, the same numbers as the tide tables, centered on the selected time. These are predictions, not live readings. Tap the station again or the chart’s × to close; tap the tool again to hide the markers.

NOAA / CHS Currents

Official current-prediction stations from NOAA in the US and CHS in Canada, drawn as arrows that point with the flow and animate as you scrub time. Tap an arrow for a chart comparing the official prediction against this app’s model at that spot — a good cross-check at important passages. If you have been viewing one of the app’s higher-resolution regional models, its line joins the chart too; a model with no meaningful signal at that station is left off rather than drawn flat. No arrows? Zoom in.

NOAA PORTS®

Live sensors in major US harbors. Tap a marker for observed water level, currents, wind, and air readings over the past 24 hours, plus about 6 hours of predictions where available. PORTS always shows real time; it does not follow the time bar.

Routes

Plan a passage through the currents. Sign-in required. 1. Open the tool and choose the route type: Place waypoints, Optimal Departure, Open saved route, or Upload .gpx (on desktop these are the Manual / Departure / Import / Saved chips). 2. Tap the map to drop waypoints in order; drag any waypoint to adjust. To check a waypoint’s distances or delete it, open its card (desktop) or press and hold the waypoint on the map (phone). 3. Enter your boat speed. 4. Set the departure time in the Departure field beside the speed input. 5. Tap Compute Route: a panel opens with depart, arrive, and transit time plus current, wind, and rough-water charts along the way, and the map and time bar jump to your departure time. 6. Drag on the charts to slide your boat along the track and the map follows. The expand button in the panel’s top corner switches between one chart at a time and all charts at once. The arrow in the panel’s top-left corner goes back to editing: your waypoints stay on the map, so adjust the speed or departure time and compute again. 7. Save with the disk icon; reopen later from the Saved chip (desktop) or Open saved route (phone), where the trash icon on a row deletes a saved route. The panel’s × closes the tool.

Optimal Departure

Finds the departure with the shortest transit. On desktop, choose the Departure chip in the route panel; on a phone, choose Optimal Departure when opening the route tool. Place waypoints and enter your speed as usual, pick a window of ±2 to ±6 hours around the departure time, and tap Find Optimal Departure. The app times the passage at every 15-minute departure in the window, then opens the route panel at the best departure with a Saved stat showing the minutes gained, and moves the map clock to match. The top chart plots transit time against departure time: the green dot is the fastest departure, the bracket shows the minutes it saves over your rough departure, and dragging along the curve previews any other start; release to load that plan.

Saved Places

Bookmarks for map views. Sign-in required. Open the tool, tap Save current view, and name it. A crosshair pin then drops on the map: drag it onto the current you care about — the live readout under it previews that spot — and tap Save gauge. That gauge point is where the place reports live conditions from; Skip saves the place without one. Tap a saved place in the list to fly back to it. Each row has four controls: the ☰ handle drags to reorder the list (this is also the order on the start screen), the crosshair sets or adjusts the gauge point (blue once set), the house makes it your Home, and the trash deletes. On a phone, the bookmark button on the map opens the same list. Saved places follow your account to every device.

Start-Screen Conditions

On the start screen, Saved opens a live dashboard of your places drawn over a quiet map of their surrounding waters. The open card shows conditions at its gauge point right now: a large arrow pointing where the water is flowing — sitting exactly over that spot on the map behind it — with the speed, the next slack time, and a tide curve marking now and the coming high or low. The tide curve shows timing and shape only, never heights. Swipe up or down on the card (or tap another place in the list) and the view fades to the next spot; View on map opens the app there. A place without a gauge point shows a hint instead of readings — set one from the Saved Places tool.

Navigate

A live dashboard for when you are underway; location access required. GPS position, speed, and heading on a compass dial, plus Current Effect, the predicted current along your heading, green when it pushes you and red when it is against you. iPhones may also ask for motion access for the compass. Close with the ×.

Trips

Record your actual track. Sign-in required. Tap New Trip, then ▶ to start recording. ❚❚ pauses, and while paused that button becomes Stop & Save. Name the trip and save; it opens in review with current, wind, and rough-water charts along your track. Drag on them to replay the trip. Reopen anytime under Saved Trips; the × on a row deletes it immediately.

Logs

A logbook pinned to the map. Sign-in required. Tap New Log: add a title (required), notes, and a photo from camera or library. Position is your GPS spot by default, with the current there captured automatically, or tap Choose on Map and pick the spot yourself. In a saved log: Add to Personal Map pins it to a map only you see, Add to Public Map shares it with every user, and Delete asks before removing.

Markers

On/off switches in the Markers section of the settings panel.

Navigation Aids

Real buoys, beacons, and lights drawn with proper chart shapes and colors, on any basemap. Zoom in: lights show first, buoys and beacons closer in, names and light characteristics closer still. US waters. Drawn for reference; they are not tappable.

Personal Markers

Pins for the log entries you have added to your personal map. Tap a marker to open the log. Visible only to you; sign-in required.

Public Markers

Log entries the community has published. Anyone can browse them, signed in or not. Tap a marker to read; other people’s logs are read-only.

Weather Buoys

Live offshore buoys from the NDBC network. Tap one for its latest report: wind, gusts, waves, air and water temperature, pressure. Latest reading only; buoys do not follow the time bar.

Time Control

Time Bar & Scrubber

Everything on the map is drawn for one moment, and the bar at the bottom of the screen chooses it. The line at the center of the bar is the selected time: drag the bar and the timeline slides underneath the line, and you can keep dragging to roll hours or days into the past or future; the date appears at each midnight so you always know what day you are on. Tap anywhere on the bar to bring that time under the line, and an amber tick marks the real present. You can also step ±1 hour with the arrow buttons (±1 day on Sea Surface Temp), tap the time field to type an exact date and time, or use Jump to Now in the settings panel to snap back to the present. Free accounts view today, about ±3 hours around now; the PRO-marked stretches of the bar and other calendar days require an Unlimited subscription.

Display & Units

The pill rows near the bottom of the settings panel. These choices are remembered on your device, and follow your account everywhere when you are signed in.

Appearance

Dark, Light, or Night. Night redraws the map and data in reds only, to protect night vision at the helm.

Currents Display

Particles animates the flow; Arrows shows a fixed field of vectors for a traditional chart look.

Gridlines

A latitude/longitude grid over the map. Major and Minor are separate switches; tap each on or off, or stack both. Off clears them.

Color Scale

Fixed uses one 0–5 kt palette everywhere. Auto stretches the palette to the fastest water in view so gentle flows stay visible. The legend updates to match, and its floor lifts (≤) when there is no slack water on screen.

Units & Speed

Units sets Imperial (feet, °F) or Metric (meters, °C) for depths and temperatures. Speed is its own switch, kts or the land unit — shown as mph with Imperial units and km/h with Metric — so you can keep metric depths and still read knots. Distances follow the speed choice: nautical miles with kts, statute miles with mph, kilometers with km/h.

Clock

Shows every time in the app in a 12-hour or 24-hour clock: the main clock, the time bar, route and trip times, and station charts.

Account & Unlimited

Account & Sign-in

Tap the person icon at the bottom of the settings panel to sign in with Apple, Google, or your email. An account is free. It unlocks Routes, Saved Places, Trips, and Logs, and keeps your routes, trips, logs, saved places, and settings in sync on every device. Signing out removes that synced data from the device.

Actual Currents Unlimited

Real-time analysis is free and fully featured. Unlimited removes the time limits: view currents and tides for any date and time, past or future, to plan next weekend’s passage or replay last month’s ebb. Monthly or annual, with a 7-day free trial. The gold Unlimited button in the lower-left corner of the map opens the details. Already subscribed? Open the panel and use Sign in / Restore purchases.

1. The Ocean Model

Our predictions begin with ADCIRC (Advanced Circulation), a high-resolution ocean model developed at UNC and Notre Dame, widely used by NOAA and the US Army Corps of Engineers. ADCIRC solves the shallow water equations — the fundamental physics of how water moves — on an unstructured triangular mesh of over 2 million nodes.

The shallow water equations describe three things: conservation of mass (continuity) and the balance of forces in two horizontal directions (momentum). Here are the full equations:

i. The full shallow water equations

Continuity (mass conservation)

∂ζ∂televation
change + ∂(Hu)∂xE-W flux + ∂(Hv)∂yN-S flux = 0

x-Momentum (east-west)

∂u∂tacceleration + u∂u∂xadvection + v∂u∂yadvection − fvCoriolis = − g∂ζ∂xpressure
gradient − 1ρ∂P_a∂xatm.
pressure + τ_sρHwind
stress − τ_bρHbottom
friction

y-Momentum (north-south)

∂v∂tacceleration + u∂v∂xadvection + v∂v∂yadvection + fuCoriolis = − g∂ζ∂ypressure
gradient − 1ρ∂P_a∂yatm.
pressure + τ_sρHwind
stress − τ_bρHbottom
friction

u, v = depth-averaged velocities • ζ = surface elevation • H = total depth (h + ζ)
f = Coriolis parameter • g = gravity • ρ = water density
P_a = atmospheric pressure • τ_s = wind stress • τ_b = bottom friction

ii. Simplify for a tidal-only simulation

No weather forcing: atmospheric pressure gradients and wind stress drop out.

x-Momentum

∂u∂tacceleration + u∂u∂xadvection + v∂u∂yadvection − fvCoriolis = − g∂ζ∂xpressure
gradient − 1ρ∂P_a∂xatm.
pressure + τ_sρHwind
stress − τ_bρHbottom
friction

y-Momentum

∂v∂tacceleration + u∂v∂xadvection + v∂v∂yadvection + fuCoriolis = − g∂ζ∂ypressure
gradient − 1ρ∂P_a∂yatm.
pressure + τ_sρHwind
stress − τ_bρHbottom
friction

iii. Rearrange to solve for velocity over time

East-west velocity

∂u∂tacceleration = fvCoriolis − g∂ζ∂xpressure
gradient − τ_bρHbottom
friction − u∂u∂xadvection − v∂u∂yadvection

North-south velocity

∂v∂tacceleration = − fuCoriolis − g∂ζ∂ypressure
gradient − τ_bρHbottom
friction − u∂v∂xadvection − v∂v∂yadvection

Surface elevation

∂ζ∂televation
change = − ∂(Hu)∂xE-W flux − ∂(Hv)∂yN-S flux

These are solved at every node in the mesh, at every time step, producing velocity time series u(t) and v(t) at each node.

Note: ADCIRC does not solve these equations in their standard form. It reformulates the continuity equation into a Generalized Wave Continuity Equation (GWCE) — a second-order wave equation that eliminates spurious numerical oscillations inherent to finite element methods on large unstructured meshes.

2. From Simulation to Constituents

The raw time series above is accurate but impractical — gigabytes of data that only covers the simulated period. To make predictions for any date, we need to extract the underlying tidal patterns.

A harmonic analysis is performed on each node's time series. Because the tidal frequencies are known precisely from astronomy, a least-squares method can decompose the complex signal into its individual wave components:

Least-squares decomposition

signal(t) = Σ A_i · cos(ω_i · t − φ_i)

ω_i = known tidal frequency (from astronomy) • solve for A_i (amplitude) and φ_i (phase)

This is similar to how a prism splits white light into its component colors — the complex tidal signal is split into its component waves. The result at each node is a set of amplitudes and phases that compactly encode the full tidal behavior.

3. The Tidal Constituents

Each constituent corresponds to a specific gravitational cycle of the Moon and Sun. Together they capture the dominant patterns in tidal currents — like individual notes that combine to form a chord.

The 8 constituents extracted at every mesh node:

M2 — Principal lunar (12.42h period) — the dominant tide
S2 — Principal solar (12.00h)
N2 — Larger lunar elliptic (12.66h)
K1 — Luni-solar diurnal (23.93h)
O1 — Principal lunar diurnal (25.82h)
P1 — Principal solar diurnal (24.07h)
M4 — Shallow water overtide of M2 (6.21h)
M6 — Shallow water overtide of M2 (4.14h)

At each mesh node, ADCIRC provides the amplitude and phase of every constituent for both the east-west (U) and north-south (V) current components.

4. Predicting Currents at Any Time

To calculate the current at a specific moment, we use harmonic synthesis — summing the contribution of each constituent separately for the east-west (U) and north-south (V) components:

Harmonic synthesis — east-west current

U(t) = \sum_{i} f_i \cdot A^{u}_{i} \cdot \cos\!\big(V_i + u_i - \phi^{u}_{i}\big)

Harmonic synthesis — north-south current

V(t) = \sum_{i} f_i \cdot A^{v}_{i} \cdot \cos\!\big(V_i + u_i - \phi^{v}_{i}\big)

U(t) = east-west current velocity at time t • V(t) = north-south current velocity at time t
f_i = nodal correction factor (Moon's 18.6-year orbital wobble)
A^u_i, A^v_i = amplitude of U and V current for constituent i
V_i = astronomical argument (positions of Moon & Sun at this moment)
u_i = phase adjustment from the nodal cycle
φ^u_i, φ^v_i = local U and V phase offsets from the ADCIRC model
(Note: f and u here are standard tidal notation, distinct from the Coriolis f and velocity u used above)

Current speed & direction

\text{Speed} = \left(\, U(t)^2 + V(t)^2 \,\right)^{\!1/2}

\theta = \operatorname{atan2}\!\big(U(t),\; V(t)\big)

Direction θ follows nautical convention: 0° = north, 90° = east, 180° = south, 270° = west.
This is the direction the current is flowing toward.

The astronomical calculations (V, u, f) are computed using the TTide library, which tracks the precise positions of the Moon and Sun for any date.

5. From Numbers to Flow

The result is a U (east-west) and V (north-south) velocity at every mesh node. These velocities are then:

• Converted from m/s to knots
• Interpolated across triangles using barycentric coordinates so particles can flow smoothly between nodes
• Visualized as animated particles that follow the current — faster flow means faster, brighter particles

6. What This Means

Because tides are driven by celestial mechanics, they are highly predictable. This app can calculate accurate current predictions for any date — past or future — at any of the 2 million+ points in the mesh.

However, predictions do not account for wind, storms, river discharge, or other non-tidal forces that can temporarily alter currents. Always consult official sources for navigation.

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Acknowledgements

Tidal & Current Model

NOAA Data

Canadian Hydrographic Service

Forecast & Ocean Models

Basemaps

Open Source

NOAA Attribution & Endorsement

App Guide

Getting Around

The Settings Panel

Location Search

Live Data Readout

Date Display & Calendar

Map Buttons (phone & tablet)

Map Layers

Currents

Winds

Rough Water Index (RWI)

Sea Surface Temp

Basemaps

Standard

Satellite

Nautical

Bathymetry

Tools

Virtual Currents Gauge

Virtual Tide Gauge

NOAA Tides

NOAA / CHS Currents

NOAA PORTS®

Routes

Optimal Departure

Saved Places

Start-Screen Conditions

Navigate

Trips

Logs

Markers

Navigation Aids

Personal Markers

Public Markers

Weather Buoys

Time Control

Time Bar & Scrubber

Display & Units

Appearance

Currents Display

Gridlines

Color Scale

Units & Speed

Clock