How Slackwater works
Real math, public data, computed on your device. No black box, no server deciding what the tide is doing.
Tides from NOAA harmonic constituents
The tide at any US station is the sum of many slow, predictable cycles driven by the moon and the sun. NOAA publishes each station's harmonic constituents: an amplitude and a phase for every constituent, with names like M2, S2, K1, and O1. Slackwater evaluates that harmonic series directly on the device to draw the tide curve and to find the highs, the lows, and slack.
The same engine powers the app and the tide pages on this site, so the times you see here match the times you see in the app.
Computed on your device
There is no server round-trip to fetch a prediction. The constituents live on the device, and the math runs there too, so the tide curve works with no signal at the coast, which is exactly where signal dies. Even the web pages you are reading were computed by the same engine, a TypeScript port running in your browser.
Solunar, honestly
Solunar periods are windows around the moon's transits overhead and underfoot, plus moonrise and moonset, that many anglers use to time the bite. The app computes them from ephemeris math, the same kind of calculation that predicts where the sun and moon are in the sky.
An honest framing: solunar theory is a planning heuristic that plenty of anglers swear by, not settled science. Slackwater computes the windows precisely and never pretends they are anything more than that.
Private by design
No account, no ads, no tracking. Your spots and settings live on your device and sync only through your own iCloud, never through a developer server. The one optional feature that touches the network is marine conditions, and it is off unless you want it.
Accuracy and limits
Tide predictions are astronomical. Weather is not. Storm surge, wind setup, and barometric pressure all shift the real water level away from any prediction, ours or NOAA's. In calm conditions, Slackwater's times typically agree with NOAA's published tables to within minutes, because both come from the same harmonic constituents.
Predictions are estimates for reference only. They are not for navigation or life-safety decisions. Always consult official NOAA tide tables and nautical charts.