A marina management app has to know which boat fits which slip.

A slip is a size, a depth and a plug

Most people looking for a boat slip management app want one screen: which slips are open this weekend, and which of those can take the boat that is calling on the radio. That screen needs fields the general purpose tools do not have.

Length is the first trap. The number an owner gives you is often the registered length, and the slip has to hold length overall: bow pulpit, swim platform, davits and the dinghy hanging off the back. Marinas charge on the space they give up, so the record needs both numbers and a note saying which one the price uses. Beam decides whether the hull fits between the finger piers, and on a wide catamaran it decides more than length does.

Depth is the field that separates a harbour management app from a car park. Charted depth in United States waters is referenced to mean lower low water, so the water you actually have is that depth plus whatever the tide is doing. NOAA publishes tide predictions per station and makes the datum mandatory on the request, which is why the same spot can be quoted as two different numbers. If your basin dries at springs, the slip record needs a controlling depth, and the booking has to check the boat's draft against it with a margin, not against the depth at the moment somebody last looked.

The last field is the pedestal. Thirty amp and fifty amp service are not interchangeable, and a boat given the wrong one is a booking that fails at the dock, in the dark, on a Friday. Store the amperage, whether water is on that finger, which side the finger is on, and whether the meter is read for billing.

NOAA CO-OPS Data API documentation, predictions and datum parameters

The second booking is the one that has to fail

Two stays collide when the arrival is before the other departure and the departure is after the other arrival. Written exactly that way, the turnover day works: a boat leaving on the third and a boat arriving on the third do not collide, because the ranges touch rather than overlap. Compare ranges. The other honest option is a row per night, which turns a collision into a duplicate key and makes the rule impossible to forget.

Where the check runs matters more than how it is written. If the app loads availability, the office spends two minutes on the phone, and the booking is then written without asking again, two people can both be told the slip is free. The check has to happen at the moment of writing, against the shared record, and it has to refuse rather than warn. That is the same guard behind a campground app, where pitch length and hookup amperage decide which sites were ever candidates in the first place.

Marinas have one advantage over hotels: they reassign. The office is not selling slip B12, it is selling a night on the dock, and it will shuffle boats when a 60 foot transient calls at four in the afternoon. So the useful query is not whether B12 is free, it is which slips fit this boat and are free every night, which is what the tool above answers. Build reassignment in from the start, and keep a record of who moved which boat and when, because somebody always asks.

One honest note, since this is the first thing people ask an AI builder. Nobody here built a marina app with Newly and watched it refuse a second overlapping booking, so this page does not claim that it enforces one for you. What you get is an ordinary React Native and Expo project you own, which means the guard is ordinary application code: yours to specify, and yours to test. Ask for the rule in plain words, then try to book the same slip twice from two phones before you trust it with a season.

What the boat record has to hold

The slip is half the record. The boat is the other half, and a marina needs more of it than a name on a cleat. A hull identification number is the closest thing to a stable key: twelve characters with no hyphens, slashes or spaces, made up of three for the manufacturer identification code, five for the manufacturer's serial number, which never uses the letters I, O or Q, and four that carry a date and the model year. Store it as given and validate the length.

Then the paperwork the marina is actually exposed to: the registration or documentation number, the insurance certificate with its expiry date and the cover as written, the owner plus a second contact who is allowed to move the boat, and whether anyone lives aboard. The expiry date is the field that earns the app its keep, because it is the one that goes stale quietly. A list of certificates lapsing in the next thirty days is worth more than any dashboard.

Keep the stay, the boat and the slip as three joined records rather than one booking row with the boat's details copied into it. Owners sell boats, boats get renamed, and the same hull comes back next season into a different slip. Anything the marina hires out by the day, dinghies, kayaks, dock carts, deserves the same treatment as an equipment rental app: one record per item, and the same check at the moment of writing.

33 CFR 181.25, hull identification number format

It gets used on a dock, in wind, with wet hands

The office may run on a laptop, but the app is used walking a finger pier holding a bow line. That decides the design: a slip map readable at arm's length, large targets, arrival and departure as one tap each, and a photo of the transom attached to the stay so the next person on shift knows which boat is which.

Meter readings, pumpout entries, fuel sales and damage notes are all the same shape: a timestamp, a slip, a boat, a number or a photo, and the person who recorded it. Resist the urge to give each one a bespoke screen. One record type with a category covers most of what a marina writes down in a season, and it is the only version anyone will keep using in October.

Then the part every dock discovers late. The far end of the pier is a dock with no signal, so reads have to work from what is already on the phone, writes have to queue, and anything unconfirmed has to look different on screen. What cannot be faked offline is the booking guarantee, because two phones that cannot reach the shared record cannot agree with each other. Let a dockhand record an arrival offline. Do not let two of them promise the same slip.