Boat speedometer app: GPS speed over ground

Written by Muneeb Rana · Last updated 26 October 2026

Quick answer: A boat speedometer app shows GPS speed over ground, useful on small boats without instruments. Speedometer PRO displays MPH or KM/H; convert to knots with our tool. It does not measure speed through water, so currents make the two differ. Keep the phone dry and secure, and never rely on it as your only navigation aid.

TL;DR

• Speed over ground from GPS.
• Convert to knots.
• Currents change speed through water.
• Waterproof case.
• Not a marine instrument.

Boat speedometer setup

Open the app before leaving the dock, place the phone in a waterproof case with sky view, and start a trip.

Knots

Use knots to mph; 1 knot = 1.852 km/h. See what is a knot.

Over ground vs through water

A current pushing you adds to GPS speed; a log measures speed through water. Both are useful.

Safety

Carry proper marine charts, lifejackets and a VHF radio where required. A phone is a helper only.

Planing speed and hull speed

Small displacement boats have a practical maximum often estimated as hull speed, roughly 1.34 × the square root of the waterline length in feet, giving knots. A 25-foot waterline gives about 6.7 knots. Planing boats can exceed it once they rise onto the plane. A GPS speedometer is a handy way to see where your boat settles at different throttle settings.

Reading the effect of current

Run the same stretch upstream and downstream at the same throttle and compare GPS speeds. Half the difference is a rough estimate of the current, and the average of the two is close to your speed through the water. This simple test helps when planning fuel and time on tidal rivers.

Keeping the phone working

Heat, spray and glare are the enemies. Keep the phone out of direct sun when possible, since phones can dim or pause when they overheat. A floating waterproof case and a lanyard prevent the most common loss: the phone going over the side.

Speed rules on the water

Many harbours, marinas, rivers and lakes have speed limits or no-wake zones, often posted in knots or mph depending on the country. A GPS speedometer helps you keep to them, especially when a slow-looking speed is actually too fast for a crowded mooring. Local harbour authority rules apply, and they can change by season.

Currents

GPS shows speed over ground. In a current, your speed through the water differs.

Compass

The compass shows magnetic heading; metal on boats can disturb it.

Related: Speedometer use cases · Knots to MPH converter · What is a knot in speed? · Speedometer with compass · Speedometer offline: does it need data? · Online compass · Average walking speed · Average running speed

Get Speedometer PRO on Google Play · Download on the App Store · Set it up before you move; never handle your phone while driving.

Sources

• Knot (unit) (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
• GPS Accuracy (GPS.gov): gps.gov (accessed 26 October 2026)
• Compass (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
• Hull speed (Wikipedia): en.wikipedia.org (accessed 26 October 2026)