--- title: "Boat Speedometer App – Check Boat Speed in Knots (Free)" description: "Check boat speed over ground (SOG) in knots with GPS Speedometer for iPhone. Record trips, distance and basic speed statistics. Free download." source: "https://speedometer.app/boat-speedometer-app/" image: "https://speedometer.app/images/features/boat-speedometer-app.jpg" --- # Boat GPS Speedometer with Knots Display See your boat’s GPS Speed Over Ground (SOG) in knots on your iPhone. Record trips and review basic speed and distance statistics. SOG describes movement over the ground. It can differ from speed through the water because of currents. GPS Speedometer does not measure Speed Through Water (STW) or calculate Velocity Made Good (VMG). [![Download on the App Store](/_image/?href=%2F_astro%2FDownload_on_the_App_Store_Badge_US-UK_RGB_blk_092917.CVyK0T4N.svg&w=192&h=64&f=svg)](https://apps.apple.com/app/apple-store/id901574501?pt=100782816&ct=web.speedometer.app.marine&mt=8) Free iPhone download, with optional PRO features. [Try the browser speedometer →](/online-speedometer/) ![Illustration of an iPhone with GPS Speedometer beside the compass at a sailing yacht’s helm.](/images/photos/boat/01-sailing-helm.webp) ## Choose the tool for your trip 01 ### A quick browser reading Open the online speedometer, choose knots, press Start and allow location access. It needs a usable location reading and does not save trips in the iPhone app. [Try the browser speedometer →](/online-speedometer/) 02 ### Record trips in the iPhone app Use the native app for GPS speed and saved trip summaries. Core recording and basic summaries are free; detailed analysis and sharing require PRO. [See free and PRO features →](#app-features) 03 ### Need another marine measurement? For speed through water, VMG or navigation, choose equipment or software that provides those functions. This app is a supplementary SOG display. [Understand the measurements →](#speed-on-water) ## GPS Speedometer on a boat Real photos of the app in use, with km/h selected. Knots and MPH are also available in the unit settings. ![iPhone held upright in a protective pouch aboard a boat, showing 23 km/h in GPS Speedometer.](/images/photos/boat/real/portrait-kmh.jpg) Portrait display with current speed and trip statistics. ![iPhone held sideways in a protective pouch aboard a boat, showing 21 km/h in GPS Speedometer.](/images/photos/boat/real/landscape-kmh.jpg) Landscape display with speed and trip statistics side by side. ## SOG, STW and VMG measure different things A GPS reading tells you how fast you are moving over the ground. Choose the measurement that matches the question you want to answer. SOG Speed Over Ground: speed relative to the Earth’s surface. This is the GPS speed shown by the app. STW Speed Through Water: speed relative to the surrounding water. The app does not measure it. VMG Velocity Made Good: the component of velocity along a chosen direction, such as toward a waypoint. The app does not calculate it. ### A following current changes SOG 5 knThrough water + 2 knFollowing current \= 7 knOver ground Illustrative arithmetic, not a measured trip: 5 kn through the water plus a 2 kn current gives 7 kn over ground only when both act along the same straight line and in the same direction. 1 knot is 1 nautical mile per hour, exactly 1.852 km/h. [Read the SOG and STW guide →](/blog/boat-speed-test-gps-knots/) ## What the iPhone app includes Use the free core features to check speed and keep a basic trip record. Choose PRO when you need its additional tools. ### Free core features - ✓GPS speed in knots, MPH and km/h - ✓Current, maximum and average speed, plus distance - ✓Trip recording, saving and basic saved-trip summaries ### Optional PRO features - ✓Detailed route playback and saved-trip charts and analysis - ✓GPX export and trip sharing - ✓Picture-in-Picture speed display over other apps PRO is available by subscription or one-time purchase; both unlock the same PRO features. Check Apple’s purchase screen for your region’s current price. ![Illustration of GPS Speedometer in Picture-in-Picture over a marine chart at a powerboat console.](/images/photos/boat/02-powerboat-pip.webp) Picture-in-Picture over other apps is available with PRO. ## Set up the iPhone app before departure 1. 1 ### Choose knots Download GPS Speedometer and select knots in the app’s speed-unit settings. 2. 2 ### Allow location and secure the phone Enable location access, including Precise Location where available. Use a suitable mount and keep a clear view of the sky. Wait for a usable reading. 3. 3 ### Start a trip recording Start recording when you want to save the trip. Afterwards, open the saved trip for its basic summary; detailed playback and charts require PRO. [Change speed units →](/guides/change-speed-units/) ## Conditions that affect the reading ### Location and reception Location permission and usable data are required. Obstructions, low speed and device processing can affect the reading. ### Offline core tracking The native app’s GPS speed and trip recording work without internet when location access and a usable GPS signal are available. Maps and connected features may be unavailable offline; weather requires PRO and internet. ### Phone, mount and power Follow the device, case, mount and charger manufacturers’ instructions. Plan for water exposure, heat and battery use, and position the display without obstructing other equipment. ![Illustration of an iPhone with GPS Speedometer in a protective case on a kayak.](/images/photos/boat/03-kayak-foredeck.webp) ## Boat speedometer questions Does the app show speed over ground or through water?+ It shows GPS Speed Over Ground (SOG), including knots. It does not measure Speed Through Water (STW) or calculate Velocity Made Good (VMG). Which features are free?+ Core GPS speed, units, maximum/average speed, distance, trip recording, saving and basic summaries are free. Detailed route playback, charts, GPX sharing and Picture-in-Picture require PRO. Can I record a trip without mobile coverage?+ Yes, the native app’s core speed and trip recording work without internet, with location permission and a usable GPS signal. Maps may be unavailable offline. Weather requires PRO and internet. Can a recording continue when I leave the app?+ GPS can continue while a trip is actively being recorded or Picture-in-Picture is in use. PiP requires PRO. When recording ends, open the saved trip to view its basic summary. Can it replace marine instruments or navigation equipment?+ Use it as a supplementary SOG display. No fixed precision or superiority to another instrument is guaranteed. Use suitable equipment or software for STW, VMG, navigation or dedicated racing analysis. Can I check boat speed without downloading the app?+ Use the separate browser speedometer: choose knots, press Start and allow location access. It needs a usable location reading. It does not save trips in the native app; use the iPhone app to record and review them. ## Keep a record of your next boat trip Download GPS Speedometer for iPhone for SOG in knots and free basic trip summaries. ![Speedometer App QR Code](/_image/?href=%2F_astro%2Fqr-code-acquisition.CL72vcG5.png&w=640&h=640&q=85&f=webp) [![Download on the App Store](/_image/?href=%2F_astro%2FDownload_on_the_App_Store_Badge_US-UK_RGB_blk_092917.CVyK0T4N.svg&w=192&h=64&f=svg)](https://apps.apple.com/app/apple-store/id901574501?pt=100782816&ct=web.speedometer.app.marine&mt=8) Free download · Optional PRO subscription or one-time purchase GPS Speedometer is a supplementary speed and trip tool. It does not replace marine navigation equipment, charts or instruments required for your vessel.