Boat Speed Test (Knots, MPH, KM/H) — Real-Time GPS Speed for Marine Use
Measure your boat speed over ground using GPS in your browser. No apps. No hardware. No signup. Just allow location and go.
Shows: Speed Over Ground · Max speed · Average · Distance · Heading · GPS accuracy
Works on iPhone, Android, and tablets. Waterproof case recommended.
What This Tool Does
This tool measures your boat's Speed Over Ground (SOG) using your phone's GPS receiver.
- ✓Displays speed in knots, mph, and km/h
- ✓Works on motorboats, sailboats, yachts, kayaks, and jet skis
- ✓Runs entirely in the browser — no app to install
- ✓Functions offshore after initial GPS lock
Marine GPS units do the same thing. This tool does it for free using your phone.
Why GPS Matters on Water
Water has no lane markings, no roadside trees, no visual reference. Your perception of speed is wrong. A boat doing 20 knots feels slower than a car doing 20 mph because there's no parallax from objects passing by. On calm water, 30 knots might feel like 15. In chop, 15 might feel like 40.
GPS provides:
- 📍Speed Over Ground (SOG) — How fast you're moving relative to earth
- 🧭Course Over Ground (COG) — Your actual track, not where the bow points
- 📊Consistent data — Unaffected by current, wind, or perception
Knots vs MPH vs KM/H
| Unit | Used By | Conversion |
|---|---|---|
| Knots | Sailboats, yachts, commercial vessels, charts | 1 knot = 1.852 km/h |
| MPH | Jet skis, bass boats, US freshwater | 1 knot = 1.15 mph |
| KM/H | EU regulatory, some freshwater contexts | 1 knot = 1.852 km/h |
Why knots? A knot equals one nautical mile per hour. A nautical mile is one minute of latitude. This makes navigation calculations simpler on charts.
Types of Speed in Marine Use
- SOG — Speed Over Ground
What GPS measures. What this tool displays. Your velocity relative to earth. This is what matters for navigation, route timing, and fuel planning.
- STW — Speed Through Water
Measured by paddlewheel or impeller sensors. Shows how fast water flows past the hull. Useful for sail trim and detecting current.
- Hull Speed (Theoretical Limit)
For displacement hulls: Hull Speed (knots) = 1.34 × √(waterline length in feet). A 36-foot boat has a hull speed of about 8 knots.
This tool measures SOG. That's what you need for navigation and trip planning.
Currents and Tidal Streams
Currents affect your SOG. Understanding this matters for trip planning:
If your STW is 12 knots and the current is 3 knots against you, your SOG drops to 9 knots.
If that current is with you, your SOG jumps to 15 knots.
GPS shows SOG — which is what matters for ETA and navigation. But remember: the same engine power produces different ground speeds depending on current direction.
Boat Class Speed Benchmarks
Typical speeds by vessel type:
| Vessel Type | Typical Cruise | Typical Top |
|---|---|---|
| Sailboat (cruising) | 5–8 knots | 8–10 knots |
| Trawler / Displacement | 7–9 knots | 10–12 knots |
| Center Console | 25–35 knots | 40–50 knots |
| PWC (Jet Ski) | 25–45 knots | 55–65 knots |
| Offshore RIB | 35–45 knots | 50+ knots |
| Bass Boat | 40–50 knots | 60–70 knots |
| Kayak | 3–4 knots | 5–6 knots |
| Rowing Shell | 4–6 knots | 8–10 knots |
Supported Vessels
Sailboats
Knots preferred
Motorboats
Cruisers, runabouts
Yachts
Any size works
RIBs
Fast and GPS-friendly
PWCs
Jet Ski, Sea-Doo
Kayaks
Paddle speed tracking
Why Measure Boat Speed?
- Trip Planning
Know your actual cruising speed to estimate ETA. Tides and currents affect SOG. GPS shows the real number.
- Fuel Consumption
Planing boats change fuel burn by speed zone. At 8 knots: displacement. At 20-30 knots: planing. At 35+: max burn.
- Racing and Training
Sailors track VMG and tacking angles. Powerboat racers verify top speed.
- Legal Compliance
Harbor speed limits, no-wake zones, marina restrictions. GPS logs are evidence.
- Performance Verification
"Does my 90 HP outboard really do 34 knots?" "Is my new prop faster?" This tool answers those questions.
GPS Accuracy on Water
GPS works well on water, but conditions matter:
- Flat horizon, open ocean
- No buildings or terrain
- Stable cruise speed
- Clear skies
- Heavy chop
- Jet ski acceleration bursts
- Hardtop cabins, biminis
- Fjords and canyons
GNSS Doppler: At cruising speed, modern GPS calculates velocity using satellite Doppler shift, not just position deltas. This is more accurate than low-speed positional updates.
Offshore Functionality
Once GPS locks, internet is optional.
GPS receives signals directly from satellites. It doesn't need cell towers. After the page loads and GPS connects, you can go 50 miles offshore and the tool keeps working.
- 🌐Internet: Required for initial page load only
- 📡GPS: Required for all functionality
- 🌊Offshore: Works after initial lock
Device Support
- iPhone / iPad
Excellent GPS. Multi-constellation GNSS with sensor fusion.
- Android Flagships
Strong performance. Qualcomm modems match Apple.
- Budget Android
Usable with occasional jitter.
- Laptops
No GPS hardware. Speed stays at zero.
Frequently Asked Questions
Does this work offshore with no cell signal?+
Does it measure SOG or STW?+
Can I use it for racing?+
What accuracy should I expect?+
Does it work on jet skis?+
Will it drain my battery?+
Can I export my data?+
Safety
Small boats at speed are dangerous. Some real points:
- ⚠️At 30+ knots, chop can eject passengers. Wake and rogue waves cause violent impacts.
- ⚠️At 40 knots, you cover 20 meters in one second. Debris appears fast.
- ⚠️Bow-high planing blocks forward sight. Bow-down digs into waves.
- ⚠️Cold water reduces survivability. A fall at speed into 15°C water gives minutes of useful movement.
This tool shows your speed. Staying alive is your job.
Coming Soon
Technical Notes
- •Requires 1 Hz+ GNSS update rate
- •Speed Over Ground via Doppler shift at cruising speed
- •Distance calculated using Haversine formula
- •Speed smoothed via rolling average filter
- •Constellations: GPS, GLONASS, Galileo, BeiDou
- •Works offline after initial page load