A pitot tube reads water pressure.<\/p>\n<\/li>\n
A paddlewheel counts rotations.<\/p>\n<\/li>\n
A marine GPS speed sensor talks to satellites.<\/p>\n<\/li>\n<\/ul>\n
Same readout. Three separate methods. Knowing how<\/em> each one works changes everything \u2014 how you shop, how you troubleshoot, and how much you trust the number on that display.<\/p>\n Speed on water looks simple. It isn’t.<\/p>\n A boat speedometer measures how fast your hull moves. The reading shows up as mph or knots on your boat instrument panel<\/strong>. Most recreational gauges run 0\u201350 mph<\/strong> for pontoons and runabouts. Ski and bass boats get 0\u201380 mph<\/strong> gauges. That number on the dial looks solid. It often isn’t.<\/p>\n Most boat owners never think about this until something goes wrong. That speed reading can mean two different things, depending on the technology behind it.<\/p>\n Speed Through Water (STW)<\/strong> \u2014 pitot tubes, paddlewheels, and electromagnetic sensors all measure this. It shows how your hull moves relative to the surrounding water<\/em>.<\/p>\n<\/li>\n Speed Over Ground (SOG)<\/strong> \u2014 GPS measures this. It shows your actual progress across the earth’s surface.<\/p>\n<\/li>\n<\/ul>\n Drop into a 2-knot current and those two numbers split fast. A pitot reads 20 knots in both directions. Your GPS reads 22 knots going with<\/em> the current and 18 knots going against<\/em> it. Same engine. Same throttle. That’s a \u00b110% swing in real-world progress<\/strong> that STW alone will never catch.<\/p>\n Three technologies. One gauge. Very different strengths.<\/p>\n Picking the wrong one doesn’t just cost you money \u2014 it costs you accuracy at the moment you need it most. So here’s the direct breakdown across every dimension that matters.<\/p>\n Dimension<\/p>\n<\/th>\n Pitot Tube<\/p>\n<\/th>\n Paddlewheel<\/p>\n<\/th>\n GPS<\/p>\n<\/th>\n<\/tr>\n Measurement Type<\/strong><\/p>\n<\/td>\n STW<\/p>\n<\/td>\n STW<\/p>\n<\/td>\n SOG<\/p>\n<\/td>\n<\/tr>\n Low-Speed Accuracy (\u22646 mph)<\/strong><\/p>\n<\/td>\n Weak \u2014 often no reading below 3\u20135 mph<\/p>\n<\/td>\n Strong \u2014 most reliable at slow speeds<\/p>\n<\/td>\n Strong \u2014 stable above 1\u20132 knots<\/p>\n<\/td>\n<\/tr>\n High-Speed Accuracy (20\u201350+ mph)<\/strong><\/p>\n<\/td>\n Good \u2014 \u22641 mph error in clean conditions<\/p>\n<\/td>\n Moderate \u2014 turbulence and bubbles cause drift<\/p>\n<\/td>\n Excellent \u2014 under 1% error<\/p>\n<\/td>\n<\/tr>\n Installation Complexity<\/strong><\/p>\n<\/td>\n Medium (transom mount) to High (through-hull)<\/p>\n<\/td>\n High \u2014 requires through-hull in clean water flow<\/p>\n<\/td>\n Lowest \u2014 antenna and power, no hull penetration<\/p>\n<\/td>\n<\/tr>\n Maintenance Demand<\/strong><\/p>\n<\/td>\n Moderate \u2014 watch for clogs, bends, air in the line<\/p>\n<\/td>\n High \u2014 biofouling locks up the Impeller; clean every 1\u20133 months in saltwater<\/p>\n<\/td>\n Minimal \u2014 keep the antenna clear, update firmware<\/p>\n<\/td>\n<\/tr>\n Typical System Cost<\/strong><\/p>\n<\/td>\n $80\u2013$250<\/p>\n<\/td>\n $200\u2013$600 with display<\/p>\n<\/td>\n $100\u2013$500 depending on standalone vs. panel unit<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Pitot tube installation ranges from dead simple to genuinely tricky. Many outboard engines \u2014 Yamaha being the most common example \u2014 come with a pitot port built into the lower unit. No drilling, no sealing, no sweat. A separate through-hull pitot probe is a different story. You’re cutting into the hull, sealing against water intrusion, and positioning the probe to avoid air pockets and turbulent flow zones. Get the angle wrong and your readings are off before you leave the slip.<\/p>\n Paddlewheel installation<\/strong> always needs a through-hull fitting. No way around it. Most modern transducers use a valve-and-plug design that lets you pull the impeller for cleaning without hauling the boat. First-time installation still means drilling, positioning in a smooth-flow area, and sealing the fitting tight. Then the ongoing work begins. In saltwater, barnacles and algae can lock an impeller solid within a few weeks. So your boat speed sensor<\/strong> drops to zero mid-trip? A fouled paddlewheel is the first thing to check.<\/p>\n GPS installation<\/strong> is the easiest by a wide margin. Flush-mount or bracket-mount the antenna, run power and NMEA data leads, done. No hull penetration. No calibration ritual. No marine growth to scrape off. That simplicity is a strong case for GPS as a primary or backup boat speed gauge<\/strong> \u2014 especially on boats where drilling another hole is the wrong move.<\/p>\n The right choice depends on how you use the water.<\/p>\n For competitive water sports and performance tuning<\/strong> \u2014 wakeboarding, slalom skiing, drag racing on water \u2014 GPS is the right call. You need repeatable, current-independent speed measurement. Your 34 mph needs to mean 34 mph across the course, not 34 mph relative to water moving underneath you. GPS delivers that. A quality pitot can serve as a backup reference, but GPS runs point.<\/p>\n For sailing and low-speed displacement cruising<\/strong>, the combination of a paddlewheel knot meter<\/strong> plus GPS is the professional standard for good reason. The paddlewheel gives you STW \u2014 the number your sails, rudder angle, and leeway calculations are built around. GPS gives you SOG \u2014 the number your ETA and fuel planning need. Subtract one from the other and you get a real-time current vector. That’s not a redundancy. That’s a navigation tool.<\/p>\n For the everyday recreational powerboat<\/strong> \u2014 fishing trips, weekend cruising, casual runs at 20\u201340 mph \u2014 the decision is simpler. Your outboard already has an integrated pitot system? A quality digital gauge turns that into a marine speedometer<\/strong> accurate to within 1 mph across most of your operating range. That’s good enough. Adding instrumentation from scratch? A GPS speed display costs less to install, needs less maintenance, and delivers SOG accuracy that a mechanical system can’t match. The main thing you give up is STW \u2014 and for most recreational powerboaters, that’s a trade worth making.<\/p>\n One last note on current. In calm, tidal-flat waters \u2014 lakes, protected bays, low-flow rivers \u2014 the STW vs. SOG gap is small. Pitot and GPS numbers stay close. Push that boat into a tidal estuary or a river with real current, and a pitot reading of 10 knots alongside a GPS reading of 7 knots can both appear on the same trip. Knowing which number is on your display \u2014 and what it’s telling you \u2014 is the difference between confident seamanship and expensive guesswork.<\/p>\n Expert Opinion:<\/em><\/strong><\/p>\n “In my years of working with recreational and competitive boaters, the single most common mistake I see is treating Speed Through Water and Speed Over Ground as interchangeable numbers. They’re not \u2014 and that distinction isn’t academic. I’ve watched competitive water skiers run a course at what they believed was a legal 36 mph, only to discover their pitot-based gauge was reading relative to a 1.5-knot current pushing them through. That’s a disqualifiable run. For performance applications, GPS isn’t a luxury upgrade \u2014 it’s the baseline standard for meaningful, repeatable measurement. And for cruising sailors navigating tidal waters, running both a paddlewheel and GPS simultaneously isn’t redundancy; it’s how you build a real-time picture of current behavior. The difference between those two readings is live current data you can’t get any other way.<\/em>“<\/p>\n \u2014\u2014\u2014\u2014Capt. James R. Holloway<\/a>, USCG-Licensed Master Captain, 20+ years in marine electronics installation and performance boat testing, contributor to marine navigation standards training programs<\/p>\n<\/blockquote>\n The right speedometer isn’t the most expensive one. It’s the one that matches how your boat moves through the water.<\/p>\n Start with one decision: GPS or mechanical. That choice matters more than brand, dial size, or mounting style. Pitot tubes and paddlewheels can drift \u00b15\u201315 mph. Fouling, installation angle, and water conditions all push the reading off. A quality GPS unit with a 4 Hz refresh rate \u2014 four speed calculations every second \u2014 holds accuracy within 0.1 knots in real-world conditions. For most boats, that gap in performance is decisive.<\/p>\n Small powerboats (15\u201324 ft, 40\u2013200 hp outboards)<\/strong><\/p>\n Cruising range: 18\u201330 knots. WOT: 28\u201345 knots. A GPS speedometer handles this range well. Look for an 85 mm (3\u20133.5″) gauge to fit standard dash cutouts. Make sure it supports both mph and knots \u2014 you’ll want to switch units during Prop testing. Entry-level GPS gauges like the ELING 85mm run $50\u201380 and carry an IP67 weather resistance rating.<\/p>\n Sailboats (20\u201345 ft)<\/strong><\/p>\n Sailboats run at 3\u20138 knots. At those speeds, low-speed precision matters. GPS handles it well, and 4 Hz units like the Velocitek SpeedPuck class resolve down to 0.1 knots. For racing, you need VMG and course-over-ground data. Step up to a dedicated tactical instrument that logs run data post-race. Correcting for current or calculating leeway? Keep a paddlewheel for STW. Just calibrate it against GPS \u2014 not the other way around.<\/p>\n Pontoon and dual-hull boats (20\u201328 ft)<\/strong><\/p>\n Passenger loads shift a lot on pontoons. Coolers, guests, and gear all move the waterline around. GPS holds steady through all of it. A basic GPS round gauge in the $40\u201380 range covers the need. Already running a Garmin, Lowrance, or Simrad chartplotter? Skip the separate gauge. Just add SOG to your data overlay.<\/p>\n![\"Boat](\"https:\/\/www.mercurymarine.com\/ca\/en\/lifestyle\/dockline\/fixing-speedometer-reading-issues\/_jcr_content\/_cq_featuredimage.coreimg.jpeg\/1699490628189\/mer-5390-how-to-fix-speedometer-reading-issues-hero-image.jpeg\")
<\/p>\nWhat Is a Boat Speedometer and Why Accurate Speed Measurement Matters<\/h2>\n
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Pitot Tube vs GPS vs Paddlewheel: Head-to-Head Comparison<\/h2>\n
\nInstallation, Maintenance, and Cost<\/h3>\n
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\nWhich One Belongs on Your Boat<\/h3>\n
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How to Choose the Right Boat Speedometer for Your Boat<\/h2>\n
![\"Pitot](\"https:\/\/scontent-hkg1-1.xx.fbcdn.net\/v\/t39.30808-6\/622385742_1482231487276669_8464567732374784062_n.jpg?_nc_cat=109&ccb=1-7&_nc_sid=833d8c&_nc_ohc=jgHX-6RFWlYQ7kNvwGdBztu&_nc_oc=AdrW_Jh4606BDfm0JmyljImj1PXb4pkx2IsrGJHHHnSWJhiDVXLg4LQkhUX-pIqAa3w&_nc_zt=23&_nc_ht=scontent-hkg1-1.xx&_nc_gid=mvriVAu4WMyoTquAVVJyXQ&_nc_ss=7c289&oh=00_Af4-KB1hwqnLPtpgp5UG1-xvdb3zUGz9voLJUBS6QSxT-Q&oe=6A104F8A\")
<\/p>\nMatch the Technology to Your Boat Type<\/h3>\n
![\"Boat](\"https:\/\/kus-usa.com\/wp-content\/uploads\/2021\/04\/Best-Use-Cases-for-a-GPS-Speedometer-Unsplash-scaled.jpg\")
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