Boat Speedometer Reads Zero or Wrong: Pitot Pickups, Paddlewheels and GPS Speed

Run a phone GPS app or the chartplotter next to the speedometer on a calm day, in both directions on the same stretch, to cancel out current and wind. GPS measures speed over the ground, while pitot and paddlewheel sensors measure speed through the water, so a small difference is normal in current. A big difference, a reading of zero, or a needle that jumps around points to the problems below.

This is the fix most of the time. Find the small hole on the leading edge of the gearcase (run a fingertip along the front edge below the anti-ventilation plate; it can be hard to see). Some boats have a separate transom-mounted pickup instead.

Mercury's method: pick a drill bit that slides into the hole, and slowly work it back and forth by hand, using the fluted part to pull out weeds, sand and gravel. Don't use a drill motor, and don't force a bit that's too big; you can damage the pickup or push debris deeper.

Owners often find the pickup packed with sand after beaching, or with weed after running through grass. After winter or long storage, wasps and mud daubers are common suspects: they love small holes and tubes.

A pitot speedometer needs an airtight path. Photograph the routing, then follow the thin hose from the engine (through the rigging tube or under the cowl) to the back of the gauge. Look for:

• A hose that slipped off a fitting or a tee
• Cracks, especially where it bends or rubs
• Kinks or a hose pinched under a cable clamp or seat base

A leaking or disconnected hose makes the gauge read low or zero. Replace cracked hose with the type your gauge uses and secure it so it can't rub.

Sterndrives: the pickup is in the drive, and the speedometer hose usually has a small push-in connector or barb where it meets the drive, up near the transom assembly. When the drive has been removed for bellows, U-joint or engine work, that little connection is easy to leave unplugged or loose. If the speedometer quit right after service, raise the drive and check it first. A quick test: with the hose disconnected at the gauge, blow gently toward the engine and listen for air hissing out near the transom instead of at the pickup.

Disconnect the hose at the back of the gauge and gently blow through the hose toward the engine by mouth, or with very low air pressure, to push out water and dirt. Never blow into the gauge fitting, and never use shop air on the gauge: the pressure movement inside it is delicate.

If no air comes out at the pickup, the blockage is in the hose or pickup; clear it from both ends. Before winter storage, blowing the line clear keeps leftover water from freezing in it.

Reconnect the hose snugly. With the boat on plane, a working pitot gauge should rise smoothly with speed and settle back to zero at idle. A needle that sticks, won't return to zero with the boat stopped, or a gauge with water or fog inside is probably bad. Analog gauges and pickups come in specific speed ranges, so replace with a matching range for your boat.

Remember the limits of pitot gauges: many read nothing at idle and slow speeds, and readings can change with engine height, trim and how cleanly water flows to the pickup. Use GPS for slow trolling speeds.

On digital displays, speed may come from a pitot sensor, a paddlewheel or GPS, and you can often choose. Garmin's chartplotter manual, for example, lists a setting on Yamaha engine screens that selects the speed source. Look in your gauge's setup menu:

• Make sure the selected speed source actually exists on your boat.
• Look for a sensor fault message, and look up what it means in the gauge manual.
• If a pickup tested clear and the hose is good, the pressure sensor or its plug may be at fault, which is a dealer diagnosis on most digital systems.
• Remember that other features may lean on the same speed signal. Mercury's SC1000 digital speedometers, for example, offer a speed-based mode for SmartTow (its launch and speed control for towing skiers and tubers) as well as an rpm-based one. If speed-based features act strangely or won't engage, a clogged pickup or a wrong speed source is worth checking before you blame the control system.

Before you buy a replacement gauge: Mercury notes that when the boat builder or dealer installs genuine Mercury gauges with a Mercury engine, the gauges share the engine's warranty. If your boat is still under warranty, ask the dealer first; a covered gauge or sensor costs you nothing, and an aftermarket swap could complicate a later claim.

With the boat out of the water (or in the water, by reaching the transom-mount sensor safely from the dock or swim platform with the engine off), spin the paddlewheel with a finger. It should turn freely with no grinding or wobble. Marine growth, weed and fishing line stop wheels or make them drag. Marine Industry News, covering Airmar's sensor design, notes that growth on a paddlewheel can make readings inaccurate or stop the wheel turning altogether.

Clean it with a soft brush and mild soap. Only paint it with an antifouling paint made for transducers, if at all; regular bottom paint can jam the wheel. If the wheel is cracked or missing blades, replace the wheel or the sensor.

Paddlewheels need calibrating against a known speed. On a Garmin chartplotter, for example, it's under the vessel or NMEA 2000 device settings ("Calibrate Water Speed"), and you follow the on-screen steps while running. Garmin says that if you get a message that the boat isn't fast enough or no speed is registered, safely speed up; if the message comes back, stop and check that the wheel isn't stuck and the cable connections are secure.

Calibrate on calm water with little current, running both directions, and compare with GPS.

If the wheel spins and still nothing shows, follow the sensor cable to its plug at the display or the network. Look for corroded or loose pins, a chafed cable and, on an NMEA 2000 network, a missing terminator or power problem (see the NMEA 2000 network guide).

If your pitot clogs every season or you mostly care about how fast you're going over the ground, a GPS speedometer is a common upgrade. Gauge makers such as Faria Beede sell GPS speedometers alongside pitot models, and most chartplotters and digital engine displays can show GPS speed.

Pros: nothing in the water to clog, foul or freeze, and accurate speed over the ground. Cons: it's speed over the ground, not through the water, so it reads differently in current; it needs a clear view of the sky; and it may lag slightly during quick changes. Anglers who troll and owners tuning trim often want both: GPS for true speed and a paddlewheel for speed through the water.

If you install a GPS speedometer, mount its antenna (if separate) with a clear view of the sky and away from radar and VHF antennas, and follow the gauge maker's wiring and setup instructions.

• Avoid running the pickup through sand, mud or weed when you can.
• Rinse the gearcase after beaching and check the hole.
• At haul-out or in storage, check the pickup and paddlewheel for insects and growth. Some owners cover small openings during storage; just remember to remove anything before launching.
• Blow the pitot line clear before winter.