Units & Measurements Boat Speed

Boat speed on a fish finder is displayed in miles per hour, knots, or kilometers per hour, depending on your unit's settings. Speed data comes from one of two sources: GPS, which measures speed over ground based on position changes, or a paddlewheel speed sensor in the transducer, which measures speed through the water. Some units display both simultaneously for comparison.

For trolling, maintaining the correct speed is critical for lure presentation. Different lure types, species targets, and techniques have optimal speed ranges. Walleye anglers often troll between 1.0 and 2.0 mph. Salmon trollers run 2.0 to 3.5 mph. Bass crankbait trolling typically works at 2.5 to 4.0 mph. Even small speed variations — a quarter mph difference — can significantly affect whether fish strike or ignore your presentation.

GPS speed and paddlewheel speed diverge when current is present. If you are trolling upstream at 2.0 mph through the water in a 1.0 mph current, your GPS shows 1.0 mph over ground while your paddlewheel reads 2.0 mph through the water. For lure presentation, the paddlewheel reading is more relevant because it reflects the actual water flow across your lure. In still water, both readings are identical.

Speed data is also useful for monitoring your boat's performance. Sudden speed drops can indicate a fouled propeller, hull growth, or engine issues. Consistent logging of speed at set RPM over time helps you track boat performance changes that may warrant maintenance.