If you have ever had a spread swim perfectly going one direction and fall apart after you turn around, you have seen the difference between speed over ground and speed through water. Offshore, where currents like the Gulf Stream can run several knots, understanding this one concept will fix more lure problems than any tackle change.
What is speed over ground?
Speed over ground (SOG) is how fast your boat moves across the bottom, measured by GPS. It is the right number for navigation and fuel planning, but it says nothing about how much water is flowing past your lures.
What is speed through water?
Speed through water (STW) is how fast the boat moves relative to the water around it. Lures only respond to water flow, so STW is the number that controls lure action. A paddlewheel speed sensor on the hull measures it directly; many boats have one tied to their chartplotter or fish finder.
How current changes the picture
Imagine trolling in a 2-knot current:
- Going with the current: GPS shows 8 knots, but the water past your lures is moving only 6. Lures slow down, lose action, and may sink or wallow.
- Going against the current: GPS shows 6 knots, but lures feel 8. They can over-swim, skip, or blow out.
- Running across the current: the effect is smaller, but the spread may track to one side.
Same throttle, same lures, completely different presentation.
How to troll by speed through water
- Use a paddlewheel or STW display if your boat has one. Compare it to GPS to see current strength and direction.
- Use RPM as your anchor. RPM roughly tracks speed through water in calm conditions, so a known RPM setting is a better guide than SOG.
- Watch the lures. The spread is the final judge. If lures are dull, add throttle; if they are skipping or spinning, ease off.
- Re-check after every turn. When you change direction relative to the current, expect to adjust.
No paddlewheel? Estimate the current
Run the same RPM in opposite directions for a minute each and note the GPS speed both ways. Half the difference is roughly the current speed. If you see 9 knots one way and 6 the other at the same RPM, the current is about 1.5 knots and your true speed through water is about 7.5.
Why it matters for strikes
Gamefish key on lures that look alive and consistent. A spread that is under-speed down-current or over-speed up-current looks wrong, and fish will raise and fade without eating. Crews who adjust for current keep their lures in the strike window on every leg of a troll, which means more bites from the same water.
For target speeds by species, see our trolling lure speed chart, and learn to dial in your hull with how to find your boat's trolling sweet spot.
