GPS makes a good trolling speedometer as long as you remember what it measures: the boat’s speed over the bottom, not the speed of the water past your lures. On a straight line in slack water those are nearly the same. Troll into or with a current, make a turn, or run gear deep where the water moves differently, and the lure’s speed can be well off the GPS number.
So use GPS for consistency, to repeat the speed that caught fish, and use the rod tip to confirm what the lure is actually doing.
Why use GPS for trolling speed at all? #
Most boat speedometers are poor at trolling speeds. A pitot gauge reads water pressure, which barely changes below planing speed, so it often sits near zero. A paddle wheel works at low speed but fouls with weed, and weedy water is often where you troll. GPS has nothing in the water, and with a good fix it resolves tenths of a knot at 2 knots. Our comparison of GPS, pitot tubes and paddle wheels goes into each sensor.
How does speed over ground differ from lure speed? #
Your lures respond to water flowing past them. GPS measures the boat against the bottom. Any current sits between the two.
| Trolling direction | GPS (SOG) | Current | Water speed past the lures |
|---|---|---|---|
| Into a 0.5-knot current | 2.0 kn | Against you | About 2.5 kn |
| With a 0.5-knot current | 2.0 kn | Behind you | About 1.5 kn |
| Across the current | 2.0 kn | From the side | About 2.1 kn, and the lines drift sideways |
In that example the lures run two-thirds faster going up-current than coming back, at the same GPS reading. A lure with a narrow working range can blow out one way and die the other. That’s why many anglers speed up a little heading down-current and ease off heading up, or troll across the current instead. To measure the current you’re dealing with, use the reciprocal-run method in speed over ground vs speed through water.
Turns change lure speed across the spread #
On a turn, lines on the outside travel a bigger circle, so they speed up and rise, while inside lines slow down and sink. Plenty of trollers use S-turns on purpose to trigger strikes. The GPS number is the boat’s speed at the phone, so it won’t show what the outside planer board is doing.
Deep water can move differently #
The current at depth isn’t always the same as at the surface, especially on big lakes with a thermocline or in tidal water. Some serious trollers run speed-and-temperature probes at the downrigger weight to measure water speed where the lures are. GPS can’t tell you that; it can only tell you the boat is holding the same speed over the bottom as last time.
What trolling speed should you start at? #
These are common starting ranges, not rules. The lure maker’s recommendation and what’s working today beat any table.
| Presentation | Common starting range |
|---|---|
| Spinner or crawler harness | About 0.8 to 1.2 mph (0.7 to 1.0 kn) |
| Crankbaits | About 1.5 to 3 mph (1.3 to 2.6 kn) |
| Trolling spoons for trout and salmon | About 2 to 3 mph (1.7 to 2.6 kn) |
| Offshore skirted lures | About 6 to 9 kn |
Lure charts are often printed in mph while charts and plotters use knots. In Speedometer GPS one tap on the unit button switches between km/h, mph and knots, so you can match whatever unit your notes use.
How do you read a phone at 2 knots? #
At 2 knots the boat moves about one metre per second, while a phone’s position is only good to a few metres. If an app worked out speed purely from position changes, the number would jitter badly. Most phone GPS receivers instead report a speed based on the Doppler shift of the satellite signals, which is much steadier at low speed. Speedometer GPS uses that speed whenever the phone provides it and averages the last five one-second readings.
What that means in practice:
- A throttle change takes a few seconds to show fully. Adjust in small steps and wait about 10 seconds before judging.
- A good fix matters more at low speed. Check the accuracy indicator shows EXCELLENT (within 5 m) or GOOD (within 10 m) before you trust the tenths.
- Watch the trend, not the last digit. A tenth or two of wobble is normal; a steady drift up or down is real.
For why every GPS readout trails a change slightly, see why GPS speed lags.
A routine for holding a steady trolling speed #
- Mount the phone in a waterproof case where you can see it from the tiller or helm and it has open sky. Set the units before you leave the ramp.
- Let the fix settle while you rig, so the indicator is green before your lines go in.
- Set the throttle, wait 10 to 15 seconds, then adjust in small steps.
- Make one pass with the current and one against it, noting the GPS speed each way. The average of the two is roughly your speed through the water.
- When you get a bite, note the GPS speed and your direction relative to the current. That’s the number to repeat, more than any chart value.
- Watch the rod tips. A crankbait or spoon has a steady rhythm at the right speed. If the rhythm stops or turns frantic, check the speed and the lure.
- Use the speed alarm as a ceiling. In Speedometer GPS you can set the alarm in whole knots. Set it just above your top trolling speed and the phone buzzes and beeps when you creep over, then again every 10 seconds until you’re back under. It only warns on the high side, and whole-knot steps make it a coarse guard rather than a precise target.
If you record each pass with Start and Save, the saved trip’s speed chart shows how steadily you held your speed, and the route shows where you were when it drifted.
Will a GPS app log trolling distance? #
Not accurately with Speedometer GPS, and it’s better to know that before you rely on it. To keep drift at anchor out of trips, the app ignores steps of less than 5 metres between one-second fixes. At trolling speed you move about 1 metre a second, so most of your movement falls under that filter and the trip distance reads far short, often close to zero. The speed readout, the speed chart and the average speed aren’t affected.
If you want distance for a trolling day, multiply the saved trip’s average speed by its moving time, or measure the exported GPX track in a mapping program (saved tracks are thinned to 500 points, which smooths tight turns). A chartplotter or fishfinder with a GPS track log is the better tool if distance matters to you. More on this in boat trip distance in nautical miles.
Frequently asked questions #
Is GPS speed accurate for trolling? #
For speed over ground, yes. With a good fix, phone GPS holds tenths of a knot at trolling speeds, and it doesn’t foul like a paddle wheel. It can’t tell you the water speed at the lure, which differs whenever there’s a current.
Should I troll by GPS speed or water speed? #
Lures respond to water speed, so in a current the water speed at the lure is what really matters. GPS is still the best tool most boats have for repeating a speed, as long as you allow for the current by trolling across it or adjusting up- and down-current.
How do currents affect trolling speed? #
A current adds to the water speed past your lures when you troll into it and subtracts when you troll with it. At the same GPS reading, a half-knot current makes a full knot of difference in lure speed between the two directions.
Why does my trolling speed jump around on GPS? #
At 2 knots, small errors and waves are large compared with your speed, so the reading wobbles by a tenth or two. A weak fix makes it worse. Mount the phone with open sky, let the app smooth the readings, and judge the trend over 10 seconds rather than one reading.