Month: June 2026

A paddlewheel log and GPS measure different speeds. Knowing the difference helps you choose and understand what you see on the display.

The paddlewheel log measures speed through the water

A traditional speed log measures how fast the water flows past the hull. This is known as speed through the water (STW). If there's a current, the paddlewheel doesn't "see" it.

GPS measures speed over ground

GPS measures how far you move relative to dry land: this is speed over ground (SOG). It includes the effect of the current, which is what really brings you closer to or further from your destination.

When do you notice the difference with a paddlewheel log?

• With no current, both are practically the same.
• With a favourable current, GPS reads higher than the paddlewheel; against it, it reads lower.
• In tidal areas or strong currents the difference can be significant; in most areas it's barely noticeable.

An example: if you're sailing at 6 knots through the water with 1 knot of favourable current, the paddlewheel log will read 6 and the GPS speed log 7. With the same current against you, the GPS will read 5.

By comparing the two readings you can tell which way the current is flowing. We explain it step by step in Current with you or against you: how to tell with the speed log and GPS.

Which one suits you?

For most leisure sailors, speed over ground is the most useful: it tells you when you'll arrive. Speed through the water matters mainly when racing, to trim sails and compare performance, or in areas with very strong currents.

Advantages of a GPS speed log

• Always available: there's no paddlewheel to foul or jam.
• Zero maintenance and no holes in the hull for the speed log.
• It tells you how fast you're really progressing towards your destination, useful for working out arrival times.

sailoQ turns that GPS speed into the same signal a speed log would send, so your existing instruments display it without changing anything.

Want to forget about your speed log? sailoQ cube-micro, available in the shop.

Installing a sailoQ is simple: it's just three wires. In this guide we explain how to connect it to a Raymarine ST40 or ST60 display and how to check that the speed is shown correctly.

1. Connecting to the Raymarine ST60 or ST40

• 12 V power supply. Respect the positive and negative marked on the board. We recommend fitting a 1 A fuse and, if you like, a switch so you can turn it off.
• Speed-log output. It goes to the display's SPEED input, on the green wire's pin.
• Want to keep the water temperature reading? Leave the rest of your speed log's wires connected and replace only the green wire, which carries the speed, with the sailoQ's output.

2. Where to mount it

• Indoors, protected from moisture.
• Away from iron structures and other electrical equipment (radios, magnetic compasses…), which can interfere with the GPS.
• It can go anywhere along the cable, although next to the display is usually the most convenient.

3. First start-up: the calibration test

Every time it powers up, the sailoQ ramps the speed up to 3.5 knots, holds it for 30 seconds and returns to zero. This test lets you check that the display reads the pulses correctly:

• If your display shows 3.5 knots, everything is fine.
• If it shows a different value, it's worth adjusting the display's calibration.

After the test, the GPS needs to get a position fix. It's usually almost instant, but the first time it may take a few minutes.

4. Adjusting the calibration on the Raymarine ST60

1. Go sailing with another GPS as a reference and compare both speeds.
2. Using your Raymarine ST60 (or ST40) manual, adjust the speed-log calibration factor until they match. If you don't have it, you'll find it on the Raymarine website.

A normal detail: the sailoQ updates the speed every second, faster than many handheld GPS units. That's why it may look a bit more "lively" than your other GPS, just as a speed log would.

Download the manual

You'll find all the details in the sailoQ manual.

Want to forget about your speed log? sailoQ cube-micro, available in the shop.

If you have a paddlewheel log, this will sound familiar: you head out sailing and the speed reads zero, jumps around or gets stuck. It's not bad luck: when the speed log fails, it's almost always for the same reasons, and it happens to almost all of them.

Why the speed log fails

• Fouling and marine growth. The paddlewheel is permanently in contact with the water and, within a few weeks, it fills up with algae, barnacles and small organisms that slow it down or jam it. This is known as biofouling.
• Dirt. Floating debris, fibres or bags can jam it while you're under way.
• Wear. It's a mechanical part that never stops turning: shafts and bearings wear out over time.
• Antifouling doesn't last long. Even if you clean and paint it when the boat is hauled out, it soon gets dirty again.

Signs that your speed log is failing

• The speed reads zero even though the boat is moving.
• It reads lower than the GPS on a day with no current.
• It jumps around or is slow to react when you speed up or slow down.
• It works when you leave port and stops working after a while.

If you recognise any of these, check the paddlewheel before suspecting an electronic fault. We explain how in How to clean your boat's speed log and, if you have a Raymarine display, in My Raymarine ST60 shows no speed.

What you can do

• Clean it often. It works, but you have to pull the paddlewheel out of the hull (with its blanking plug at hand) and do it again and again.
• Replace it with another speed log. You're back to square one with the same problem.
• Forget about the paddlewheel. With a GPS system like sailoQ, the speed comes from the satellites: there's nothing in the water to get dirty.

If you're interested in the difference between measuring with a paddlewheel and with GPS, we explain it in GPS speed log or paddlewheel log: what each one measures.

Want to forget about your speed log? sailoQ cube-micro, available in the shop.