Month: July 2026

You head out sailing, look at the display and the speed reads 0.0 knots. Or it jumps around for no reason, or gets stuck on one value. It's one of the most common faults on Raymarine ST60 displays (and on the ST40), and it can almost always be fixed. Here's how to find the cause step by step.

First: is it only the speed that fails?

A Tridata display shows speed, depth and temperature. If the depth works normally and only the speed fails, the display is most likely fine and the problem may be on the speed log side: the paddlewheel, the transducer or its cable. If several readings fail at once, check the power supply and the instruments' SeaTalk connections first.

Bear in mind that in many installations the water temperature is measured by the speed log transducer itself. If speed and temperature fail at the same time, suspect the transducer or its cable.

Symptom and most likely cause

What you see	Most likely cause
Always reads 0	Paddlewheel jammed by fouling, cut cable or faulty transducer
Reads, but drops to 0 now and then	Dirty paddlewheel that slows down, or corroded connector
Jumps around or shows impossible values	Poor connection or interference on the cable
Always reads too high or too low	Display calibration set incorrectly

1. Dirty or jammed paddlewheel (the number one cause)

The speed log paddlewheel is always in the water and within a few weeks it fills up with algae and barnacles. A little dirt is enough to slow it down or stop it turning altogether.

How to check: remove the transducer from the through-hull fitting (with the blanking plug at hand) and spin the paddlewheel with your finger. It should turn smoothly without catching. If someone watches the display while you spin it, they should see the speed go up. We explain the whole process in How to clean your boat's speed log.

2. Damaged paddles or shaft

If the paddlewheel is clean but has a broken paddle or the shaft has play, it will turn badly or not at all. Paddlewheels and shafts are usually sold as spare parts, so you don't always need to replace the whole transducer.

3. Cable and connector

Check the cable from the transducer to the back of the display: make sure it isn't crushed, cut or pinched where it passes through a bulkhead. Also look at the display terminals: with the damp on board it's common to find green corrosion on the contacts. Clean them and tighten them again.

4. Calibration

If the speed works but always reads too high or too low, it isn't a fault: you need to adjust the display's calibration factor. Compare it with a GPS on a day with no current and correct the value following your ST60 manual. Note: with current, the speed log and the GPS never match, and that's normal. We explain why in GPS speed log or paddlewheel log: what each one measures.

5. Faulty transducer

If the paddlewheel turns well, the cable is in good condition and the display still registers nothing when you spin it by hand, the transducer's internal sensor has most likely failed. In that case it needs replacing.

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What if you forgot about the paddlewheel for good?

If you're tired of cleaning, dismantling and replacing parts, there's another option: let the speed come from the GPS. The sailoQ cube-micro generates the same pulse signal as the speed log and connects to the SPEED input of your ST60, so the display keeps working exactly as before. With nothing in the water to get dirty.

It's just three wires and you can keep the water temperature reading. It's all explained in How to install and calibrate your sailoQ on a Raymarine ST40 or ST60.

The speed log paddlewheel is the part of the boat that gets dirty fastest: it lives in the water, in the shade of the hull, and in summer it fills up with barnacles and growth in a matter of weeks. When that happens, the speed starts reading low, jumps around or sticks at zero. Here's how to clean it without any scares.

What you need

• The blanking plug for your through-hull fitting (it usually comes with the speed log and is kept nearby).
• An old toothbrush or a soft brush.
• Fresh water and, for hard barnacles, vinegar or a mild descaler.
• Silicone grease for the O-rings.
• A cloth or towel for the water that comes in while swapping the parts.

Step by step with the boat in the water

1. Get everything ready before you start. Hold the blanking plug in your hand, next to the through-hull fitting and ready to insert.
2. Release the lock on the transducer (depending on the model, a nut, a tab or a pin).
3. Pull out the transducer and insert the blanking plug straight away. A jet of water will come in for a couple of seconds: that's normal, so do it calmly but without pausing.
4. Clean the paddlewheel: remove the growth with the brush, soak the paddlewheel in vinegar if it's heavily encrusted and rinse with fresh water. Check that it spins freely and without play.
5. Check the O-rings. If they are cracked, replace them; if they're fine, give them a thin coat of silicone grease.
6. Refit the transducer in the correct orientation (it usually has a mark or an arrow pointing forward) and put the lock back on.

If it's your first time, it's best to have someone with you and pick a calm day. And never leave the through-hull hole open: either the transducer or the plug must be in place.

At the boatyard

With the boat out of the water it's much easier: you can clean the paddlewheel and the through-hull fitting thoroughly. To keep it clean for longer, use an antifouling paint made specifically for transducers, in a thin coat. Don't use the hull antifouling in thick coats: it can slow the paddlewheel down.

How often should you clean it?

It depends on the water and the time of year. In the Mediterranean in summer you may need to do it every few weeks; in winter it lasts longer. A popular trick: if the boat is going to stay moored for a while, leave the blanking plug in and fit the transducer only when you go sailing. That way the paddlewheel doesn't foul while the boat isn't moving.

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Or stop cleaning it

If you'd rather forget about the paddlewheel, the sailoQ cube-micro takes the speed from the GPS and sends it to your instruments as if it were your usual speed log. There's nothing in the water to clean. If your speed log is already playing up, you may be interested in My Raymarine ST60 shows no speed: causes and fixes.