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Troubleshooting
Bow thruster noise and vibration: normal, or time to stop
Every thruster is loud, and most of that noise is normal. This page helps you tell the normal roar from the sounds that mean something has changed. It also names the two that mean switch it off now.
Where this comes from. Three makers’ manuals, read 20 September 2026 or earlier for the brand reviews. Sleipner’s SE20 to SE40 installation guide has the most to say about noise. The other two are Lewmar’s Gen 2 fault table and Quick’s BTQ250 and BTQ300 manual. None of them publishes a decibel figure.
At the stern. A stern tunnel thruster is the same kind of unit as a bow one, and 61 of the 140 tunnel models this site lists are sold for either end. So it is covered by the tunnel sounds described below. The units made only for the stern are external, from Max Power, Sideshift, Sleipner Side-Power, Yacht Thruster and ezDrive, and the sections below on the other kinds cover the Sideshift and DockStar ones.
The short answer
A motor with carbon Small carbon blocks inside a motor that press on the spinning part and carry the current into it. They wear down, they shed dust, and they are the usual reason an old thruster motor stops., a gearbox and a propeller in a tube are loud by design. The tunnel carries the sound through the hull, so it is loudest in the forward cabin.
Sleipner’s guide says its propeller does not cavitate at working speed. It blames cavitation noise on how the tunnel was installed. If the noise is new, look for growth or debris at the tunnel mouth.
Water pushed through the tunnel turns it while the thruster is off. Sleipner’s fix is a recess or a deflector at the tunnel mouth.
Lewmar’s fault table says to check that it is not touching the tunnel wall.
The motor running while the bow does not move, and a knock that was not there yesterday.
Name the sound in four steps
- Check that the bow moves when the motor runs. If you hear the motor but no rush of water, and the bow stays put, the drive has failed. Stop using it and read the two sounds that mean stop.
- Listen for a knock or a rattle. A new knock means something is in the tube or the propeller is touching it. The section A rattle or a vibration has the maker’s checks.
- Listen for a crackle or hammering. That is cavitation. Whether it has always been there or is new tells you where to look. The cavitation section explains both.
- Note whether it happens with the thruster switched off. A hum or whir from the bow at speed is the propeller turning in the flow. The section The hum under way covers it.
None of these? A steady roar while the bow moves is the normal noise, and the next section explains it.
The normal roar
Three things make the noise. A DC motor with carbon brushes runs flat out. A gearbox turns the drive through a right angle. And the propeller sits in a tube that acts as a sounding board through the hull.
You only hear it for a few minutes at a time. On a Sleipner SE you get 2 to 3 minutes at most before the motor needs to cool. On a Quick it is about 5 minutes before the panel stops it. The run-time page has every maker’s limit.
The roar is loudest in the forward cabin, over the tunnel. It is the same roar the day the unit is fitted as ten years later.
A A motor with no carbon brushes: electronics switch the current instead. Nothing wears inside the motor, it runs cooler, and it costs more. or A thruster with a controller between the battery and the motor, so the joystick sets how hard it pushes rather than just on or off. The motor and the mounting are the same. Only the control differs. unit is quieter. Sleipner’s catalog sells its E-series partly on that. The Sleipner review has the claim in the maker’s words.
Cavitation, and where Sleipner says it comes from
Bubbles forming on a propeller blade when it is working too hard or running too near the surface. The propeller spins and the push disappears. is a harsher sound than the roar. It is a crackle or a hammering as water turns to bubbles at the blades and the bubbles collapse, and it comes with lost thrust.
Sleipner’s guide makes a claim about it that is worth quoting whole:
“A Sleipner thruster propeller does not produce cavitation at working speed. Therefore, any cavitation and cavitation noise in the tunnel will be caused during improper tunnel installation.”
The guide’s explanation is the tunnel mouth. A sharp-edged mouth blocks the outer part of the tunnel, so the propeller works in a smaller tube and gives less thrust. It also sends turbulent water onto the propeller, which costs more thrust and adds noise.
What a good mouth looks like. A mouth rounded into the hull by at least a tenth of the tunnel diameter lets the water in cleanly. Sleipner puts the thrust that rounding earns at 30 to 40 percent of the total.
Quick says the same in its own manual: rounded ends prevent turbulence and cavitation and keep the noise to a minimum.
A tunnel that is too short causes it too, in Sleipner’s account. The water reaches the propeller before it has straightened.
If it has always made the noise, the tunnel was fitted that way. The placement page and the install page show what a properly rounded mouth looks like.
If the noise is new, something is in the tunnel. Growth or debris at the mouth does what a sharp edge does, and the barnacles page covers it.
The hum under way
A hum or a whir from the bow at speed, with the thruster switched off, is the propeller turning in the flow. Sleipner’s guide says so directly:
“The thruster propeller can spin (passively) producing noise while sailing or cruising as water is forced through the tunnel.”
The guide goes on to warn that water forced through the tunnel at speed can damage the unit. That happens in a turn or as the bow bumps into waves.
Its fix is a recess or a deflector at the tunnel mouth, which the placement page describes. A boat that runs over 25 knots gets an extended deflector.
The guide gives a simple test. From ahead of the boat, you should not be able to see the back face of the tunnel.
The spin itself does no harm. It tells you that water is going through the tube instead of around it, and that forced water is what the deflector stops.
A rattle or a vibration
Lewmar’s fault table has a row for it: “Thruster noisy and vibrating?”
It lists two checks. The first:
“Check propeller is not touching the tunnel wall.”
The second applies to the 140 and 185:
“140-185 models - Check hub height is correct, see section 3.4.”
The gap between the blades and the tube wall is small on every make. That is why the manuals are so particular that the gear leg goes in on the tunnel’s centerline.
Any of these turns that small gap into a knock:
- a hub that has shifted
- a bent blade
- something wedged in the tube
A vibration that arrived with a knock is the same fault, felt through the hull. A vibration on its own, with the sound otherwise normal, is a propeller out of balance. That means a broken blade, or growth on one side of it.
External and retractable units
No tunnel means no tunnel roar and no cavitation off a square mouth. What is left is the propeller and the mounting, and each maker names what to check.
- DockStar has one troubleshooting row for vibration: take the propeller off and refit it turned through 180 degrees, and check the cap and blades for damage or debris. Its maintenance table adds that nicked or rough blade edges should be sanded smooth with fine sandpaper, and the prop nuts checked tight. The manual (v.260305) is linked from the DockStar review.
- Sideshift says nothing about sound and one thing about rattle: inspect the motor straps and hardware periodically for excessive play, and replace the strap kit if the motor is not secure. So when a knock starts on an external unit, check the straps first. Its manual also asks for a rinse and a check of the propellers for weed and line.
- A retractable’s propeller sits in a tube once the leg is down, so Lewmar’s two checks above apply. Two more sounds belong to the leg. Sleipner’s retractable manual warns that a unit set too shallow draws air from the surface, which cuts thrust and raises the noise. A leg that fights its hatch shows as an error code. Quick’s BTR panel tries three times against an obstruction, then flashes red once if the leg would not close and seven times if it would not open (BTR manual).
The two sounds that mean stop
The motor, and nothing else. You hear the roar without the rush of water, and the bow does not move. The drive has failed, at the pin, the coupling or the propeller. Lewmar’s instruction is in capitals: “DO NOT continue to run thruster.” The shear-pin page says what to do next. A thruster that will not stop at all has its own page.
A knock that was not there yesterday. Something is in the tube, or the propeller is touching it, and every run grinds it.
Switch off at the main switch and look in the tunnel before running it again. The troubleshooting page has the order to check things in.
A third sound comes from the switch that feeds the motor, not from the thruster. A An electrical switch worked by a small current that closes a large one. In a thruster it connects the battery to the motor when you press the joystick, and it is a common part to fail. Solenoid, relay and contactor are three names for this one part. chatters as the voltage sags under load. That is the sound of a battery bank or a cable that is too small.
The troubleshooting page starts there, and the wiring calculator reads the cable you have.
Sources: Sleipner’s SE20 to SE40 installation guide (cavitation, water deflection), Lewmar’s Gen 2 manual (fault finding) and Quick’s BTQ250 and BTQ300 manual.