A diesel drowns out almost everything else in the drivetrain. Swap it for an electric motor and suddenly you hear the saildrive gears, the propeller, the cooling pump and the controller — sounds that were always there, plus some that are new. Most are harmless. A few are early warnings of a bearing, seal or coupling that will fail expensively if you ignore it.
This guide gives you a repeatable method: describe the noise precisely, isolate which component makes it, then check that component. It applies to electric saildrives (purpose-built ones and conversions that put an electric motor on an existing Yanmar or Volvo leg) and, for most of it, to shaft drives too.
Safety before you start
- Isolate the battery before touching anything electrical. Switch off the main isolator, then wait for the controller's DC-link capacitors to discharge (check the manual — often a few minutes) and confirm 0 V with a meter across the controller's DC terminals.
- Never put hands near the propeller or coupling while the system is armed. An electric motor makes no sound before it moves, and a throttle bumped by accident starts it at full torque. Remove the key or disable the throttle when working near the drive.
- In-water tests at the dock: make sure the boat is securely moored, with spring lines, before running in gear.
- The saildrive seal is part of the hull. If you see water around the leg's base inside the boat, or milky oil, treat it as a hull integrity issue, not a noise issue.
Step 1: describe the noise precisely
"It makes a noise" isn't something you can diagnose. Record it on your phone, with a note of the conditions. A free spectrum analyzer app shows the noise's pitch as a number, which is the single most useful clue.
Answer these questions for each noise:
| Question | Why it matters |
|---|---|
| Does it change with RPM? | Tracks a rotating part (motor, gears, prop) |
| Does it change with load — in gear at the dock vs. lightly loaded? | Gear mesh, cavitation and controller whine all rise with torque |
| Is it there when the system is powered but the motor is stopped? | Controller, contactor, DC-DC converter, cooling pump or fan |
| Does it continue while coasting down after you cut the throttle? | Mechanical (bearing, gear, prop) vs. electromagnetic |
| Is it different in forward vs. reverse? | Gear tooth wear and loose prop nuts often show in one direction |
| Is it there under sail with the motor off? | Free-wheeling prop, leg bearings, regen setting |
| Is it once per revolution, or a steady tone? | A knock once per revolution usually means the prop or coupling |
Matching pitch to the part
If you have the frequency from a spectrum app, compare it to these. With the motor at a steady RPM:
- Shaft rate: RPM ÷ 60. At 1,500 rpm that's 25 Hz. Imbalance, a bent blade or misalignment shows here (or at twice this).
- Propeller blade rate: prop RPM ÷ 60 × number of blades. On a saildrive, prop RPM is motor RPM divided by the leg's reduction ratio, which the drive's manual gives.
- Gear mesh: teeth on the input pinion × motor RPM ÷ 60. Usually a few hundred Hz — the classic saildrive whine.
- Motor electrical frequency: pole pairs × RPM ÷ 60. Magnetic hum often appears at twice this.
- Controller switching frequency: fixed, and it doesn't change with RPM. It's the high, thin whine that gets louder with current. It's set in the controller: Kelly's KLS-N, for example, runs at 10 or 20 kHz, and its manual recommends 20 kHz where quiet running matters.
You don't need exact numbers. What matters is whether the pitch follows RPM, stays fixed, or follows current.
Step 2: isolate the source
Run these tests in order; each one rules out a group of parts.
Test A — powered, motor stopped
Arm the system but leave the throttle in neutral. Anything you hear now is not the drivetrain: controller cooling fan, coolant pump, DC-DC converter, the contactor closing (a single clunk at power-up is normal), or a charger running. A rapid chattering contactor is not normal — it means the coil voltage is dropping or the controller is repeatedly cutting out. Switch off and investigate the battery and pre-charge circuit; see the DC safety guide.
Test B — coast-down
Bring the motor to the RPM where the noise is clearest, then pull the throttle back to neutral (with regen braking off, if your controller has it).
- Noise stops instantly: it's electromagnetic — controller switching, motor windings, or rough commutation.
- Noise fades with the RPM: it's mechanical — bearings, gears, coupling, propeller.
Test C — listen at each component
Use a mechanic's stethoscope (or the handle of a long screwdriver pressed to your ear) on the motor bearing housings at each end, the motor-to-leg adapter, and the saildrive leg housing. The loudest point is usually the source. Keep fingers and clothing well away from the coupling.
Test D — forward vs. reverse
Gear wear usually shows on one side of the teeth, so it's louder in one direction. A clunk only at the change of direction points to backlash or something loose (Step 3).
Test E — out of the water
When the boat is hauled out, spin the propeller by hand with the battery isolated. Feel for roughness, tight spots and free play. If it's practical, remove the prop and run the motor briefly at low RPM: a noise that disappears without the prop was the prop, its hub or its interaction with the water.
Step 3: what each noise usually means
| What you hear | Most likely cause | What to check |
|---|---|---|
| High, thin whine, fixed pitch, louder with current, stops instantly on coast-down | Controller switching (PWM) | Normal if unchanged since commissioning. Some controllers let you raise the switching frequency above hearing range, at some cost in heat |
| Whine that rises with RPM, louder under load, continues in coast-down | Saildrive gear mesh | Oil level and condition; compare with your baseline recording |
| Growl or rumble rising with RPM, continues in coast-down | Bearing — in the motor or the leg | Stethoscope to locate; spin by hand for roughness; check motor end-bearing temperature |
| Washboard-like hum from the motor that develops over months | Bearing damage from stray current through the bearing (fluting) | Motor bearings; whether the motor frame and shaft are properly grounded per the maker's instructions |
| Clunk on engaging or reversing | Backlash, loose prop nut, worn spline, worn coupling or damper, loose motor mount | Prop nut torque and locking; play at the prop with the motor locked; mount bolts; controller torque ramp |
| Knock once per revolution | Bent or fouled blade, line around the prop, damaged folding-prop hinge, coupling misalignment | Haul out or dive; inspect the prop and hub; check alignment of a conversion adapter |
| Vibration at one RPM band only | Resonance in the mounts, adapter or a hull panel | Mount condition and torque; whether it moves with RPM changes; consider softer or stiffer mounts |
| Rough, buzzing running at low RPM with high torque | Rotor position sensor (Hall, encoder or resolver) fault or misalignment, or a loose phase connection | Controller fault log; phase currents balanced?; torque on phase cable lugs — and look for discoloration |
| Crackling, gravel-like sound at high power or in reverse | Cavitation | Prop size and pitch for the motor; reduce power; check for damage on the blade surfaces |
| Steady hum or "singing" from the prop at certain speeds, motor on or off | Vortex shedding from the blade trailing edges | A prop shop can add an anti-singing edge |
| Slapping or knocking at low RPM from a folding prop | Blades not fully opening at the low RPM of an electric drive | Prop maker's advice for the minimum opening RPM; a feathering prop may suit better |
| Gurgling or rising whine from the cooling system | Air in the coolant loop, failing pump | Bleed the loop; check pump current and flow; check motor and controller temperatures |
Electric motors make noise diagnosis easier in one way: the controller logs what the motor is doing. If your controller or GX device logs RPM, phase current and temperature, check those alongside the noise. A bearing or fouling problem shows up as more current for the same RPM than your baseline. A position sensor problem often shows up as fault codes or unbalanced phase currents. The Cerbo GX motor drive guide covers getting that data, and the CAN log analyzer can decode it.
Saildrive leg checks
The leg is where most mechanical noise ends up, and it is easy to check.
- Gear oil level and condition. Check it with the boat level. Clean oil is clear amber. Milky or grey oil means water ingress past the lower shaft seals — stop using the drive and haul out. Fine metallic glitter on the dipstick or drain plug magnet means gear or bearing wear.
- Shaft play. With the battery isolated, try to rock the prop shaft up-down and side to side. Rotational free play from gear backlash is normal; radial play is not.
- Diaphragm seal and hull opening. Look for water, weeping or cracked rubber where the leg passes through the hull. Manufacturers specify a replacement interval for this seal — Volvo Penta's is seven years, as Practical Boat Owner reports it — so check when yours was last done.
- Anodes and corrosion. A consumed anode leaves the aluminium leg unprotected; pitting around the leg's lower housing can eventually reach the seals. Electric installs need particular care: stray DC current from a fault in the boat's wiring corrodes legs fast. See the DC safety guide.
- Conversion adapters. If an electric motor is bolted onto an existing diesel leg, the adapter plate and coupling must hold the motor shaft exactly in line with the leg's input shaft. Misalignment shows up as a once-per-revolution knock or vibration and wears the input seal and bearing. Check the adapter bolts and coupling after the first few hours of running and at each service.
Motor checks
- Bearings: with the motor uncoupled and isolated, spin the shaft by hand. It should turn smoothly; you may feel the magnets' gentle cogging on a permanent-magnet motor, which is normal and even. Grittiness or tight spots aren't.
- Mounts: check bolts and the rubber elements for cracks, collapse or oil damage.
- Phase and DC cable connections: with everything isolated and discharged, check lug torque against the maker's spec. A loose phase lug causes rough running, heat and eventually a burnt terminal. Look for discolored insulation.
- Temperature: after a run, an infrared thermometer on each bearing housing should show similar temperatures. One end much hotter than the other points to that bearing.
When to stop and get help
Stop running the drive and investigate before your next trip if you find:
- milky oil or water around the saildrive's hull opening
- a contactor chattering, or the controller cutting out repeatedly
- a burning smell, or discolored cable insulation
- a sudden new knock or a big rise in vibration
- motor or controller temperatures climbing higher than usual for the same power
Make your next diagnosis easy: record a baseline
The most useful thing you can do is record what "healthy" sounds like. Soon after commissioning, or after a service, at the dock in gear:
- Record 30 seconds of sound at three steady RPMs (low, cruise, high), with the phone in the same place each time.
- Note DC current, phase current (if your display shows it) and motor temperature at each RPM.
- Note the gear oil color and level.
Repeat this every season. Changes in pitch, loudness or current for the same RPM show you problems long before you'd hear them yourself.
These suppliers cover the most common power ranges (6–25 kW) for sailboat auxiliary propulsion. Match the motor to your kW bucket and drive type (shaft or saildrive).
Saildrive and shaft-drive motors purpose-built for sailing yacht conversions. Available from 6 kW to 25 kW with integrated motor controllers.
Integrated electric propulsion systems with app connectivity. Offers Deep Blue motors from 5 kW upward for serious auxiliary use.
Lightweight pod and outboard motors ideal for smaller auxiliary builds and dinghy conversions.
US-based electric marine propulsion systems focused on sailboat shaft-drive conversions.
Links go to manufacturer websites for reference.
Sources
Checked on 7 November 2026:
- Kelly Controls — KLS-N user manual V2.9: PWM frequency options (10 or 20 kHz) and the quiet-running recommendation
- Practical Boat Owner — "Volvo saildrive seal replacement: How often do you need to do it?", Stu Davies, 28 February 2022: the seven-year interval in Volvo's manual
The frequency formulas (shaft rate, blade rate, gear mesh, electrical frequency) follow directly from the rotation speeds and need no outside source.
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