Unreliable Bow Thruster Side-Power SP240 TCI's : A Captain's Troubleshooting Guide (No Fools, Just the Facts)

When a bow thruster starts responding "half the time," you need to take action right away. Here's a hands-on method for diagnosing and restoring a DC (12/24V) brushed-motor thruster the most common fault I encounter is... worn carbon brushes.

Quick Facts (read before opening the hold)

Typical symptoms of coals at the end of their life

• Relay clicks but motor does not start (or starts randomly).

• Back to life after a few attempts… then complete breakdown.

• Smell of heat, black dust around the engine, slight drop in

power before failure.

Security

• Cut off the general power supply to the thruster (circuit breaker, battery switch).

• Ventilate the hold, remove rings/watches, gloves + glasses.

• The thruster is a large consumer (hundreds of amps): avoid

any wild bypass.

Tools

• Multimeter (measurement under load), DC current clamp, set of

keys/bits, marker/labels to identify cables, brush

nylon, lint-free cloths, fine sandpaper (600–1000), cleaner

contact, dielectric grease, set of carbon brushes adapted to the model,

gaskets/O-rings if available.

Express decision tree

1. Nothing happens and no “click”

→ Look on the control side (fuse, button, joystick, circuit breaker, unpowered relay).

2. Clear “click” of the relay but no rotation

→ Measure the voltage at the motor terminals during the request:

• Voltage close to battery and no rotation → internal open circuit

(carbons, blocked brushes, very dirty collector).

• Voltage dropping → faulty resistance/connection upstream or

mechanically braked engine.

3. Low rotation / growl

→ Check for voltage drops on the cables, oxidized terminals, tired battery. If no problem → carbon brushes/collector.

Step-by-step procedure (quick diagnosis at the dock)

1. Visual & tactile check (1–2 min)

• Place your hand on the relay when actuated: it should click.

• Look at the cables + motor terminals: traces

warm-up/verdigris? Pod spinning = to be restarted.

2. Measurement under load (3–5 min)

• Multimeter directly on + and – of the motor.

• Operate a command (left/right).

• 12/24 V present without motor starter → suspect no. 1: carbon brushes/collector.

3. Relay crossover test (2–3 min)

• If the edge has an identical relay, try a temporary exchange

(without forcing the terminals).

• If the symptom does not change, forget the relay/control.

4. Inspection of heavy connections (5–10 min)

• Disconnect the thruster battery.

• Remove, clean, tighten all the power terminals of the engine and the

relay.

• Retest. If identical, open the motor.

Opening the engine & checking the carbon brushes

Purpose: To confirm the wear of the carbon brushes and the condition of the collector.

1. Clean removal (10–20 min)

• Cut and label all cables.

• Mark the orientation of the motor (mark with a felt pen).

• Check if there is an O-ring between the motor and the base (provide

replacement if marked).

2. Access to the coal holder (5–10 min)

• Remove the rear flange.

• Observe: residual length of the carbon brushes, condition of the springs, free

sliding in their guide.

3. Wear criteria

• Short coal, chipped edges, spring at the end, abundant dust.

• Collector (segmented copper): blackened/burnt, deep streaks, mica

(insulation between segments) flush or protruding.

Expected conclusion in your case: heavily worn carbon brushes → random contact → intermittent failures then total failure.

Refurbishment (the part that changes everything)

1. Replacing the carbon brushes

• Fit original carbon brushes (quality/grade suitable for the engine).

• Check that it slides freely in the carbon brush holder and that the

springs press correctly.

2. Cleaning the collector

• No coarse grit paper: use 600–1000 smoothly, rotation direction.

• Dust carefully (contact cleaner + gentle air).

• If the mica is prominent (rare in pleasure craft), lightly remove it with a fine scraper

(if you master it; otherwise abstain).

3. Reassembly

• Seals/O-rings: check, replace if marked.

• Tighten to the manufacturer's torque when available

(otherwise “firm + ¼” on small screws, without tearing).

• Reconnect the cables exactly as marked.

4. Soft bedding-in

• If possible, run empty for a few seconds (power supply

protected by a suitable fuse) to seat the carbon brushes.

• Avoid long requests on the first try under load.

5. Load testing

• At the dock, short left/right impulses.

• Monitor the battery voltage (do not lower it excessively).

• Check the intensity at the DC clamp: consistent and symmetrical values in

both ways.

Points of attention (classic traps)

• Contact returning after a tap on the crankcase: this is typical of a

coal at the end of the run → do not rely on it to leave the port.

• Overuse: many thrusters are in intermittent service (from

the order of 2–3 min cumulative before cooling). Follow the instructions.

• Forgotten ground cable: a weak ground is enough to cause the

power and burn the coals faster.

• Saline oxidation: protect terminals and relays with a thin layer of

dielectric grease after reassembly.

Preventive maintenance (which avoids hassle)

• Annual inspection: open the rear engine cover to check

length of coals and dust.

• Cleaning the collector if traces are visible, without removing too much material.

• Tightening of heavy connections at the start of each season.

• Stock on board: a spare set of carbon brushes, an identical relay, fuses

adapted.

Expected result

After replacing the carbon brushes + cleaning the collector + reconnecting the connections, a healthy propeller starts up again cleanly and symmetrically in both directions. This is exactly the behavior I aim for before authorizing a maneuver in a crosswind or in a tight space.

“Port departure” checklist (30 seconds)

• Left/right test 1 s each.

• Listen for the relay click and the increase in speed. (within the limit of the

possible, I am aware that on a 25 meter yacht, listening for the

bow thruster is more complex from the flybridge or the

wheelhouse!)

• Correct propellant battery voltage.

• No hot smell / abnormal noise.

Hope this tutorial provides you with some solutions!

#yacht #cannes #sidepower #breakdown