Yacht generator light loading and wet stacking

A yacht generator that spends long periods carrying only a small electrical load may develop wet stacking. Thick black liquid can then appear around exhaust joints, sometimes looking like used lubricating oil.

That appearance does not by itself prove engine damage. This guide explains how loading affects exhaust conditions, which load figures apply to specific generator guidance, what crew can record safely, and when controlled load testing or a sizing review is needed.

Why prolonged light loading causes wet stacking

Caterpillar states that extended operation below 30% load can cause wet stacking on the generator sets covered by its guidance. Kohler gives the underlying mechanism for the marine models covered by TP-7044: normal loads of 30% or more keep the diesel exhaust hot enough to prevent water and fuel vapour from condensing.

When a set remains lightly loaded, that condensation can mix with carbon, unburnt fuel or lubricating oil. The resulting black fluid may escape through joints at the exhaust manifold, turbocharger or exhaust pipes. Because it can resemble used lubricating oil, its source should be identified rather than assumed from appearance alone.

Prolonged light loading has consequences beyond external staining. Kohler identifies these possible outcomes:

• Cylinder-wall glazing. This is one of the conditions associated with prolonged light loading in Kohler's operation guidance.
• Fuel-on-water. This is another possible result identified for the covered marine generator models.
• Crankcase-oil dilution. A change in oil condition, particularly alongside persistent exhaust liquid, requires engineering attention.
• Wet stacking. Carbon mixed with unburnt fuel or lubricating oil may become visible at exhaust joints.

A wet-looking exhaust is not necessarily evidence of an engine defect. Caterpillar nevertheless warns that long-term underloading can accelerate component wear, so the operating profile should not be ignored.

Reading the signs without jumping to conclusions

The useful evidence is the combination of load history, exhaust condition, oil observations and electrical behaviour. A single visual sign does not establish the whole diagnosis.

Location matters when recording the symptom. Note whether the liquid is emerging at the manifold, turbocharger or pipe joints, and provide clear photographs where these areas are accessible. Do not touch an exhaust component while the set is running because the surfaces may be hot.

Soot should also be recorded, but neither soot nor black fluid should be treated as automatic proof of damage. The load log gives the engineer the operating context needed to assess the symptom.

Apply the load figure for the installed generator

There is no single load percentage in the source guidance that applies to every marine generator. The correct figure comes from the operating manual for the installed model.

For the marine models covered by Kohler TP-7044, the minimum load profile is 30%, while the ideal profile is 70% or more. Caterpillar's wet-stacking guidance identifies extended operation below 30% as the risk condition for the generator sets it covers.

Northern Lights uses a different sizing rule of thumb: continuous load should be at least 50% of generator capacity. It also notes that two smaller sets can sometimes be advantageous. This is a sizing consideration, not permission to apply the same figure to every existing installation.

The practical approach is to compare the yacht's recorded operating load with the model-specific manual. Crew should avoid creating a target by averaging or combining figures from different manufacturers. Guidance for Kohler generators and Cummins Onan generators must remain tied to the models and documents it covers.

A sizing review must consider simultaneous loads and motor-starting loads before equipment is changed. If the yacht's routine demand leaves the generator below its documented operating profile for extended periods, the system needs an engineering review rather than an improvised loading routine.

Exercise runs and controlled load tests

Running a generator with no load does not prove that it can deliver its rated capacity. Cummins specifies that a post-service proof test should use a load-test panel, applying progressively greater load until full rated output is reached.

That controlled test is different from a routine crew run. It checks the set as load rises rather than assuming that starting and idling demonstrate satisfactory operation. If voltage or frequency cannot be held, the generator trips, or an exhaust leak appears, call an engineer.

For an infrequently used generator covered by one Cummins schedule, the instruction is to exercise it for at least two hours each month at approximately half rated power. Cummins says this run drives off moisture, relubricates the engine, replaces stale fuel and removes oxides from electrical contacts. Apply that interval and load only where it matches the installed generator's documentation.

Cummins also recommends running an Onan marine set for at least 30 minutes before a trip under the loads expected during that trip, without overloading it. This gives crew a documented opportunity to record alarms, voltage and frequency before departure. It does not replace a progressive full-output proof test after service.

What crew can check safely

Crew checks should concentrate on observation and accurate records. They should not involve touching the exhaust or changing equipment to force a load target.

• Record the normal load. Log generator kW or percentage load during routine operation. Flag extended periods below the minimum profile stated for the installed model.
• Inspect accessible exhaust joints. With the set running, look for black liquid or soot around accessible joints. Do not touch the manifold, turbocharger, pipes or other hot exhaust components.
• Carry out the documented pre-trip run. For an Onan set where the Cummins guidance applies, run the expected onboard loads for at least 30 minutes without overloading the generator.
• Record electrical behaviour. During that pre-trip check, note alarms, voltage and frequency. The record is particularly useful if output changes as onboard loads are added.
• Describe the evidence clearly. Record where fluid appears, whether it persists after correct loading, and whether the oil level or oil condition changes. Photographs can be submitted through the generator service booking form.

Do not treat a clean no-load run as proof that the set will carry the yacht's demand. Capacity is demonstrated by applying load progressively, as specified for the post-service proof test.

When to call an engineer

Arrange an engineering assessment when the evidence moves beyond a temporary visual symptom or when controlled loading exposes unstable operation.

• Black liquid continues after correct loading. Persistent discharge needs assessment, particularly if the source is an exhaust joint.
• Oil level rises or oil condition changes. Kohler identifies crankcase-oil dilution as a possible outcome of prolonged light loading.
• Voltage or frequency will not remain stable. Call if the set cannot hold either value during a controlled load test.
• The generator trips under the test. Do not assume that successful no-load running proves available capacity.
• An exhaust leak is present. Keep clear of hot components and arrange inspection.
• The operating profile needs to change. Generator sizing or load-profile changes require calculation of simultaneous and motor-starting loads before equipment is altered.

Blueward Marine in Palma de Mallorca can assess wet-stacking symptoms, operating loads and controlled generator tests through its marine generator service. Book at /book with fault details and photographs, or call the 24/7 emergency line on +34 971 251 374.