High Moisture Readings on a Yacht Rudder – But Was It Actually Wet?
During a recent survey of a Catalina 445, I came across an interesting example of why moisture meter readings should never be interpreted in isolation.
The yacht had previously been inspected ashore and the underwater hull had generally returned reasonable moisture readings. The rudder, however, was a different story.
The Initial Finding
Using a Sovereign moisture meter, I mapped the rudder blade systematically from top to bottom.
The upper section of the rudder returned relatively low readings, generally around 10–12. As I worked down the blade, the readings gradually increased through the high teens and low twenties, reaching approximately 25–27 over much of the lower section.
At first glance, this could easily have been interpreted as moisture having entered the rudder and migrated downwards.
That would certainly be a plausible scenario on many older GRP rudders.
However, a moisture meter is only one inspection tool.
Percussion Sounding Told a Different Story
The complete rudder blade was percussion sounded.
The response remained firm and consistent throughout, with no obvious hollow or dull areas suggesting delamination or separation of the internal structure.
There was also no visible swelling, distortion, cracking, movement around the rudder stock, rusty staining or evidence of water weeping from the blade.
Structurally, the rudder appeared sound.
This immediately raised the question: were the elevated meter readings really showing moisture inside the rudder?
Mapping the Rudder
To investigate further, the rudder was divided into a simple grid using chalk.
Moisture readings were recorded at regular intervals across the blade.
This produced a clear visual pattern showing the readings progressively increasing towards the lower section.
The process was useful because it allowed the readings to be compared with:
the rudder construction;
the position of the stainless-steel rudder stock and internal reinforcement;
changes in the surface coating;
and the percussion sounding results.
One thing became particularly noticeable.
The yacht had an epoxy-based Coppercoat antifouling system fitted to both the hull and rudder.
The Coppercoat on the upper part of the rudder was noticeably lighter in colour, becoming progressively darker towards the lower half of the blade.
Interestingly, the darker coating broadly corresponded with the areas giving the highest meter readings.
Coppercoat and Moisture Meters
Coppercoat contains a very high concentration of metallic copper powder suspended within an epoxy resin system.
A moisture meter does not directly measure water.
Instead, it reacts to changes in the electrical or dielectric properties of the material beneath the sensor.
This means that metal, conductive coatings, variations in laminate thickness, internal reinforcement and certain coating systems can all influence the displayed result.
The rudder also contained internal stainless-steel reinforcement, providing another possible source of interference.
At this point, there were several possible explanations for the readings:
Genuine moisture inside the rudder.
Influence from the epoxy/copper coating.
Influence from the internal stainless-steel structure.
Variations in laminate and internal filler thickness.
A combination of several of these factors.
Rather than assuming the worst, further investigation was carried out.
Removing the Coppercoat
Small localised test areas of Coppercoat were removed.
The difference was significant.
Once the Coppercoat was removed, the moisture meter readings reduced noticeably.
This provided very useful evidence that the coating itself was influencing the meter.
However, there was still one question remaining.
Was the internal core actually wet?
The Diagnostic Pilot Hole
With permission, a small controlled pilot hole was drilled through the outer laminate in an area that had previously shown one of the higher moisture readings.
This was the most useful part of the investigation.
The internal filler/core material was completely dry.
There was no free water.
There was no damp material.
There was no staining or evidence suggesting that moisture had accumulated inside the rudder at the test location.
The apparently “high moisture” reading had therefore not represented a wet core.
The Conclusion
The final evidence was quite clear.
The rudder had:
elevated moisture meter readings over the lower blade;
excellent percussion sounding throughout;
no visible evidence of structural deterioration;
significantly lower meter readings after removing the Coppercoat;
and completely dry internal material when physically checked.
The elevated meter readings were therefore substantially influenced by the coating system and possibly by the internal construction of the rudder.
This does not mean that every rudder displaying high readings should automatically be considered dry.
It means the opposite.
Every finding has to be investigated properly.
Why Moisture Meter Readings Need Interpretation
Moisture meters are extremely useful surveying instruments, but they are not water detectors.
A number displayed on a meter should never be used on its own to condemn a rudder, deck, hull or other GRP structure.
Good surveying requires several methods to be used together:
visual inspection;
moisture mapping;
percussion sounding;
knowledge of the vessel's construction;
consideration of coatings and internal structures;
and, where justified, limited physical investigation.
In this particular case, simply looking at the meter readings could have resulted in a perfectly serviceable rudder being incorrectly described as saturated.
The additional investigation told a very different story.
A Useful Surveying Lesson
This was a particularly good reminder that survey instruments provide information — they do not make the diagnosis.
The surveyor still has to understand why the instrument is giving that information.
On this Catalina 445, the combination of Coppercoat, epoxy resin, internal stainless-steel reinforcement and varying rudder construction produced readings that initially looked concerning.
A few careful checks proved otherwise.
Sometimes the most important part of a survey is not finding a defect.
It is proving that something which initially appears to be a defect actually isn't.
Yacht Surveys Greece Independent yacht surveys throughout the Ionian Islands and western Greece. Pre-purchase, insurance, condition and valuation surveys.
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