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Using Moisture Meters to understand hull condition

Osmosis damage and moisture damage in the hull can be a huge and costly nightmare for yacht owners. That's why we strongly recommend that our Pre-Purchase Survey clients undertake Hull Moisture Readings (especially for yachts that are more than around 10 years old).

Moisture Meter on a yacht's hull

How to prepare for Hull Moisture Readings

We recommend that the yacht is hauled out of the water and remains on-the-hard for two weeks prior to the hull survey.

This is because we want to ensure the readings of our instrument are as accurate as possible. According to research by Surveyor David Pascoe, "Random testing of boats just hauled from the water (with bottoms that are dry to appearance) demonstrates conclusively that residual moisture within paint layers, the gel coat or CSM layers will produce erroneous readings most of the time." In other words, if your boat is wet because it was just recently floating in the water - or if there was intense rainfall - the moisture meter readings are probably telling you about how wet the boat is, not how much moisture is in the hull.

How should Moisture Meters be used? And how do they work?

During the Pre-Purchase Survey, the surveyor does a visual inspection of the hull and then proceeds to take moisture readings at dozens and dozens of points throughout the hull exterior, including the rudder.

The process of using a moisture meter may appear simple. The surveyor just sticks it against the hull and reads the measurements, right? Wrong.

Any good surveyor will use the moisture meter as one of many factors he or she takes into account when considering the condition of the hull, including visual clues, the use of sound and resonance, and a deep knowledge of how different hull materials differ, among many others.

It's important to remember that Moisture Meters are just a part of the picture and to really understand hull condition and the likelihood of repairs in the future, you need to combine Moisture Meters with a lot of experience and understanding since they do not tell the whole story.

Most Moisture Meters are of the capacitance type. These meters typically have two electrodes or sensors, one that transmits an AC signal and another that receives it. Because water has a much higher dielectric constant than air or fiberglass, the difference in strength between the transmitted and received signal can be measured and displayed on the meter.

Being able to spot signs of osmosis early is key to avoiding big headaches and costly repairs.

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