Moisture Meter Readings: What's Safe For Boats?

Moisture meters are an important tool for boat owners and buyers to assess the condition of the hull and deck of a vessel. They are also used by surveyors to determine the moisture content of a boat, which can indicate potential problems such as rot or delamination. While moisture meters can provide valuable information, it is important to interpret their readings with caution as there are many factors that can affect the accuracy of the results.

Moisture meters come in three types: capacitance, probe, and free-field effect. Most meters are of the capacitance type, which have two electrodes that send and receive an AC signal to measure the moisture content. Other types of meters use resistance or free-field effect to measure moisture. It is important to note that moisture meters are typically designed for wood and may not provide accurate readings for fiberglass boats.

When using a moisture meter, it is crucial to establish a baseline reading and compare it to other areas of the boat. A reading of 0-15% on the CT-33 meter is considered dry for fiberglass, while readings above 35-40% indicate the need for attention. Additionally, factors such as temperature, bottom paint, glycols, and core types can influence the accuracy of the meter.

To ensure accurate and reliable results, it is recommended to work with experienced professionals who have a thorough understanding of moisture meters and can interpret the readings correctly. Moisture meter data should be used in conjunction with other non-destructive test methods, such as percussive testing and visual inspection, to make trusted and informed decisions about the condition of a boat.

Moisture meters don't actually read moisture content, they read capacitance

A moisture meter is a simple device that can be a helpful tool when diagnosing and repairing a blistered hull. However, it is important to understand that moisture meters do not directly measure moisture content. Instead, they measure capacitance, which is the capacity of a material to store energy. This reading is then converted into a moisture reading.

Moisture meters come in two main types: pin and pinless. Pin moisture meters, also known as resistance meters, work by pushing two or more pins directly into the material being measured. A direct current travels from one pin to the other, and the resistance to the current is measured and correlated to the moisture content. Pinless moisture meters, on the other hand, use electromagnetic field technology (EMF) to measure moisture content without piercing the material. They emit an electrical wave through a sensor pressed against the surface, creating an electromagnetic field that behaves differently depending on the amount of moisture present.

While moisture meters can be useful, it is important to recognise their limitations. For example, they require a flat measuring surface and sufficient material beneath the sensor to obtain an accurate reading. Additionally, the orientation of the meter head, the presence of condensation or metal, and the thickness of the material being measured can all impact the accuracy of the reading. Furthermore, moisture meters cannot detect frozen water or provide an accurate prediction of an osmotic problem when there are no visible blisters.

When using a moisture meter to assess the condition of a boat, it is crucial to understand the different materials used in its construction, as readings can vary depending on the material. For instance, a fully saturated fiberglass hull will have no more than 2.5% to 3.5% moisture, while timber typically sounds an alarm at 20-25% moisture, which is often the fibre saturation point. Additionally, the presence of bottom paint, gelcoat, or core material can impact the accuracy of the reading. Therefore, it is recommended to scrape away the bottom paint and place the meter directly on clean gelcoat. Taking readings in multiple areas, both above and below the waterline, can help identify areas of concern and track changes in moisture levels over time.

In conclusion, while moisture meters can be a valuable tool, it is important to understand that they do not directly measure moisture content. Their readings should be interpreted with caution, taking into account various factors that can influence their accuracy. When used correctly and in conjunction with other diagnostic tools, moisture meters can provide valuable insights into the condition of a boat.

Moisture readings on a marine survey are often vague and difficult to interpret

Firstly, different types of moisture meters can produce varying readings. For example, the Tramex Skipper Plus measures moisture content on a scale of 0-30%, while other meters may use a different scale or display readings as a bar graph. It is important to understand the specific meter being used and how to interpret its readings.

Additionally, the experience and skill of the surveyor can play a significant role in the accuracy and interpretation of moisture readings. According to an article by Practical Sailor, approximately 40% of surveyors do not fully understand moisture meter operation. This lack of understanding can lead to inaccurate readings and misinterpretations. It is crucial to ensure that the surveyor is knowledgeable and experienced in using moisture meters.

Furthermore, the condition and history of the boat can also impact the accuracy of moisture readings. For instance, boats that have recently been hauled out of the water or exposed to rain may produce false readings due to residual moisture in the gel coat or paint layers. Similarly, salt residue on the hull or deck can affect the readings, requiring a thorough cleaning before an accurate measurement can be obtained. Moisture meters can also be influenced by the presence of metal fittings, bottom paint, and other factors that can interfere with the signal.

To overcome these challenges, it is recommended to calibrate the moisture meter and familiarize oneself with its specific instructions. Taking multiple readings at different locations on the boat and comparing them can also help identify inconsistencies or false positives. It is also essential to consider the context of the readings, such as the boat's age, construction, and maintenance history.

In conclusion, while moisture readings on a marine survey can be vague and difficult to interpret, a knowledgeable surveyor who understands the limitations and proper use of the moisture meter can provide valuable insights into the condition of a boat.

Moisture meters are most reliable when used by someone who has experimented with them and carefully considers what they're doing

• Moisture meters were originally designed for other industries such as timber, roofing, and building trades, and may not be specifically calibrated for fiberglass boat hulls.
• The readings from a moisture meter need to be interpreted correctly. On fiberglass, a fully saturated hull will have no more than 2.5% to 3.5% moisture, while moisture meters designed for timber often sound an alarm at 20-25% moisture. Therefore, the percentage of moisture indicated by the meter may not directly correlate to the actual amount of moisture in the laminate.
• The thickness of the laminate can impact the accuracy of the moisture meter. Thicker laminates may lead to lower readings, even if there is a significant amount of moisture present.
• The presence of certain materials, such as metals or carbon fiber, can interfere with moisture meter readings. It is important to be aware of the materials used in the boat's construction before interpreting the results.
• Environmental factors, such as temperature, humidity, and recent rainfall, can also affect the accuracy of moisture meter readings. For example, moisture meters should not be used on laminate that is below freezing or on surfaces with salt residue.
• Moisture meters should be calibrated properly and regularly to ensure accurate readings.
• It is important to understand the limitations of moisture meters and combine their readings with other assessment methods, such as visual inspections, laboratory tests, and the expertise of experienced marine surveyors.
• The user should be familiar with the boat's construction, materials, and potential sources of moisture intrusion to interpret the moisture meter readings accurately.
• Using the moisture meter in a scanning mode by moving it across a sizeable area can help avoid random false readings.
• Taking moisture readings from the inside of the hull, where the inner skin is not submerged and should be dry, can provide more reliable indications of a wet core.

Moisture readings should be higher towards the keel of a boat

This is also why it is recommended to take moisture readings from the inside of the hull, as the inner skin is not submerged and should not be wet. This means that any wet readings obtained on the inner skin are a positive indicator of a wet core.

It is also worth noting that moisture readings taken on boats that have just been hauled out of the water are highly unreliable and prone to false readings. This is because residual moisture within paint layers, the gel coat or CSM layers can produce erroneous readings.

Moisture meters can be used to check for water in the bilge

• Moisture meters are typically designed for other industries such as timber, roofing, and building trades. While they can be used on boats, the readings need to be extrapolated and interpreted differently.
• The accuracy of moisture meters depends on factors such as the density of the material being tested, recent rainfall or immersion, type of exterior finish, temperature, and humidity.
• Moisture meters should be used on the inside of the hull rather than the outside for more reliable results. The inner skin of the laminate is usually dry and not submerged, so any wet readings indicate a wet core.
• It is important to clean the surface and ensure it is dry before taking measurements with a moisture meter. Residual moisture, salt residue, or condensation can affect the readings.
• Moisture meters have varying depths of penetration. Thicker laminates may result in lower readings, even if the same amount of water is present.
• False readings can occur due to voids, delamination, or resin-starved laminate. Scanning a larger area can help avoid random false readings.
• Moisture meters cannot distinguish between water and metal. Metal fittings, anchor chains, water hoses, or fuel tanks in contact with the hull can lead to false positive readings.
• Different types of moisture meters and brands may yield different readings on the same boat. It is important to understand the operating principles and limitations of the specific moisture meter being used.
• A moisture meter reading should not be the sole factor in assessing the condition of a boat. Other factors such as blistering, swelling, cracking, and the integrity of the FRP bond should also be considered.
• A moisture meter reading of 0-15% on fiberglass is considered dry. Readings above 35-40% indicate the need for attention, as water has likely ingressed.

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