Partial surveys (or limited scope surveys) usually relate to a particular point of concern. A frequent concern Solent Marine Surveys is asked to investigate is the moisture level to a hull laminate below the waterline, or to elsewhere such as a rudder, bulkhead, transom or within a cored deck.
To do this we take moisture readings using a range of moisture meters depending on the situation and conditions. Moisture meters are generally building/construction industry based or derived, so the relevant range of the readings for GRP are essential to know accurately for each meter. Because we specialise in GRP/FRP constructions this allows us to invest more in different types of moisture meters. All meters behave and measure slightly differently, and it is useful to you as either an owner or buyer to understand the differences between them.
The moisture readings must be interpreted not just in isolation but within context, and with knowledge and experience. Multiple factors other than high moisture might affect readings, including internal moisture such as bilge water, condensation, or flexible water tanks. Electrical cables under the deck or alloy plates laminated within a deck can also have an affect.
Sometimes, despite having been ashore for some time, moisture readings below the waterline can remain elevated. This happens particularly on older vessels built before the improvement of modern resins and can mean that any epoxy coatings have reached the end of life and may now be retaining moisture.
In some cases when moisture readings are high within a cored structure, whether a balsa cored deck or hull, or a plywood reinforced transom; we may advise that core samples are taken. This provides more confidence in the level of the moisture content and allows inspection of the condition of the structure in addition to the moisture readings alone. For most vessels however this is not necessary.
We will of course advise if any future course of action is required in light of any elevated moisture readings.
Taking moisture readings
When we are asked to take moisture readings to the hull below the waterline we use a template. The template divides the hull into 10 “stations” of approximately even spacing fore and aft. Sample areas are then randomly chosen from near the waterline, centreline or ‘mid bilge’ position on each side, plus the rudder.
We can use up to four different moisture meters, but generally there are three that we use regularly: A Sovereign MK II moisture meter for shallow readings, a Protimeter Aquant moisture meter for deeper readings and a Tramex Skipper Plus to help confirm any deeper readings.
In common with other moisture meters, none of the meters indicate the actual percentage of moisture within any GRP/FRP laminate. By choosing the appropriate meter for any given task the nature of any issue, such as moisture within the gelcoat or epoxy coatings, or moisture within the core, can be more accurately determined.
Sovereign MK II moisture meter
The Sovereign MK II moisture meter is a capacitance type meter using a scale of 0 to 25. It is used to indicate the moisture level near the surface of the laminate, up to around 2 to 3mm in depth. This meter has, in addition to the standard pad for readings on a flat surface, two spikes or probes which can be inserted into any fibrous material such as balsa core or plywood and is the preferred meter for taking moisture readings on any core samples.
The Sovereign Quantum (which we can also use) was intended as the replacement meter for the Sovereign MK II, combining the ability to read both on shallow or on deep settings. However, the “shallow” reading function is not particularly shallow, and the “deep” reading function not particularly deep, thus reducing it’s effectiveness for either function. So unless a specific request for comparable numbers with previous readings from this meter is required, we tend to favour specific meters for specific tasks, often defined by the depth of measurement required.
Protimeter Aquant moisture meter
The Protimeter Aquant moisture meter uses a comparative scale of 0 to 999 and is less susceptible to surface moisture than all other meters known, measuring moisture up to approximately 22mm deep in the laminate. This meter tends to be the “go to” meter, and is particularly useful on either wet decks (from dew or rain) or on a wet hull, perhaps during a hoist and hold survey. The ranges relevant for GRP vessels are: moisture measurements below 90 are considered ‘dry’, over 100 ‘elevated’ and up to approximately 150 becoming increasingly ‘high’.
Tramex Skipper Plus moisture meter
The Tramex Skipper Plus moisture meter can either be used on the 0 to 100 “comparative” scale, or on the 0 to 30 scale which can also be used to indicate percentage of moisture in hardwoods.