article and the two months of research that preceded it were prompted
by the numerous phone calls and e-mails that I receive from boat
buyers asking this question. They have had surveys performed on
boats they propose to buy on which the surveyor used a moisture
meter on the hull. The surveyors gave them the results of the meter
readings, but were apparently unwilling or unable to provide a comprehensive
explanation of those results; at best, these people told me, the
surveyor’s advice was vague. This, of course, left the boat
buyer with more questions than were answered.
that time I knew little more than what any other surveyor already
knows, mainly that meter readings can be difficult to interpret
at best, and unreliable at the worst. This is not the meter’s
fault, rather it is the result of a wide range of unknown factors
that can influence the performance of the meter, as well as the
manner in which the surveyor uses it. Of all the meters I have investigated,
virtually none of them provide any information for their use on
composite boat hulls. This prompted me to do some testing on how
meters actually perform with a wide range of materials and circumstances.
Some of the major questions to be answered were:
the density of the material being tested affect the meter readings?
There are a wide range of differing types and quality of plastic
resins being used. Moreover, it is well established that laminate
density and quality is highly variable owing to the fact that
it is a hand made product.
- Can a meter
be reliably used on a hull that has just been pulled from the
water? Will a recent rainstorm affect its readings?
- Does the
type exterior finish such as gel coat, paint or bottom paint affect
- Can a meter
be used to successfully detect water in a core?
and humidity affect the results?
- Will a meter
read through a laminate to detect water on the inside of a hull,
such as water in the bilge, and will these produce false readings?
are just a few of the questions that can be raised about the use
of moisture meters, and which I set about trying to answer. It must
first be pointed out the major difficulty of obtaining the right
kind of laminate samples to be tested since I can’t go around
cutting up boats to get what was needed. Therefore, the range of
proper samples significantly limited what I was trying to accomplish.
The range of samples was limited to what I could scrounge from the
dumpsters of boat builders, boat yards and pieces cut from wrecked
boats. While additional work was performed directly on boat hulls,
the difficulty with working directly on boats is that the thickness
and quality of the laminate remains unknown, whereas with a sample
the edges can be examined under magnification to determine its thickness,
density as well as porosity and the inevitable voids.
can be large or small, water-filled or not. Taken out of a hull
side below waterline, this void was filled with water.
whether cored or solid, contain millions of voids, due in part to
air entrapment as well as problems with out-gassing during cure,
plus just plain sloppy workmanship. Further, the skin-out layer
of chopped strand mat or chopper gun as it is sometimes called,
is well known to be extremely porous and will absorb large amounts
of water. This has been directly linked to the cause of blistering.
This is by far, the major cause of most water absorption by laminates.
Structural laminates, those containing the major structural fibers,
are far less prone to absorbing water.
leaves us the question of what is the location of the water that
our meters are reading? Is it in the gel coat, the CSM layer, the
structural laminates, the core or all of theses? Clearly, when working
on a boat hull, there is no way of knowing. What with the reintroduction
of cored boat bottoms, the most critical question that the surveyor
wants to answer is whether water has gotten into the core because
this can be fatal to boat hulls.
ultimate horror story for boats: Outer skin of hull is removed
to display water-soaked balsa core.
area is around a chain plate with a plywood doubler (center).
Both the plywood and balsa are equally deteriorated by acid
corrosion. Even though it is darkened at lower right, the
balsa is equally wasted at upper left.
on Recently Hauled Boats
received a phone call from a fellow who just had a boat surveyed
and was handed a report that said, "Entire bottom has elevated
moisture readings." The boat had been hauled only several hours
before the readings were taken. Two weeks later I was asked to double
check the readings by the owner. The readings I got, inside and
out, all were dry.
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. This was determined against taking readings
randomly on boat bottoms that had been sitting on shore for much
longer periods that produced aggregate readings that were much lower
than the just hauled group. Even so, the on shore boats still produced
large numbers which the meter said were wet. What was notable about
this later group was the fact that the wet readings tended to be
lower toward the keel whereas the just hauled group were more uniformly
wet all over. The conclusion here is inescapable: metering boats
just hauled from the water is highly unreliable and prone to produce
next question to be answered is one of how gel coats behave. Are
gel coats more prone to water absorption? Is there a difference
between gel coat below the water line versus above? Tests were performed
under varying climatic conditions. Moisture meters will not read
frozen water; place the meter on a piece of ice and the reading
is zero. Humidity levels were proven to have little effect except
on wood. The difference in meter readings on a laminate at 40% humidity
versus 90% was only 4 points on the meter scale.
decks of dozens of boats were tested immediately after rainfall.
Here, the wet gel coat was dried with a towel and measured immediately.
It was very much a surprise that readings were almost always near
zero, even on old boats with weathered gel coat. From this we conclude
that gel coat above the water line that is not subjected to water
pressure absorbs very little water. Thus, the use of a meter to
detect wet cores beneath the outer skin on things like decks and
stringers will produce reliable results.
salt residue affect meter readings? The answer to that is absolutely
yes. If there is salt residue on a dry deck or hull it will affect
readings by as much as 50% on an apparently dry surface. On the
other hand, wiping a surface with a clean cloth removes the residue
sufficiently that the reading is affected by no more than 10%.
might think that capacitance meters would read poorly on rough uneven
surfaces like the insides of hulls and non skid decks. In fact,
the meter reads remarkable well on moderately rough surfaces. Even
so, one should try to obtain the smoothest surface possible.
Deep Do Meters Read?
of course, is a critical question for surveyors which, unfortunately,
our testing produced no hard and fast answers. This is largely due
to the fact that the quality and consistency of the material being
tested cannot be known; we are testing an essentially unknown material.
However, our findings did reveal some important results. These included
the fact that the denser the material, the more capable the meter
was at reading through it. The meter would readily read through
a half inch of vacuum bagged laminate to water on the other side
of that laminate, whereas reading through a hand lay up laminate
produced results that were around 25% lower. A meter will not read
water through any kind of dry core. The principle here is that entrapped
air space blocks the instrument’s signal in the very same
way it does with ultra sound.
implication of this factor is that false readings can occur if the
meter is placed over a void, delamination or resin-starved laminate.
The material behind it can be soaking wet and the meter will not
pick it up on that spot. Fortunately, random false readings can
be avoided by using the meter in a scanning mode. That is, don’t
take "spot" readings but move the meter around a bit.
The meter works very well by dragging it across the surface over
a sizeable area. This will defeat random false readings unless there
is massive delamination, in which case that is detectable by other
of the problem in answering this question comes from the fact that
we cannot determine whether we are reading moisture behind the
laminate versus within the laminate. This is the fundamental
problem with trying to meter hulls for wet cores; the meter won’t
tell us what part is wet, the skin, the core or both. A factor that
affects this so greatly is with immersed hull areas, water pressure
is exerted against the hull skin, and the deep it is, the more pressure.
This probably explains why we find more wet readings in lower hull
sections than in higher sections.
moisture meter does not read through this 3/4" cored sample
from a hull bottom that is sitting in a tray of water.
Passing the Limitations
there are some techniques by which we can make the meter more useful
in our quest to evaluate hull cores. The first and most important
of these is to use the meter on the inside of the hull rather than
the outside. The inner skin of the laminate, of course, is not submerged,
and therefore should not be wet. Thus any wet reading obtained on
the inner skin – after assuring that the surface is clean
and dry – will be a positive indicator of a wet core. This
is bolstered by the fact that inner skins are usually about 30%
to 50% thinner than outer skins, so there is less opportunity for
the skin to affect the reading.
though less reliable, method is to search for areas on the hull
bottom that read dry. This idea is based on the premise that if
the hull skin is universally absorptive, the bottom should read
universally wet. If there are a number of areas that turn up relatively
dry, this becomes a fairly strong indication that certain areas
of the core are wet. This should be confirmed by doing at least
some limited testing from the interior.
readings can be produced by a number of conditions that include
excessive amounts of CSM on the outer surface, the use of absorptive
fairing materials, ablative bottom paints and paints containing
metals and oxides of metals, with copper topping the list of metals.
Be careful and skeptical of readings taken around keels, struts,
rudders and other areas that may contain fairing.
in the bilge or trapped in hull areas will definitely affect readings
taken on solid laminates. On the other hand, water cannot be detected
through a dry core of balsa or foam, no matter what its thickness
is. Watch out for things like box stringers that may be full of
water, bilge sumps and the like. Before metering the outside of
a hull, we should be familiar with what is on the inside and where.
Keep in mind that the meter will read right through " of solid
glass even though the glass itself is dry.
dry readings can be obtained after a core has completely rotted
or otherwise broken down. At this point there may be no water saturated
material that is in contact with the outer or inner skin and the
meter reads low when, in fact, the core is wasted away. This has
proved to be a problem with older boats that have very thick skins
that don’t deflect much. I’ve had numerous cases where
sounding did not reveal a wasted or delaminated core and neither
did the meter pick up any wetness. Old Bertrams and Hatteras are
good examples of the kind of boats where this happens. The key here
is that such badly wasted cores will usually leave other telltale
evidence such as brown water stains leaching out of the laminate
at various points, particularly at the low point of any structure
such as a deck or cabin top.
had a recent case with a 46 Bertram where the bridge deck was flexing
and brown water stains were found along the edges of the overhang,
clear proof of a bad core. But when I put the meter on that deck,
it read dry in most places! Reason: the core was completely gone
and all the water had drained out. Had I relied on the meter alone,
I would have been in real trouble.
idea that high moisture readings are a precursor to blistering needs
to be laid to rest. The correct way to state the matter is that
high moisture may result in blistering.
It’s been known for decades that boats built with good quality
material have little tendency to blister, so just because we get
high readings on a hull doesn’t mean that it will blister.
Low quality resin combined with excessive use of CSM is the primary
cause of blistering.
Nature of Cored Structures
the nature of cored structures is important to understanding how
and why they can become water saturated and how to detect these
conditions. This begins with the fact that any structure that is
curved can only employ a core material that is cut into small blocks,
which is what is allows for bending and conforming to the curve.
Solid sheets of core material are generally not used because of
the problem of assuring continuous contact and bonding to the skin
that has already been laid up. The gaps between the blocks are known
as "kerfs" while the loose weave fabric holding it together
is called a "scrim." The side of the coring that goes
up against the skins already laid into the mold is known as the
"blind side". This is because the laminators cannot see
through the core material to see how well it is making contact with
the outer skin.
one can see from this description, assuring uniform contact with
the core is a major problem except when vacuum bagged, a process
that assures good bonding. Thus, any hand lay-up cored structure
can be assumed to contain what are called "never bonds,"
those areas that do not bond to the skins. The strength of balsa
is that this material has extremely good cleavage strength; the
wood cells are shaped just right so as to suck up resin and create
a very strong bond, whereas foam makes for a much weaker bond due
to its round, cell shape.
wood is highly absorptive – as are all woods – on the
end grain axis, whereas perpendicular to the cell length or grain
direction it has been shown that balsa does not absorb water well.
Like any piece of wood, it sucks up water on the ends but not from
the face surfaces. That’s why it is referred to as "end
grain balsa"; the wood is cut so that end grain is facing out
and in contact with the resin. Were it not for the kerfs in the
balsa or foam, neither material would transmit water throughout.
Unless vacuum bagged, it is impossible to assure that the kerfs
are completely filled with resin, and it is these unfilled kerfs
that permits water migration through all cored structures.
some foams, and Klegecell is one of them, are very good at transmitting
water along its surface.
is what hydraulic erosion does to balsa which is reduced to
a slurry of fibers.
migration is aided, particularly on hull bottoms, by the forces
of water against the hull. This creates a pumping, hydraulic effect
that serves to distribute water throughout the core. If the void
areas in the core are completely water-filled, the hydraulic action
will become so powerful that it will reduce the core to a slurry
of fibers or mush. This, then, is a total catastrophic core failure
after which the skins are likely to fracture.
bagging goes a long way toward eliminating the water transmission
problem, even if water does enter the core at some point. This is
because the vacuum forces resin down into the kerfs thereby blocking
them as a means of, or channel for, water migration. Thus, water
entry into the core becomes very unlikely to migrate or spread.
Yes, it can result in deterioration of the core at the point of
entry, but the area is likely to remain small and unlikely to threaten
the entire hull as with a non vacuumed hull.
has been shown to break down through three different mechanisms,
hydraulic erosion, fungal degradation or rot, and a lesser known
and understood form of corrosion. As oxygen starvation can cause
severe corrosion of metals, it can also cause corrosion of wood.
The acid destruction of paper has long been a problem for historians
and anthropologists as natural acids cause paper to self destruct.
Paper, of course, is made from wood cellulose, both of which are
vulnerable to acidic destruction. Our investigations have shown
that balsa cores in hulls are most often degraded by means of closed
cell corrosion, caused by the lack of oxygen within the core, causing
the water to become acidic.
you didn't think that foams can absorb water, check out this
test sample that was placed in a tray of colored water for five
minutes. Notice how the water climbs and creeps along the surface.
caused the degradation of the balsa is readily assessed by its color.
Fungal attack will turn the wood dark brown or black whereas corrosion
causes little color change, or turning it only a slightly darker
shade than its original color. Hydraulic erosion turns the wood
into a slurry of loose fibers with almost no color change, unless
accompanied by fungi.
this same type of acidic corrosion affect foam cores? The anecdotal
evidence so far suggests that it can, although I have not seen any
recent examples. Many formulations of polyester are vulnerable to
corrosion, but most foams today are PVC based and so far have not
been shown to be particularly vulnerable. Ultimately, it’s
only going to be the test of time in application that will prove
the point, for the fact is that we do not yet know.
core materials have been improving in recent years, becoming stronger
with better cleavage strength. One product of noticeably improved
properties is ATC Corecell on which we have done some limited testing.
One of its virtues, besides improved strength, is its reduced tendency
to transmit water along its surface via the capillary effect. Testing
with Kledgecell and several other types revealed that when placed
in a tray of water, the water would creep along the surface of the
material over considerable lengths. This ability to migrate water
along its surface is troubling and appears to be caused by the size
and shape of the cells that promotes capillary effect much the same
as wood cells do. Corecell produced almost an almost zero creep
Prospects for Balsa
on its historical performance, balsa has many admirable properties
but it should not used unless in conjunction with vacuum bagging.
Labor costs today are so high that it cannot be used reliably with
normal hand lay up operations, plus there are too many people designing
boats today who are utterly ignorant of proper design and lay up
techniques. A fairly large number of yachts have been built of vacuum
bagged balsa core (mainly custom or semi custom boats) that it can
be reliably stated that vacuum bagging in conjunction with proper
design will preclude the severe problems that have occurred of late.
However, a new generation of foams may ultimately render balsa obsolete
and not the best choice of materials.
is a proprietary product that is made up of a thin sheet of fibrous
material that resembles the absorbent material we see at the bottom
of packaged fresh meat. It is very absorbent of resin and when wetted
out properly makes for a very strong laminate and has been in use
for about twenty years. The saturated dimensions are only a few
millimeters. We know of no known problems with this material when
properly utilized. Contrary to what we expected, our testing of
it indicates a notable lack of a tendency to absorb water even when
material has its widest application in smaller boat hulls and is
rarely found in larger boat hulls. It is not normally used in conjunction
with any other type of core material. Delamination occurring with
the use of this material is unknown to us. The material is sufficiently
porous that it will prevent a moisture meter from reading water
through a laminate containing the material, even when placed in
a tray of water. Neither does the material show any tendency to
wick water through it when the edges are exposed to water. Coremat
seems to be one of the unrecognized success stories of the boat
building materials industry.
or Spray Cores
little is known about putty or spray cores. In fact, when we query
surveyors about what they know of it we find that most aren’t
even aware of its existence. Putty cores are usually a polyester
paste that is sprayed into a mold after the outer skins have been
laid in, used mainly in small boats up to about thirty feet. The
thickness of the putty is usually about 3/16" or slightly less
and often comprises the major part of the hull thickness, and is
used mainly on the hull sides. In many instances we have found that
the putty comprises over 50% of the hull thickness. Of the half-dozen
samples we obtained of the material from broken up boats, we found
two problems: (1) the material is universally weak and, (2) it exhibited
adhesion problems. We have had at least five cases of massive delamination
in addition to what was found with hurricane damaged boats where
some hull sides became completely separated. However, the material
has not yet shown any tendency to water absorption, probably because
it is not used on bottoms.
meters can be used to reliably determine if hull cores are wet,
but not, as surveyors expect from the outside of the hull. The only
reliable method is from the inside of the hull. The surveyor will
need to make certain that the inner skin is dry and has not been
recently submerged, areas such as the bilge, and any other area
that may trap and hold water, such as outboard of stringers, even
if it presently appears dry. The presence of water stains will usually
tell the story.
meters can be used reliably anywhere from about a foot above the
water line, including on decks shortly after a rain, so long as
the surface is outwardly dry. Care should be taken to be sure that
there is not a residue of salt on the surface. If there is, use
a slightly damp cloth to clean it.
tests also show that the meter can even work on some non skid deck
surfaces such as the leveled diamond pattern, although clearly the
reliability of such readings is going to be somewhat reduced.
meter is most reliable in the hands of a person who experiments
with it and carefully considers what he’s doing. It is least
reliable in the hands of a person who thinks that it always produces
infallible results. The reason that it does not is because we subject
the instrument to limitless varying conditions for which there are
no absolute answers. In other words, it takes part science and part
reporting meter readings the surveyor should identify the type of
meter he is using. He should also avoid reporting numbers as a percent
because that is inaccurate. It is OK to say that readings were,
for example, 24 points on the Trammex scale, but do not express
it as a percent.
June 13, 2004