Spend
time at a boat yard and you will be sure to hear the tap,
tap, tap of a marine surveyor. “What are you looking
for?” is a question I hear with regularity. At a boat
show recently I heard a broker complain about a surveyor
who “showed up with a small hammer and tapped everywhere.”
Here is why I do it.
Tapping,
or percussion testing, is an extremely useful non-destructive
inspection technique. Tapping is not limited to boat yards;
it is used throughout the world and in many different fields.
It is included in the inspection process of stealth technology
planes and space shuttles. It provides information that
is not available visually and usually causes no damage,
except for perhaps a small localized loss of bottom paint.
Its
primary use in a boat yard is determining the condition
of fiberglass laminates. It is also useful with wood and
other construction materials. In fiberglass or composite
boats, tapping allows detection of delamination. Fiberglass
boats and their component parts, are made of several layers
of lamination. Gelcoat, coring and fiberglass material with
resin are among the most common types of laminates. A separation
or “disbond” can occur at any level. Percussion
testing allows detection of separation between the gelcoat
and the first fiberglass laminate (skin coat). It can also
detect delamination within any of the layers of the fiberglass,
including delamination caused by osmosis (blisters) and
disbanding of the layers adjacent to the core.
The
vast majority of boats are manufactured with a cored deck.
Hulls, stringers and other parts may also be cored. Coring
is usually balsa, foam or plywood, sandwiched between layers
of fiberglass. The fiberglass encasement on either side
of the deck coring can delaminate. Thus, percussion testing
both the top and bottom of the deck, where accessible, is
a useful inspection technique. Fiberglass bulkheads are
usually tabbed (fiberglass taped) to the hull sides. Tapping
the fiberglass tabbing can determine if the tabbing is properly
attached. Occasionally the tabbing does not bond (usually
to the plywood bulkheads and I have seen vessels in which
the fiberglass tabbing attachments (secondary bonds) were
never well made. On a few occasions, this has left virtually
all of the bulkheads “adrift” and grossly weakened
the hull structure. Properly made tabbing can become detached
due to an external force from impact, heavy weather operation
or even over tensioning a sailboat’s rigging (usually
a backstay).
In
vessels that use organic coring material, and there are
many, the most type is balsa or plywood. This wooden coring
can deteriorate (fungus deterioration or dry/wet rot) and
the deteriorated coring can often be detected by tapping.
A properly made deck cored with balsa sounds solid and sharp
when tapped; if the balsa is deteriorated the same deck
sounds soft or dull. Deteriorated coring can also be detected
by tapping stringers and transoms, particularly on smaller
boats.
Tapping
wood boats or wooden components also allow detection of
deterioration. Bulkheads are often constructed of plywood
which is covered with fiberglass, upholstery or carpet;
tapping can reveal secrets that even the owner was unaware
of. Occasionally when tapping deteriorated wood, this inspection
technique becomes destructive.
Many of the things detected by tapping are minimal including
“voids”. Voids are air bubbles trapped just
below the gelcoat or skin coat. Tapping a void is distinguishable
by the higher pitch. The area around the void will have
a lower report than the thin skinned void. The voids are
cosmetic and a nuisance occasionally but are rarely ever
significant structurally.
The
tapping can be done with virtually anything. Depending on
the material and thickness of the component which is being
tapped variances can be found using a finger nail, edge
of a coin, butt of a screw driver or of course the preferred
tool, a hammer.
Hammers
can be made of bronze, steel, plastic or any hard material.
As long as an audible report is made when the device taps
the component being inspected. The difference in the sound
made is what the surveyor is listening for. I have found
metallic hammers give the most discernible reports, but
I primarily use plastic hammers above the waterline. If
an audible difference is determined I will also tap in the
area a second time using a small metal hammer.
Percussion
testing should be non-destructive. Occasionally voids will
open, paint will flake and small hammer marks are accidentally
made. I attempt to leave no indication of my survey and
surveyors should be aware of any damage being caused by
their percussion testing. One fateful survey began with
a hammer becoming lodged in a rotten wooden boat. It was
the first tap of that area of the boat and the last tap
of that day with a hammer. I kept tapping, but used my finger.
While
percussion testing is an excellent use of a non-destructive
inspection technique it is but one of many inspection techniques,
which are used in the process determining the condition
of a vessel. Along with percussion testing many other methods
are used in assessing the condition of a laminate. Flexing
under foot, visual discoloration or cracks, indentations,
crackling sounds when stepped on, concave or convex areas,
discoloration are other signs of potential problems. Moisture
meters can be used to help assess the condition of laminates
and, when necessary, destructive techniques can be employed.
