Hull Inspection and Materials Flashcards

When performing a non-destructive test on a steel hull to determine plate thickness, which method is most commonly used by marine surveyors?

Answer: Ultrasonic Thickness Measurement (UTM)

Ultrasonic Thickness Measurement (UTM) is the standard non-destructive method for gauging the thickness of a vessel's hull plating. It works by sending a high-frequency sound wave through the material and measuring the time it takes for the echo to return, which directly corresponds to the plate's thickness. While RT, MPI, and DPI are also NDT methods used in marine surveys, they are for detecting flaws like cracks and surface defects, not for measuring material thickness.

A surveyor is inspecting a 15-year-old fiberglass (GRP) vessel that has been kept in warm seawater. They observe numerous small blisters below the waterline that, when punctured, release an acidic, vinegar-like fluid. What is the most likely cause of this phenomenon?

Answer: Osmotic Blistering

The scenario described is a classic presentation of osmotic blistering. This occurs when water molecules slowly permeate the semi-permeable gelcoat and react with uncured resins or other soluble materials within the fiberglass laminate, creating an acidic solution that forms blisters under pressure. This process is accelerated in warm water conditions. Galvanic corrosion affects metals, and delamination from impact would typically present as a larger, more localized area of damage.

During a survey of an aluminum-hulled vessel, which of the following findings would be of GREATEST concern regarding potential accelerated corrosion?

Answer: A bronze through-hull fitting installed without proper isolation

A bronze through-hull fitting in direct contact with an aluminum hull creates a galvanic cell, where the aluminum (the less noble metal) becomes the anode and corrodes at an accelerated rate in the presence of an electrolyte like seawater. Proper installation requires isolating dissimilar metals. Minor scratches, uniform oxidation, and a functioning anode system are less critical concerns; in fact, the anode system is designed to prevent this type of corrosion.

A surveyor is using the percussion (hammer) testing method on a cored fiberglass deck. A sharp, solid sound is heard over most of the area, but a dull thud is noted near a stanchion base. What does this dull sound most likely indicate?

Answer: Delamination or water-saturated core material

Percussion or 'tap' testing is a common method to identify voids or delamination in composite structures. A solid, sharp sound indicates a well-bonded laminate, while a dull, hollow, or 'dead' sound suggests a void, delamination between layers, or a water-saturated (and potentially rotted) core material. The area around deck fittings like stanchions is a common location for water intrusion into the core.

Which of the following is a primary reason for using cored composite construction in a fiberglass hull instead of solid fiberglass?

Answer: To achieve a higher stiffness-to-weight ratio

Cored composite construction, which sandwiches a lightweight core material (like balsa or foam) between layers of fiberglass, is used to create panels that are significantly stiffer and stronger for their weight compared to a solid laminate of the same weight. This allows for a lighter, more efficient, and often faster vessel. While it can sometimes be cost-effective, its primary engineering purpose is to improve the stiffness-to-weight ratio, not simply to cut costs or increase weight. Cored hulls are susceptible to water intrusion if not properly sealed.

When inspecting a wooden vessel, a surveyor finds soft, white, fibrous-looking wood around several keel bolts. This condition is most indicative of:

Answer: Freshwater rot

The condition described, often called 'angle hair,' is a classic sign of freshwater rot. It occurs when fresh water (from leaks via decks or fittings) is trapped against the wood, allowing rot-producing fungi to thrive. This deteriorates the cellulose in the wood, leaving behind the softer lignin and creating the characteristic soft, fibrous appearance. It is a serious structural concern, particularly around critical fasteners like keel bolts.