Hull Inspection 101: What You Need to Know

What is a Hull Inspection

The ‘hull’ refers to the watertight exterior of the ship. Either open ceilinged or enclosed with a deck, the hull is an integral part of a ship structure, since it is the first line of defense against leaks. With ship hulls covering such a large surface area and being in direct contact with the water, hull inspections are vital and should be conducted regularly. Due to the variety of related components, situations, and potential defects, there are two general categories of inspections.

GVI Inspections

In broad categories, inspections can be broken into General Visual Inspections (GVI), and Close Visual Inspections (CVI). A GVI is a simple visual inspection of overall condition with a report following the inspection to report on the status of the structure.

GVIs are conducted from a moderate distance and provide a general overview of the hull status. Typically completed at approximately an arm’s length away from the component, operators are searching broadly for any defects or issues. While a mirror may be required to enhance visual access to exposed areas, a GVI is conducted under normally available lighting conditions. Daylight, flashlight or ROV lights are usually the only light used for GVI and sometimes nearby structures or objects may have to be moved out of the way to gain access.

A GVI looks to detect a number of things including:

1. Physical Defects
2. Biofouling (Accumulation of organics)
3. Cracks
4. Discoloration

CVI Inspections

CVIs are crucial for hull maintenance. A detailed and close up visual examination, a CVI is used to identify areas requiring more in depth inspections. More specifically a CVI is an extensive examination of a specific area, section or component to detect and potentially identify damage, failure or irregularity.

CVIs typically make use of additional equipment to enhance the quality of the inspection. Normally natural lighting is supplemented with a direct source of targeted light such as auxiliary lights or flashlight. In particularly murky or turbid waters, imaging sonar tools can also be utilized.

Furthermore, CVI often entails surface cleaning to gain clear access to all components of the area being inspected. Specific tools such as probes, gauges and measuring tools are also often used to acquire further insight into the hull or structure. Finally, more extreme access procedures may be required to ensure close inspection to all aspects of the component being inspected.

The qualifications applicable to CVI can be both PCN or CSWIP. A CVI is suitable for a number of structures and components including:

1. Hull
2. Anchors
3. Propellers
4. Sea Chests

Read more about the difference between GVIs and CVIs here.

What ROVs Are Used for Modern Hull Inspections?

Modern hull inspection ROV systems replace diver-led surveys for many vessel classes, particularly during UWILD operations and routine hull condition assessments.

Deep Trekker’s current inspection lineup includes:

• SPECTRA – High-power inspection ROV with stereo vision for accurate 3D mapping and hull surface modelling, and structured hull scanning across large vessel surfaces. A seven-thruster configuration supports angled station holding and high-force lateral movement against current.
  
• REVOLUTION – Six-thruster system with rotating camera head and sonar integration for complex hull geometry, under-keel navigation, and low-visibility inspection environments.
• PIVOT – Compact inspection ROV with tooling tilt-platform for rapid angle adjustment during general hull surveys and port-side inspections.
• PHOTON – micro-ROV designed for confined hull zones such as sea chests, thruster tunnels, and intake structures where access clearance is limited.

Each platform supports GVI and CVI workflows depending on inspection requirements.

Inspection Tactics

Dry Docking

Hull maintenance is crucial to maximize the expected life cycle out of a vessel. However, external inspections can be difficult since the majority of the ship will be submerged. Dry-docking is a traditional method of conducting comprehensive hull inspections. This method is extremely time consuming and expensive to perform.

The process involves:

• Heavy lifting infrastructure
• Dock personnel coordination
• Significant downtime
• Risks include structural instability during blocking and high operational cost.

To dry-dock a vessel, the entire ship must be removed from the water and placed onto blocks. For large ships, this can be a complex and dangerous procedure, requiring a crew, heavy equipment, and a qualified dockmaster. In the event of a failing dry-dock block, the entire ship can capsize, resulting in catastrophic damage and danger to the crew. Due to the high cost, time usage, and potential complications, many are now opting for Underwater Inspections in lieu of Dry Docking.

UWILD

UWILD also known as, Underwater Inspection in Lieu of Dry-Docking, is a safer and more efficient alternative to the more conventional dry-docking method of marine vessel inspection. UWILD inspections entail inspecting the submerged part of the vessel while still in the water. Occasional dry-docking will always be necessary, however underwater inspections can help reduce the frequency, resulting in long-term time and cost savings.

To conduct a UWILD, the owner of the vessel must first ensure that they qualify. If there are outstanding recommendations for repairs to propellers, rudders, stern frames, sea valves or other underwater structures, dry-docking may be the only suitable option (ABS 2015). Once qualified, UWILDs have been performed by ABS certified divers traditionally. The divers will navigate under the hull of a ship to conduct visual inspections as needed. To further elevate the efficiency of UWILDs over dry-dockings, new equipment such as ROVs are rapidly becoming more popular to eliminate the need for dive teams.

Class societies such as ABS define eligibility requirements, including:

• No unresolved underwater structural defects
• Compliance with inspection intervals

ROVs are increasingly used in UWILD programs to replace diver-only inspections due to:

• Reduced operational risk
• Consistent video documentation
• Repeatable inspection paths

To learn more about UWILD best practices, check out our full article here.

How Do Ship Owners Select the Right Hull Inspection ROV?

Selection depends on vessel class, inspection depth, hydrodynamic conditions, and required inspection outputs (video-only vs mapped data).

Typical mapping:

• Large commercial vessels: REVOLUTION or SPECTRA for full-bottom coverage, higher thrust output, and integration with sonar and navigation systems.
• Routine port inspections: PIVOT for rapid deployment, visual inspections, and light tooling applications.
• Confined geometry areas: PHOTON for restricted access zones such as sea chests, intake grates, and thruster tunnels.

Key selection factors:

• Thruster configuration and thrust-to-weight ratio for maintaining position and heading under current load, especially during lateral hull tracking
• Camera resolution and optics (e.g., 4K sensors, low-light performance) for identifying fine defects such as coating breakdown, pitting, and weld anomalies
• Payload compatibility for integrating imaging sonar, ultrasonic thickness gauges, CP probes, and navigation systems
• Stability and control architecture for maintaining consistent standoff distance during inspection passes
• Data requirements, including whether inspections require simple visual documentation or spatially referenced outputs for reporting and repeatability

How Do Dead Reckoning and ROV GPS Improve Hull Position Tracking?

ROV positioning systems enable consistent and repeatable hull survey execution, particularly for large vessels where manual tracking is insufficient.

• ROV GPS establishes a surface reference position prior to submersion, allowing operators to define starting points and vessel-relative positioning.
• Dead reckoning calculates subsea position using heading, velocity, and time-based movement data when GPS signals are unavailable underwater.

These systems support:

• Repeatable scan paths by enabling operators to follow consistent inspection routes along hull plating
• Geo-referenced defect logging, associating corrosion, coating loss, or damage with approximate spatial coordinates
• Improved inspection coverage control, reducing missed areas during long hull passes
• Cross-survey comparison, allowing condition tracking across multiple inspection intervals

In practice, this reduces operator dependency and improves consistency across inspection teams and survey cycles.

What Is 3D Sonar SLAM in Hull Inspection Surveys?

3D Sonar SLAM (Simultaneous Localization and Mapping) is a spatial mapping method that uses sonar data to generate real-time 3D models while simultaneously tracking ROV position.

Unlike optical systems, it operates effectively in zero-visibility or high-turbidity environments.

Applications include:

• Hull geometry reconstruction where visibility is obstructed by sediment, biofouling, or low light
• Detection of deformation, protrusions, and coating discontinuities based on surface variation
• Spatial mapping of inspection areas, enabling structured coverage and revisit capability
• Pre-dry dock planning, providing measurable hull condition data prior to vessel removal

SPECTRA is the only platform in the Deep Trekker lineup capable of supporting 3D Sonar SLAM, due to its stereo camera, processing capability, and stability required for consistent data acquisition.

This capability is used in higher-spec inspection programs where mapped outputs, not just video, are required for analysis and reporting.

How Hull Inspections are Performed

Components to Inspect

Knowing the complete list of components to inspect on each vessel is integral for a thorough inspection. Ships can have a great variety of different designs, so pre-identifying the number and location of each key component can help reduce the time needed for inspections. The main vessel components to inspect are:

• General hull
• Rudders
• Propellers
• Thrusters
• Shell Plating
• Sea Chests

Potential Concerns

Qualified inspectors should know exactly what imperfections to be looking for during a hull inspection. This allows for potential issues to be caught quickly and ensures that no glaring issue is left unaddressed. The main concerns to watch for are:

• Discolouration
• Cracks or Leaks
• Corrosion
• Fouling from Aquatic Life
• Dents or Grooves
• Lodged Debris
• Any other irregularities

Evaluating Components

Inspection services/programs, surveyors, and the Coast Guard all follow different grading procedures. All inspections should evaluate the general condition of the accessible parts of the hull structure, condition of coatings and corrosion prevention, and overall cleanliness of spaces and tanks within the hull.

The respective inspector will then assign whether the ship is in need of repair, requires further monitoring, or in good condition. For Coast Guard inspections, security inspections for contraband may also be conducted. This extra step monitors for potentially dangerous items, illegal goods, or unregistered crew members.

Why are Hull Inspections Important?

1. Ensure Structural Integrity

Evaluating ship hulls for structural integrity is essential for crew safety as well as logistic efficiency. Finding and addressing cracks, leaks, dents, corrosion, discoloration, or other irregularities ensures that all vessels on the water are in safe operating condition. According to the 2021 Safety & Shipping Review, there were 49 large class ships lost and 2,703 shipping incidents that year alone. With physical damage being the leading cause of incidents (40%), a large portion of these dangerous and costly situations may have been avoidable with thorough inspections/maintenance.

2. Improve Port Security

Waterways are becoming an increasingly popular method of contraband transportation due to the low cost and lighter security in comparison to air. Seizures of cocaine aboard commercial ships and private vessels world-wide more than tripled over the past three years, to 73.2 metric tons in 2019 from 22.4 metric tons in 2017 (source). In many situations, the ship crew will be completely unaware of the illegal goods aboard. Conducting dedicated security checks during hull inspections is an essential part of preventing dangerous substances from entering countries.

3. Discover Invasive Species

Approximately 42 percent of threatened or endangered species are at risk due to invasive species (source). If undetected, many invasive species like Zebra Mussels, Green Crabs, or Killer Algae can cling to the side of ship hulls and unintentionally be distributed around the globe. Once established in a foreign ecosystem, invasive species quickly reproduce and cause irreparable harm to the natural environment. This carries a heavy burden on biodiversity as well as aquaculture production. If these species can be identified and removed during hull inspections, these concerns can be eliminated before they happen.

4. Monitor Paint Status

Monitoring and maintaining paint integrity is a proven method of reducing overall maintenance costs. Antifouling paint is commonly used on ship hulls as protection against corrosion, organic growth, and improved water flow for better efficiency. After identifying degradation points in exterior vessel paint, inspectors can advise for reapplication to prevent corrosive damage or the entry of invasive species.

5. Optimize Maintenance Scheduling

Being knowledgeable of the quality status of any large asset is essential for effective maintenance scheduling. Without consistent inspections, scheduling maintenance is an estimation at best. In a best case scenario, this can lead to overspending on unnecessary treatments/repairs. At its worst, this may result in the neglect of deteriorated components, causing an incident.

Benefits of Using a ROV for Hull Surveys

Emergency Response

If a vessel collides with an object on the seafloor during operation, an emergency inspection is needed. This is to ensure that the vessel is still safe to be aboard and does not compromise the safety of the crew. Traditionally, ships would be required to navigate to the nearest dry-docking port immediately for a visual inspection. This yields a high cost, potential danger, and renders the vessel completely useless for an extended period.

With the introduction of ROVs, ship crews are able to quickly deploy the vehicle to survey for damage in mild to moderate currents. Requiring no certifications to operate, any qualified crew member can use the ROV to inspect for damage and evaluate the integrity of the hull. This limits dry-docking to only necessary situations, while simultaneously putting no crew members in danger for an underwater inspection.

Record inspections

ROVs are equipped with the ability to record video and save images instantly to either SD cards or external computers/hard drives. This enables for proper evaluation after the initial inspection rather than relying on a single passover of a diver. Rotating camera heads allow operators to orient the camera vertically and simply drive the ROV forward for a seamless video of the entire ship bottom. HD camera technology combined with integrated flood lights allows for even minor irregularities to be recognizable through the ROV.

Thickness Testing

Dealing with corrosion is an integral part of ship upkeep. Conducting regular thickness tests is the key to optimizing maintenance schedules and minimizing vessel downtime. Along with sonar, ROVs can also integrate with ultrasonic thickness gauges. This enables them to evaluate the thickness of steel on tanks, hulls and other metal surfaces. The probe simply presses on the surface to provide effective measurements even through paint coatings or marine growth.

Mauverbility / Navibalty

Powerful thrusters, small form factors, and adjustable control sensitivities result in an extremely easy to maneuver inspection system. ROVs that are configured with vectored or vertical thrusters provide superior navigation through currents and also enable multiple planes of movement for comprehensive scans of the hull.

Sonar

Maritime environments can make obtaining clear visuals difficult, particularly at ports where frequent movement can stir up silt from the bottom. While most ROVs come standard with high quality lighting, turbid conditions can render camera visuals useless. In these instances, ROVs can easily be equipped with imaging sonars to provide clear perspective even through the murkiest environments.

Minimize Risks to Divers

The most important benefit of utilizing an ROV for hull inspections, is the elimination of danger for divers. Diving under a large vessel can put divers in extremely precarious positions. Container ships are divided into seven sizes: small feeder, feeder, feedermax, Panamax, Post-Panamax, New Panamax and ultra-large. Ultra-large vessels can measure up to 160ft wide and 50ft deep. With no method for immediate resurfacing, equipment failure during an underwater inspection can be a lethal situation.

Additionally, even when a ship is anchored, it still retains the ability to adjust positions with the current. If a diver is underneath conducting an inspection, a shift can result in a direct collision with the diver. On top of the specific dangers of navigating under ships, divers are also at risk of the regular concerns of pressurization during diving. When an ROV enters the water in lieu of a human, all of these risks are effectively eliminated.

Maximizing Hull Inspections with Deep Trekker

Deep Trekker offers three different ROV models to suit any budget. All three models are equipped with powerful thrusters, high definition cameras, intuitive controls, and are extremely portable. Trade spending thousands of dollars on professional dive teams or dry-docking for one of Deep Trekker’s comprehensive ROV packages.

The DTG3

For quick eyes underwater, the DTG3 is the most portable and cost-effective ROV from Deep Trekker. It can battle up to 2 knot currents and houses a 270 degree panning HD camera for smooth scans underneath ship hulls. Unable to effectively integrate with sonar due to its compact two-thruster design, the DTG3 is best suited for calm, clear environments.

The REVOLUTION ROV

The REVOLUTION ROV from Deep Trekker utilizes six thrusters in a larger, more robust body for optimal power through complex environments. The four vectored and two horizontal thrusters enable three different planes of movement to navigate in unorthodox patterns while maintaining a clear visual.

This larger design is best suited for hull inspections when combined with the M3000D imaging sonar. This is the best-in-class option for extremely detailed, up-close inspections through murky conditions. The enhanced 4K camera is also recommended for identifying smaller defects, and the thickness gauge or CP probe can also be integrated for hull structural evaluations.