Underwater Hull

The underwater hull is defined as the area of a ship’s hull from the keel to the top of the boottop. This area is almost always immersed in seawater and is subject to marine growth.

The boottop is defined as the black area between the minimum load waterline and 12 inches above the maximum load waterline. This area can be immersed in seawater and exposed to the topside weather environment. Hull appendages such as rudders and struts, are prone to erosion.

Typical Coatings Used

Underwater Hull and Appendages

The underwater hull and appendages are coated with an anticorrosive and antifouling paint system. In general, underwater hull coating systems consist of an anticorrosive coating and one or more antifouling topcoats. The anticorrosive component of the coating system protects the substrate from the seawater, while the antifouling paint is necessary to help reduce drag in the water. Underwater hull coating systems are categorized into one of the following coating types:

  • Foul-release coatings which do not contain biocide
  • Coatings containing biocides which ablate or self-polish
  • Coatings containing biocides which do not ablate or self-polish

The coating system’s class is determined by the substrate of the ship’s hull.

  • Class 1 – Paint systems for use on rigid, fiberglass, wood, or metallic substrates, other than aluminum
  • Class 2 – Paint systems for use on aluminum substrates
  • Class 3 – Paint systems for use on elastomeric substrates

The grade of the coating system is determined by the volatile organic compound (VOC) content of its ready-to-apply condition. The application of the coating system is defined by the required service life interval (3, 7, or 12 years, or a minimum of 2 years for high-speed vessels attaining 40 knots or greater).

MIL-PRF-24647, Paint System, Anticorrosive and Antifouling, Ship Hull is the performance specification that provides requirements for ship hull anticorrosive and antifouling paint systems. The specification is available for download from the ASSIST Quick Search website.


The black paint used on the boottop is a MIL-PRF-24647 antifouling paint. The haze gray paint applied above the boottop is a MIL-PRF-24635 polysiloxane or silicone alkyd paint.

Surface Preparation

According to NAVSEA Standard Item (NSI) 009-32, the underwater hull, including the appendages and surfaces up to and including the boottopping, is considered a critical coated area. To ensure proper coating adhesion, special attention should be paid to surface preparation of this area. All surface preparation work must be accomplished in accordance with NSI 009-32.

Depot Repairs

NSI 009-32 is the governing document for preservation requirements. Only those coatings specified in Table One of NSI 009-32 are permitted. All underwater hull coating system repair work must be accomplished in accordance with NSI 009-32.

Ship’s Force Repairs

Ship’s force is not permitted to make repairs to the underwater hull coating system.

Underwater Hull Cleaning

Underwater hull cleaning is periodically accomplished to:

  • Maintain effective leaching rate of antifouling coating
  • Improve fuel economy through reduction of hull friction resistance
  • Restore sonar system performance by reducing self-noise
  • Prepare damaged or abraded surfaces for application of underwater curing compounds

Specific instructions on underwater hull cleaning are provided in NSTM Chapter 081, Waterborne Underwater Hull Cleaning of Navy Ships.

New Construction Ships

New construction ships are painted in accordance with the ship build specification for that class of ship.