HTMS Hanhak Sattru 312

Koh Tao’s newest east-coast wreck dive

The HTMS Hanhak Sattru 312 is one of two former Royal Thai Navy fast attack craft purposely sunk off Koh Tao in September 2023, transforming the island’s east coast into a world-class wreck diving destination.

Resting in the calm waters of Aow Mao Bay, just north of Laem Thian, the 44.9-metre vessel sits upright on a sandy seabed at depths of 22 to 24 metres, with the highest point of the wreck reaching up to 13 metres below the surface.

For Black Turtle Dive divers, the Hanhak Sattru opens a new chapter in Koh Tao’s wreck diving heritage. Alongside its sister ship, HTMS Suphairin 313, and the established HTMS Sattakut, MV Trident and MV Unicorn, Koh Tao now offers the most diverse wreck diving portfolio in the Gulf of Thailand.

Brief history of HTMS Hanhak Sattru 312

The HTMS Hanhak Sattru began service in 1976, built by Singapore Shipbuilding and Engineering Company Limited as a high-speed attack craft for the Royal Thai Navy’s Gulf Patrol Fleet.

Across more than four decades of active duty, the vessel was equipped with sophisticated weaponry of its era, including Gabriel surface-to-surface missile launchers, a 20mm anti-aircraft cannon at the bow and a 50-calibre machine gun at the stern.

Together with HTMS Suphairin 313, the two sister ships formed part of Thailand’s coastal defence posture, conducting joint operations with the Royal Thai Naval Special Warfare Division.

After 42 years of operational service, the Hanhak Sattru was decommissioned in 2018. Rather than being scrapped, the vessel was transferred to the Department of Marine and Coastal Resources (DMCR) to begin a new mission as part of the Underwater Learning Park and Marine Ecosystem Rehabilitation Project, an initiative focused on Surat Thani Province’s coastal waters.

Preparation for sinking

The transformation from warship to artificial reef is a meticulous process. Every fluid was drained, every loose fitting removed, and any material that might harm the marine environment stripped from the vessel. Hatchways and access points were carefully cut and reinforced to allow safe diver penetration while preventing entanglement risks.

In September 2023, after months of environmental preparation, the Hanhak Sattru was towed to its final resting position at the mouth of Aow Mao Bay and scuttled in a controlled sinking. The ship descended upright onto the sandy seabed, exactly as planned.

Diving the HTMS Hanhak Sattru 312

Site location and conditions

The Hanhak Sattru lies approximately 60 metres north of Laem Thian on Koh Tao’s east coast, at the mouth of Aow Mao Bay. Its sheltered position is one of the wreck’s defining features.

While Koh Tao’s most famous wrecks sit on the west coast and become a little rougher during the south-west monsoon, the Hanhak Sattru offers consistently calm conditions during the same period, making it a wreck dive suited to May to September.

What to expect underwater

Approaching the wreck from the descent line, divers first encounter the silhouette of the bow rising from the sand. The intact superstructure, including the bridge, mast and gun positions, gives the wreck a clear sense of purpose. The 20mm bow cannon and aft 50-calibre machine gun mount remain visible.

Visibility on the east coast of Koh Tao typically ranges from 10 to 20 metres at this site. Currents are usually light, though stronger tidal flows can pass through Aow Mao Bay during peak tidal exchange.

Skill level and certification requirements

The Hanhak Sattru is suited to PADI Advanced Open Water Divers and above. Divers must hold a Deep Diver certification or have completed the deep adventure dive of the Advanced Open Water course before exploring the lower sections of the wreck.

Penetration of the wreck is reserved for divers with PADI Wreck Diver Specialty certification. While the ship has been prepared for safe penetration, internal navigation, line laying and emergency procedures all require formal training. Black Turtle Dive offers the PADI Wreck Diver Specialty as a two- to four-day programme, often using the HTMS Sattakut and the new wrecks together as training sites.

Marine life around the Hanhak Sattru

Even within months of sinking, the Hanhak Sattru began attracting marine life. Steel surfaces are colonised first by encrusting organisms — barnacles, hydroids and bryozoans — followed by algae, soft corals and eventually hard coral recruits. This succession will continue for decades, ultimately forming a fully developed artificial reef ecosystem.

Resident and visiting species

Within the first year of deployment, divers have already reported sightings of:

• Schooling pelagic species — yellowtail barracuda, chevron barracuda and great barracuda use the wreck as a hunting and resting station
• Reef predators — malabar grouper, brown marbled grouper and giant trevally patrol the structure
• Rays — the Jenkins whip ray, a known resident of nearby HTMS Sattakut, is increasingly observed at the new wrecks
• Reef fish — schools of fusiliers, snappers, sweetlips and the full Gulf of Thailand reef community are establishing residence
• Macro life — early-stage colonisation by nudibranchs, shrimps and small crustaceans is well underway in the wreck’s nooks and shaded sections

As the wreck matures, sightings of larger pelagic species and cephalopods are expected to increase, mirroring the ecological progression observed on the older HTMS Sattakut over its decade-plus on the seabed.

A new habitat — the conservation story

From decommissioned warship to thriving reef

Artificial reefs created from purpose-sunk vessels deliver measurable conservation benefits when carried out responsibly. The Hanhak Sattru is a textbook example of this principle in action. The wreck provides:

• Hard substrate for coral recruitment in an area where natural rocky surfaces are limited
• Three-dimensional structural complexity that supports ten to twenty times the fish biomass of an equivalent area of bare seabed
• Refuge habitat for juvenile fish from open-water predators, supporting nearby natural reefs through population connectivity
• Pressure relief for popular natural reefs by attracting divers to alternative sites

The Hanhak Sattru and Suphairin together represent one of the most significant conservation investments in Koh Tao’s recent diving history.

Climate resilience and ecosystem function

Steel-hulled artificial reefs are notably more resilient to climate stressors than natural coral reefs. They do not bleach, they cannot be killed by elevated sea temperatures, and they continue providing structure and habitat even when adjacent natural reefs experience disturbance. As ocean conditions become more variable, these stable structures play an increasingly important role in maintaining ecosystem function across the wider seascape.

Citizen science and monitoring opportunities

Black Turtle Dive supports the long-term monitoring of Koh Tao’s artificial reefs through the Conservation Diver Certification programme, our position as a PADI AWARE Foundation 100 AWARE Partner, and our ongoing collaboration with marine research initiatives in the Gulf of Thailand.

Divers participating in our conservation courses contribute to fish population surveys, coral recruitment monitoring and debris removal at sites including the new wrecks.

Wreck diving courses at Black Turtle Dive

PADI Wreck Diver Specialty Course

The four-dive PADI Wreck Diver Specialty teaches the skills needed for safe wreck exploration: hazard assessment, penetration planning, mapping techniques, safe navigation inside enclosed spaces, line laying and emergency procedures. The new wrecks are now part of our standard rotation alongside the HTMS Sattakut.

Other relevant courses

The PADI Advanced Open Water Diver course is the minimum entry point for diving the Hanhak Sattru, with deep and navigation adventure dives forming part of the certification. The PADI Deep Diver Specialty extends maximum recreational depth to 40 metres, opening up exploration of the deeper sections of both new wrecks.

For technical divers, PADI Tec Sidemount and PADI Tec 40 courses offer the configuration and skills necessary for extended-bottom-time wreck dives.

Plan your dive

The HTMS Hanhak Sattru 312 is part of Black Turtle Dive’s standard fun diving rotation as well as our wreck specialty training. Whether you’re working towards your first wreck certification or simply want to explore one of the Gulf of Thailand’s newest artificial reefs, our experienced team will guide you safely through this remarkable site.

Book with us now.