HTMS Hanhak Sattru Wreck, Koh Tao: Depth & Marine Life

HTMS Hanhak Sattru is one of Koh Tao's two newest wreck dives, a former Royal Thai Navy attack craft sunk in September 2023 to become an artificial reef and training site. This Koh Tao wreck lies on the east coast at the mouth of Mao Bay (Ao Mao), which is why you will also hear it called the Mao Bay wreck, Aow Mao wreck or simply Hanhaksattru 312.

The ship sits upright on a sandy bottom, about 44.9 m long and 7 m wide, with the highest point of the wreck at roughly 13 m. It is a deeper, more serious dive than most of the east coast, so most operators keep it for Advanced Open Water divers and above. Its sister ship, HTMS Suphairin, was sunk on the west side near No Name Pinnacle.

HTMS Hanhak Sattru dive site map

2Wreck hull, 22 to 24 m
3Bridge and mast, top 13 m
4Bow and cannon
5Mao Bay slope, 4 to 20 m

Usual route: Drop onto the wreck at 20 to 24 m, circle the hull, bridge and bow, then move up the Mao Bay reef slope to finish in the shallows. Drawing based on local dive briefings: approximate, not to scale and not for navigation. Your guide's briefing on the day always comes first.

Getting to the HTMS Hanhak Sattru dive site

This is an east coast site on the quiet side of the island, south of Hin Wong, and you can only reach it by boat. Most dive centres send you by taxi to Hin Wong Pier, where a shuttle boat takes over for a ride of around 20 minutes to the mooring.

Because of the transfer, it makes for a half-day or full-day outing rather than a quick hop from Mae Haad. Book ahead and ask your centre when the next trip runs, since the wreck is only dived when the east coast is calm.

Diving HTMS Hanhak Sattru: the route

You usually drop onto the wreck first. The bow comes up out of the sand as you descend, and from there you work along the hull toward the bridge and mast, which are the highest structures on the ship at around 13 m. The hull itself rests on sand at roughly 22 to 24 m, so the dive is a gradual climb from the deepest part back up to the shallows.

The bow cannon and the stern machine-gun mount are both still in place, along with the railings and ladders. After you have circled the wreck, the dive typically moves up the sloping Mao Bay reef, where boulders and coral patches take you back to the shallows for your safety stop.

Underwater layout

Bow and cannon: the first thing you see on approach.
Bridge and mast: the highest point, around 13 m.
Hull on sand: resting at about 22 to 24 m with open sand on all sides.
Stern gun mount: a good landmark for the far end of the ship.
Mao Bay slope: a sloping reef of boulders and coral patches for the ascent.

The ship's exact compass heading is not confirmed, so the map on this page is a schematic rather than a survey.

Marine life at HTMS Hanhak Sattru

For a wreck this young, the fish life is already impressive. Yellowtail, chevron and great barracuda hang above the deck, with fusiliers, snapper and sweetlips around the hull. Malabar and brown marbled groupers and giant trevally have moved in, and queenfish are sometimes seen passing through.

Under ledges and along the sand you can find lionfish, moray eels, blue-spotted stingrays and young Jenkins whiprays. Look closer and you will spot nudibranchs, shrimp and small crustaceans, plus sea whips and the first patches of coral growing on the ladders and deck gun. The wreck gets a little richer every year.

History of the wreck

Hanhak Sattru is a former high-speed attack craft of the Royal Thai Navy's Gulf Patrol Fleet. It was built in the mid-1970s, served for about 42 years and was decommissioned in 2018. It was then handed to the Department of Marine and Coastal Resources for an underwater learning park and marine ecosystem rehabilitation project in Surat Thani Province.

Before it was sunk, fluids and harmful materials were removed, and hatchways were cut and reinforced so that trained divers can enter safely. Koh Tao's two navy wrecks went down on 7 and 8 September 2023, with Hanhak Sattru sinking on the second day.

Conditions and best time to dive

The best months are May to September, when the south-west monsoon leaves the east coast calm. October to December brings the north-east monsoon, and the wreck is often off the schedule then.

Visibility is typically 10 to 20 m, though it can dip to around 8 m on a poor day. Current is usually light, but stronger tidal flow can pass through the bay at peak exchange, so expect your guide to plan around the tide.

Tips from local divers and safety notes

Penetrating the wreck needs a Wreck Diver specialty or equivalent. The cut hatchways make entry possible, but it is still an overhead environment.
Descend near the bow so you see the whole length of the ship on approach.
Nitrox is a good idea with the hull at 22 to 24 m.
Bring a torch for shaded areas and interiors, and look for lionfish under ledges.
Watch for sharp edges and entanglement hazards, which are normal on a young wreck.
Check your depth limits before booking: Advanced or Deep certification is recommended.