The dangers of large ships deviating from a passage plan when in extreme weather has been highlighted in the sentencing of a captain at court this week.

Captain Yongyu Li pleaded guilty in May to one charge of operating a ship  - the Spinnaker SW - in a manner causing unnecessary danger or risk when he altered course without further assessment and navigated the large ship dangerously close to Portland Island while underway to Tauranga to load cargo.

The charge was filed by Maritime NZ.

Prior to its departure on February 14, 2023, the vessel was anchored off the Mahia Peninsula to shelter from the impact of Cyclone Gabrielle.

Maritime NZ investigations manager John Maxwell says the 175 metre vessel was en route to Tauranga as the east coast was being hit by the cyclone. The country was in a national state of emergency while the vessel was under way.

"Due to the heavy conditions at sea, the captain decided to change the route without further assessment, navigating the vessel close to Portland Island.

"This was an incredibly risky decision. Navigating a large bulk carrier close to the island meant there was a risk it would get into dangerously shallow waters, removing the safety net of deeper water should something have gone wrong with the vessel, such as a loss of power.

"The decision of Captain Li to alter course without further assessment of whether the route was safe put the crew, vessel, and the environment at risk.

If something went wrong, the conditions would have hampered a potential response as well," says John.

The original passage plan for the vessel was for it to go around Portland Island at a safe distance. This allowed for the significant swells and heavy weather anticipated by the crew.

After its departure from anchor, the vessel was recorded as encountering six to seven metre swells. The heavy weather caused the ship to roll, and the captain made the decision to navigate the ship closer to the Mahia Peninsula and Portland Island.

"This change in route took the vessel close to shallow water, passing two spots with depths of 9.4 and 10m.

"This is very shallow for a bulk carrier of this size, and would have only left it with a couple of metres of clearance between the vessel and a potential grounding."

Captain Li’s was convicted of operating a ship in a manner that caused unnecessary danger or risk under s 65 of the Maritime Transport Act 1994 and sentenced on July 19, 2023, to a fine of $1875, which included a discount of 25 per cent for his early guilty plea.