The illegal trade in ships' fuel is a lucrative business, and more people have been caught stealing, selling and buying such diesel, say police.
From January to July this year, 60,000 litres of oil changed hands illegally, 10 times more than the whole of last year.
In total, the oil was worth more than $70,000.
The diesel was siphoned off boats while they were moored in Singapore waters, and later sold to vessels outside the territory through a middleman.
During a briefing yesterday, Police Coast Guard (PCG) Deputy Commander (Special Duties) Lim Kim Tak said the jump was mostly due to one of the four cases this year, where a whopping 50,000 litres of oil had changed hands.
While an average boat can hold only 20,000 litres of oil, offenders will usually modify the boats' fuel tanks illegally so they can hold more.
The police arrested 26 men last year for such offences. Up to July this year, 32 men have been caught.
Of the 58 men arrested, 55 were Indonesians, and three were Singaporeans in their 50s.
The Singaporeans - a shipping company director, an assistant pub manager and an unemployed man - had provided the middlemen with cash to buy the oil, for a cut of the profits.
Assistant Commissioner Lim, the PCG deputy commander, said the coast guard is doing checks on many boats entering Singapore waters. The vessels in question usually have pumps and long hoses on board to transfer the oil, he said, speaking at the PCG's Brani base in southern Singapore.
The coast guard also works with the Immigration and Checkpoints Authority and the Maritime and Port Authority to detect such offences.
Private companies also police their own vessels.
Mr Fazail Sapto, for instance, a crew coordinator from coastal company Yeng Tong Construction, oversees 16 tugboats with five to six crew members each.
He conducts surprise checks and reviews camera footage. His company also uses a system to track the movements of his boats.
So far this year, he has reported two cases to the police after finding oil waiting to be sold on his boats.
Those found guilty of theft of bunker oil face seven years in jail and a fine. Those who buy the stolen oil could be jailed for five years and fined.