In The Maltese Falcon, how did the ship named La Paloma catch fire?

It is Joel Cairo who spots the arrival of La Paloma in the shipping news of the San Francisco Call-Bulletin. He knows that that the captain of that ship was friendly with Brigid O'Shaughnessy in the Orient and guesses that she got Captain Jacobi to bring the Maltese falcon to San Francisco for her after she had stolen it from General Kemidov and then double-crossed Caspar Gutman who had hired her to get it for him. When all of the principal characters are gathered in Sam Spade's apartment towards the end, Caspar Gutman is compelled to tell Spade a great deal of the truth because the detective insists on having a "fall guy" to take the rap for the murders of Miles Archer and Floyd Thursby. That will be Wilmer Cook, Gutman's loyal bodyguard and hired assassin. In Chapter 19, "The Russian's Hand," Gutman explains:

"Mr. Cairo and Wilmer and I went to call on Captain Jacobi and were fortunate enough to arrive while Miss O'Shaughnessy was there....finally, by midnight, we had persuaded Miss O'Shaughnessy to come to terms, or so we thought.....En route, she and Captain Jacobi and the falcon slipped completely through our fingers."

"You touched off the boat before you left?"

"Not intentionally, no, sir," the fat man replied, "though I dare say we--or Wilmer at least--were responsible for the fire. He had been trying to find the falcon while the rest of us were talking in the cabin and no doubt was careless with matches."

"That's fine," Spade said. "If any slip-up makes it necessary for us to try him for Jacobi's murder we can also hang an arson-rap on him." 

So while Gutman and Cairo were arguing with Brigid and Captain Jacobi, Wilmer Cook was trying to find the statuette below decks and was using matches for lighting. This is the best explanation of how the ship caught on fire. Evidently Spade only intends to use Wilmer as the "fall guy" for two murders: those of his partner Miles Archer and of Floyd Thursby. Wilmer actually did kill Thursby in front of his hotel on Geary Street. Gutman ordered that killing to deprive Brigid O'Shaughnessy of a fierce and loyal protector, think that would scare her into coming to terms with him. Wilmer was also responsible for killing Captain Jacobi, who was trying to bring the falcon to Spade at Brigid's request. But Spade wants to keep the case against Wilmer simple and not bring in the falcon at all. At least that is what he is letting the others think. He is letting Gutman believe that he will let him buy the statuette and get away from San Francisco with it, which is why Gutman is revealing so much information to him and double-crossing Wilmer.

Dashiell Hammett had many years of experience as a private detective. One of his ways of dealing with criminals was to get them turned against each other. This technique can be seen in another Hammett crime novel, Red Harvest (1929), set in a crime-ridden town called Personville, which locals call "Poisonville." Hammett was the best writer of the so-called "Hard-boiled school" of crime writing in the 1920s and 1930s. This was due to his talent and his actual experience as a private detective.