It is Harqu-s-sufun حَرْقُ السُّفُنِ in this instance which means "burning (to set on fire) the ships" in infinitive form.
. َحَرَقَ السفن Haraqa s-sufune is in simple past tense (he burned the ships) and transitive because it has an object (sufun-ships).
I see, Harq wasn't a verb but a maSdar (مصدر). The online dictionary suggested one of the readings - Harq - burn and I took it for a verb (simple past tense) but this is a noun. Sometimes the part of speech is not obvious in that dictionary. http://qamoos.sakhr.com/