When could a merchantman defeat a warship?

How many merchantmen picked fights with warships and won?

Not many, but a large letter-of-marque East Indiaman with roughly thirty guns could defeat a smaller privateer or warship that attacked after misjudging it as an easy prize.

Occasional merchant victories depended on the size of the opponent and a mistaken assessment of the target. A raider carrying about twenty guns could find itself facing a large East Indiaman with thirty or more. A vessel sailing alone under a letter of marque could also have a crew prepared to fight and take prizes. Several apparent fourth rates together could present the numerical threat Linois feared at Pulo Aura, even though a single merchantman could not match his third rate.