In modern English we do not expect the preposition of after aboard, but here in Ether 6:4 the of appears to be intentional. According to the Oxford English Dictionary, the word aboard appears to have been originally borrowed from the French à bord, and the à was interpreted as the equivalent of the English preposition a (which derives from the preposition on), thus leading to the expression “on board”. In the printer’s manuscript, the two-word spelling a board suggests that the word board could be treated as a separate noun, thus implying the need for the preposition of.
Again according to the OED (under section B of aboard), the prepositional use of aboard (referring to being in or on a ship) is equivalent to “on board of ”. One could thus interpret the expression here in Ether 6:4 as the same as “they got on board of their vessels”, but of course in modern English we expect the of to be missing with on board as well as aboard (thus “they got on board their vessels or barges”). Even so, there is some evidence in the last three centuries (at least) for the expression “to get aboard of a ship”, as in the following examples from and from Literature Online :
The possibility remains, of course, that the of in the Book of Mormon expression “got aboard of their vessels” is an error in the printer’s manuscript (and perhaps even earlier in the original manuscript). Unfortunately, we have no other examples of aboard or on board in the Book of Mormon, so it is difficult to decide here if an error has occurred. There is one occurrence of aboard in the King James Bible (“we went aboard”, in Acts 21:2), but in that case no noun phrase follows aboard. The most reasonable solution here in Ether 6:4 is to accept the unusual usage with the of rather than an emendation that cannot be directly supported by either usage or errors elsewhere in the text. The of presents an unexpected but possible reading and will therefore be maintained in the critical text.
Summary: Maintain in Ether 6:4 the difficult reading “they got aboard of their vessels”; the use of the of is unexpected here but appears to be intended.