In this passage, four instances of the postmodifier thereof were deleted in the 1920 LDS edition. Traditionally, these instances of thereof have been interpreted as referring to the Jaredite vessels, which are called barges (in verses 6, 16, and 18). On the other hand, Hugh Nibley has interpreted these four instances of thereof as referring to an air chamber, but never explicitly named, that each barge is implied as having had. For Nibley’s arguments, see pages 276–277 of An Approach to the Book of Mormon, second edition (Salt Lake City, Utah: Deseret Book, 1964). Each air chamber would have had a hole in its top and bottom, thus obviating the difficulty of having a hole in the bottom of the barge itself (as implied by the second instance in verse 20 if thereof refers to barges).
On the other hand, we have already seen that the word thereof is sometimes used vacuously in the original text of the Book of Mormon. In other words, in some cases it appears that the thereof has no particular referent. For discussion of this problem of the unattached thereof in the Book of Mormon text, see under Alma 46:12. As far as the four instances in Ether 2:20 are concerned, the critical text will restore them; they are clearly intended, even though the referent doesn’t seem to be fully determinable (or if the referent is barge, then the thereof doesn’t quite work).
Besides the four instances here in Ether 2:20 where the 1920 LDS edition deleted the thereof, there are three more that have been deleted in the book of Ether (namely, in Ether 6:2, Ether 14:2, and Ether 14:17). See under each of those passages for specific discussion of the thereof and why it was deleted in later editing.
Summary: In Ether 2:20 the four instances of the word thereof will be restored; these were deleted in the 1920 LDS edition, probably because the referent could not be readily determined.