Immediately I thought, What am I missing? I looked up cubits in a reliable Bible Dictionary. The standard builder's cubit in the Bible is about 17.5 inches long. Okay, how was that translated into Latin, from which the Anglo-Saxons would have derived their text? The Vulgate says, cubitorum. So, Jerome knew how to translate the term. Then what did the Anglo-Saxons mean by a fæðm?
OE fæðm means "outstretched arms, embrace," etc. It is equivalent to the ancient Greek orguia, which also means "outstretched." While not an exact measurement, it was usually considered to be about six feet.
Now, the OE translators consistently use fæðm to translate cubitus. Can it be that they're just saying "300 x 50 x 30 something-or-others"? Was the fathom the only unit of measurement they could find in OE to use? Or did they not know how much a cubit was? Had that knowledge been lost by the time the monks began translating into OE? Did they have to constantly remind their hearers that "fathom" didn't mean fathom as they were used to thinking, but rather a smaller unit? Or did they actually mean to convey the idea that the ark was larger than the biggest supertanker afloat today?
We wonders, aye, we wonders.