15/04/2020
This is one of those confusing sentences that are called "garden-path sentences". It may seem hard to believe, but this is a perfectly grammatical sentence that obeys all the underlying grammar and syntax rules, it's just a little weird to process. But bear with me, I'll break it down for ya. We seem to rule out the sentence because we initially analyzed ‘old’ as an adjective describing ‘man’, something that we thought is a noun. As you go on processing the rest of the sentence, you don’t seem to find any verb, which ultimately leads to a dead-end and thus the sentence is ruled out. However, if you rewind your processing and reinterpret ‘man’ as the verb of the sentence and the ‘the old’ as a collective noun which functions as the subject of the sentence, your reading of the sentence will be just right. In other words, ‘the boat is manned by the old (people)’ is the interpretation you're looking for. Our grammar blueprint is satisfied now, as it processed the sentence with no question marks. I hope this helped. Cheers 😀
