"Is this a guess ? Or do you have some basis for it ? "

Logic.

They didn't have canals with locks as far as I know, so it was natural rivers they were using. You couldn't sail a boat that big upstream with a sail that small so something must have been pulling it.

They may have had tracks for horses, but knowing the Romans, big manpower specialists, they had people pulling as well.

Yes, the Dutch guy was good as well, but that was also cause they'd excavated the only other know example previously.

Dave L