If many many factors are just right, the rotating prop or damaged shaft could contact/breach the hull.
It is very, very unlikely though. This method is probably the easiest and safest way to disable a ship after it is a few hours of days under way.
Thanks, that does seem unlikely. That was the only thing I could think of, but it seemed like if the saboteurs really wanted to sink the boat, and had access to do this, they’d have done it. The reaction in the article seems hystrionic to me, and geared toward all their readership who, like me, don’t know much about boats.