Organisers of a Gaza-bound flotilla said Israel has sabotaged a second ship.
An activist told Israel's Army Radio that the engine of an Irish ship was damaged.
Earlier this week, activists said Israel damaged the propeller of an Irish ship in the Greek port of Piraeus.
Israel has not commented on the allegations.
About 10 ships are due to set sail this week from Greece. It's unclear if the damaged boats will delay things.
Israel imposed a naval blockade after Hamas militants overran the Palestinian territory in 2007.
It said it will not allow the flotilla to reach Gaza.
Last year, an Israeli raid on a similar flotilla killed nine activists on a Turkish vessel. Each side blamed the other for the violence.