En route to the harbour at Islana, north of Ragusa, to rendezvous with "Viribus Unitis" and "Prinz Eugen" for the coordinated attack on the Otranto Barrage, "Szent István" and "Tegetthoff" attempted to make maximum speed in order to catch up to the rest of the fleet. In doing so, "Szent István"s turbines started to overheat and speed had to be reduced to . When an attempt was made to raise more steam in order to increase to "Szent István" produced an excess of smoke. At about 3:15 am on 10 June, two Italian MAS boats, "MAS-15" and "MAS-21", spotted the smoke from the Austrian ships while returning from an uneventful patrol off the Dalmatian coast. The MAS platoon was commanded by "Capitano di corvetta" Luigi Rizzo, who had sunk the Austro-Hungarian coastal defense ship in Trieste six months before. The individual boats were commanded by "Capo timoniere" Armando Gori and "Guardiamarina di complemento" Giuseppe Aonzo respectively. Both boats successfully penetrated the escort screen and split to engage each of the dreadnoughts. "MAS-21" attacked "Tegetthoff", but her torpedoes failed to hit the ship. "MAS-15" fired her two torpedoes successfully at 3:25 am at "Szent István". Both boats evaded any pursuit although "MAS-15" had to discourage the Austro-Hungarian torpedo boat "Tb 76 T" by dropping depth charges in her wake. "Tegetthoff", thinking that the torpedoes were fired by submarines, pulled out of the formation and started to zigzag to throw off any further attacks. She repeatedly fired on suspected submarine periscopes.