Boem was part of the breakaway which held off the charging peloton in the finale of the 200-kilometres route from Civitanova Marche to Forli.
“I’m really, really happy. Of course, it was unexpected to win today as it was the easiest stage but 20 kilometres to go I had a really good advantage... I could not let the opportunity go,” Boem told Eurosport. Alberto Contador (Tinkoff-Saxo) enjoyed an uneventful day to retain the overall leader’s pink jersey, with Porte’s bid for the maglia rosa hindered by the mishap 5km from the finish.
The Australian, who began the day 22 seconds behind in third, suffered a late and costly mechanical, losing 47 seconds to the Spaniard to slip to fourth.
Porte’s fortunes would have been worse but for the sportsmanship of compatriot Simon Clarke.
The Orica-GreenEdge rider gave Porte his wheel to help to minimise time losses. Porte wrote on Twitter: “Not the best of luck for me today but guess it could have been worse. Ready to fight all the way to Milan!
“If that’s not Aussie mate ship then what is? Punctured and clarkey gave me his front wheel.”
Clarke told cyclingweekly.co.uk: “He punctured and obviously needed a hand. It could have cost him the Giro.”
Porte now trails Contador by one minute nine seconds overall, with Fabio Aru (Astana) the nearest challenger, three seconds behind.
Contador, who suffered a dislocated shoulder last week, played down rival Porte’s time loss, with plenty of racing remaining before the finish in Milan on May 31.
“It was anything but an easy day,” Contador said.
“Every day at the Giro can hold surprises in store. The other day it was my crash.
“Today Richie had an unfortunate puncture.
“It is always annoying to lose time that way although, with everything that lies ahead of us, the time lost is very small.”