Iranian Boats Challenge US and Royal Navy Ships in Strait of Hormuz - can Iran really close the strait?

On 4 March 2017,an Iranian fast attack speedboat got within 600 yards of the USNS Invincible,a Special Missions ship of the US Navy-led Military Sealift Command,a US defence official revealed.The Invincible,which is crewed by civil service mariners,is a missile surveillance ship.It has two radomes:a large one on its stern,and a smaller one on its bridge area.These contain dual band radars that can track ballistic missiles.The US had imposed sanctions on Iran when it tested a new ballistic missile in February,so conceivably this most recent harassment incident was in response to that.
In the first phase of the incident,an Iranian Revolutionary Guard Corps Navy fast attack speedboat got within 600 yards of the Invincible and stopped dead in the water,blocking the Invincible's path,so that it had to steer around the Iranian boat to avoid a collision.Part two of the drama unfolded when another Iranian fast attack craft got within 350 yards of one of three Royal Navy ships escorting the unarmed Invincible,and the British ship,too,had to change course.Both Iranian obstructive actions were considered unsafe and unprofessional.The Iranian vessels ignored repeated attempts to contact them.*
In addition to its ballistic missile test in February,Iran has also recently installed a Russian S-300 missile defence system that can shoot down third and fourth generation fighter aircraft,but not fifth generation such as the F-35 Lightning II.*
Iran has made several threats to close the Strait,through which about 20% of the world's oil supply flows;but can it actually do so?In 2012,US Joint Chiefs Chairman General Martin Dempsey said that Iran has invested in capability that could,in fact,for a period of time block the Strait of Hormuz,which narrows at one point to only 54 kilometers/32.4 miles across (one estimate said Iran could close or impede the Strait for up to a month).At the same time,however,we've invested in capabilities to ensure that if that happens,we can defeat that.*
The US Military Sealift Command exists to support the warfighter across the full spectrum of military operations.MSC provides on-time logistics;strategic sealift;as well as specialised missions anywhere in the world,under any conditions,24/7,365 days a year.With unified resolve,MSC says,we boldly sail the world's oceans to provide essential assured logistics and service support to the warfighter as the nation's premier maritime transport organisation.*
The current commander of MSC is Rear Admiral Dee L. Mewbourne,USN.Commander,MSC Central is Captain Edwin D. Kaiser,USN.Some of the MSC ships have blended civil service/military crews;others,such as Invincible have purely civil service mariner crews.