The Marine Institute's 'Holland 1' Remotely Operated Vehicle (ROV) spent much of the day surveying the sub-sea terrain around Blackrock Island as the search for two missing Coast Guard crew members continues.

Ciarán Smith and Paul Ormsby have been missing since their helicopter crashed on 14 March.

The ROV is being operated from the Irish Lights Vessel, the Granuaile, which returned to Blacksod Bay today.

It left the area earlier this week, in order to transport the wreckage of the Rescue 116 helicopter to Galway.

A Coast Guard spokesperson said the survey had covered much ground and with favourable conditions it was expected to continue for several hours tonight and into the early part of tomorrow morning.

The ROV is being used to carry out a detailed examination of the seabed around the area where the aircraft wreckage lay for several weeks.

Search crews want to comb the seabed in an effort to see if there is any indication where the two men are.

Meanwhile, more than 80 boats are now expected to take part in a major sea surface search for the crew men on Saturday.

Mr Smith's relatives had called on those with an intimate knowledge of the sea conditions to assist in the operation to find the two men. 

A number of boats will gather close to the site of the tragedy early on Saturday morning. They will fan out from there in a northward direction, carrying out a detailed survey for up to 20 miles off shore. 

Lifeboat crews from Árainn Mhór, Bundoran, Sligo, Ballyglass and Achill will be coordinating the search effort.

An area stretching from Achill Island as far north Árainn Mhór will be covered by the fishermen. They will be allocated specific sections of sea to survey and will report back to RNLI coordinators on shore. 

The Killybegs Fishermen's Organisation and the Irish Fish Producers Organisation will also take part, with several large vessels involved. They will carry out surveys further out at sea, as a huge area is combed for any trace of the two missing men. 

Organisers have appealed to all those taking part to put safety first and are stressing that all vessels that put to sea must be in good condition and that all crews are fitted with personal flotation devices.

In addition to the sea search, further shoreline and aerial monitoring is expected to take place from north Mayo, all the way along the Donegal coast.

The LÉ Ciara has taken over from the LÉ James Joyce as the on-sea coordinator of the search effort.

Garda and Naval Service divers remain on standby to carry out further dives, as the need arises. 

While Saturday's search will involve a huge number of boats, fishermen and locals along the coastline in question are being urged to stay vigilant at all times, for any trace of Winchman Smith and Winch Operator Ormsby.