HMS Magpie surveys Portsmouth Harbour

Britain’s second smallest warship will spend the summer helping its largest: scouring every inch of Portsmouth Harbour so it’s safe for HMS Queen Elizabeth to use.

HMS Magpie – also the UK’s newest commissioned warship – will spend more than three months surveying the harbour and its approaches to make sure they remain safe for use by the Royal Navy’s two new gigantic aircraft carriers, both based in Portsmouth.

More than three and a half million cubic metres of clay, sand and gravel was excavated as part of £100m improvements to the harbour’s infrastructure to accommodate the 65,000-tonne warships – each one with a draught of 11 metres (36ft).

Now harbour and naval chiefs want to ensure that the sands and mud on the seabed hasn’t shifted – posing a danger chiefly to the carriers (HMS Prince of Wales is due to join her older sister in the base later this year) but also other harbour users.

One of the biggest challenges is how busy the harbour is

Lieutenant Commander Will Alexander RN

It’s only the second mission carried out by Plymouth-based Magpie, which joined the Royal Navy last year – the first of more than 30 new small craft being supplied to the fleet by Atlas.

After painstakingly surveying the waters around Barrow – where the nation’s nuclear submarines are built – at the end of last year, Magpie is giving Portsmouth and Solent the same thorough treatment.

Her built-in and towed sonar systems will scan every inch of the working part of the harbour to provide 3D imagery and an understanding of the seabed like never before.

Once work inside the harbour and main approaches is finished, Magpie will shift focus to the wreck site of the Mary Rose to see if there’s anything of significance left on the seabed from Henry VIII’s flagship.

And then she’ll investigate the wreck of a French galley, lost around the same time as the famous British flagship off the northeast coast of the Isle of Wight.

This is the first visit to Portsmouth by Magpie, which replaces long-standing survey motor launch HMS Gleaner, retired last year after 35 years’ service.

The catamaran is typically crewed by just four people on a survey patrol, while the remaining half a dozen crew occupy offices in Fort Blockhouse in Gosport where they will process and analyse the reams of data collected. Magpie’s sonars alone are expected to gather half a terabyte of information – in very shallow waters the sensors can collect perhaps 800 readings per second.

“One of the biggest challenges is how busy the harbour is,” explained Commanding Officer Lieutenant Commander Will Alexander. “There are around 230,000 movements every year, and it’s especially busy in the summer with pleasure craft.

“Being in command of Magpie is a hugely rewarding job. She’s a great boat to drive, there’s a degree of autonomy as a small ship, and wreck surveying is really interesting.

“But most importantly, what we do has a direct impact on the ships and submarines of the Fleet, surveying waters so that they can use them safely.”

Before beginning her harbour mission, the ship was presented with a battle honours board by Atlas marking the achievements of the previous eight vessels to bear the name, most famously the WW2 sloop/frigate which successfully hunted U-boats alongside legendary sub killer Johnny Walker and later became the Duke of Edinburgh’s sole command between 1950 and 51.