'Spitfire of the sea': Replica of WW1 torpedo vessel created in Portsmouth completes sea trials

As the replica takes its first trip out on the water, ITV Meridian reporter Chlöe Oliver has been to see the volunteers who are bringing the history to life

A replica of the Coastal Motor Boat (CMB), one of the most lethal small vessels used during WW1, has been created in Portsmouth.

Despite its size, the small but speedy boat single-handedly sank a 7,000 tonne Russian warship, 'Oleg', in 1919.

It's taken volunteers seven years to complete the 40 feet long vessel using original plans and century-old boat building methods, with contributions from the Imperial War Museum and National Maritime Museum. 

It took to the waters in the solent for the first time in 100 years last week.

Blanche Thornycroft holding a recording disc used to test model boats at Steyne House on the Isle of Wight, the Thornycroft family home Credit: Family handout

The CMB was designed in 1915 by The Thornycroft Company, a shipbuilder experienced in designing, building and racing high speed boats in the years before the first World War.

Owner, Sir John Isaac Thornycroft and his children, John Edward, Tom and daughter Blanche all contributed to the design of the CMB.

Despite being one of the first three women to be admitted to the Royal Institution of Naval Architects in 1919, it’s only recently that Blanche has begun to receive recognition for her contribution to naval engineering.

  • Hamo Thornycroft, volunteer and great nephew of CMB4 architect Blanche Thornycroft

Its stepped hull was a revolutionary design feature that allowed it to reach impressive top speeds of nearly 40 knots, allowing it to slip by enemy defences.

Such was its significance, sea trials for the CMB were held at night in order to keep it a secret from enemy agents.

Equipped with torpedoes, speed and agility, the CMB made for a highly efficient attack boat during WWI and beyond, influencing high-speed boat design across the world, including the much larger and widely-used gunboat; a staple vessel in naval history.

Made of wood, the CMB had limited defences against enemy fire, making the crew among the bravest in the Royal Navy at the time.

CMB4 famously sank a 7,000-ton Russian cruiser (Oleg) - over 1,500 times its weight - with a single torpedo in a daring solo mission led by Lieutenant Agar, shortly after his SIS assignment.

(R-L)Team lead David Griffiths and volunteers Mike Scott, Hamo Thornycroft, Tim Deacon, Mike Finch and Steve Dawson with CMB4R Credit: Portsmouth Naval Base Property Trust

A team of volunteers ranging from family members of the original boats' conception, to apprentices, to Ukrainian refugees have worked with the Portsmouth Naval Base Property Trust to bring the history alive.

David Griffiths is the shipwright and project manager of the CMBD4. He said: "We've been building this for something like seven years since its inception. We applied for funding for the libel fund for projects of military significance and committing ourselves to building it.

"We were able to secure copies of original plans without which we wouldn't have been able to do it."

He continued: "It's about 4.5 tonnes, it has a stepped hull there's a point where the hull has a different in height and at the back of the boat its squared off so when it starts running it lifts it up and hydroplanes.

"And at the same she was the fastest boat in the navy."

The motor boat in 2023 taking part in sea trials on the coast off of Portsmouth

The coastal motor boats were originally built all over the country, including at Camper and Nicholson's Yard in Gosport.

Hannah Prowse, CEO of Portsmouth Naval Base Property Trust, said: "In building the replica we've created something people can see and test how they manoeuvre.

"Because of her size we can't get the public on her, but next year people can go out alongside her in motor gun boat and watch it on the water for the first time in over 100 years."

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