Turning the tides: Could hidden sea power energise our coastal towns?

In a secret location in the Highlands of Scotland, there's a fast-flowing river that's the key to understanding the massive power of water.

It's where engineer David Mummery found himself more than a decade ago.

With a length of cord tied tightly across the river, he swam out, gripped firmly, and let the unstoppable current rush over as he sunk his head below the surface.

"As an engineer we do the maths and stuff," he says, "but sometimes you can get a lot more from actually touching the water and feeling it".

As he lay down in the powerful rapids, he visualised a new type of small-scale tidal energy generator.

It's an innovation he says could change lives for people in coastal communities.

"I'd be under the river, feeling the turbulence of the water rushing over me.

"Trying to envisage how this turbine needed to interact with that environment - to understand the way the water was working."

Mummery's idea became the Swimmer Turbine - a 50kw tidal generator named after his experience in the river.

Anchored to the sea floor, the fiberglass and steel device lies invisible below the water's surface. It can generate enough electricity to power about 30 homes.

He began tests in the river in early 2010s. On a patent lawyer's advice he found a secret location, far from curious Highland tourists and their cameras.

"You're trying to be quite discreet because you're working with new technology," he says.

It was in the first years of his company, Flex Marine Power, long before Britain was gripped by the soaring costs of an energy crisis.

The size of the Swimmer Turbine is what sets it apart from large tidal projects dotted around Scottish waters.

Scotland is a world leader in the sector at sites including:

Nova Innovation - The world's first tidal stream array, deployed in 2016 in Bluemull Sound in Shetland

MeyGen - The world's largest tidal array, with four large 1.5MW turbines on the seabed of the Pentland Firth

Orbital O2 - The world's most powerful floating turbine, generating 2MW, anchored at Fall of Warness in the Orkney Islands

Mummery's turbine is ultra compact and low cost compared to these giant machines. Fishermen and local workers can build and install it themselves from their own village harbour.

"Can you imagine like an Ikea furniture kit? You get a shipping container and you'll open it up and it will be about 20 components.

"You'll bolt them all together and then you've got your turbine ready.

"There'll be a launch plan, you follow these instructions, then you'll be in the water."

Mummery's work did not always focus on small-scale power. Originally from Guildford, he studied mechanical engineering at Surrey University.

Before founding Flex Marine Power, he was a sought-after consultant in the UK energy sector. For a time he specialised in nuclear work and decommissioning, designing robots for pulling material out of contaminated tanks.

He moved north after falling in love with a girl studying at Glasgow University. Jo became his wife and they now live in the city suburbs with three young children.

Conveniently, Mummery's relocation came at a time when Scotland was establishing its reputation as a centre for marine power expertise.

In 2010 he took a secondment at Scottish Power Renewables on the ground-breaking Pelamis project in Orkney.

The 180m-long (591ft) Pelamis P2 - nicknamed the sea snake - was the first deep-water, floating wave energy machine to generate power for the national grid.

Ultimately, Pelamis went into administration in 2014 , but the ill-fated device remains celebrated as a milestone in marine power innovation.

"The industry is littered with these examples of fantastic technologies and teams that do brave things that take decades," Mummery says.

"But at some point the market doesn't step up and they falter. Some really good ones have lasted the course, but others have not and it's sad."

Mummery says the UK has "phenomenal waters for tidal power" and he loved being a part of major projects - but couldn't shake a discomfort.

His work felt incongruous to the many small coastal communities he found himself in.

"I'd be going out on boats around Scotland doing sonar surveys with all these really expensive pieces of kit.

"But the equipment was all really big, utility-scale stuff that would weigh hundreds of tonnes, shipped by people from the mainland.

"All the people I was meeting in harbours or in the pub were fishermen. And they were knowledgeable and they were interested.

"I thought 'we've got to do this in a way that they can take the lead'."

In 2011 he left big projects behind to found Flex Marine Power and focus on his turbine concept.

He spent hours at his desk sketching, working out how to make the technology smaller and lighter.

He hired a student employee and found a sleek, glass office space outside Glasgow - which he quickly discovered wasn't entirely suited to an engineer and his tools.