Image caption The turbines are placed on the seabed

On paper, it looks like a blindingly obvious idea: take a version of a wind turbine and plant it on the seabed so that its blades spin in the flow of the tides and so generate electricity.

Unlike wind, the tides are totally predictable for decades ahead.

The turbines, well below the waves, are also out of sight and probably out of mind. And the tidal currents are of course utterly carbon-free.

For an island nation surrounded by some of the world's most powerful tides, optimistic estimates say this form of power could - and should - play a big part in keeping British lights on.

It is one reason why Scotland has been described as a Saudi Arabia of renewable energy potential.

Well, I've been to the baking Saudi oil fields and it was hard to conjure up a resemblance during a visit this week to Orkney, the front line of tidal energy research to the north of the Scottish mainland.

A launch took us between the islands where the waters surge at high speed between the Atlantic Ocean and the North Sea and back again every six hours.

The first challenge is the weather. This is an unbelievably harsh environment in which to build anything Tidal turbine 'performing well'

Harnessing this massive source of energy looks like a no-brainer but will be a lot harder than laying a pipeline in the desert.

Tough climate

The first challenge is the weather. This is an unbelievably harsh environment in which to build anything, let alone manage a vast fleet of tidal machines beneath the waves.

As we lurched through a heavy swell along the shores of the tiny island of Eday, icy winds racing at up to 40mph brought a succession of heavy showers of rain, sleet and even hail. In the middle of May.

We were being taken to see one of the latest devices to go through the trial of everything Orkney could throw at it: a Norwegian turbine called the Hammerfest 1000, a giant three-bladed propeller perched atop nearly 1000 tons of steel structure sitting on the seabed.

Except that we couldn't see it because it is well below the surface, deep enough to avoid any shipping.

Only the ghostly images from a remotely-operated vehicle - a robotic submarine - confirm that the giant machine is down there, spinning in the turbulent sea.

This turbine is being tested by the energy firm Scottish Power. It was chosen because it had survived off Norway for half-a-dozen years without falling apart. In an infant industry, that counts for something.

This must be a little like the pioneering days of steam or aviation

Scottish Power's plan is to deploy ten of the devices off Islay next year and then, later, up to 100 in the Pentland Firth.

As the boat heaves in the waves and the gusts tear at our waterproof clothing, I shout questions to the company's senior man on board, Keith Anderson.

The most obvious is one about scale, and it is something that relates to the dozen or so different marine renewable technologies now being tested in Orkney.

If each Hammerfest machine delivers its advertised 1MW of power, then wouldn't you need 1000 of them to hope to match the output of a typical gas or coal-fired power station?

Could one really imagine great armies of turbines scattered across the ocean floor?

Predictability challenge

Mr Anderson thinks you can. "The real aim," he says, "is to establish the predictability which you get with tidal power, and to feed that into the energy mix which includes the less predictable sources like wind or wave.

Image caption Installation is a huge challenge

"The whole point of this device is to test that it can produce power, and we believe it can, and to show it's robust and can be maintained.

"We believe the UK is in a fantastic place to capture all the advantages for manufacturing and investment."

Maybe he is right but by this stage we're sheltering near the stern of the boat, clinging to the railings and wedging our boots against coils of rope to avoid sliding on the wet deck.

Weird inventions

So the first challenge is survival. And the European Marine Energy Centre (EMEC), established in Orkney in 2003, is hosting trials of a range of weird and wonderful inventions so that companies can investigate which of them can cope.

When I first visited EMEC in 2009, only a handful of technologies was being tested.

Now all 14 of its 'berths' - areas of sea connected by cable to the shore - are booked, a sign of growing interest in this fledgling source of power.

More significant is that the list of companies involved in this work has been transformed from a collection of relatively small and little-known concerns, bravely struggling with the elements and unconvinced investors, to a roll-call of some of the biggest names in engineering and energy.

Rolls Royce and Kawasaki Heavy Industries are among the giants now exploring the development of tidal power.

Voith Hydro, makers of the vast turbines fitted to gargantuan dams like the Three Gorges in China, is also involved.

Siemens is now backing SeaGen, the first commercial tidal system, deployed at Strangford Lough in Northern Ireland.

The attraction for most is a gold rush of generous subsidies.

Each unit of power fed into the grid from a marine renewable machine earns about five times more than power generated by a fossil fuel.

The question is whether this will create a mature and viable set of technologies, and how soon.

New revolution

This must be a little like the pioneering days of steam or aviation: the earliest creations have passed the first credibility test and now the big powers of industry are getting interested.

As we roll and lurch back to shore, most people on board, including this reporter, felt more subdued than at the start of the journey, and more admiring of the teams determined not just to endure Orkney's wild seas but to harness them.

A final thought: if this particular industrial revolution does take shape, and these machines multiply across the ocean floor in an unprecedented change in the seascape and the way we get our power, we'll need a new word to describe them.

'Farms' wouldn't quite serve for a collection of a thousand giant machines.

Earlier, I mentioned 'armies' but maybe that's too militaristic.

'Hordes' is perhaps slightly pejorative. 'Fleets' is suitably marine but these things won't move as ships do.

There is no rush however: deployments on this kind of scale are at least a decade away, probably more.