In the second of our occasional summer series of tales from the riverbank,

Stephen Lewis spends a day on patrol with the 'river police'.

"We have just had a blue light fitted, and a siren," says Ray Duckworth with obvious pride, standing with feet planted as we move away from the moorings at Naburn Locks. "A lot of people used to say they couldn't hear us. They've got no excuse now."

We're on the Ebor Light II, a trim ocean-going launch modified to British Waterways specifications in which waterways patrol officer Ray and York harbour master Martin Stilliard patrol the inland waterways connected to the Ouse.

It's a beautiful Sunday - the busiest day of the week for the river patrol, when most boat users are likely to be out and about. We motor quietly up the Ouse from Naburn Locks towards York, swatting away the insects which flit over the brown surface of the water.

In places, where low-hanging willows crowd either bank, we could almost be nosing upstream along a tributary of the Amazon, so enclosed is the water.

Then the banks open out, and along both sides are moored gleaming river cruisers: visible symbols of the river's importance as a waterway.

Ray and Martin are not, strictly-speaking, police officers at all. They don't have powers of arrest and are employees of British Waterways, not of North Yorkshire police. Their uniforms are more for show than anything.

But it is they who enforce the by-laws governing use of boats on the Ouse, the Foss, and other inland waterways in North Yorkshire.

There's actually an awful lot you can't do while messing about on the river. No swimming, for one. No speeding, for another. No causing a nuisance, no throwing rubbish - not even any washing of animals, if you would be so inclined.

Some of these, of course, the 'river police' take more seriously than others.

Many of the big cruisers moored alongside the Ouse between Naburn and York are capable of top speeds of up to 30 knots - nearly 40mph.

On a narrow river like the Ouse, where the speed limit varies between five and six knots, a boat travelling at such speeds can cause havoc.

The main problem, Ray says, is the wake or wash left by the boat. The faster the speed, the deeper and stronger the wash.

"We've got a lot of boats moored along the river, tied to cleats," he says. "If there is a boat speeding, you get a big wash. The moored boats can be banged about, the moorings ripped off, some real damage done. If there is someone on board cooking, they could easily get a pan of boiling water in the face."

To demonstrate the effects of a big wash Martin, at the helm of the Ebor Light II, accelerates when we reach a quiet stretch. The stern of the boat buries itself in the water: a hump of brown water rises behind and waves spread out from either side astern, pounding the banks of the river.

It's not only moored boats that are at risk. A heavy wash caused by a speeding boat, Ray says, can also over time cause serious erosion to the river banks - and even be a threat to livestock.

"You tend to get sheep having a drink beside the river down near Selby. I've talked to people who tell me they've seen sheep dragged in by the wash. It's fairly common to find dead sheep in the river."

Speedsters aren't common, Ray concedes: but just occasionally the urge to open up one of those powerful cruisers proves irresistible.

If you do get caught speeding, though, there is no escape. The Ebor Light II comes equipped with a state-of-the-art laser speed gun - and it doesn't lie. If you're caught, British Waterways will prosecute.

Ray is also keen to crack down on swimming in the Ouse. Not because he's a killjoy. It is, he says as we pass through York heading north, simply dangerous. The water may look inviting on a hot summer's day: but it is deep, the water is cold and, especially near the main York bridges, there can be a strong undertow.

"We had a fatality near King's Staith last year," he says. "The water isn't particularly clean and if you get sucked under you can be completely disoriented. You may not know where the surface is: and the cold can affect you very quickly."

So the Ebor Light makes a point of seeking out any swimmers and ordering them out of the water. Few argue.

The river patrol's bread-and-butter work, though, is keeping an eye out to ensure all the pleasure craft using the river are licensed. They're like the Ouse and Foss's very own traffic police.

When Ray, a strapping former sergeant with West Yorkshire Police, first joined British Waterways 18 months ago, nearly one in ten boats on the Ouse, Foss, and the Ripon, Selby and Pocklington canals were unlicensed. Now, it's fewer than one in 200.

Still, as the Ebor Light patrols between Naburn and York, Ray keeps an eagle eye fixed on the gleaming cruisers moored on either bank of the river.

He has a checklist of all the 700-or-so boats licensed to use or moor on the Ouse and the other waterways which are his responsibility. Any spotted without the proper licence will be photographed, and the owners contacted with a demand they pay up at once.

It's not, Ray insists, a trivial matter. "The licences generate a lot of revenue for the waterways - and it is money that can be spent on amenities for the boat-users," he points out.

His interest in the boats, though, goes beyond just checking they're properly licensed. After 18 months on the job he knows many of the boats - and even many owners - by sight.

At one point south of York we pass a boat moored amongst the willows hanging low over a bank. Ray doesn't recognise the person standing on deck and, suspicious, asks Martin to slow the Ebor Light to investigate.

Martin takes the boat in a broad, 275 degree turn, nudging her bows gently towards the riverbank.

As we get nearer, though, the boat's owner puts in an appearance. He recognises Ray and waves. "Oh, it is you," Ray says, waving back cheerfully. "So long as it is you, that's fine. I just don't want anybody nicking it." And with a final wave, the Ebor Light continues on her way.

PICTURE: York harbour master Martin Stilliard checks for speeding boats with his laser gun