The Sir Walter Scott will return to service in early summer 2008

The coal which powers the boilers on the Loch Katrine-based Sir Walter Scott is set to be replaced with bio-diesel.

The vessel's final coal-powered journey on the loch, made famous by Scott's Lady of the Lake poem, will take place on Sunday.

Despite the change, the vessel, which carries 75,000 passengers a year, will remain steam-powered.

Work to replace the boilers will begin in November and is due to be completed by early summer 2008, when the ship will return to service.

Earlier concerns that a change of fuel could pose a pollution threat to the fresh water loch, which is Glasgow's main water supply, were dismissed by owners Scottish Water.

A spokesman said: "There is no risk whatsoever.

"The bio-fuel is stored in a sealed container within the hold with limited risk of escape. In the unlikely event of a discharge, the type of fuel would carry no risk to the water supply.

"A full risk evaluation has already been completed to ensure Scottish Water is entirely happy with the use of bio-fuel.

"The added benefit of bio-fuel is that it reduces the sooty deposits currently experienced by the coal-powered engine, so reducing the environmental pollution."

'Real benefits'

Gordon Allan, director of the Steamship Sir Walter Scott Trust Limited, said the conversion would help secure the ship's long-term future.

He said: "It will bring real benefits in terms of efficiency and cleanliness to the operation of the ship, as well as minimising the level of atmospheric pollution, which is so important in an area as special as Loch Katrine."

Based in the Loch Lomond and Trossachs National Park, the Sir Walter Scott is 110ft long and 19ft wide.

The vessel is the only surviving single screw-driven steamer in Scotland.