The typically Scottish weather might not have helped, but it was still full steam ahead.
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Nothing could deter the 19th century steamship from successfully completing one of its first sailings after an extensive three-year refurbishment.
A delegation of travel agents from Poland, Hungary, and the Czech Republic sat side by side with a school party from France in the new covered front deck - specially built to cope with the weather.
The historic pleasure boat's £1.3m refurbishment also includes a rebuild of the superstructure, a refurbishment of the Victorian steel hull, a complete overhaul of the original triple expansion engine, and a unique conversion from coal to Scottish biofuel.
The SS Sir Walter Scott, built in 1898 to sail the loch that supplies Glasgow's drinking water, is believed to be the UK's first "green" passenger vessel.
Argent Energy of Motherwell supplies the biofuel, which is a blend of recycled tallow (animal fat) and used cooking oil. A spokeswoman said: "It is less toxic than table salt and as biodegradable as sugar, and would cause no harm to the environment in the unlikely event of a spill.
"That is why it is good for use on waterways."
Gordon Allan, of the Steam Ship Sir Walter Scott Charitable Trust, set up with the support of the Scottish Government when it began leasing the vessel from Scottish Water in 2006, said: "We were keen to create a greener image for the ship but did not want to go down the route of sourcing specially grown rape seed oil."
Built by William Denny & Bros of Dumbarton, the steamship was named after the novelist who first visited Loch Katrine as a young apprentice in his father's law practice and whose Lady of the Lake, published in 1810, immortalised the area and made it a magnet for tourists.
The coal that was regularly delivered to Trossachs Pier and shovelled into the ship's coal bunkers created a black plume of smoke that would belch from its funnel as it set off.
Its last sailing as a coal-powered ship was in 2007, and its maiden voyage on biofuel on April 11 marked the final transfer of ownership from Scottish Water to the trust. Now, only a small puff of white steam emerges from a chimney.
The Sir Walter Scott's engineer Malcolm Styles, a former officer with the Merchant Navy, oversaw the engine room's transformation from sooty to slick. The two old coal bunkers have been ditched and replaced by a tank containing 4.5 litres of biofuel.
Two new Cochran Wee Chieftain boilers, made in Annan, Dumfriesshire, have been installed and the crank shaft and bearings have been cleaned up.
"The engine was a mess when I took over," said Mr Sykes. "Now it's so much more efficient it's like a completely different ship."
Up on deck, we clip swiftly up the 530ft deep loch at the same speed it always travelled. Steel ingots planked deep down in the steel hull compensate for the lighter aluminium superstructure. Air powered fans that force the steam through the engine's cylinders make only the slightest sound.
Ken Veitch, a retired school teacher from Northumberland, was entranced. The 66-year-old is a direct descendant of Rob Roy MacGregor, the famous folk hero who was born at GlenGyle at the head of Loch Katrine.
But his longed-for first voyage on the steamship when he was just six years old was stopped by wet weather, and he has dreamed of returning ever since.
"This is a pilgrimage for me," he said. "My great-aunt Madge Menzies MacGregor, who lived at Bridge of Allan, was a fervent Scots nationalist and keen to take me on the boat. I was bitterly disappointed when she said we couldn't go on because of the rain. Now I'm fulfilling a promise to myself that I've kept for 60 years.
"I'm particularly delighted that I'm sailing on the same boat I would have sailed on all those years ago. It's a credit to the people who have kept the Sir Walter Scott going, and saved her from being scrapped.
"I was so worried when I saw that it was raining today, but I'm very glad I came. It's so quiet and magical, and exactly the way I'd imagined it all those years ago. Everybody should do this. It stops you thinking about your mortgage and your health.
"Sailing on the Sir Walter Scott is a very spiritual experience."
Steamship has sailed on loch for more than a century The steamship Sir Walter Scott was built in 1898 by William Denny & Bros of Dumbarton as an "excursion passenger launch" for the Loch Katrine Steamboat Company. She has never sailed any other water. The Sir Walter Scott is 110ft long and 19ft in beam. After being built, the steamship was conveyed in sections from Dumbarton along the River Leven, towed to Inversnaid, carried overland by horse-drawn wagons and reassembled at Stronachlachar on Loch Katrine. Half of the total bill of £4269 was related to the cost of delivery. Following a £1.3m refurbishment, it now uses Scottish biofuel and is believed to be the UK's first "green" sailing vessel. Loch Katrine, where the steamship operates, has been supplying Glasgow's drinking water since 1859.