In a big shed off the M8 in Glasgow, Mabel is declaring her love for Frederic.

She urges him to stay. He says he must go but will return. And then the orchestra starts. This is the first time that all the cast and musicians of the Scottish Opera and D'Oyly Carte production of The Pirates Of Penzance have performed together, and there's a lot of intensity and love in the room. And naughtiness. And wordplay and swordplay. And peg-legs and parrots. And a man who rhymes "hypotenuse" with "lots o' news". It is classic Gilbert and Sullivan. It is G&S old-school: silly and soaring.

Sitting behind the orchestra, listening closely to Mabel and Frederic's duet, are some of the young members of the cast including three performers for whom this epic tour of Pirates will be a kind of grand finale, a valedictory stomp around the country in pirate boots. For the past nine months, Katie Grosset, Andrew McTaggart and Ronan Busfield have been on Scottish Opera's emerging artists programme, which gives them a year of full-time work to help launch their careers, and Pirates, which starts next week, will be their farewell tour. It will also be a colourful launch into the excitement – and fear – of the rest of their lives as opera singers.

After the rehearsals finish for the day, all three gather on the sofas at the front of the building to talk about the production, and their descriptions trip out quickly and fluently, a little like the words of a modern major-general. "Pirate joy" is the phrase Grosset uses. "Fun on a stick," says McTaggart. "Who hasn't worn a bandana or drawn on a beard and pretended to be a pirate?" he adds.

However, what this exuberance hides are the realities of the 10-week tour on which they are about to embark. In all, there are 58 dates and eight shows every week, which for an opera singer is at the top end of the sort of the pressure they should be putting on their voices. It's also a personal challenge for all of them: throughout the tour, they will be living in each others' pockets in a range of digs and flats. "Let's see who comes out friends," says Grosset.

Of all three, it is mezzo soprano Grosset, 27, who has had the most practice at touring. In this production, she is Edith ("Stay, Frederic, stay!") and it will be her fifth tour this year (she pretty much hasn't been home since January). She also knows that the upcoming dates – which start in Glasgow and take in Aberdeen, Edinburgh, Inverness, Manchester, Bristol and Cardiff – will be testing. "It is a very long tour," she admits.

All three agree the trick to surviving is looking after yourself with the techniques that sometimes get opera singers labelled divas: wearing a scarf to avoid sudden changes of temperature, that kind of thing. All three are also at an interesting stage in their training. They're all in their late twenties – young, in opera years – but, as 27-year-old baritone McTaggart points out, they have also had years of practice getting their voices, and their bodies, strong.

"When we sing," he says, "our voices are doing some of the work but so are the bodies and muscles that we've developed so we don't need to shout. We've built all that up, so eight shows a week is a push and unusual, but for a West End show it's normal, and we're working with D'Oyly Carte so that's why we're doing it. I'm looking at it as a test of stamina and sanity."

In this production, McTaggart is playing Samuel, the Pirate King's lieutenant ("Pour, oh pour, the pirate sherry!") and, like Grosset and Busfield, passionately defends Gilbert and Sullivan against the opera snobs and accusations that it is light, easy and lowbrow. Grosset in particular is a big fan and compares Pirates to a Disney film such as Toy Story in which there are jokes for kids and jokes for grown-ups. Her 90-year-old Aunt Hilda will see this production, she says, and she can imagine her sitting in the stalls and laughing at every filthy nuance.

"I would encourage the snobs to listen to the words and look at the costumes," she says. "And it's old-fashioned humour – I love Laurel and Hardy and Monty Python and that's what's on stage with Gilbert and Sullivan. It's not dense or intellectually demanding, but Mabel and Frederic have the most wonderful and hilarious relationship, and there are also the most incredible, tender moments. When I was watching their duet, tears were streaming down my face. Two minutes later, I was crying with laughter."

Busfield, 28, who is a member of the chorus and understudying Frederic ("Now for the pirates' lair!") is just as passionate about G&S and says there is a lot of misplaced prejudice about their work because so many amateur companies perform the operettas all year round.

"G&S has encouraged a rich amateur performing tradition in the UK and we should be proud of that," he insists. "We've all been part of G&S at some point – I met my fiancee through an amateur performance of G&S. The music is easier to sing and it's accessible, but it's clever and difficult to do very well."

Busfield also thinks Gilbert and Sullivan sometimes suffers from being in the shadow of its Victorian values: all the stuff about being a gentleman and doing your duty. The answer, he says, is to look for the parts of G&S that have endured and are still relevant: love, relationships, gossip.

Which means we're all agreed then: Gilbert and Sullivan is still challenging, relevant and worth it. What Scottish Opera's emerging artists are not so agreed upon is the blurry line dividing traditional opera from some of the more crossover material performed by the likes of Alfie Boe and Katherine Jenkins. Busfield admires Boe and says he would take the chance to do what he has done, while Grosset, on the other hand, is a little impatient at the mention of Jenkins. "It does get frustrating if a taxi driver asks you what you do and you say opera singer, and he says 'Oh, so you're like Katherine Jenkins then?'. She has a different job – it's like comparing a policeman to a fireman."

However, what the debate over Jenkins and Boe illustrates is that Grosset, McTaggart and Busfield are all already thinking hard about their future and what kind of work they will do after their year with Scottish Opera ends in August. Opera work is hard to find and they constantly have to work at finding it. Money is also a constant worry.

"As well as the battle of having to be sure of yourself and positive about what you're doing, there's also the financial battle when you are studying for this long," says Grosset. "We are very fortunate to be in an employed position. If you're freelance, you're not always going to be employed unless you're lucky."

Like the other emerging artists – like any opera singer – Grosset is constantly auditioning and working on possible projects two or three years ahead. After the end of her year with Scottish Opera, she will be doing further training at the National Opera Studio in London, which takes on 12 singers a year and works with the six top UK opera companies. Scottish Opera will pay her fees but she still needs to find sponsorship for her living expenses.

Busfield and McTaggart are in similar positions: they live in 2013 but, like old Communist states, they've always got three or five-year plans on the go. The first thing Busfield is going to do after his time at Scottish Opera is get married. He will then be working with Scottish Opera again in August on a project still to be announced. McTaggart, meanwhile, is staying on with Scottish Opera for another year as an emerging artist.

In the meantime, there are a few more days of rehearsals left before the first performance of Pirates at the Theatre Royal in Glasgow on Wednesday. Tomorrow there's another run-through with the orchestra and then on Monday there's the first dress rehearsal – something all three are looking forward to.

"Everybody is itching to get their costumes on now," says Grosset. "We've made the cake. Now all we have to do is ice it."

The Pirates Of Penzance is at Theatre Royal, Glasgow, Wednesday-Saturday and then on tour around the country. For more information, visit www. scottishopera.org.uk