Despite her lack of speed, Mr Thomson said Tuna was a "nice little boat".
The 48-year-old now owns a fleet of three towboats that work on the Kaipara harbour. He said the best thing about his job was the "great bunch of guys" he worked with.
Mr Thomson was about 25 when he started working commercially at sea in 1989.
"I was deckhand on the rubbish boat - that's where I started. A pretty unglamorous job, but quite a lot of fun."
But his family's seafaring history goes back a lot further than that.
His paternal grandfather ran a schooner through the Pacific and he and Mr Thomson's great-uncle were both master mariners.
He said the authors of 1932 novel Mutiny on the Bounty, Charles Nordhoff and James Norman Hall, had travelled around the South Pacific on his grandfather's sailing vessel.
"He was a foreign-going master, his brother was a foreign-going master, and dad was into tugs and stuff - that's how I got into it. It looked like a lot of fun, and it has been."
On Monday, he reckons there will be about 12 people heading out on Tuna, which might weigh the little tug down a bit. "That's why we're not winning, you see."
Monday's tugboat race starts at 10am from Princes Wharf. Boats will race to Orakei and back.
They will then be on display off Queens and Captain Cooks Wharves before being berthed at the Tidal Steps off the Karanga Plaza.