This is a Napier Deltic engine.
It’s a two stroke opposed piston diesel which works like this, multiplied by three or six:
The Deltic is very smooth running and was chosen for use in minesweepers because it was less likely to set off acoustic mines than more conventional units.
This is HMS Quorn
She’s a Hunt class minesweeper in use by the Royal Navy. She and her sister ships were built in the early 1980s and were the last ships to use the Deltic. Their other claim to fame is the use of fibreglass hulls, also chosen as being less likely to set off mines.
Sadly, the days of the marine Deltic are numbered. HMS Quorn has escaped thus far but four of the eight Hunts remaining in the UK have been upgraded which involves hauling out the Deltics and replacing them with less unusual engines sourced rather surprisingly from Caterpillar, an engine supplier not so often encountered on this side of the Atlantic. Once all of the UK Hunts have been refitted there will still be a couple in service with the Greek navy which so fas I know still proceed with the characteristic buzzing noise of the Deltic.
Want to hear a Deltic? They were also used in trains and some have been preserved.
Like this one: