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The British are known to have been an early adaptor of pump jet propulsion technology with the Swiftsure SSN class utilizing it starting in the early 1970s (and first trialed on the HMS Churchill). So the follow-on Trafalgar class naturally received it too. But for whatever reason, the lead boat, the HMS Trafalgar, went to sea with a conventional screw, and stayed that way throughout her entire career. It is the only boat of the Trafalgar class with a convetional screw. Naturally if you're looking to model any one of the pump jet Trafalgar variants you won't need this kit.
The screw I've drawn up is designed for the one and only 1/350 scale Trafalgar kit as produced by Airfix. It is drawn as a straight replacement and is vastly more accurate than what comes in the Airfix kit. I've combined kit parts 37B and 38B into one part that gets bonded to the hull trailing edge. You also get your choice of two screw boss trailing edge (see CAD image). The screw itself is on display at the Royal Navy Submarine Musem in Portsmouth, UK, though the screw boss trailing edge has been removed. Without knowing what it could look like, I've speculated something similar to what we've seen on the Resolution class SSBN screw (also on display in Portsmouth), though I suspect it was an evolution more advanced than that (perhaps along the lines of the US Navy's LA class'), hence why it's not on display.
In addtion, all sbmarines of the Trafalgar class have appeared at one time or another with hull mounted countermeasure ejector chutes just aft of the sail. These seem to vary from sub to sub, and I have drawn up what appear to be the typical 5-hole per side ejector layout (1 large, 4 small) as seen on the HMS Trafalgar.
The ejector chutes are designed to be posted in from the back side and are flanged that way. The flange could certainly be removed and they be posted in from the opposite side. You'll need to cut a 17.8 mm long x 4 mm tall rectangular hole in the hull located 7 mm aft of the side vent trailing edge, 5.25 mm down from the top of the hull.