Diesel engines of the period were unreliable and not very powerful so steam turbines were proposed instead to meet the RN's requirement.

Most of the earlier British submarines had been single-hulled vessels built by Vickers, and the Navy was interested in evaluating other designs.

At the same time Laurenti prepared a design with geared steam turbines having a speed of 18 knots on a surface displacement of 856 tons.

The upper portion of the double hull was controlled free-flooding while the rest was devoted to watertight 'baling flats', ballast and fuel tanks.

[4] In marked contrast to contemporary Vickers designs much attention was paid to safety arrangements, including her extensive subdivision.

[4] Swordfish could carry 102 long tons (104 t) of fuel oil, which her builders estimated gave her an endurance of 3,000 nautical miles (5,600 km; 3,500 mi) at a speed of 8.5 knots (15.7 km/h; 9.8 mph) on the surface.

[7] Captained by Commander Geoffrey Layton, her post-completion trials lasted for five months as she was used to evaluate steam power for submarine use.

She proved to be very unstable while surfacing, presumably because she could not pump the water out of her controlled free-flooding spaces quickly enough in the upper part of her double hull.

[5] These problems, coupled with the fact that she was too slow to work with the fleet as originally envisioned, meant that she was impossible to make into an effective warship, and she was laid up after her trials.

She was rearmed with a pair of 12-pounder[Note 2] (3 in, 76 mm) guns and depth charges, weapons more suitable for her new role as an anti-submarine patrol boat.