She was a Special Service Vessel (also known as Q-ships) whose function was to act as a decoy, inviting attack by a U-boat in order to engage and (if possible) destroy it.

Penshurst fought a number of engagements against German U-boats during her service, and was successful on two occasions, destroying UB-19 in November 1916, and UB-37 in January 1917.

Penshurst was built in 1906 as a cargo steamer, and had an uneventful peacetime career before the start of World War I.

For almost a year she had little success; during 1916 the Imperial German Navy had scaled down their U-boat operations against commerce around Britain, and there were few contacts in this theatre.

Grenfell's crew went through their "abandon ship" evolution, putting out boats manned by a "panic party", while Penshurst stopped, waiting for the U-boat to come closer.

However the U-boat declined to come closer, and with it partly hidden in the glare of the setting sun Penshurst opened fire.

The following day on 30 November Penshurst, having changed her appearance and moved to a different part of the English Channel, came upon a U-boat, UB-19 attacking the steamer Ibex.

However, the U-boat made to cross Penshurst's bow, continuing to fire, causing damage and several casualties over a twenty-minute period.

On 2 July, back in service and under a new commander, Lieutenant C Naylor, Penshurst was again in the Southwest Approaches when she fell in with another U-boat.

However UC-72 was not fooled by this lacklustre response, and Penshurt was not fully under control, so Naylor refrained from sending off his boat party, but opened fire with all guns.

The boat, U-110, submerged in order to let Penshurst come into range, and just after midday fired a torpedo, which hit her in the engine room.

Naylor sent his boat crew off, but U-110 remained under for two hours, examining the ship, until she surfaced off Penshurst's bow and commenced shelling.

She scored some hits, but U-110 submerged again, and, when a Royal Navy submarine chaser arrived around 4 pm, made off, having suffered little damage.

Penshurst was one of the Royal Navy's most successful Q-ships, fighting eleven engagements over a two-year period, and destroying two U-boats and damaging several others in that time.