Today, every ship, submarine, shore establishment and Naval Air Squadron has its own unique badge. A ship's crest, not badge (see note) as I am often informed,  and their name represents a piece of history in that it is not the first time that the badge or name would have appeared on a Royal Navy ship or submarine. For as long as there has been a navy ships have been named after other ships that previously existed. The Trafalgar class submarine HMS Turbulent for instance is the fifth unit to bear the name Turbulent. Ships names are usually allocated to the same class of ship, a good example of this is the name HMS Ark Royal that apart from the first that was 690 tonne 38 gun ship built for Sir Walter Raleigh all of the subsequent five have been aircraft carriers.

The first HMS Ark Royal did not have a crest like the present Ark Royal but was identified by its figurehead as almost all ships were until the creation of steam powered ships. Although HMS Warrior, the first steam powered iron-clad warship that was launched in 1860, had a figurehead the new advancement in propulsion and protection forced a change that was soon to see the abandonment of the figurehead in warship design. The disappearance of the figurehead caused ships to find a new way of uniquely identifying themselves and soon all sorts of designs of crests proliferated. The adoption of crests by ships was done without direction from the Admiralty and thus there was no uniformity in the design of badges. Many Captains would fund the creation and design of their ship's badge with mixed results. Captains who had their own family Coat of Arms would assign this as the badge for the ship that they commanded and take the Arms away from the ship when their command ended, causing the ship to create a new crest with which to identify herself. Extract from: http://www.royalnavy.mod.uk/server/show/nav.3650 The Navy say its a badge when on clothing, a crest is shipborne! Well, as 99% of the planet call them crests, then I suppose they win the vote ? I found out (March 2017) why different vessels have different shaped crests/badges. Thanks to Joe for asking the question. Here is the solution:

As we all know Royal Navy ships at one time had figureheads until these became too costly and were discontinued. Some captains and ships officer designed there own unofficial ships badges until 1918 when an Admiralty Fleet Order stated that all ship would now display an Admiralty designed badge. There were four different shapes Circular, Pentagonal, Shield and Diamond. Circular for Battleships and battle Cruisers, Pentagal for Cruisers,Shield Destroyers and diamond for all other ships and shore establishments. In 1940 an Admiralty Fleet Order Stated that all ships and establishments would now only have circular badges. Some ships have kept their old badges, ships being built to replace older ships have passed the badge over to new commission as it seems was the case with Walkers own WREN. Some ships dont have badges such as submarines and unarmed auxilaries such as boom defence vesells such as Barbastel which I served on. The emailer is quite right in saying that Sheffield had two types of badge pentagonal and circular, she had circular when I served on her in 1955 which was arrows laid out as lattice under the Admiralty Crown. From Ray Holden, formerly career sailor..