The Royal Navy Captain's rank shoulder strap or board epaulette is a distinctive insignia worn on the uniform to denote the rank of Captain in the Royal Navy. Here is a short description of the Royal Navy Captain Rank Shoulder Strap (Epaulette):

• Insignia: A Royal Navy Captain's shoulder strap or epaulette typically features a single row of gold lace, referred to as executive curl, on a navy blue background.

• Rank: The epaulette signifies the rank of Captain, which is a senior officer rank in the Royal Navy.

• Background Color: The background color of the epaulette is navy blue, in keeping with the standard color for Royal Navy uniforms.

• Size: The size and dimensions of the Captain's epaulette may vary slightly, but it usually includes a single gold stripe of lace with the executive curl.

• Epaulette Attachment: These epaulettes are typically attached to the shoulder of the uniform and are used to distinguish a Captain from lower-ranking officers.

Please note that uniform regulations and designs can change over time, and there may be variations in the specific design elements or materials used for Captain's rank insignia. For the most current and accurate information, it's advisable to refer to the latest Royal Navy uniform regulations or consult an official source.