The yacht was moored at its berth when the fire started at approximately 1.30 am on September 6.
Two fire brigade teams attended the scene and neighbouring yachts were forced to evacuate the area as fire teams began to tackle the blaze. Four crewmembers were on board when the fire began.
The fire was extinguished after three hours but the damage to the yacht was severe. Siempre eventually capsized and began sinking.
A statement by Private Yacht Group, which manages the vessel, said: "The fire started on the aft deck and quickly spread to the internal areas, but the cause is as yet unknown."
"All crew safely evacuated the vessel and were unharmed, however, the Tansu built vessel that was launched in 2010 and refitted in 2020/21 is a total loss and has sunk. Efforts are now being made to salvage the vessel and limit any environmental damage."
Siempre was originally launched in 2010 under the name Ceylan and was most recently sold in July 2020.
The semi-displacement yacht was designed in-house at Turkish shipyard Tansu Yachts and featured a steel hull and aluminium superstructure. Twin Caterpillar C-32 diesel engines provided a top speed of 17 knots.
Siempre could accommodate up to 10 guests and was available for charter in the Mediterranean.