The Duchy of Savoy was not able to deploy a proper naval policy until the acquisition of Sardinia in 1720. In early times, the Savoyard Sardinian ensign was a white cross on a red field, with the "FERT" cypher on the cantons, either to distinguish it from Malta's own ensign and probably to make it closer to the traditional civil ensign of Sardinia itself (a red cross on a white field, with a black head on each canton). At some point within the XVIII century, this white cross was placed on the canton of a plain blue field.
Sources: «Flags of The World» (FOTW) website:Duchy of Savoy | «War Flags Through the Ages & Around the World» website:Kingdom of Sardinia
Sizes: ensign W12 x L18 mm, mast flags W8 x L12 mm, jack W6 x L9 mm, pennant W4 x L48 mm.