Hi Margaret,
From the illustration it would appear to definitely be one of the Sinclair/St.Clair armorial achievements, I have two entries in Burkes General Armory which would be possibly applicable. The first is the Earl of Orkney and of Caithness whose arms were quarterly 1 and 4 - Orkney and 2 and 3 - Caithness. The blazon for each of those are as follows:
For the Earldon of Orkney - Azure a Ship at anchor oars in saltire and sails furled within a double tressure flory counterflory Or.
For the Earldom of Caithness - Azure a Ship under sail Or the sails Argent.
These two feudal coats are found marshalled with Sinclair of Rosslyn [Argent a Cross engrailled Sable] in different ways by the Earls of Orkney and their descendants. No Crest or Supporters given.

The second record is a lot closer to the actual illustration that you have provided, but the motto is completely different. The blazon for these arms are for Lord Sinclair originally descended from the eldest son of the last Earl of Orkney. Through an heiress, the succession passed to the St. Clairs of Herdmanston, and by a new patent, their paternal relatives were brought in as successors, strangers in blood to the original line. This blazon reads:
Quarterly 1st and 4th Orkney as above; 2nd and 3rd Caithness as above, en surtout Argent a Cross engrailed Sable.
Crest: A Swan Argent ducally gorged and chained Or.
Supporters: Two Griffins 'proper' armed and beaked Or.
Motto: Fight

I put the tincture of the Griffins inside quote marks because, not having seen one, I'm darned if I know what the actual colouring of this mythical creature might be ;-))