From http://www.allefriezen.nl/, Minute-deeds in 1838, 1839, Notarial Archive
Rudolf Durks North Werff residing in Woudsend
Amount: annual rent 87 fl
Apparently kofschips were for inland and coastal navigation. Here is what a Kofschip looked like. Pretty impressive. The painting is in the Fries Scheepvaart Museum, a maritime museum in Sneek (the big town in the Tjalsma neighborhood).