Cornelis was merchant and sawmill owner. His sawmill was called "Gruawe Eend" which he biught from his uncle Aris de Boer in jan 1770. In feb. 1767 he bought the "paltrok" "de Deutel" alias "het Herderskind". In 1776 he expanded with "de Herderin" which geocraphically was placed behind Breedwater te Koog. Cornelis was also accountant and owner in a whaling company. Later he bought a whale oil refinery together with his uncle Aris de Boer. This was a long term wish as these men had been interested in whale fishing around Greenland for many years.

They had 2 whaling vessels built, "de Hopende Visser" under command of Commodore Cornelis Riekels, and the "Cornelis en Neletta" under command of commodore Adriaan de Groot. The whalers were active until 1792.

In 1798 Cornelis bought the mill "de Dikkert" (which today still exists as restaurant).

After the 4th war with England the whaling industry grinded to a full stop.