Gerrit Femssen served the Admiralty of the Noorderkwartier. We know he served in the First Anglo-Dutch War and was at the Battle of the Sound, in 1658.
We know three ships he commanded. In 1652, he commanded the Wapen van Enkhuijsen The first listing credited her with 30 guns and a crew of 110 men. There is another listing which credited her with 34 guns while still having a crew of 110.
In 1656, Gerrit Femssen commanded the Gouden Leeuw in the blockade of Danzig, under the command of Jacob van Wassenaer. The Gouden Leeuw carried 48 guns and had a crew of 155 men. Her dimensions were: 134ft x 34ft x 13.5ft.
In the Battle of the Sound, Gerrit Femssen commanded the old ship, the Eendracht, built in 1639. She carried 38 guns and had a crew of 125 men. Her dimensions were: 130ft x 32ft x 12ft.
We know a few facts about his service in the First Anglo-Dutch War. On July 26, 1652, attended a council of war on Tromp's flagship, following the storm off the Shetlands [1DW1, p.393]. At the Battle of the Kentish Knock, Gerrit Femszoon commanded the North Quarter ship, Wapen van Enkhuisen, which carried 30 guns and a crew of 110 men [Ball, p.366]. There was a document that listed the Noorderkwartier ships that were part of the "36 Ships of 1651": Gerrit Semssen [Femssen] was captain of the ship, 't Wapen van Enkhuisen, which carried 34 guns and a crew of 110 men. The ship is a national ship ('s Lands schep). There is a notation on the list that the ship "was cruising between Texel and Ushant, and afterwards served under Lt-Admiral Tromp's flag, and being engaged in the last action with the English, came in damaged to Enkhuisen; is now lying ready to return to Texel". The ship appeared on a list of ships belonging to the thirty-six ships of 1651. [1DW4, p.315].
On November 2, 1653, Admiral Wassenaer reported that Gerrit Femszoon's [Femssen] ship, from Enkhuisen, was lying off the Texel, with other ships [1DW6, p.159]. This was after the strong storm that swamped or wrecked may Dutch warships, on their way home while convoying a merchant fleet that had put into Norway.