Power take-off device and wave energy converter unit comprising such power take-off device

A power take-off device for use in a wave energy converter unit having a buoy, the power take-off device having:

• a power take-off hull connectable to a mooring device,
• the power take-off hull being connected to or connectable to the buoy, preferably by means of a mooring line, and
• a power extracting device connected to the power take-off hull and adapted to extract power as the buoy moves with the waves, by applying a control force within a predetermined range,
• wherein
• the power take-off hull comprises a first shell and a second shell, the first shell being movable relatively to the second shell, the first and second shells together defining an inner volume,
• wherein a seal is provided to separate the inner volume from water outside of the power take-off hull, and
• wherein the differential pressure between water surrounding the power take-off hull and the inner volume is used to, during operation, provide a nearly constant pre-tension force from the first and second shells being pushed together by surrounding water pressure.