NOVELTY - The apparatus includes a floating vessel (3) arranged to pitch on an ocean swell, ballast masses within the vessel, and a pendulum mass, particularly a wheeled mass (65) movable along a flat flexible steel beam which can be flexed to a shallow curve (63) or to a more pronounced curve (64) within the vessel. The oscillation periods of the vessel and the pendulum mass may be simultaneously and continuously varied to resonate with the ocean swell.    USE - Apparatus for converting wave energy into electrical power.    ADVANTAGE - The apparatus is suited to high energy wave fields, since the apparatus is designed to operate in the sweet spot of high energy ocean swells with heights in range of 1 to 8 meters and periods of 5 to 16 seconds. The use of a pitching system enables the apparatus to capture both heave and surge energy. The apparatus is also intrinsically robust to provide damage resistance, and is fully sealed from the ocean environment due to absence of external moving parts. Energy capture efficiency is high since the vessel and the inertial engine including the ballasts and pendulum mass may be tuned rapidly and continuously to a resonant condition within an expected range of swell periods. The vessel can be submerged while the engine can be applied with brakes or locked down under predicted extreme conditions to enable the apparatus to survive in such conditions. The apparatus is also low cost in construction relative to power output, since the apparatus uses common materials and standard parts which are simple to manufacture. The apparatus also offers low installation costs relative to power output, since a power cable is laid from the shore station to transmit power to the shore, while one or more mooring lines are attached to the sea floor, and the vessel is towed in place and attached to the lines and the cable. The apparatus is also low in maintenance cost relative to the power output, since the apparatus uses the sealed vessel and since the vessel and engine are designed for minimum sideways stress. Downtime is reduced by reducing incidence of damage and by avoiding the need to tow the vessel to the port for maintenance. Electricity produced can be sold at low costs due to reduced costs of construction, installation, operation and maintenance. The apparatus is environmentally acceptable since the apparatus does not present a visual intrusion, is positioned at a considerable distance from the shore, offers a low profile construction, and minimizes possibility of ocean contamination due to the sealed vessel structure. Efficiency of wave energy conversion is improved by simultaneously and continuously varying the oscillation periods of the vessel and pendulum mass to resonate with the ocean swell. Power output can be increased by building a wider vessel carrying a heavier total inertial mass.    DETAILED DESCRIPTION - The apparatus includes a floating vessel (3), particularly a containing vessel, arranged to pitch on an ocean swell, ballast masses within the vessel, and a pendulum mass, particularly a wheeled mass (65) movable along a flat flexible steel beam which can be flexed to a shallow curve (63) or to a more pronounced curve (64) within the vessel. The curvature of the steel beam is varied by moving the rollercoaster wheel arrangements (66) at the beam ends along the overhead beam end rails (67) and vertically moving the lifts fixed below at the center and longitudinal midpoint to the beam. The added mass moment of inertia of the vessel can be varied by continuously varying the rigid immersed length of the vessel in the pitching plane. The mass moment of inertia of the vessel is also varied by varying continuously the distance of the ballast masses from the pitching center of the vessel. The relative motions of the pendulum mass and the vessel are converted into electricity. The oscillation periods of the vessel and the pendulum mass may be simultaneously and continuously varied to resonate with the ocean swell.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic side views illustrating the variation in circular curvature defined by the flexible steel beam on which the pendulum mass moves.    Floating vessels (3)    Shallow curve of flexible steel beam (63)    Pronounced curve of flexible steel beam (64)    Wheeled mass (65)    Wheel arrangements (66)    Overhead beam end rails (67)