lubricant may lead to a reservoir for maintaining a constant supply as piston 21 strokes up and down in the preferred embodiment a plurality of seals 22 surround the circumference of piston 21 in the embodiment shown in fig1 and 2 seals 22 engage an interior surface of cylinder 19 a piston chamber is defined by piston 21 seals 22 and cylinder 19 in the embodiment shown in fig1 and 2 a plurality of seals 26 also extend from cylinder 19 to sealingly engage rod 23 cylinder 19 is connected on its lower end to a brace 27 by a pin 25 in the preferred embodiment pin 25 is spherical so as to allow pivotal rotation not only in the plane containing the drawing but also in a z plane perpendicular to the plane containing the drawing brace 27 in this embodiment is secured to deck 11 and the lower ends of cylinders 19 are located approximately at the same level as deck 11 each cylinder assembly 17 inclines relative to riser 15 and deck 11 in the embodiment shown in fig1 and 2 the upper ends of rods 23 are closer to riser 15 than the lower ends of cylinders 19 rods 23 are secured by spherical pins 29 to a top frame 31 top frame 31 is mounted to a tension ring 33 that is clamped or otherwise secured to riser 15 for movement therewith the radial distance from the axis of riser 15 to upper pins 29 is less than the radial distance from the riser axis to lower pins 25 the angle of each cylinder assembly 17 relative to.