teeth 26 fig6 which engage respective teeth of rack chocks carried by jacking assemblies 30 fig3 conventionally there is one jack assembly for each chord member 22 horizontal and inclined braces or trusses 32 rigidly interconnect the chords 22 the chords 22 are located at apexes of the triangularly shaped legs 12 of course the number of chords and the shape of the legs are not limited to the embodiment shown in the drawings each leg 12 is provided with the jacking assemblies 30 for moving the leg vertically with respect to the hull 14 the legs 12 move from a raised position when the jack up unit is in transit and the legs 12 are supported by the hull 14 to a lowered position when the legs 12 support the hull 14 the lowered position is illustrated in fig1 each leg 12 may be provided with a spud can 34 for bearing against an ocean floor and for supporting the jack up unit the jack assemblies 30 are retained against vertical displacement by the hull 14 as shown in fig2 a typical jack up unit has nine jacking assemblies 30 three assemblies per leg with one located at each leg chord 22 of the triangularly shaped legs 12 each elevating jack assembly 30 is provided with four pinions which operationally engage teeth 26 of racks 24 associated with the legs 12 the jacking system also includes a system of guide plates installed above and below the pinions the guide plates act as a horizontal restraint for the drilling unit as they deflect under harsh environmental conditions turning now in more detail to fig4 6 the leg guide system of the present invention is shown to comprise a plurality of edge guides 36.