maintain an angle of 10 u00b0 creating a negative lift arrow a which will therefore push the spikes 16 into the seabed and immobilise the space frame 10 as will be described subsequently positioning lugs 46 mounted on a central shaft 48 provided a stop for locating lobes 44 of the hydrofoil 12 such that the hydrofoil 12 cannot rotate further than the 160 u00b0 shown in fig2 a to 2 d by rotating the central shaft 48 through slightly greater than 180 u00b0 say 200 u00b0 the negative lift becomes positive lift arrow b and the space frame 10 will rise through the water so that the tidal turbine 90 can be recovered on the vessel not shown fig4 shows in more detail the mechanical assemblage of hydrofoils 12 with space frame 10 the hydraulic motor 30 for actuating the positioning gear is equipped with a drive shaft 32 that is utilised for rotating an indented positioning gear 42 or a toothed gear wheel the positioning gear 42 is solidly attached to a central shaft 48 which passes through a bore provided in the larger end of each hydrofoil 12 a section of which is show on fig4 the bore of the hydrofoil 12 is provided with a pair of diametrically opposed and inwardly projecting hydrofoil locating lobes 44 the central shaft 48 has a pair of diametrically opposed and outwardly projecting positioning lugs 46 each one of which selectively co operates with one of the respective pair of diametrically opposed hydrofoil locating lobes 44 thus by rotating the drive shaft 32 the hydraulic motor 30 actuates or rotates the position gear 42 which in turn rotates the central shaft 48 the positioning lugs 46 will contact the locating lobes 44 and carry them.