an anti cavitation plate 9 mounted above the propeller 8 in order to synchronize the steering movements a tie bar 10 pivotally connects the anti cavitation plates 9 to each other according to the invention a shock absorber in the form of a piston cylinder device 11 joins one drive 3 to a fixed portion of the boat transom 1 in the example shown in fig1 and 2 the piston rod 12 of the shock absorber is joined to the anti cavitation plate 9 of the right hand drive via a ball joint 13 while the cylinder 14 is joined to the left hand drive shield 4 via a ball joint 15 lying on the tilting axis of the drive thus the shock absorber 11 is not affected by the tilting movement in the single drive model shown in fig3 the ball joint 15 of the cylinder 14 is mounted directly on the boat transom 1 the shock absorber 11 can have the same construction in principle as those used for automobile wheel suspensions i e it can have a piston provided with valved oil channels said piston being displaceable in a cylinder filled with oil the resistance to displacement being determined by the constrictions in the channels tests.