Prop-shaft couplings need regular maintenance to make sure they stay secure. When the bolts on the coupling become loose or disconnect all together, this can cause extensive damage. The worst-case scenario is the engine being thrown off its mountings potentially damaging the stern gland, prop-shaft, coupling, gearbox and engine mounts, plus any external components that may become trapped when the engine shifts. The cost to rectify can go into the thousands.
Unfortunately, this kind of occurrence is still relatively common due to boaters overlooking maintenance in this area. Everyone knows filters and fluids need maintaining, however ensuring bolts are tight tends to be less recognised. The worst part is, this is one of the simpler maintenance tasks and easier to identify than a loose engine mount or slack fan belt.