Mystery solved - and personal prejudices reinforced.
With the boat I inherited a plastic prop (you know where this is going, don't you!). It was used from a shingle beach and replaceable blades seemed a good way to mitigate the inevitable dings.
The plastic boss "fuses" in the hub have broken. I may have hit something at one point - there is a lot of rubbish in the river at the moment.
The really bad point is the fact that the exhaust ports were occluded by the aft portion of the propeller.
A steel propeller has been added to the shopping list.