I was cruising on the Upper Avon recently when I had to take evasive action to avoid being sunk by a very fast travelling narrow boat, unfortunately the prop grounded fo a few seconds. This caused a strong banging somewhere under the rear of the Volvo AQ100 Leg if the revs went above 2000 rpm. At 2500 rpm the banging stopped and the boat appeared to have lost all drive and lost speed. Reducing the engine revs to idle and then opening up to 1800rpm all drive was restored, however the banging at 2000 rpm and loss of drive were still there. With the boat out of the water the cause of all the trouble became obvious. At the centre of the prop boss is a rubber bush and this could rotate at higher engine revs and so loose drive. Water is a great lubricant. Also the prop had been pushed slightly forward into the sacrificial anode and destroyed it. Fitting a new anode and replacing the prop cured all problems. The old prop has gone away for repair. Thought that this might be of some interest to someone.