Rudder vibration when motoring.

[Saltram31]

My yacht has developed a vibration from the rudder when motoring. I have owned her for 10 years and have not had this issue before. The rudder is mounted on 3 pintals and has a tiller. There is no vibration when the engine is in gear while tied to a pontoon.
I have recently moved her to a swing mooring which is in a quiet part of the harbour and initially thought the boat could have been hit by another boat, but there is no sign of any damage or marking to the gel-coat around the stern area. Last year I replaced the cutlass bearing and flexible coupling so doubt it is related to the stern gear. Any ideas would be welcome.

 

[Tranona]

Obvious thing is worn pintles if is recent. Possibly fouled propeller. Unlikely to be anything to do with the stern gear changes.

 

[Seajet]

Check for any barnacles or weed on the prop; more likely, has something nasty like weed or flotsam - could be rope etc - floated downstream onto the prop - has it got a cutter ?

As a nasty idea I hope doesn't apply, if the boat is on a drying mooring could the prop or shaft have settled on some unpleasant hard bit ?

Certainly sounds like the symptoms of something round the prop' though - I once had to snorkel under my father's Centaur to clear the prop', which had picked up weed on the way into Cowes; the engine and boat vibrated badly and was lucky to make 3 knots - despite having a cutter.

 

[Saltram31]

Thanks for the replies. I checked the pintals when changing the stern gear and they were in good order. I coated the prop with Velox but did notice it did not adhere well as it came off in patches after about 3 months so prop fouling is a possibility.
Seajet she is a long keel so the mooring is supposed to be deep water but heard from a bloke who keeps his boat near to mine that during the strong Northly winds, the channel emptied at low water more than usual so the boat hitting the ground seems likely. I'll get her lifted out and have a look.

 

[Seajet]

Skatt,

a check is always good but with a long keel I'd think it extremely unlikely there's any bending or damage to the prop and shaft - as I feared - even if she has touched the bottom - still as said no harm in a check - apart from maybe to your wallet !

My money is on collected weed, which may even have dropped off later.