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Rudder rattle

The higher I rev my engine, the more my rudder rattles. At about 20000 its rattling so hard that the handle shakes violently and I have to decrease revs. The engine is a brand new and newly fitted beta 38. Very professionally fitted and I have no concerns about that part of that. But why would my rudder handle rattle so much?

Asked by: Carl Young  | 11.28am, Thursday 1 August

WW says:

There could be several reasons- the design of the rudder might not be that hydrodynamic, so water flow around it is poor, leading to pockets of turbulence, which give a rattle to the blade. A poorly designed blade mya have too little (or too much) "balance" area (the bit of the rudder which is forward of the rudder stock). If the rudder is made of too thin plate, it is liable to judder, as are thicker rudder plates which have no fairing on them on the leading edge.
Another possibility is that the propellor is too close to the rudder, so that increase in power leads to increased local turbulence around the blade. This can also happen where the swim design of the boat is poor- such as too short, not smoothly tapered, not having a sharp/fine end to the swim (a section of flat plate is the worst option here), having a very low skeg below the hull to mount the rudder on, etc.
The rudder blade itself may be bent below the water, which can increase vibration. Also, the lower cup bearing the rudder stock sits in my be damaged, while the top bearing may be loose, though this is more likely to happen because of the vibration, rather than causing it.
If the propellor is incorrectly sized, or has insufficient clearance between the blade tips and the counter/weedhatch, then this can increase ventilation (and possibly cavitation at high revs) which may again increase turbulence.
Alas, it can be hard to remotely diagnose the actual problem. Do you have pictures of the boat out of the water at all? This might help. If you do, please email them through 9and any further details- possibly a picture of the inside of the engine compartment/rudder, etc.) to mark.langley@waterwaysworld.com if we can be of any more help.

Mark Langley  | 11.59AM, Thursday 1 August

It might seem obvious, but if there is something wrapped around the propeller, the water flow will be disrupted sometimes causing a rattle. Check the prop is clear of rubbish.

Rupert Smedley  | 11.07AM, Sunday 11 August

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