Thanks to all who have responded.
My counter is always under water, at standstill the whole counter is about 2
inches under water, and underway a bit more.
The problem only exists in shallow water. Deep water running is fine,
additionally no rudder vibration in deep water.
Rudder vibration in very shallow water (K&A shallow pounds) starts not far
up from tickover and gets worse as revs go up.
My propellor size was calculated by the boat builder and the engine supplier
and they both came up with the same figure of 20 x 14. I checked this
against the Excel prop calculator (has anyone still got a copy of this as I
have lost mine) and it came out with similar figures.
My stern does not have a standard V shape, it has been flattened off to
allow fitting of the internal hydraulic motor and the external pod (which
means I do not have a standard stern gland). The flat portion is 190mm x
590mm and the prop shaft is in the centre of this. The actual prop is 210mm
from this flattened stern and the rudder starts 240mm behind this.
Having checked the rudder whilst on the slipway there is no bend and whilst
in air the rudder centres from either direction and so I assume it is
correctly balanced. The portion of rudder before the stock is 70mm at bottom
and 175mm at top and behind the stock is 520mm and 455mm respectively,
whether this proportion is correct I am unsure.
Whilst the boat was being built I asked my boatbuilder whether the
flattening off of the stern would cause me any cavitation problems and he
was unsure as he had never attempted this type of fit previously.
Is there anyone out there that has hydraulic drive and a similar fit to me
(Nanni engine and Peachment Hydraulics), if so do you have any problems
similar to me????
Terry
nb Hakuna Matata
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