My rudder contained 15 litres of oil until I drained it (I discovered two tiny holes where the oil was escaping. I've welded the holes up, but I've been told that water is just as good as oil to dampen vibration noise. Any experts out there? I don't want to have to fill the rudder with oil again-it would take ages and create an oil catastrophy.
I assume your boat is a steel hull. The rudder being built of steel plate would have too much buoyancy if not filed with liquid. Oil is the best rust preventive for filling it up; don't know about antifreeze with water. john
My understanding of yacht design is that, generally, rudders on monohull sailing yachts should exhibit neutral to slightly positive bouyancy.
I suggest that calculation is required to determine the bouyancy of the hollow steel rudder. I think that most such rudders would have a negative bouyancy, and at best, would be neutral. Steel weighs approx. 490 lbs./cu. ft. Sea water weighs 64.4 lbs./cu. ft. Remember that for the rudder to be considered at least neutrally bouyant, the bouyant force provided by the immersed volume of the rudder must offset the entire dry weight of the rudder, including the weight of all the remainder above the water line (stock, head, etc. I have never seen a steel rudder of any design on a sailing yacht that exhibited positive bouyancy.
Filling the rudder with liquid only detracts from the bouyancy of the rudder.
It is possible to offset the undesireable effects of negative bouyancy of the rudder as it affects steering, whilst not changing the bouyancy of the rudder. This is done by adding a counterweight. On my boat, tiller steered and having a stern hung rudder, adding a small weight near the inboard end of the tiller reduces the negative effect (though not to zero) of the negative bouyancy of the steel rudder. This increases the load on the rudder bearings, but only slightly. My rudder is hollow, weighs approx. 132 lbs. dry, and has approx. 60 lbs. bouyant force (by calculation). Fortunately, the greater mass of the steel is not far from the axis of the stock, so the negative effect on steering is tolerable.