I have just carried out a few rough calculations.
If the tiller is 1 metre long.
If you can exert a pull about the same as your weight on the tiller about 220lbs or 100Kg.
The torque on the rudder stock is T = F x L.
This is T = 100 x 1 = 100Nm
If the arm at the bottom of the tiller stock is say 3″ or 75mm long – just a guess I haven’t measured it.
Then the force on the wire is F = T / length
Wire force = 100/0.075 = 1333Kg.
If we use 5mm stainless wire it has a safe working load of 1448kg. I think the breaking load is 14210kg.
So I reckon that at the most the helmsman can apply a load which is just within the safe working load.
However on a mooring, when the blade is raised, and the helm locked, that safe working load will regularly be exceeded by shock loads as the boat swings between wind and tide. It clearly isn’t reaching the breaking load of the wire – or you would come back to the boat to find a broken wire.
If anyone is unsure about the force that a raised rudder can exert on the helm try motoring backwards, with the blade raised. Hold that tiller centrally and firmly or you will lose control of the tiller.
I think this is the cause of your problems.
Check my calculations and take measurements to see how far out I am – but I think my values are in the correct order of magnitude.
Trevor