In renewing the downhaul line,and digging out the knot from the recess,I noticed that due to this activity over the years the mahogany core was no longer sealed either in the side recess or in the somewhat enlarged and irregular drilling that accepts the line.On a boat that dries,where the rudder is typically left hoisted,this means that the recess and drilling are constantly (while afloat) below the waterline and any exposed core constantly wetted.This seems much less than ideal to me even though the timber is clearly resistant and in our case without cracks,the surface merely being 'aged'but easy to clean back.Rather than attempting a multi coat waterproof seal thro the difficult access,and wanting to maintain easy renewal,I used poured West resin to line the recess and reform the drilling,using a centred 'plug' of timber broomhandle for the recess and a spare 'nylon' rigid tube (ex foam aerosol)initially as a 'poker' and then as a former to maintain a guidepath for the drilling.
Whatever the solution adopted it's worth checking if your core is fully sealed.