Seek the no-brainer first
We've had this sort of leaking with Cherubini 44s, particularly those that were modified at new-build stage because many buyers did not agree with my dad's design and insisted that a prop in an aperture was *better* than the off-center arrangement (which is actually better) my dad (and Herreshoff) preferred. On the aperture boats the propwash was running straight up the rudder shaft and entering over the top of the gasketed flange there. I have heard of this on H30s and Raiders (which use the same Buck-Algonquin rudder-shaft packing gland) as well.
The C44 owners all suspected the prop shaft's stuffing box and kept tightening it ad infinitum. We 'fixed' this by specifying Tides Marine UHMW dripless bearings. The bearing arrives cut exactly to the shaft diameter and has to be hammered down onto it-- after that, all movement gradually wears it to a perfect-clearance fit. I wouldn't use anything else on an inboard-powered boat.
(There is another type of 'dripless' bearing whose name begins with P-- I forget what it's called-- but it's not as good as the Tides Marine one, though it is much cheaper.)
Two lessons got taught from this:
1. Don't mess with a perfect design.
2. Doubt the silliest, easiest and most-likely thing first, rather than relying on your preconceived notions of what *should be* wrong. :naughty: