Thanks Al,

The shoe is outside the hull, it fits into a "boss" on the bottom of the hull that is the same shape as the top of the rudder (see pic.).
Four large flat head machine screws fasten it through the hull (about 3/4" thick here) with washers and nuts on the other side.

The shoe has a circular groove cut in it around the main tube presumably for sealant.

Unfortunately I can't get McMaster Carr to deliver here in Canada, but I will try to source some silicone gasket.

Will a gasket seal the machine screws as well ?

Cheers,
John.