Generally the porcelain insulators were not strong enough to withstand the forces in the backstay. We used to make them from Delrin rod in which we drilled two holes. We also milled material from the holes with a depth of half the backstay wire to the end of the rod so that it had a snug fit. See the rather primitive drawing to give an idea.
Polyoxymethylene (POM), also known as polyacetal, is a polymer that mainly has an industrial application because of its hardness. The substance is also known as the Delrin monopoly, developed by DuPont. Delrin is often used for parts in machines as a replacement for metal. The advantage is that it does not oxidize and that no oil or ball bearings are required for smooth mechanical mobility.
On medium wave frequencies propagation occurs mainly with the help of the ground wave. For a good range a solid earth connection is then essential. On wood and glass boats this is a problem. A not so ideal solution, but better then nothing, is to install a Dynaplate on the hull. It is a porous bronze construction which must be kept clean and certainly not painted!
Distance between holes: 63,5mm
On larger yachts I have also seen larger, 1x1 foot, gold plated earth plates with a wrinkly surface. The idea is that the uneven surface has a larger earthing area.