You know me. I would use the same alloy as the bushing. Close isn't close enough. Ask your expert if he knows what galvanic corrosion is, yes? - then will he guarantee the installation?
It isn't usual to have a bushing down there on the shoe. If the hole is drilled precisely 3/4s inch diameter and 3/4s inch deep and the shaft end is repeated exactly, minus a micron, you have, I believe, and Pearson certainly believed,
a good long lasting fit. If one has to have a bushing, I would think a thin washer of delrin would be ok. If you make the strap gudgeon openable as part of your yearly maintenance, a new washer could be slipped in fairly easy. You'ld have to disassemble the tiller head.
I'ld guess that ANY thin material you'ld be putting down there would have to be renewed regularly.
The socketed rudder shaft in the hole has no place to go. Can't hardly move downward. Might argue that it can't even wear any if it is fit to the specs. I know in my bones that the married couple have to be totally compatible, preferably the same stock. Otherwise they will be exchanging electrons not vows
[You know I don't think a bushing is necessary or even an upgrade. If you keep the specs as above, the bearing surface of the added bushing would be too small, too narrow to act as a 'bushing'.
(If you turned the shaft down to 1/2" you could make a quarter inch washer for the shaft for more surface for the flange to sit on. I can't see that a smaller stub on the end of a 1" shaft is a good thing on the barn door type of rudder we have.)
The 3/4" shaft end turns only on its circumferance in the bottom of the ideal hole right where the drillpoint's V shape begins. Ideally the shaft here shares support with the flange that rests on the shoe. In an ideal world the shoe has exactly the correct machined surface at exactly 90 degrees to the shaft. But this is probably more luck than reality. Would think that the heel and the shaft being a quarter of a degree out a washer/bushing would be reduced quickly to useless. Arguable that this may be a reason for the added piece.
IMCO such a small inconsistency mates in to something very stable because of the relative softness of the copper alloy. IE the rudder shaft stabilizes itself. (Such closeness demands exact same alloy in the two parts!!!) The shoe is itself the bushing with not much friction at all.]
[2) Agree that a bushing is a sleeve that lines a hole? So that's the arguement, that the hole in the heel of the shoe is self bushed. And works in this case because it is, or should be, the same alloy.
However, when you talk with an engineer you might be designing in a bushing in the glass reinforced plastic shoe for the rudder shaft to turn in. It would keep the hole from elongating. I think all the polys (carbonate - ethylene - propylene - etc) are too soft and won't be made hard by the fiber. There are a zillion plastics. At the moment that Marelon stuff is unbelievably tough and hard and proven in the marine env. It might be comparable to bronze, maybe. And it might be found by another name if the patent has run its course.]
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later Edit: If anyone persues an alternative non-metal material for the rudder shoe...GAROLITE (available at McMasterCarr.com) would certainly
be a phenolic/glass contender as the substitute. The mass of the existing cast rudder shoe would translate, imco, to an equal mass
of G-10 - and is a much harder material than nylon or acetal.
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