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Re: Islander 28 propeller location from shaft support?
"I bought the boat with an electric motor,"
Could easily be read to mean you bought it with an electric--but are in the process of restoring the proper diesel engine. Since the usual prop would be relevant for the usual diesel engine, not the electric.
Either way, David Gerr has a "Propellor Handbook" out there. From the displacement of the boat, the engine speed, etc. there are formulas for figuring the correct prop to use. That would be another way to go, and to verify differences from the diesel setup. Although I'd figure, the same prop at the same prop speed should be used for the same boat, regardless of what was driving the prop.
A good prop maker might even know what they've sold for the boats before.