ORIGINAL: Rhus
About your statement regarding "lower rpm"; Is it normal for the rpm to be lower in general, or is it because you typically bump the prop size up that you get lower rpm? I was under the impression that Davis Diesel claimed a slight rpm increase.
Thanks.
Others have already answered the lower RPM question, but one additional thought: Some older long stroke diesel designs run better on large props. They were designed to peak at a lower RPM and will break if run way past their designed RPM. Famous engine man George Aldrich used to say, "Never under-prop a diesel".
When running an old diesel, I would consult data that suggests RPM range and prop sizes. As a rule, I like an 8x6 or 9x4 for a .15 diesel. I have run my old David Andersen twin stack (~1952) with a 9x6.
Diesels (purpose-built or conversions) of modern construction can usually handle a wide range of props. If in doubt, use what the manufacturer of the engine or conversion suggests.
George