Gary, I dont think there is a rule as such, just guessing, but the PT boat props may all go round the same way for economy and speed (of construction)..ie, the motors were all the same and so to make one prop go the other way would have meant another set of gears?

For a fast model,there should be a handling benefit by having the two outer props differently handed, or you may like to keep it original and overcome the little handling quirks buy rudder twiddling and/or having permanent offset ballast or rudder set. This would be more fun I am thinking, especially in reverse when the difference will be more marked…

Must be a million bods out there who have done it both ways and will be flooding this post with wisdom (arguably) <everyone argues about something like this>

Ashley

Simple answer is : "The motor needs to rotate in the correct direction to suit the propellor."