Why does my two engines have mirrored propellers?

[Maika]

When I bought my boat, the previous owner had mounted the port propellers on starbord drive and visaversa. This made the boat go in reverse when pushing the handles forward and visaversa.
The boat was beached, propellers where swithed to oposite side, and the controllers now worked as they should.
What is the reason for the drives having this setup?
Is there some advantages in having "mirrored propellers", or is it maybe just that the one gear has been turned around to make it easier to mount/run the cabels controling it?
(I am looking at replace them with electric engines/drives, and wonder if this is something I would think of when ordering propellers.)

[RonaldL]

The wheel effect is reversed

[GordMay]

Except in rare instances, twin inboard engine, shaft drive propellers are always counter-rotating [turn in opposite directions], so that the torque, created, by each, balances the other out [asymmetric thrust].
Generally, this is accomplished at the transmission [both engines rotate counter-clockwise (LH)].

When viewed from astern, in forward, the starboard [Right Hand] prop’ turns clockwise; the port [Left Hand] prop’ turns counterclockwise, so that both are turning ‘outboard’ [direction at top of prop disk].
Conversely, if the port prop turned to the right, and the starboard to the left, that would be ‘inboard’ turning.

With the normal ourboard turning prop’s, the rudders should be set just [max 2.2 shaft diameters] outboard, of the prop’ shaft centerlines.

On single engine shaft drives, LH props like right turns, and handle better, whereas RH props would like left turns.

Dave Gerr goes into great detail, in his book, “Propeller Handbook” [The complete reference for choosing. installing, and understanding boat propellers]:
Free ➥ http://mojaladja.com/upload/The-Prop...-Dave-Gerr.pdf