Engine shaft length for Mac - 20 or 25 inches

[normo]

Many/most engines offer two shaft lengths, 20 and 25 inches.
1. Which is the correct length for the 26X
2. How can you check this with a tape measure i.e. what do you measure?

[DLT]

20inch


15" is referred to as short shaft, and is typically really small boats.
20" is referred to as long shaft, and is the vast majority of boats.
25" is referred to as extra long shaft, and is typically limited to offshore fishing boats.

[Terry]

I noticed most, if not all Mac M's & X's have a 20" shaft, but even with it installed at max lowest position the anti-cavitation plate (on my Honda) is just about an inch, if that, below the hull keel. From what I have read it should be 1-2" lower so I suppose it is in the range, but I have to wonder if I have something to gain by having it a full 2" below the hull especially in turns and at full throttle as I notice the plate is out of the water at higher speeds. The 25" shaft could be positioned a a few inches higher and still allow for a full 2" or more below the hull therefore keeping the prop fully submerged even in turns and avoid the ventilation problems that occur. Any thoughts on this?

[Robert]

one to two inches cavitation plate below the hull is supposed to be where you want it for best efficiency top speed etc...
..
However, some props may be able to suck air under certain conditions, this can be corrected by cupping the prop or lowering the outboard.