OB fitting onto transom

[Steve509926]

Am I missing something or just being a bit thick!
After seeing the vids that Fenlander posted re: losing outboards, I thought I better bolt the OB onto the transom before my maiden voyage next week.
I read through the Mercury manual, which says that the AV should be in line with or no more than 25mm below the bottom of the boat - happy with that and understand it. Go to fit the OB and I need to pack the transom with a 25mm block to bring it to level I need. Had a look at posts on here and nearly everyone with a SIB says that the OB mounting needs packing of 25mm to bring it up to the correct height, again I understand why.
What I don't get is, if OB shafts are a standard height ie: 15" & 20" why are transom on SIB's made 25mm to short?, (sorry for mixed measurements, I'm of that generation!)

[Fenlander]

It's just one of those things. Don't bolt on until water trials have been undertaken. Take a couple of packers with you and find which height... if any... is needed.

[Steve509926]

Thanks for the advice.

[NeilJ]

And if your worried about it jumping off in the mean time, attach a safety line between boat and outboard.