Long shaft / short shaft outboard question

[Mankato-Ice-Man]

I feel so stupid for asking this, but here goes anyway....

What is the difference between a long shaft and short shaft motor in inches? Is it a set amount, does it vary by motor type?

I am looking for a motor for my boat and I want to know that I need a long shaft motor. One of the motors that I am looking at, the owner is selling it for his father and he does not know if it is labled a long or short shaft motor, but he was going to measure it and give me the measurement. Now all I have to do is figure out what to do with that measurement.

Should I measure the transom of my boat?
How many inches should the prop hang below the bottom of the boat when the motor is mounted on the boat?

Thanks.
Jeremy

[Surface Tension]

Transom hight will be 15(takes a short shaft) or 20"(takes a long shaft)
Cavitation plate on the lower unit should be even with bottom of boat so measure the motor from the plate to the motor mount braket. These sizes are standard with all brands.

[Mankato-Ice-Man]

S.T.-
Thanks a ton for the quick and perfect answer.
Jeremy