is it a short shaft or long shaft?

i have an older 13 foot deep v boat. i just traded for a 1972 25 hp. the guy wasnt sure if it was a short shaft or long shaft. the transom on my boat is 15 inches. i thought a longshaft boat was a 20 inch transom. if i do have a long shaft motor which i was told i do. if i raised the motor 5 inches the prop would be i littile above the bottom of the boat. i was told the main bigger planing fin was supposed to be even with the bottom of the boat. i have seen pictures that it is lower than that on other boats. quite a bit of water sprays up in the back when running it. the plaining fin is a couple inches below the bottom of the boat now. thanks for any help.

 

Short = 15"
Long = 20"
X-long = 25"

The big antiventilation plate right above the prop is generally mounted level with the keel, for engines up to about 50hp. If it is too high the prop will just suck air in and lose almost all power; if too low, the motor shaft will add a lot of drag and performance will be sluggish.

 

Is this for sailboats? Are shaft lengths different for sailboats, because I'm thinking of purchasing a 20" 5hp Nissan for a 22' South Coast, and I'm wondering if it's still the same as a long shaft or do I need an x-long shaft?