If it is a horizontal fin you're referring to that looks like it's been added -- congrats -- you are the proud owner of a hydrofoil (or whale tail, or known by various other names).
Search for threads here under hydrofoil and you will discover the lineup is long and distinguished on the con side. Generally people feel these are a marketing ploy to sell stuff to people to make them think their boats will handle better, get up on plane quicker, stay on plane at slower speeds, increase fuel economy, cure world hunger, make our politicians smart, and basically cure all manner of ills.
Since you have it, try it out. Then remove it and try out the boat again and see if it made any difference. If you have acces to the previous owner ask them what they hoped the thing would do for them. It may be that a set of Smart Tabs would do what it is supposed to do. Or it may be you are perfectly happy without it.
Oh, and welcome to iboats (and Yammy ownership -- I love mine) Post us some pics of your rig.
