Im going to be getting a jack plate for the boat here in the next 6 weeks and I have one rather simple question about them?. Im looking at 6", 8", and 10" all with 7.5" travel. What purpose woudl a 10" serve over a 6". (Generally speaking)

First, if the hull is still under warrenty, check and see what the manufactor will allow. Most have limits as to what they will warrent.

The more setback from the pad the jackplate gives the more bow lift it gives. Too much setback and you will have a problem controlling the porposing.

As for what size you need for your boat, it depends on the amount of built-in setback the boat has and the length and weight of the boat. Some boats, like your newer Rangers, have a whole lot of setback already built in so you don't need to add bunches more to get more lift. Others, like a Bullet have very little so they add up to 14" of jackplate to get the lift they want and be able to run neutral trim.

For a starting point on bass boats, I usually figure 1.25 - 1.5 inches per foot of boat length from the trailing edge of the pad to the leading edge of the motor. This is with the pad leveled by holding a level on the bottom edge of it and cranking the tounge jack and the motor trimed level with a level sitting on the anticav plate and measuring between the two. If it's a heavy hull, I figure 1.5", lighter hulls I figure, I go with 1.25" per foot. On my 18.5' Stratos I'm running a 8" Jackplate with a total offset of 27.5". On my 20" Javelin I'm running a 10" jackplate and thinking about adding a little more if the prop work I'm having done doesn't give the lift I need.

Setback and prop work hand in hand. You need to do your homework on your hull and find what most people run for optimal performance and what prop they are running. Also, if they are running a factory prop or one they've have had tunned for their hull. Most of your best running boats are not running stock, factory props. They send them off to Rich Boger (one of the best) or one of the other prop tunners and have them custom tunned for their boat.

Forgot to mention why run a jackplate. I gets the prop back away from the hull turbulance and in clean water. This gives the prop better bite and is much more efficient. It's all part of finding what's known as the sweet spot. That's getting the right amount of setback and the right prop height for your hull. When it's right, it will lift the bow, fly the hull with no problem, and drive great.