hi,

lowering your jack plate will create more lift and hoist your transom and hull up out of the water, creating less wetted surface on the hull.

raising your jack plate will allow the transom and hull to settle deeper into the water, but also allow the prop to slip more - inceasing rpm and sometimes speed.

you just have to experiement on the water.

read this - it will help:

http://www.bbcboards.net/zerothread?id=715239