IMHO - there is no need to add a jackplate for your situation except for making it easy raising and/or lowering the motor. An 18' 6" boat with a 90 hp probably won't gain much if anything. The hight of your motor as it relates to best performance is purely dependant on the hull design. From what I can see from the specs at Tracker, you don't want to be running your motor too high expecially with an aluminum prop as the closer to the top of the water the prop runs the more likely you'll begin to have blade stress and possible cracks that can result in throwing a blade - and that's not fun at any speed. Skeeter mentioned a pressure guage - best piece of equipment you can have!! To get a true evaluation of your situation we need to get several bits of information: 1) Motor height in relation to the pad (the pad is the back running surface closest to the transome) - measure the distance between the prop shaft (the tip of the nose cone) and the bottom of the pad. 2) Speed at WOT (wide open throttle) - I think you said 47 3) Prop pitch - I think you said 21 4) water pressure - that will be the hard one for you since you don't have a guage and 5) Your RPM at WOT. With this we can calculate your prop slipage and give you an idea of how high you need your motor set. You also may want to consider contacting your local prop shop - one that customizes props as they may need to adjust your leading or training edges to give you more bow or stern lift depending on your needs. Keep in mind the more of the boat you can get out of the water, the less drag you have and the faster you can go. In my situation, once I'm trimed out at WOT, I'm running on the back 18" of a 21' boat and the prop.