What do jackplates do?
Response by Bass Rogue - Dated 10/21/1998
Jackplates or transom jacks do two things. First, they move the motor back away from the boat which puts the prop in cleaner water. In other words, the prop is moved away from the turbulence caused by the boat's hull. In the cleaner water, the prop bites better and pushes the boat better. Second, jackplates allow the motor to be raised as a way to reduce drag which results in a faster boat. The part of the motor below the surface of the water creates significant drag.
Jackplates are adjusted by raising the motor a little bit at a time until the prop is just on the verge of breaking loose. The prop breaks loose when the top of the it starts pulling air instead of water. When that happens, the rpm jumps and the motor becomes noisier. Starting with the prop set at the factory height, adjust the motor up 1/2 inch at a time until the point is reached where the prop just breaks loose. Then lowered the motor 1/4 inch at a time until it doesn't break loose anymore. If you adjust the prop higher than this point, you'll start effecting the long term reliability of your lower unit. One way to overcome that problem is to run a surface prop. They cost about $350.
If you really adjust the motor too high, you can vent the water pick-up holes in the lower unit and overheat your motor. If you are running wide open and this happens, you can easily damage your motor. That's a good argument for a water pressure gauge with a jackplate.
Manual jackplates for V-6's run between $200 and $250. So, what will you get out of all of this? Probably 3 to 5 mph on the top-end, a little better gas mileage, and a lot of hassle finding that optimum height point. Also keep in mind, that point varies depending on your load (fuel, number of people, live well fill).