This article is from the Fishing bass FAQ, by Bass Rogue with numerous contributions by others.
- Bass Rogue
Jackplates or transom jacks do two things. First, they moves 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).