Well I'm not a marine engineer but that thing is going to be a BIG drag when under way. I might even make your depth transducer act odd due to the disturbed water. The web site does not have any "works on boats up to xx feet long" or power requirements so it is hard to tell if this thing would even be up to a 42' boat with a big keel and rudder resisting it.
I know you have a 42' boat and docking can be a challenge but with a little dedicated practice you can "park" her like a pro. I'd suggest getting into 10-15' of open water and dropping anchored floats (balloon on a string with a hunk of metal for an anchor) port and starboard, fore and aft to make an imaginary slip to practice docking on.
if you don't want to go to the trouble of floats then try practicing turning the boat within her own length (aka the bat turn). This is a great skill to have in your haversack of "stuff I can do" when trying to get out of that tight fuel dock that is surrounded by thin water.
FWIW
