As Capt. Dave said, I have a 15 HP 4-stroke Yamaha as a kicker on my 2510 Parker. I installed it as a get home motor if the big motor were to fail. But, I have found it to be very useful for trolling for flounder when the current wasn't moving, and I have used it for trolling for stripers when I was not in a crowd. I usually keep the kicker motor headed straight ahead and then use the big motor for steering. I have to make gradual turns unless I go back to the kicker motor and speed it up and turn it.
Get the LONG SHAFT motor. It will keep the prop in the water better in rough seas while keeping the powerhead out better.
I will try to remember to run a speed test the next time I go out on the Parker and will post the results.
Tom