Re: 4 Stroke Upgrade?
The lake restriction is the deciding factor and deal killer on a kicker, the other concerns aren't that big of a deal.
There's more than enough room and the cost of connecting the two is $50.00, if you add other controls it'll go up a little more. I have a 19' CC with a 200hp Evinrude and I use one of three small motors on it 6hp, 8hp and 9.9HP, it works well with each one, but the 6 is too small for faster trolling or rougher water.
The cost of a 4S like the Yamaha T8 (8HP high thrust) with PT&T and electric start new is about $2,500 other brands are less, let?s say that you add another $500 for other items and you're at $3,000 and you added about 125 lbs with everything.
A new 115 or 120 is going to be around $10,000 ( I haven't looked recently at the cost, so it could be different) then add rigging it, plus gauges and controls. The weight gain will be similar to that of the kicker so there's no advantage there.
Don't get me wrong either option works and if where you plan to boat won't allow your current main motor you don't have a choice, or if what you really want is a new main motor then go for. The reason a I went with the kicker option is you said the main motor ran great and there was no mention of it's age (20 years isn't that old for that motor though) so it sounded like there was no real need to replace it other than the fumes.
Here in the North West virtually every fishing boat has a kicker for trolling, few use the main motor although many of the bigger new engines will troll pretty well these days. The kicker allows for more speed control, less fuel usage, quieter operation, a back up for getting home and puts less hours on a very expensive main motor.