Four stroke
I have a 9.9 Yamaha 4 stroke,long shaft, electric start on my S2 7.3 (24 ft). This motor was on the boat when I purchased it used four years ago. It runs very quietly and smoothly and gets good mileage on regular gas. Previously, I had a manual start Evinrude 9.9 2 stroke on my Venture 25.As to how long the shaft should be, the answer depends on the boat. On my S2, the cavitation plate is about 3 inches below the water (Yamaha recommenda at least 2 inches) and it has not been a problem. On the Venture, the cavitation plate was 2 inches below the water and it ran fine, except in heavy chop. The boat would "hobby-horse" through the chop and the motor would come out of the water, rev high, then plunge back into the water. I am not sure that an inch or two of more depth would have made any difference on the Venture.The 4 stroke is great from the standpoint of not having to mix oil with the gas, but it is so much heavier than a 2 stroke, that it can be a problem. First, the outboard bracket for a 2 stroke is usually smaller and ligher, with smaller springs to lift it than the same for a 4 stroke. Make sure you current bracket is adequate. Next, I am unable to lift my Yamaha up on the bracket, even with the heavy springs. The best I can do is tilt it out of the water. On full tilt, the motor comes all of the way out, with about 2 inches of clearance between the motor's foot and the water. Have your new motor trial mounted then try lifting and tilting it to see the clearances.On electric vs manual start, the electric does a good job, as long as you have a good battery and your cables are not too long. I have had to manually start the motor when the battery went dead. This requires removing the motor cover (and on the Yamaha, an inner cover over the flywheel), wrapping the emergency start rope around the starting sheave on the flywheel and giving it a good steady pull. If the only problem is the dead battery, the motor starts right up. By the way, the motor is a 1989 model and has never been rebuilt (only routine maintenance). I have no idea how many hours it has on it, but I would guess less than 1500. Runs great!