Just because you change the way you use your boat doesn't necessarily mean that you need to change the length of your motor's drive shaft. Converting your long shaft outboard motor to a short drive shaft does, however, mean changing more than just the drive shaft. Before you start the conversion, you need to be sure your boat can handle a shorter shaft.
Short Shaft, Long Shaft
An outboard motor comes with short drive shaft, a long drive shaft or an extra-long shaft. The drive shaft is supposed to be long enough so the bottom of the anti-cavitation plate, the shelf-like projection above the propeller, remains in the water when you're running the boat at full speed. A short-shaft outboard has a drive shaft that's 15 inches long; long-shaft outboards have a 20-inch drive shaft. The extra-long shaft is 25 inches long.
Measure the Shaft Correctly
The length of the shaft isn't the height of the motor, nor is it the height from the bottom of the motor cover to the bottom of the motor. The shaft length is measured from the top of the boat's transom to the bottom of the motor's anti-cavitation plate for motors that clamp onto the boat, or from the top of the motor mounting to the bottom of the anti-cavitation plate for motors bolted onto the boat's stern.
What Has to Change
If you decide to go with the shorter shaft, the exhaust tunnel -- the part of the motor between the motor's top cowling and the lower unit, where the gears and propeller are contained -- that's on the motor now will be to long. This means you'll have to change the exhaust tunnel as well. Not only the exhaust tunnel will require replacement. If you change to a short drive shaft, you'll need to change the other shaft that runs through the exhaust tunnel, as it will be too long as well -- you'll need to install a new, shorter shift rod.
An Alternative
You might wonder what happens if you change the drive shaft and your boating needs change -- perhaps you decide you want to fly across the water with family instead of spending your boating holiday fishing with friends. Rather than make the change to the short shaft, you can install a jack plate, sort of an elevator for your motor, to raise it so you can get into those shallows where the fish live. If you've already made the change to the short drive shaft, the jack plate not only raises the motor, but it lowers the motor as well, effectively adding up to six inches to the depth of the propeller.