..effectively measured from the motor clamps where it fit onto the bracket down to the anti ventilation plate just above the propeller area. When the motor is installed the anti ventilation plate, sometimes improperly called the anti cavitation plate, should be at least one inch under surface of the waters. When you are under power the boat will tend to squat, the stern will go down, and the motor will be more immersed in the water. This is not bad for the speeds you will go.

I would suggest that other factors intercede in the search for shaft length at times, however. When you go forward to work the foredeck the stern will come up and the propellor will likely come out of the water at least partially. When you motor over waves of some signifigance, the same can happen.

So sometimes a regular shaft for a 15" transom will work fine depending on the height above the water surface at 15" but sometimes you will prefer to have the 20" shaft to keep the propellor in the waters. I tend to go with the longer shaft length as I weigh 220 and when I go forward the bow dips on a boat that size.

Measure the height of the bracket top to the water when fully extended down to the "running" position. If it is 15" or less, likely a regular shaft will do as they are rated for 15" transoms. If it is over 15", I'd go with a long shaft rated for a 20" transom.

Your experience may vary.