There are lots of variables here -- diameter of tubes, amount of kick, type of mount, typical load being carried, whether an adjustable "jackass" is being used, how hard the tubes are, even how heavy the driver is.
Actual data. I've used a long shaft on my 14' NRS E-140, a 16' NRS cat, a 16' Avon Pro, an older 16' Sotar, an 18' Avon Spirit, and an 20' Jack's El Tigre Grande.
In each of those cases a short shaft would not have worked without figuring out how to lower the transom quite a bit, which has the downside of making the controls harder to reach. But with smaller tubes a short shaft can work. Some people just let air out of the rear chambers to let the tube sag, and get the motor closer to the water.
A forum community dedicated to whitewater kayaking, boating, and rafting enthusiasts. Come join the discussion about safety, routes, gear, models, styles, gear swaps, reviews, accessories, classifieds, and more!