20" is the preferred. If you are planning on using it in calm waters it might be ok. If not you might consider getting an extension for your lower unit, which would require some mods I would assume.
You want that lower unit to stay in the water and with the short shaft the lu is more likely to come out of the water on swells and waves. Shifting weight inside the boat can cause it as well. With the 20" there is no worries in that dept.
Just so you don't have to, please allow me:
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If not you might consider getting an extension for your lower unit.
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My wife has been trying to talk me into this for 10 years ;D