Why is shaft angle important?

Shaft angle affects the maximum propeller diameter.  The steeper the shaft angle the further it projects below the hull. 

A shaft angle of zero – parallel to the waterline – is the most efficient since thrust is straight aft and water flows to the propeller from straight ahead.  In practice this is difficult to install and allow sufficient room for engine and gear box in the hull.

The vast majority of shaft installations are between 8 and 14° degrees.

Shafts with angles greater than 15 degrees begin to introduce significant variable loading to the propeller blades because the upper blade, as it rotates up, is actually receding from the onrushing water, while the lower blade, as it rotates down, is moving forward into the slipstream.  This results in uneven blade loading which can cause vibration and or cavitation. 

The performance efficiency between a shaft angle of 5 degrees and 15 degrees is very little.