SHAFT   ANGLES

A zero degrees shaft angle (parallel to waterline) is the most efficient in theory, since thrust is straight aft and water flows to the propeller from straight ahead. In practice, it is very difficult to install like that and leave enough room for the engine and gearbox inside the hull.

From the desktop of

Leonardo Montoya

The performance or efficiency between a shaft angle of  0° and 5° is very little, so is the same from 5° to 10°, however, shafts with angles greater than 15° begin to introduce significant variable loading to the propeller blades, this is 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, the result is uneven blade loading that can cause vibration and/or cavitation.

In addition to aperture size, shaft angle affects the maximum propeller diameter. The higher the shaft angle for a given engine location, the further below the hull bottom the propeller shaft will emerge from its bearing. Thus, the greater the propeller diameter can be. This is a particularly important on twinscrew crafts.

In a new design, or in any major refit and repowering, some thought should be given to the possibility of increasing diameter by increasing shaft angle, within reasonable limits.

An example of performance losses regarding the shaft angle:

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