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Shaft Angle

Shaft angle is a very important variable which will affect the efficiency of any propeller.

Basically this is driven by two variables - each of which will impair motoring performance - again of any propeller type.

Firstly - The propeller will deliver thrust up the drive shaft which will be at an angle to the waterline. This thrust can be resolved into two vector components. One parallel to the water line which will generate forward thrust - the other a vertical component which will serve no purpose other than to attempt to lift the vessel from the water. This component provides no forward thrust and is effectively wasted.

As the shaft angle increases the relative thrust of these two components will change. At low angles - say below 10º - the vertical or wasted component is relatively insignificant. ( The sine of the angle for those of a mathematical bent ).

However as the angle increases beyond  ~ 10º - the forward component starts to reduce quite quickly and at 45º is exactly half the shaft thrust with a collateral reduction in propeller efficiency manifesting as reduced vessel speed.

Note as stated that this analysis is quite independent of the type of propeller and applies equally to all propeller installations

Saildrives have the drive shaft installed parallel to the waterline, and it is for this reason that they will always produce superior motoring performance compared to a shaft installation at any angle. In simple terms they have no vertical component and thrust is transferred 100% to forward motion.

Secondly an additional effect is delivered by any propeller that is not receiving streamlines on it's forward face at 90º to the plane of rotation.

Imagine a propeller where the tip angle of the blades is 20º and the shaft is installed at a 20º angle to the waterline.

Now one side of the propeller will " see " the water entering at 20º + 20º or 40º, whilst the other side will " see " the water entering at 20º - 20º = 0º.  The high tip angle will likely lead to stalling or cavitation at the tip on one side and excessive thrust whilst the opposite side will generate little or no thrust. This is a scenario to be avoided as it will generate an excitation in the shaft leading to vibration and a noisy installation.

We also need to consider that generally the buttock lines of most hulls will be rising somewhat at the stern sections above the propeller. In this situation - the water will be following streamlines that will be following the buttock lines of the hull in which case we should strictly evaluate shaft angle not to the water line but to the buttock lines above the propeller. If these are rising, this will only serve to exacerbate the above efficiency loss by delivering an even higher angle of effective streamlines to the face of the propeller.

As a general rule for all propellers - high shaft angle installations or those much above ~ 10º  relative to the buttock lines above the propeller will inevitably result in reduced efficiency and a loss of potential motoring capability.