Y = the minimum clearance between the tip of the propeller and the underside of the hull and should be 10% (or greater) than the propellers overall diameter (X).
The purpose of a propeller is to convert the power and torque provided by the engine into thrust and the design should deliver the most efficient forward / ahead movement of the vessel. It will also provide backward / astern movement of the vessel but depending on propeller type this can be compromised in favour of forward or ahead efficiency.
The propeller then needs matching to the power & speed of the engine and to the water line length and displacement of the vessel to ensure both engine size and fuel consumption are kept to a minimum but are still capable of achieving the maximum theoretical hull speed. Generally a large diameter propeller rotating slowly is more efficient in converting available power and torque into thrust than a smaller diameter, faster rotating one.
Additionally, for many reasons owners often want their vessel capabilities tailored for different needs which also affects propeller sizing and selection so there is no single correct answer or solution to suit all needs;
Racing yachts for example may wish to opt for smaller, lighter engines with more limited horse power as the engine is primarily only used to reach or leaving moorings and is not required to achieve maximum theoretical hull speed. Propellers will be selected that deliver the least amount of drag when under sail rather than optimum efficiency when under power.
A motor sailor or yacht used in heavier seas and or stronger tides may often opt for an engine with up to 30% more power to ensure an adequate safety factor is available and easily be capable of achieving maximum theoretical hull speed in harsher conditions. Propellers will be selected for high efficiency when under power and drag when under sail is not such a primary consideration.
In some vessels a propeller may be “sized” to absorb and transmit the full engine power and torque at a specific engine speed rather than the maximum rated engine speed (RPM) it is capable of. Vessels with this criteria are known as being ‘over-propped, engine fuel consumption and noise are reduced but the full maximum rated speed (RPM) will not be able to be achieved whilst in gear.
For displacement hull yachts, propellers are generally specified with 2 or 3 blades and available as fixed, folding, feathering or Autoprops. All propeller types have benefits and compromises in one form or another and the selection made would be based on vessel type, use intended, engine power, efficiency, drag, maintenance and cost.
Our standard recommended range of propellers for the SeaProp 60 Saildrive are: 2 Blade Folding, 2 Blade Fixed, or 3 Blade Fixed.
Propeller Inefficiencies, Problems and Specialist Support
Some of the highly technical problems that can occur that will require specialist support for are:
- Propeller Blade Loading
- Propeller Cavitation
- Propeller Slip
To get a better understanding of the benefits and compromises of each propeller type please read or download the articles listed below or contact a propeller specialist.