Technical area

We have developed our own propeller calculation software, based on polynomial methods.
From the input data, the hull drawings and the hull resistance curves, we are able to calculate the optimum propeller for each configuration.
Beyond determining the most suitable geometry, this programme lets us calculate the ideal working speed for commercial vessels, the thrust and efficiency curves, the bollard pull, and the cavitation risks linked to overpressure and to excessive tip speeds.
As well as defining the optimum propeller for each configuration, this software lets us determine propeller shaft diameters according to the torque to be transmitted or to classification society rules, their vibration frequencies, and the position and number of bearings — ensuring full consistency throughout the propulsion driveline.

The efficiency of a marine propeller depends on its diameter. In theory, the closer the propeller diameter is to the optimum diameter calculated for the boat's configuration, the better its efficiency will be.
Unfortunately, in many cases that diameter cannot be reached, because the propeller aperture is too small (the distance between the propeller shaft axis and the hull) or because the reduction ratio was poorly chosen.
In every case we recommend a clearance between propeller and hull of at least 15% of the propeller diameter, to guarantee correct operation.
That figure should be increased to 20-25% for propellers running in tunnels.

The number of blades on a propeller is determined by several factors.
1 – The application. A two-blade propeller is preferred to reduce propeller drag (on sailing yachts).
A three-blade propeller suits motor boats or heavily powered sailing yachts.
A four-blade propeller is chosen for commercial vessels or heavily powered pleasure boats.
Finally, a five-blade propeller is preferred to reduce vibration.
2 – The propeller calculation. Depending on the boat's configuration, it can be necessary to increase the number of blades in order to increase the blade area ratio and so limit cavitation risks.
In some cases, increasing the number of blades can also improve propeller efficiency.
Finally, where vibration has to be reduced, a four- or five-blade propeller is preferred.

The pitch of a propeller is the theoretical distance it advances in one revolution. This figure does not account for slip. Slip occurs because the propeller works in a liquid rather than in a solid medium, and it is what explains the difference between the propeller's actual advance per revolution and its theoretical pitch.

The blade area ratio of a propeller, referred to as Fa/f or DAR, is the ratio between the area of the disc described by the propeller diameter and the area of the blades.
Increasing the blade area is directly related to the power the propeller has to transmit and to cavitation risks.

The skew of a propeller is the degree of curvature of the median line of a blade.
Today virtually every propeller has some skew, and propellers with strongly curved blades are commonly called "skew back".
This feature reduces vibration and, in some cases, cavitation problems.

To determine a propeller's direction of rotation, compare it to a screw.
Place the propeller on the ground and imagine you want to screw it into the ground.
If you have to turn clockwise, it is a right-hand rotation propeller.
If you have to turn anticlockwise, it is a left-hand rotation propeller.

Rake is the angle at which the propeller blades are inclined aft.
Standard propellers generally have a rake of between 0 and 10°.
Rake is often used to clear the propeller from a very bulky, rounded sternpost, so as to optimise the propeller diameter and improve water flow to the blades.

Several tapers exist on the market, the most common being 10% (1/10), 8.33% (1/12) and 6.25% (1/16). Our standard tapers comply with ISO 4566 and have a 10% taper.
To calculate the taper of your propeller, subtract the small diameter of the cone from the large diameter, multiply the result by 100 and divide by the length of the cone.
Example:
Large cone diameter: 50 mm (generally the propeller shaft diameter)
Small cone diameter: 38 mm
Cone length: 120 mm
Taper: (Ø50 − Ø38) × 100 / 120 = 10%

Cavitation is a phenomenon linked either to the way the propeller works — through its geometry and design — or to an external parameter that disturbs the flow of the fluid.
When the propeller is running, the blade behaves like an aircraft wing, creating pressure on the pressure face (the aft face of the propeller) and suction on the forward face.
When the suction reaches a certain limit, the water turns to vapour, creating bubbles and even pockets of gas on the blades.
Cavitation can cause partial destruction of the blades, vibration and/or a sharp drop in propeller efficiency.

Propeller shafts are made of stainless steel for water-lubricated shaft lines, and of carbon steel for oil-lubricated ones.
Several grades of stainless steel exist on the market, but the grade used must combine good mechanical properties with good resistance to galvanic corrosion and to pitting.
Our design office is at your disposal to determine the best grade for the torque to be transmitted and for your application.