How propellers are calculated?
We have developed a propeller calculation software on the basis of polynomial calculations.
Depending on the input data, of hull designs, curved hull resistance, we are able to calculate the optimum propeller for each configuration.
In addition to determining the most suitable geometry, this program allows us to calculate the ideal working speed for professional boats, thrust and efficiency curves, the bollard pull and the risks of cavitation-related pressures and excessively large peripheral speed.
This software allows us, in addition to defining the optimum propeller for each configuration to determine the shafts propeller diameters according to the torque to be transmitted or the regulations of classification societies, their vibration frequency, the position and number of bearings, thus ensuring perfect consistency in the kinematic chain.
How to determine the diameter of my propeller?
Performance of marine propellers depends on the diameter of the latter. In theory, the greater the diameter of the propeller approaches the optimum diameter calculated based on the vessel's design, the better its performance.
In many cases, unfortunately, this diameter can’t be reached due to the space too limited (distance between the axis of the propeller shaft and the hull) or a bad choice of reduction ratio.
In all cases, we suggest a clearance between the propeller and the hull of at least 15% of the impeller diameter to ensure correct operation.
This value should be increased to 20/25% for the propellers working in tunnels.
How many blades for my propeller?
The number of propeller blades is determined by several factors:
1 - Application
A two-blade propeller will be favored to reduce the drag of the propeller (for sailboats).
3 blade propellers are more suitable for motor boats or highly motorized sailboats.
A propeller 4 blade will be chosen for workboats or highly motorized pleasure boats.
Finally, a 5 blades propeller will be favored to reduce vibration.
2 - The calculation of the propeller
Depending on the configuration of the vessel, it may be necessary to increase the number of blades to increase the area factor to reduce the risk of cavitations.
In some cases, the fact of increasing the number of blades can also increase the propeller efficiency.
Finally, in case of need to reduce vibration, propeller with 4 or 5 blades will be favored.
What is the pitch of a propeller?
Propeller's pitch corresponds to the theoretical advance for a revolution. This advance does not take into account the slip phenomenon. This is due that the propeller does not work in a solid medium but in a liquid. This phenomenon implies the difference between the actual advance of the propeller for a turn and the theoretical propeller pitch.
What is the propeller DAR?
The surface of propeller blades, called Fa/f or DAR is the ratio of the disk surface corresponding to the diameter of the propeller and the blade surface.
Increasing the surface of propeller blades is directly related to the power to be transmitted by the latter and the risk of cavitations.
What is the skew of a propeller?
The skew of a propeller corresponds to the greater or lesser curvature of the median line of a blade.
Currently, almost all propellers are designed with skew and propellers with very curved blades are commonly called 'skew back' '.
This feature reduces vibration phenomena and in some cases cavitations.
How to determine the direction of rotation of my propeller?
To determine the direction of rotation of a propeller, it is necessary to compare it with a screw.
If you put the propeller on the ground, imagine that you wanted to screw it into the ground.
If you should turn clockwise, it is a right hand propeller.
If you need to turn counterclockwise, there is a left hand propeller.
What is the rake of a propeller?
The rake is the angle of inclination of the propeller blades rearward.
Standard propellers usually have a rake between 0 and 10 °.
The rake is often used to disengage the propeller of a very large stern and rounded to maximize the diameter of the propeller and enhance the water supply.
How to determine the type of taper of my propeller?
Different taper existing on the market, the most common being 10% (1/10), 8.33% (1/12) and 6.25% (1/16). Our standard tapers correspond to the ISO 4566 standard and a taper of 10%.
To calculate the taper of its propeller made the difference between the large diameter of the taper and the small diameter multiplied by 100 and divide by the length of the taper.
Large taper diameter: 50mm (generally corresponding to the diameter of the propeller shaft)
Small taper diameter: 38mm
Taper length: 120mm
Taper: (Ø50 - Ø38) x 100/120 = 10%
What is cavitation?
Cavitation is a phenomenon related to the operation of the propeller by its geometry and design or to an external parameter which disturbs the flow of the fluid.
When the propeller is in operation, the blade responds as an aircraft wing by creating a pressure on the pitch side (rear side of the propeller) and a depression on the front side.
When the negative pressure reaches a certain limit value, the water turns to steam thereby creating bubbles and gas pockets on the blades.
Cavitation can cause partial destruction of the blades, vibrations and / or a significant drop in the efficiency of the propeller.
Which material for my propeller shaft?
The propeller shafts are made in stainless steel for water lubricated shaft lines, and steel for oil lubricated.
Various types of stainless steel are available on the market, but it is important to use as stainless steel with good mechanical characteristics and good resistance to electrolysis and 'pitting' phenomenon.
Our research department is at your disposal to determine the best choice of stainless steel depending to the torque to be transmitted and your application.
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