Propeller selection

Propeller selection

The number of happy owners of yachts and boats is constantly growing. This means that the number of water-powered vessels is also increasing, because modern yachts and boats are equipped with engines of various designs, power and operating principles. One thing unites them - the problem of optimal selection of the propeller. A properly selected propeller can increase the engine's resource and reduce its noise, while significantly reducing fuel consumption. The issue of propeller replacement, its modernization or repair has always been relevant among yachtsmen, whether it is the owner of a yacht leased in yacht rental, yachts for own needs water recreation or sporting achievements.

The abundance of propeller types on offer, made of various materials, confuses inexperienced yacht and boat owners. But some tips can help you make the right choice.

The optimum propeller should match the motor and boat. Even those who have never gone to sea know that motor boats do not have an analogue of the gearbox, which is familiar to all motorists. The gearbox of the outboard motor, in the absence of a gearbox, gives out a constant gear ratio to the shaft. Since the engine speed has a speed limiter, the correct engine load depends on the correct propeller, optimal use of its power, which, in turn, minimizes fuel consumption. You can check how the existing propeller fits the boat. To do this, measure the revolutions in "full throttle" with the maximum and minimum load, while the engine speed should not exceed the limit values recommended by the manufacturer.

If the motor is "not picking up" the speed - it is necessary to install the propeller with a smaller pitch, if there is an excess of the speed, then the propeller pitch needs to be increased.

Propeller performance depends on many factors:

  • the propeller material;
  • the number of blades, and the quality of the propeller balancing;
  • propeller manufacturing method.

Propeller Materials

A wide selection of materials for the manufacture of propellers, makes it possible to realize any idea of the owner of a yacht or boat, who wants to see his vessel, easily planing on the sea surface. The most common metals used in making propellers are steel, brass and bronze. Propellers made of steel, stainless steel, non-ferrous metals, aluminum and even polymers are able to satisfy the most meticulous yachtsman. Alloy from non-ferrous metals for screws is called "cunial". Such an alloy, consisting of 80% copper, with the addition of nickel and aluminum (5% each), and 10% of other metals, can be in water for decades, absolutely not corrosive. Remelting takes place at a temperature of 3200 degrees.

Stainless steel screws

They have proven themselves perfectly in the process of real operation for decades. Stainless steel propellers actively resist corrosion, suitable for repair and subsequent operation. High-alloy steel guarantees long-term use. A negative characteristic of the screw is considered to be its high cost, which arises due to the particular complexity of casting and processing.

Steel screws

Material for steel screws, steel alloys st25L and st20. Due to this, the steel propeller has a completely affordable price, it hardly wears out in fresh water (when operating in seawater, periodic maintenance is required). An additional advantage of such a propeller is the ability not to wear out the hull of a steel vessel. In the case of replacing an aluminum screw with a steel one, it should be borne in mind that the steel screw must be taken in increments of 1 '' less than the aluminum one.

Brass propellers

Expensive, but well-proven propellers, made from brass alloys, giving the brass propeller good corrosion resistance. These propellers are made by manufacturers who keep the alloy composition secret, which greatly complicates the propeller repair. In case of deformation or damage of such a screw, it is very problematic, it is almost impossible to find the solder necessary for work. Aluminum propellers

Aluminum propellers are easy to handle, inexpensive and have excellent hydrodynamic characteristics. An additional plus is that the impact of the aluminum propeller on an underwater obstacle will not damage the vertical and propeller shaft, gear parts, due to the loss of more fragile blades. Insufficient strength of aluminum propellers limits their use only with engines of low power.

Polymer propellers

The simplicity of making such screws (up to printing on a 3D printer) ensured their low cost. But during operation, despite the lightness and full corrosion resistance, polymer propellers showed their fragility and complete impossibility of repair. Probably, the development of new technologies will give manufacturers a polymer of greater resistance, but now polymer screws are most often used as replacement screws in case of loss or breakage of the main screw.

Further, to select a propeller with a certain number of blades, it is necessary to clearly determine what result you want to get in the end. If you need a stable minimum planing speed, lifting capacity and a large stop - surely 4 blades will be just right. If the owner is more important to light speed - 3 blades will be the best solution. But it must be remembered that due to the greater emphasis of the four-bladed propeller, the engine speed will be approximately 100 rps less than when using a three-bladed propeller. If we fully consider the question of the correspondence of the propeller to the motor and the case, you can deduce a certain gradation:

  • Heavy propeller. With such a propeller, the engine is not able to develop full speed, getting into planing is difficult. The optimal solution is to decrease the step;
  • High-speed propeller. The set of maximum revolutions and speed is achieved only with a low load, with the upper position of the hydraulic lift ("trim");
  • Universal propeller. The motor develops maximum rpm with minimum load, allowing you to enter planing at full load;
  • Cargo propeller. Allows you to go on planing with a full load, with some loss of speed. The set of maximum speed is reached already at an average load;
  • The propeller is too light. The motor significantly exceeds the permissible speed, while the boat is greatly underspeed. Exit: replacing the propeller with a propeller with a large pitch.

Practice has shown that three-bladed propellers are most widely used. The use of four-blade propellers is less common, and is most often used when high thrust is needed, and the installation of an oversized propeller is prevented by the gearbox design. Two and five-bladed propellers are practically not found in everyday use.

The most popular propellers have between one and four blades. After manufacturing, each screw needs to be balanced. For balancing two-blade and single-blade propellers, special expensive equipment is required, which can only be afforded by companies that serially produce such propellers. Three-bladed propellers are much easier to balance, but certain equipment is still needed for this. A significant disadvantage of three-bladed propellers is slight unbalance at the slightest damage.

Dynamic and hydrodynamic balancing of four-bladed propellers is close to ideal. Once manufactured or repaired, their statistical balancing is sufficient for correct operation.

One of the main conditions for guaranteeing long-term operation of the propeller is high-quality casting. Engineering thought is in constant search for the ideal geometry of the propeller, in the search for new materials and technologies for its manufacture. In a custom-made propeller, design begins with the selection of criteria - a set of factors that serve as the boundary conditions for its design. These factors can be the shape of the ship's hull, which limits the diameter of the propeller; the angle of inclination of the shaft can be taken into account (the smaller it is, the higher the useful component of the stop).

According to experts 2yachts, there are cases when boundary conditions collide. For example, a diesel engine requires a large propeller diameter, and in such cases, the bottom may rise in the stern or even tunnel formation. An error in the design of the propeller, a large angle of inclination of the shaft, poorly made balancing, bends and poorly processed surface of the blades are the reasons for the occurrence of the cavitation process.

Cavitation is the formation of bubbles in a fluid flow, followed by their collapse, accompanied by noise and micro-hydraulic shocks. With a small area of the blades, the pressure during their rotation decreases so much that the water flowing around the blade literally boils, releasing steam bubbles. Small bubbles merge into large - cavities. A stronger vacuum leads to the discharge of bubbles into a solid cavity, which contributes to disruption of flow continuity. Over time, immense local pressure causes the blades to chip out, which is called cavitation corrosion. The boat starts to vibrate, move in jerks, the speed of the boat stops increasing with a further increase in the number of revolutions - these are signs that the propeller is subject to destruction under the influence of cavitation erosion. The material from which the propeller is made does not matter before the destructive effects of cavitation. To avoid such consequences, propellers are tested in a cavitation tube. The design of the installed propeller is fundamental to achieving high efficiency.

Therefore, when replacing or installing a propeller on his boat, the yachtsman should not be guided by the advice of friends and his own desires, but use the advice of an experienced specialist.

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