- 01.12.2019
- 707

# How to find the right propeller for your yacht

The correct selection of a propeller for a yacht is an important condition for the efficiency of the power plant, and this process can be compared with the choice of rubber for a car. For some reason, many yacht buyers are sure that the manufacturer has already installed the most efficient propeller on the vessel, but this is not so - ** specialists at the yacht are guided by average performance**, having no idea what conditions the vessel will be used in ... The correct choice of the propeller will determine the frequency of its rotation, yacht speed, acceleration intensity, speed of reaching planing and other indicators.

With an incorrectly selected propeller, a vessel, even with the most powerful engine, will not be able to show its maximum speed and carrying capacity. Since the ship's propeller is its propeller, interacting with the external environment, then when it runs aground, it is subject to increased loads, from which it sometimes breaks. All propellers are characterized by pitch, dimensions, disc ratio, number of blades, material of manufacture, hub dimensions, etc. It is known that steel screws give a speed increase of 5-7% and do not corrode in salt water. ** They are more durable ** in comparison with aluminum and can easily withstand blows on firewood, the bottom or prolonged rotation in the sand. An aluminum propeller can wear out over time from friction against the sandy bottom, and the geometry of its blades can change from collision even with minor obstacles - flooded snags, bottles, etc.

The choice of propeller is an individual matter, while it is desirable that the vessel be equipped with a tachometer. The ratio of the diameter of the blades to the area depends on the number or size of the blades. Although an increase in the area of the blades increases the area of action of the forces pushing the vessel, it increases the friction against the water. It is pertinent to make a comparison here with the tires of a car - wider wheels also increase friction, although they contribute to better cross-country ability. ** If it is necessary to increase the carrying capacity ** of the vessel - a larger diameter propeller is installed on it, it increases the thrust at the same speed. When speed is required from the vessel, propellers with increased pitch but smaller diameter are chosen.

## Diameter

The larger the diameter of the propeller, the higher its pulling properties and the lower the speed of the vessel. On rented in rental of high-speed yachts usually put propellers with blades tapering to the tips - thus reducing friction and achieving high speed of movement of light yachts.

## Step

The pitch of the screw is the most important indicator, and it should be chosen especially carefully. The pitch expresses the distance traveled by the propeller in one full revolution in a dense environment without slipping and affects the maximum RPM - the smaller it is, the higher maximum RPM can be developed by the yacht's engine. ** Pitch calculation is needed to achieve optimal maximum speed**, planing and proper motor operation without undue wear. There is a general rule of thumb: a 1-inch change in propeller pitch leads to a 150-200 rpm change in engine speed.

When calculating propeller performance, many factors will need to be taken into account, but in general, if you want to know the characteristics of a simple propeller for a planing yacht with an outboard motor, you can get by with a minimum of values. You will need to know the maximum speed (indicated in the vessel's passport), engine speed and gear ratio.

** For example ** let's take a Suzuki DF 325 outboard motor:

- Gear ratio: 2.29: 1;
- Range of working revolutions, rpm, 5300 - 6300 in 60 seconds;
- The maximum speed is 68 km per hour.

First, you need to divide the indicators of the maximum shaft rotation by the gear ratio: 6300 / 2.29 = 2751 rpm. The step is calculated according to the formula H = 750V / n, H = 750 * 68/2751 = 18.54 - round up to 19. ** Specialists 2yachts it is recommended to purchase several screws ** with a step close to the calculated one. Next, the vessel's speeds are measured, the engine speed is observed.

## Number of blades

An increase in the number of blades reduces efficiency, but contributes to less runout of the propeller on the shaft. It is known that 4-blade propellers accelerate a yacht much better than 3-blade propellers and are suitable for planing at lower speeds (which saves fuel); in general, yachts over 7 meters in length behave better with a 4-blade propeller. The use of a 5-blade propeller can only be justified on fairly large or sporty yachts.

** And finally**: do not save, the propeller is not the thing that you can earn or save on, only a few tens or hundreds of dollars can separate you from the best propeller, which will allow you to use it most optimally yacht motor power - don't be afraid to spend it.

The money spent will result in comfort on the water and fuel economy, which is much more important.