The correct selection of boat propeller for outboard motor

Choosing a propeller is not just about buying an accessory, it's about fine-tuning the entire boat system for specific tasks. Many boat owners make the mistake of thinking that a standard propeller, complete with the motor, is the perfect solution for all conditions. regular-rotor Often represents a compromise, averaging the performance for the average load and standard height of the transom.

It's the geometry of the blades that determines whether your engine can reach maximum power, how the boat will behave when planing and what the final fuel consumption will be. Incorrect geometry can lead to overheating of the power unit or, conversely, to a lack of revs, which in the long run is detrimental to the piston group. In this article, we will discuss in detail how to find a balance between speed and thrust.

So first, you need to figure out what you want to get out of your boat, and some anglers are looking for the maximum speed to move quickly between points, and others are critically important to pull heavy loads or to move against the current. boat-setting.

The main parameters of the propeller

Each screw has a marking consisting of two key numbers, for example, 13.25 x 19. screw-pieceIt's a slug that directly affects the traction performance and the motor's ability to pull a heavy boat into plane, and the larger the diameter, the more water it captures in one rotation, creating more force.

The second number is screw-stepThe theoretical distance that a boat will travel in one full rotation of the propeller in a solid environment without slipping. An increase in pitch by one unit usually results in a change in maximum engine speeds of about 150-200 rpm. This is a critical parameter for getting into the operating power range.

Three-blade models are standard for most outboard motors, providing a good balance between speed and economy. Four-blade options often provide smoother running and better handling at low speeds, but can slightly reduce top speed due to increased drag.

Effect of screw material on specifications

Aluminum screws are cheaper and more plastic — they bend when impacted, protecting the gearbox, but lose geometry. Stainless steel is stronger, holds the pitch better and allows you to make thinner blades to reduce drag, but when hit hard, the load is transferred to lower unit.

The effect of step on speed and speed

The main purpose of the selection is to make the engine work in the range of maximum torque and power specified in the instructions (usually 5000-6000 rpm for two-strokes and 5500-6500 for four-strokes). If the step is too large, the engine will not be able to spin to the desired speed, detonation and overheating will occur. If the step is small, it will be torsion, which is also dangerous for the engine life.

There is a rule of thumb: changing the pitch of the screw by 1 inch (2.54 cm) changes the engine speed by about 150-200 units. For example, if your engine with a 19 pitch gives 5200 rpm and a passport maximum is 5500, then you need to reduce the pitch. Going to the screw in steps of 17 or 18 will allow the engine to spin and get to the optimal mode.

But blindly following the numbers without considering the actual conditions can be wrong, speed is also affected by the bottom of the boat, the height of the engine and the load, sometimes to achieve the ideal performance requires not only a change of the screw, but also adjustment. angularity.

Load calculation and operating conditions

You can't pick a screw without taking into account the weight that comes per transom. This includes the weight of the boat itself, the engine, the fuel, the equipment and the passengers. A heavy boat requires a smaller-step screw for a confident getting on plane. If you often ride alone, it makes sense to have a second, more "speedy" propeller with a larger pitch for lighter conditions.

It is also important to consider the type of reservoir: In calm waters, the propeller requirements differ from the conditions of shallow waters or grassy areas. For difficult conditions, where there is a high risk of impact on underwater objects, it is preferable to use aluminum screws or models made of composite materials that are easier to repair or replace.

And the height of the engine is also important, because if the lower unit is buried too deep, the water resistance rises, and the motor can suffocate without giving full speed even on the right screw, which may require a screw with an increased diameter or specialized geometry of the blade, for example, with an increased area.

Table of correspondence of screw pitch and revolutions

To make it easier to navigate the screw specifications, the following table shows the approximate effect of step changes on engine speeds. Remember that these data are averaged and can vary depending on the specific model of the engine and the boat hull.

Current step (inch)	Actual turnover	Recommended action	Expected impact
21	4800 (Malo)	Reduce step to 19	Rate growth of ~400
19	5500 (Norma)	Leave it unchanged.	Optimal work
17	6300 (Many)	Increase step to 19	Reduced speed
15	6800 (Crossing)	Increase step to 17	Reduced fuel consumption

Using this data, you can roughly calculate the required step. If the motor does not reach 500 rpm to maximum, feel free to reduce the step by 2-3 units. Conversely, when you torsion, increase the pitch. theoretical It is always different from the real thing because of slippage.

The geometry of the blades and lifting force

Besides the pitch and the diameter, the shape of the blade is important. blade-lift (Rake), which describes the angle of the blade relative to the plane of rotation. High-rail propellers work better at high speeds and raise the boat's nose, improving visibility and reducing water resistance.

But large-rail propellers tend to cavitate at low speeds and in sharp turns. For heavy boats or flat-bottomed boats that have difficulty planing, low-rail and wide-blade propellers are better suited to provide powerful pull-outs.

⚠️ Attention: The use of a screw with an excessively high rack on a heavy boat can lead to a breakdown in the flow of water and cavitation corkscrew, when the motor sharply loses traction and goes into torsion.

Also worth noting is the edges of the blades: sharp, polished edges reduce resistance and improve water output, which has a positive effect on speed. Factory aluminum screws are often rough, and their rework with a file can give an increase in speed of 1-2 km / h.

Diagnosing problems through the appearance of the screw

The propeller can tell you about problems in the configuration of the boat complex, regular inspection of its condition helps to identify hidden defects, for example, if you see cavitation shells (small ulcers) on the surface of the blades, it is a signal of a violation of hydrodynamics or use of poor-quality alloy.

Bullies and chips on the edges often indicate impacts on solid objects or work in rocky soil. But if the blades are intact and the boat does not go, the problem can be a bending, even a minimal deformation of a few millimeters disrupts the balance and reduces efficiency by 10-15%.

The simple method to test the integrity of geometry is to place the screw on a flat surface and spin it, and if it rotates unevenly, jumps, or knocks on the surface, the blades are different heights or curves, and it requires repair or replacement in the workshop.

Manufacturing materials: aluminum or steel

Material choice is always a trade-off between price, weight and strength. Aluminum screws are the most common option. They're lightweight, which reduces the load on the lower unit, and quite soft. When hitting a stone, aluminum is swayed, absorbing the impact energy and protecting the more expensive parts of the engine.

Stainless steel is more expensive and weighs more, but it has exceptional strength. Steel screws do not change geometry when light impacts, which guarantees stability of performance. In addition, steel casting technology allows you to create blades of complex shape and smaller thickness, which improves streamlinedness.

⚠️ Warning: When installing a steel screw, be sure to check the condition of the shaft and nuts. Steel is harder than aluminum and when snacking can damage the lower unit thread if special lubrication and proper puffing are not used.

There are also composite screws that combine plastic flexibility with the strength of reinforcement fibers, which are often used on low-power engines and are highly maintainable, and the choice of material should be based on operating conditions: for rocky rivers, aluminum can be safer, and for high-speed boats on clean water, steel.

How often should the boat rotor be changed?

The propeller's life is not limited to time, only to condition. If the blade geometry is intact, there are no cracks and deep erosion, the propeller can last for decades, and only need to be changed if there are inexorable vibrations, cracks or if you want to change the specifications of the boat (speed/thrust).

Can I use a propeller from another motor?

Only if the landing hole (slice joint) matches the diameter of the hub, but even if mechanically compatible, it is important to consider the power of the motor, the screw from a low-power motor on a powerful engine can not withstand the load and collapse.

What is a screw slip?

It's the difference between the theoretical speed (calculated in step and speed) and the actual speed of the boat, which is a 10-15 percent slip, and if it's above 20 to 25 percent, it means the propeller is wrong, damaged, or the boat is overloaded.

Does the height of the engine installation affect the choice of the screw?

If the motor is set too high, cavitation occurs, and the effective diameter of the screw decreases, and in such cases, sometimes screws with a larger diameter or a special shape of blades (for example, with an increased area) are put in place to compensate for the loss of engagement with water.