What are the screws for outboard motors: types, materials and choice

The choice of propeller is often a key factor in the tuning of an outboard motor that can dramatically change the performance of a vessel. Many boat owners underestimate the importance of this part, relying solely on the standard propeller that comes with the power plant. However, it is the geometry and condition of the propeller that determines how efficiently the engine will deliver its power into the water.

The wrong configuration can lead to overheating of the engine, increased fuel consumption or, conversely, to the inability to achieve maximum speed. Understanding basic specifications such as pitch and diameter allows you to adapt the boat to specific tasks, whether it is high-speed planing, skiing towing or economical movement with full load.

In this article, we will discuss in detail what are the propellers for outboard motors, what materials they are made of and how to calculate the necessary parameters for your set "boat-motor". Competent approach to the choice of this part will prolong the engine life and make the operation of the boat more comfortable and predictable.

Main specifications of propellers

Any propeller has two main geometric parameters: diameter and pitch, which are usually stamped on the hub of the product and written in a format like 13x19, where the first digit represents the diameter in inches and the second digit represents the step. Rotor diameter This is the size of the circle described by the blades, and it directly affects the sweep area and the stop that the screw can develop.

Screw pitch This is the theoretical distance that a screw will travel in one full rotation in a solid environment without slipping. The larger the pitch, the higher the potential speed of the vessel, but the more stress is placed on the engine. If you put a screw with an excessively large pitch on a weak motor, it will not be able to spin it to working turns, which will lead to detonation and overheating.

It is important to understand that changing the diameter or step, even by one inch or one, respectively, significantly changes the engine speed. Increasing the pitch by 1 inch, as a rule, reduces the maximum revolutions by 150-200 units. crankshaft.

Manufacture materials: aluminum or stainless steel

The current boating market is dominated by two main propeller materials: aluminum alloys and stainless steel. Aluminum screws are the standard for most serial outboard motors, small and medium-powered. They are cheaper to produce, lighter to weigh and, importantly, when hit by a hard obstacle (stone, snag) deform or break themselves, often saving them from the effects of a hard obstacle (stone, snag). gear And a deadwell shaft from expensive repairs.

Stainless steel is the choice of professionals and powerful motors. Steel screws have a much greater strength and elasticity. When impacted, they don't bend as easily as aluminum, but simply spring back to their original shape. This property allows you to maintain the ideal geometry of the blades, which ensures stable speed and no vibration. In addition, steel allows you to create blades that are smaller in thickness, which reduces water resistance.

⚠️ Warning: Installing a steel screw on a non-designed motor (without a dedicated sleeve) can cause the energy to be transferred directly to the gears of the gearbox when impacted, causing them to collapse. Make sure your motor model allows the use of stainless steel.

There are also composite screws and special alloy products, but they are less common and are usually a specialized solution for specific racing tasks or extreme operating conditions. For most fans, the best choice is quality aluminum or, if you have a budget and the appropriate engine power, stainless steel.

Number of blades and their impact on the course

The number of blades is another critical parameter that affects traction, speed and smoothness. The standard solution for most motors is a three-blade rotor. It provides a good balance between speed, economy and handling. Three blades create less drag in the water, which allows the motor to gain speed more easily and reach maximum speed.

Four-blade propellers are often called "cargo screws." By increasing the area of the blades, they create greater emphasis on low and medium revs. This is ideal for heavy boats, pontoons or if necessary to tow a water skier. Four blades provide faster exit to the sleeving and better handling on turns, since the screw is less likely to break into cavitation when rolling, but the maximum speed of the vessel is likely to decrease.

There are also propellers with five or more blades, but this is already the lot of specialized equipment, which requires minimizing noise, vibration or ensuring operation in conditions where ordinary cavitation is unacceptable. For recreational boating, the choice is between three and four blades depending on the priorities: speed or traction.

The geometry of the blades: shape and profile

Not only the number and material, but also the shape of the blades play a role in the efficiency of the propeller. The blades can have different rounding edges, different widths and angles of attack. For example, the blades with a rounded rear edge (trailing edge) are often found on speed boat propellers, because they improve streamlining. Wide blades are characteristic of traction screws designed to work with high loads.

Particular attention should be paid to the concept blade-sweep This is the angle of the blade tilt relative to the perpendicular to the axis of rotation. Larger rake propellers (tilted backward) are better at raising the bow of the boat, facilitating early planing, but can reduce overall thrust. Small rake propellers provide a powerful thrust, which is useful for heavy boats.

There's also the concept of "cup" -- it's a small bend in the back edge of the blade, and it allows the screw to cling to water more efficiently, lowering cavitation and allowing the motor to run at higher revs with a larger pitch screw -- and having a cup is a sign of the more advanced and expensive geometry of the screw.

Comparative specifications Table of Rotors

To make the choice easier, we compare the main types of screws depending on their configuration, and this table will help you to orient yourself in what changes in the behavior of the boat should be expected when replacing regular equipment.

Parameter	Impact on speed	Effect on traction/acceleration	Impact on fuel consumption	Recommended application
Increase the step	Growth (theoretical)	Decline	Growth (underload)	Light boats, speed control.
Slow down the step	Decline	Growth	Decrease (overload)	Heavy boats, towing
4 blades	Decreased (3-5%)	Significant growth	Medium.	Pontoons, boats, water skis
3 blades	Maximum	Medium	Optimal.	Universal use
Stainless steel	Growth (due to geometry)	Stable.	Decrease (COP above)	Sport, powerful engines

From the table it is clear that there is no perfect screw "for all occasions." Maximum efficiency is achieved only when the propeller parameters strictly correspond to the weight of the vessel and the tasks that are set before it. A universal screw will always be a compromise.

Common mistakes in the selection and operation

One of the most common mistakes is to try to increase the speed of the boat solely by installing a large-step propeller, ignoring the engine's capabilities, as a result, the engine does not enter maximum power mode, the speed does not increase, and the life of the piston group rapidly declines due to the operation of a rich mixture and low revs under load.

The other extreme is using a propeller that's too small, and the engine is easy to spin to cut off, the boat seems to be flying, but the efficiency of the system drops, you burn excess fuel, you make excess noise, and you don't use the potential engine power that would go to increase speed with the right step.

⚠️ Attention: Regular operation of the engine with a screw, which does not allow the engine to reach the recommended range of maximum speeds (WOT range), is guaranteed to lead to the formation of soak on candles, piston rings and, as a result, to major repairs.

Also worth remembering is the condition of the propeller: chips, clamps and cracks on the blades disrupt fluid dynamics, causing vibration and cavitation. Even a small defect can reduce speed by 10-15%, so regular inspection and correction (if material permits) or replacement of the propeller is a mandatory part of maintenance.

What is screw cavitation?

Cavitation is the formation of vapor bubbles in the local pressure-lowering zones behind the propeller blades, and when these bubbles collapse, a shock wave occurs that destroys the blade metal (cavitation erosion) and creates a characteristic noise similar to a screw scrolling in the air, a common cause of cavitation is a damaged anti-cavitation plate or an incorrectly selected screw.

FAQ: Frequently asked questions

How do you know if the screw is wrong if you can’t measure the GPS speed?

The main indicator is the tachometer. If the full load (boat with a crew and a full tank) and the fully open throttle speed is below the minimum range WOT (indicated in the manual), the step is too large. If the speed exceeds the maximum, the step is too small. Also the sign may be the vibration of the hull or the inability to get on plane with full load.

Can I use a 15 hp propeller on a 20 hp engine?

Mechanically, the seats often coincide within the same series of engines, but this is not recommended without calculations. The screw from a smaller engine is likely to have a smaller pitch. On a more powerful engine, this will lead to the engine torsion and exit the red zone of the tachometer, which is dangerous for the engine. Always check the permissible pitch ranges for a particular model.

Should I buy a used stainless steel screw?

Buying a used steel screw is only permissible if you carefully inspect it. Steel is difficult to operate without special equipment and heating, which changes the properties of the metal. If the blades have deep holes, cracks or traces of serious editing ("bugs"), it is better not to take such a screw, as it will vibrate and can collapse at high revs.

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

Yes, it's indirectly. If the motor is too high, the propeller can capture air (cavitate) even on a straight line, and sometimes it helps to install a propeller with more blades (4 instead of 3) or a screw with more blades to stabilize the flow of water, but it's better to adjust the height of the transom.