The step of the propeller of the outboard motor: what it is and how to choose

Getting your propeller right isn't just a technicality, it's a key to determining whether your boat will fly over water or crawl over, burning off excess fuel. Many outboard owners mistakenly believe that the propeller is just a consumable that anyone can buy if they can fit the landing gear, and in practice, it's the geometry of the blades that dictates the engine's operating modes, its life and its economy.

One of the most important things is the step, and if the diameter of the screw is responsible for the overall thrust and the motor's ability to push a heavy boat, then the step determines how efficiently that thrust is converted into speed. outboard Avoid costly repairs to the piston group.

In this article, we will discuss what is hidden behind the numbers on the rotor marking, how they affect engine speed at full load and why the wrong choice can lead to overheating or, conversely, to underloading of the power unit. We will also look at practical examples of selection for different types of boats.

The physical meaning of the concept of the screw step

In the technical documentation and propeller markings, the pitch is in inches, and it indicates the theoretical distance that the propeller will travel in one full revolution in a hard, unyielding environment. Imagine screwing a screw into a tree, one revolution it travels a certain distance depending on the thread. So is the screw: if it is on the number 11, it means that in one revolution it has to theoretically move the boat 11 inches forward.

But water is not a solid, it's viscosity and fluidity, so the actual progress of a boat is always less than the theoretical progress. screw-slipFor planing boats, a sliding of 10-15% is considered normal, while for heavy displacement vessels it can reach 30-40%.

It's important to understand that step directly affects the load on the engine. A screw with a large pitch creates higher resistance to rotation. If you put a screw on a light boat with an excessively large pitch, the engine will not be able to develop the necessary speeds, which will lead to rich mixtures, swelling and overheating. Conversely, too small a step will allow the engine to spin above the limits, which can lead to the destruction of the shaky piston group.

⚠️ Warning: Operating a motor with a propeller that does not allow you to reach the WOT RPM range specified in the instructions will void the manufacturer's warranty. Always check the tachometer revolutions when the boat is fully loaded.

So the pitch of the propeller is a kind of "transfer number" of your boat, and when you change the propeller, you change the specifications of the entire engine-transmission-hull complex.

Relationship between the pitch of the screw and the engine speed

There's a direct correlation: increasing the pitch of the screw leads to lower maximum engine speeds, and vice versa. This is an axiom that every boating should know: If your passport engine is supposed to produce 5500-6000 rpm at full gas, and the tachometer shows only 4800, then the pitch of the screw is too big for this boat.

On the other hand, if the engine howls at 6,500 rpm, but the boat does not accelerate to the expected speed, then the propeller is "empty", its pitch is small. The engine runs idle, consuming fuel, but not transferring energy efficiently to the movement. Optimal selection allows the engine to reach the upper limit of the recommended range of rpm at full load of the vessel.

Each inch of a propeller's pitch typically changes the engine's speed by about 150-200 units. This figure can vary depending on the diameter of the propeller, the number of blades and the type of boat, but it serves as a good guide for initial calculations. For example, if you're short of 400 rpm to normal, you probably need to reduce the pitch of the propeller by 2 inches.

• 🚀 Increasing the pitch of the screw reduces the speed, but increases the maximum speed (if the motor has enough power).
• 🐢 Reducing the pitch of the screw increases speed, improving traction and acceleration, but reduces the "maximum speed".
• ⚖️ The ideal balance is achieved when the engine reaches the upper limit of the passport range of RPM at full gas with full load.

And we must not forget that screw-diameterOften, a change in diameter can compensate for a change in step, but to a lesser extent. The main lever of the rev is the step.

The effect of step on speed and traction

Choosing a propeller pitch is always about finding a compromise between thrust and speed. Other tasks require different propellers. If you use a trolled fishing boat, where keeping your course and pulling heavy tackles against the current is important, you need a screw with a smaller pitch, which will provide a confident exit to planing even when fully loaded.

For high-speed walks, water skis or wakeboarding, where acceleration dynamics are critical, low-stroke propellers are also often used. They allow you to quickly bring a heavy boat to plane. However, if your goal is maximum speed on a light boat with a powerful engine, increasing the pitch will allow you to realize the potential of the engine at high speeds.

It's worth noting that just putting a big-pitched propeller for speed is not going to work. If the motor doesn't have enough torque to pull the screw off, the boat will stand up, so for heavy boats with low-powered engines, increasing the pitch often leads to the opposite effect of falling speed.

The efficiency is also affected by the number of blades. Three-bladed screws usually have less resistance and give a greater maximum speed, while four-bladed ones provide better traction, stability and quicker hole-shot, but can slightly "strangle" the maximum speed.

Why is 4 blades better for a heavy boat?

The four-bladed screw has a large total area of the blades, which allows you to work more effectively with water at low speeds and at high loads, reducing cavitation and improving handling.

Table of correspondence of the screw pitch and type of boat

Although accurate selection is possible only by experimental means or complex calculations, there are averaged recommendations for different types of boats. Below is a table that will help you navigate the initial selection.

Type of boat / Task	Recommended step (inches)	Purpose of use	Expected impact
Heavy PVC/Trolling	8 - 10	Transportation of goods, fishing	Maximum traction, confident start
Universal boats	10 - 12	cruising, fishing, relaxing	Balance of speed and traction
Speedboats	13 - 15	Water skiing, racing	High maximum speed
Sailing auxiliary	8 - 9	Maneuvering in calm	Minimum resistance on the move under sail

The data in the table are relevant for medium-power engines (15-60 hp). For small power (2-9.9 hp), the step is usually less (6-9 inches), and for large suspensions (100+ hp) it can reach 17-21 inches or more.

Also worth considering is the material of the screw. Aluminum screws are more flexible and can deform when impacted, which changes their geometry. Stainless steel is stronger and allows the blades to be thinner, which theoretically improves efficiency, but such screws are more expensive and heavier.

Calculation of the required pitch of the screw

For those who like precision, there's a formula that allows you to calculate the theoretical speed or the required step, and it's based on the engine speed, gear ratio, and the desired speed.

The formula is as follows:

Speed(s) = (Motor Turns × Rotor Step) / (1013 × Transmission Number)

Where 1013 is a constant that takes into account the translation of inches into nautical miles and minutes into hours, as well as the average slip ratio. The gear ratio (Gear Ratio) can be found in the instructions for the motor, it is usually from 1.85 to 2.33.

Let's take an example: you have a 50 hp engine with a 2.08 gearbox. Maximum revs. 5500. You want to know how fast a screw will get with 11 increments.

Substitute the values: (5500 × 11) / (1013 × 2.08) ≈ 28.6 knots. This is a theoretical speed. Given the real conditions (wind, wave, fouling of the bottom), the real speed will be about 10-15% lower.

If you don't like the speed, you can mathematically pick up a new step, but remember that the formula doesn't take into account the aerodynamics of the body and many other variables.

⚠️ Note: In your calculations, remember that the actual speed is always lower than the theoretical speed because of the slip. Don't focus on numbers only, be sure to run running tests.

Typical mistakes in choosing a screw

One of the most common mistakes is to buy a screw by eye or on the advice of a parking neighbor who has a completely different boat. What works perfectly on a lightweight aluminum "cossack" may be useless on a heavy PVC boat with a keel.

Another mistake is ignoring the state of the screw. Fractures, flexes of the edges of the blades or cracks change the fluid dynamics. Even a small defect can reduce the efficiency by 10-15%. Aluminum screws can be ruled, but carefully, heating is not allowed, because the metal loses strength.

The electric motors also often forget about the engine-mountageIf the motor is set too low, the propeller runs in a turbulent stream, and the pitch no longer matters, cavitation begins. If it's too high, the propeller gets enough air in the corners.

• 🛑 Using a damaged screw for the sake of saving is a direct way to overload the engine.
• 🛑 Ignoring full loading when testing – a boat with one person and three passengers requires different thrust.
• 🛑 Trying to compensate for the weak power of the motor by increasing the pitch will only finish the engine.

The right approach is trial and error, but wisely, and having two propellers in stock, one for fishing with a load and one for cruising, is the perfect solution for the owner who wants to enjoy swimming in any conditions.

Frequently Asked Questions (FAQ)

How to know the current pitch of the screw if the marking is erased?

If the marking on the hub is not readable, you can try to measure the step physically, but it is difficult and requires special tools (protractor and ruler) to measure the angle of attack of the blade. The easiest way is to find the manufacturer's catalogue on the Internet by appearance and model number, knocked on the back of the blade, or simply buy a new screw with known parameters for tests.

Can I put a screw with a large pitch to increase speed if the engine does not go on the revs?

No, it's a big mistake. If the engine doesn't get up, it means the load is too high, and if you put a big screw in, it'll add more load, the revs will drop even more, and the engine will start running in emergency mode, and then you need to reduce the step.

Does the screw material (aluminum or steel) affect the step choice?

Indirectly, steel screws are stronger, their blades can be made thinner and with more complex geometry, which increases efficiency. So a steel screw with a step of 13 can work more efficiently than an aluminum screw in steps of 13. Often, when switching to steel, it is recommended to take a screw in 1 inch more increments to compensate for the increased efficiency and not go into torsion.

What is a cavitation bushing and how does it relate to the step?

The cavitation sleeve (or plate) is part of the lower unit over the screw. It's not directly related to the pitch, but it affects the screw. If the screw is wrongly matched (too big a pitch or diameter), the flow of water can break away from the blade, creating bubbles of steam (cavitation) that collapse with great force, destroying the metal. The right step minimizes the risk of cavitation.