Right. speed It's a fundamental parameter that determines not only the maximum speed of your vessel, but also the longevity of the power plant. Many boat owners make the mistake of thinking that the engine should be operating only at its limits, but operating outside the recommended speed range can lead to premature wear and tear of the piston group or overheating. Understanding the physics of the propeller-water interaction allows you to significantly save fuel and save engine life.
A water motor needs to clearly distinguish between the maximum speeds that an engine can develop idling without a load and the working speeds under load when the propeller meets the resistance of the aqueous environment. motor If the engine is not gaining speed, it is running in overload mode, which is fraught with detonation and candles coking, on the contrary, if the engine is easily "unwinded" above the permissible limit, there is a risk of cavitation destruction of the blades and loss of traction.
In this article, we will discuss in detail how to calculate optimal parameters, what factors affect the efficiency of the propeller and how to avoid typical errors in the selection of equipment. You will learn to diagnose traction and speed problems yourself using simple tools and matching tables.
Physics of the process: how the screw creates traction
The propeller converts the engine's torque into the boat's forward motion. screw-step The theoretical distance that a screw would travel in one complete revolution in a solid medium, but water is a liquid, and the actual movement is always less than the theoretical one due to slippage. slip or slip, and in ideal conditions it is about 10-15%.
The speed of rotation directly affects the volume of water thrown back, and the higher the speed, the more water passes through the disk of the screw, but this is where the law of resistance comes into force. When certain values are reached, cavitation begins, the formation of vapor bubbles in the areas of rarefaction, which dramatically reduces efficiency. screw-piece and its step must strictly correspond to the power and torque of the motor at specific turns.
What is cavitation in simple words?
Cavitation is the process of boiling liquid at low pressure, and the propeller blades form bubbles of gas that collapse to create micro-shock waves, and these waves gradually flush metal off the surface of the blades, forming ulcers and caverns, resulting in loss of balance and reduced efficiency of the screw.
It's important to understand that changing the geometry of the propeller or its rotation changes the engine's performance point on its performance. If you install a propeller with a large pitch, the engine will run at lower revs at the same boat speed, but it will need more torque to overcome resistance.
Operating RPM Range (WOT) and its Value
Every manufacturer of motors, whether Yamaha, Tohatsu or MercuryThe Wide Open Throttle (WOT) is not just a reference figure, but a target that a boating should aim for when selecting a propeller, and falling into this range ensures that the engine develops its declared power.
If the throttle is fully open, the tachometer is below the lower end of the range, it means that the screw is heavy. The engine is choking, cannot develop power, and operates at an increased fuel consumption per unit of travel. kneeling And the shaky-piston group increases many times over.
The ideal setting is achieved when the boat is fully loaded (passengers, equipment, fuel) the engine reaches the upper limit of the recommended WOT range.
On the other hand, if the tachometer hand is confidently going over the upper limit, the screw is too light. The engine operates in a "brute" mode, which can lead to the operation of the speed limiter or, in the worst case, to the twisting of the liners and the destruction of the engine due to excess inertial loads. For two-stroke engines, this is especially critical, since the lubrication in them depends on the number of revolutions and the mixture.
For accurate diagnosis, you need to use a calibrated tachometer, since regular instruments on the control panel can have an error of up to 10-15%. Professionals recommend using portable digital tachometers connected to a candle wire to obtain reliable data.
Relationship of step, diameter and revolutions
The choice of the screw is always a compromise between speed and thrust. Increasing the pitch of the screw at constant engine speed will lead to higher speed, but it will require more power. If the engine power is not enough, it simply will not be able to turn this screw to the desired speed. The diameter of the screw affects the area of the disk hole and therefore on the traction, especially at low speeds and when trolling.
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 (RPM). This rule helps to approximate the required step. For example, if your engine with a 9 inch screw develops 4800 rpm and the WOT range is 5000-6000, you need to reduce the speed. Increasing the pitch to 10 inches theoretically reduce the speeds to 4600-4700, which may not be enough, so you may need a step of 9.5 or 9.25 if any are available, or a change in diameter.
However, linear dependence does not always work perfectly: the shape of the blades, the number of blades (3, 4 or 5) and their profile (for example, the shape of the blades). High Thrust or PerformanceFour-blade propellers often allow better control of the boat in planing and have a smaller slip step, but may require a more powerful motor to spin.
When switching to a propeller with a large number of blades (for example, from 3 to 4) to maintain the same revolutions, the screw pitch often has to be reduced by 0.5-1 inch due to increased resistance.
Also worth considering is the material of the screw. Aluminum screws are more flexible and can deform when impacted, which changes their geometry and, as a result, their performance. Stainless steel is tougher, allows you to make the blades thinner and more efficient, but it is more expensive and heavier, which affects the moment of inertia.
Calculation of theoretical and real speed
To begin to estimate the capabilities of your boat complex, you can use a formula for calculating theoretical speed, which is based on the pitch of the screw, the engine speed and the gear ratio, and the formula is as follows:
V = (RPM Pitch 60) / (GearRatio * 185200)Where:
- 🚀 V - speed in nodes (1 node = 1.852 km / h);
- 🔄 RPM - engine speeds per minute;
- 📏 Pitch - screw pitch in inches;
- ⚙️ GearRatio - gear ratio of the gearbox (indicated in the manual, for example, 1.85 or 2.08).
This formula gives a speed in vacuum, without taking into account water resistance, boat weight and slip. The real speed will always be lower than theoretical. The efficiency of the propeller rarely exceeds 0.6-0.7. So, when you get the calculated value, experienced water turbines take 15-20% from it to get a realistic forecast.
Consider an example: 30 hp 2.08 gearbox, 10 inch pitch, 5000 rpm.
Theoretical speed: 5,000 10 60) / (2.08 * 185200) ≈ 7.78 m/s ≈ 15.1 knots ≈ 28 km/h.
Taking into account the efficiency (take 0.7): 28 * 0.7 ≈ 19.6 km / h. This value will be close to reality for a light inflatable boat.
| Type of boat | Expected Slip (slip) | Recommended speed range (plane) | Effects of loading |
|---|---|---|---|
| Inflatable PVC (NDND) | 15-20% | 25-40 km/h | High (soft bottom extinguishes speed) |
| Metal (Kazanka, Voronezh) | 10-15% | 30-50 km/h | Medium (hard case better holds plane) |
| Fiberglass boat | 8-12% | 40-70+ km/h | Low (hydrodynamic housing) |
| heavy trolling boat | 20-25% | 10-20 km/h | Critical (thrust is important, not speed) |
Factors that reduce the efficiency of the screw
Even a perfectly matched screw can be inefficient due to external factors. One of the main enemies is fouling. Algae, shells and tin sticking to the blades disrupt the hydrodynamic profile, increase weight and create additional resistance. Regular cleaning of the screw is a mandatory procedure to maintain the declared speed specifications.
The cavitation plate should be on the same level or 1-2 cm below the bottom of the boat when moving. If the motor is raised too high, the screw captures air from the surface, which leads to a disruption of the flow and a fall in revolutions. If lowered too low, water resistance increases and the risk of impacts on the bottom ground.
☑️ Diagnostics of screw problems
The technical condition of the lower unit itself also affects the power transfer: The developed paddle shaft bearing can cause the propeller to beat, which reduces the efficiency of the stop. O-rings (o-boats) must be whole so that water does not fall into the gearbox, although this affects reliability rather than instantaneous speed.
⚠️ Attention: Even a small dent on the edge of the blade can reduce the efficiency of the screw by 5-10%. Don't try to control the cast aluminum screws with a hammer - the metal can crack. It is better to replace the screw or give it to professional welding and editing.
Tuning and modernization: is the game worth the effort
Boat owners often look for ways to increase speed without changing the engine. Stainless steel screws are one of the most effective methods. Because of the strength of the material, the blades can be made thinner, which reduces resistance, and also change the geometry of the rear edge for better water discharge. These screws are usually more expensive, but give a speed gain of 2-4 km / h.
Another method is the installation of a sliding sleeve (Rubex and analogues), a special sleeve in the screw hub that changes its properties depending on the load, allowing the screw to operate in a wider range of revolutions with high efficiency. However, the effect of such a replacement is noticeable mainly in transient modes and trolling, at maximum speeds, the gain can be minimal.
Changing the number of blades is a radical step, going from three blades to four or five (as in the series). Yamaha High Thrust) increases thrust at low speeds, which is ideal for heavy boats that can not go to the plane for a long time, or for water ski catamarans, but the price will be a decrease in the maximum speed by 3-5 km / h due to increased drag.
The screw polishing myth
Smooth, mirror-polished rotor surfaces are not always a boon. Micro-sharpness helps to hold the boundary layer of water, improving grip. Excessive polishing can even worsen performance, although visually the boat looks more "sporty."
And you also have to adjust the angle of the trimming, and if you lift or lower the lower unit with a hydraulic lift or a tray wheel, you change the angle of attack of the propeller, and the right trimm allows the boat to go plane faster and reduce fuel consumption at cruising speed, actually using the same angle of attack. motor-power more rational.
⚠️ Attention: When experimenting with screws, never ignore the condition of the lower unit bushings. A sharp change of load (for example, installing a too aggressive screw) can squeeze out a worn sleeve, and the screw will simply fly off the shaft in the middle of the reservoir.
Frequently Asked Questions (FAQ)
Can I use car spark plugs in a outboard motor?
Candles for outboard motors (marking usually ends with: W or XFor example, NGK BPR6ES) have a shielded casing to suppress radio interference and a design to prevent sparks from entering the fuel-rich mixture at low revs. Car candles can cause kalil ignition and piston destruction.
Why does the engine stop when the gas is opened?
It's probably a problem with the carburetor accelerator pump system (if the motor is carburetor) or the throttle position sensors (if the injector), and it could be because the screw is too big, which creates an instantaneous load that the engine can't handle at low speeds.
How often should I change the lower unit sleeves?
The life of the bush depends on the operating conditions, on average, they are changed every 300-500 hours or when a backflip appears. If you often walk in shallow water or rocky bottom, inspect their condition after each season.
Does the water temperature affect the speed of rotation?
Yes, cold water is denser than warm water, which increases propeller resistance and engine load, and in the summer in warm water, the speed can be 100 to 200 units higher than in the early spring at the same load, which is normal and does not require a replacement screw.