The issue of transferring power from kilowatts to horsepower often arises before owners of water equipment, especially when choosing a new engine or processing documents in the GIMS. 8 kW outboard motor This is a common category of propulsion systems that provide confident plane for most inflatable boats and small boats, and understanding the real power is not only necessary for formalities, but also for the right selection of propellers that will unlock the potential of your vessel.
The exact value depends on the conversion rate used, because historically different measurement systems have evolved. 8 kW roughly 10.8 horsepowerHowever, for marketing purposes, manufacturers often round these numbers by calling motors 10 hp or even 12 hp, which can mislead the buyer about the traction specifications.
In this article, we will examine the mathematics of translation in detail, the effect of gear ratio on real thrust, and help you decide on the choice between electric and gasoline units of such power. You will learn why the rated power on the shaft may differ from the power consumed, and how this affects the speed of your vessel.
⚠️ Note: When registering with GIMS or filling in technical documentation, always use the data specified in the product passport (PS), and not theoretical calculations, since rounding in the lower direction may be more beneficial for compliance with legal norms.
Mathematics of translation: exact coefficients and formulas
To convert kilowatts to horsepower, you need to understand what kind of measurement system we use. In the technical documentation for outboard motors, the most common metric horsepower (hp) is 735.5 watts. If you divide 8000 watts by 735.5, we get the value of the value. 10.87 hp This is the most accurate figure for engineering calculations.
There is also mechanical (imperial) horsepower, common in the US and UK, where 1 hp is 745.7 watts. 8 kW turn into 10.73 hp The difference seems small, but when fine-tuning a carburetor or speed controller, it can make a difference. Most Chinese and European brands supplying equipment in the CIS are metric-based.
It is important to consider that the declared power in kilowatts is often indicated as the maximum power output on the propeller shaft. However, the actual power reaching the propeller is always less due to losses in the gearbox and lower unit. Efficiency Modern outboard motors are about 85-90%, so the actual thrust will correspond to about 9-9.5 hp of clean work on moving water.
An accurate translation of 8 kW gives 10.87 hp (metric), but the actual propeller thrust is always lower due to mechanical losses in the gearbox.
Gasoline vs. Electric: Where is the honest 8kW?
Comparison of gasoline and electric engines with a capacity of 8 kW reveals significant differences in the nature of work. Gasoline engine develops its maximum power only at high revs, usually in the range of 5000-6000 rpm At low and medium revs, its return is much lower, which requires constant gear shifting or work in a narrow range to get on plane.
Unlike the ICE, electric motor It gives you maximum torque instantly, from the first seconds of power on, which means that an 8 kW electric unit can accelerate the boat to plane faster than a gasoline equivalent of the same rated power. However, the duration of the electric motor is limited by the battery capacity, whereas gasoline allows you to run for hours when you have fuel.
- ⚡ Gasoline engines require regular maintenance (oil change, candles, carburetor cleaning), whereas electric motors require almost no maintenance.
- ⚡ Electric motors operate silently and without vibrations, which is critical for fishing on cautious fish.
- ⚡ The weight of an electric battery installation often exceeds the weight of a gasoline engine, which affects the carrying capacity of a boat.
When choosing between technologies, consider not only the 8 kW figure, but also operating conditions. If you plan long transitions against the current or wave, the gasoline option may be more reliable. For quiet fishing on lakes and small rivers, the electric option with the declared power of 8 kW will be more comfortable and environmentally friendly.
Effect of gear ratio of the reducer on thrust
The power of 8 kW is only the potential of the engine, which is transformed into thrust through the gearbox. Transfer ratio It determines how many revolutions a screw makes in one turn of the crankshaft of the engine. For motors with a power of about 10 hp (8 kW), the standard ratio is 2.08:1 or 2.15:1. This means that the engine needs to do just over two revolutions for the screw to turn once.
If you change the gear ratio by changing the gearbox, you can significantly affect the specifications of the boat. Increased gear ratio (for example, to 2.33:1) will allow you to use a screw with a large pitch, which will increase the maximum speed on straight sections, but worsen acceleration. Reducing the number will make the engine more "tractive", which is useful for heavy boats or when driving against a strong current.
Owners should remember that installing a screw with an inappropriate pitch at a fixed power of 8 kW can lead to engine overload. If the engine can not gain maximum speed (underload) or, conversely, “roars” without acceleration (overload), the resource of its units is sharply reduced. The correct selection of a pair of “reductor-screw” allows you to squeeze the maximum from the available horsepower.
How to check engine overload?
If the engine does not develop the maximum speeds specified in the instructions (usually 5000-6000 rpm) with a fully open throttle, then the screw is too heavy. If the speed is higher than normal, and the speed does not increase, the screw is too light, and the engine is idle.
Comparative table of popular models of motors
There are many models on the market that fall into the 8 kW category. Below is a comparison of the technical specifications of popular gasoline and electric units so you can estimate the range of parameters.
| Model | Type of engine | Power (kW) | Power (hp) | Weight (kg) |
|---|---|---|---|---|
| Yamaha F9.9 | 4T petrol | 7.3 | 9.9 | 38 |
| Tohatsu M9.8 | 4T petrol | 7.2 | 9.8 | 36 |
| HDX F8BMS | petrol 2T | 5.9 | 8.0 | 25 |
| Flux Marine 10R | Electrical. | 7.5 | 10.2 | 33 |
| Eltech 900 | Electrical. | 6.6 | 9.0 | 28 |
Analyzing the table, you can see that gasoline engines often have less weight at the same power, but require more space to install. Flux MarineThey offer comparable power in a compact body, but their weight increases rapidly when you add batteries. The choice of a particular model depends on what is more important to you: the weight of the engine itself or the total range.
⚠️ Note: When buying a used motor, pay attention to the condition of the lower unit. If the gears of the gearbox are worn on an 8-kilowatt engine, the real power on the screw can drop by 15-20% due to slippage and friction.
Selection of screws for the 8 kW motor: theory and practice
The right propeller is key to the efficiency of any outboard motor. For power of about 8 kW (10-11 hp), the optimal screw diameter is considered to be a range of 8 to 10 inches. The screw pitch (the distance the propeller travels in one revolution in a solid medium) usually varies from 7 to 9 inches for standard tasks.
If your boat is heavy or you often walk full load (fishermen, gear), it makes sense to install a screw with a smaller pitch. This will allow the engine to unwind more easily and bring the boat to plane, although the maximum speed will decrease. For light inflatable boats with 1-2 passengers, you can use large-step propellers to achieve maximum speed.
☑️ Screening of rotor status
The material of the screw also plays a role. Aluminum screws are cheaper and easier to repair, but they are less efficient and prone to cavitation at high revs. Stainless steel provides better efficiency and durability, which is especially important for 8 kW motors running at their limit when planing.
Operational nuances and legal aspects
Owning an 8 kW motor imposes certain obligations. In the Russian Federation, engines with a capacity of more than 10 hp (which corresponds to 8 kW) require registration with the GIMS and the right to operate a small vessel of category "M". If the manufacturer has specified a power of 9.9 hp or 10 hp, you are subject to regulation.
However, if the engine documents are artificially low to 9.8 hp (about 7.2 kW) or 5 hp, registration is not formally required. Many manufacturers offer a power downgrade service by installing restrictive plates or reconfiguring the fuel system. But it is worth remembering that if the actual engine power is 8 kW, then technical checks (speed measurement, exhaust analysis) may raise questions.
- 🛡️ Always carry your passport to the engine with certified copies, where the power is indicated.
- 🛡️ Keep an eye out for noise levels: 8kW and above engines often have stricter eco-standards in protected areas.
- 🛡️ When installing the smock wheel, take into account the weight of the motor: 8 kW is already a significant load on the stern.
Operating a motor of this power requires quality fuel. For two-stroke engines, use AI-92 or AI-95 gasoline in a mixture with oil in a ratio of 1:50 or 1:25 (depending on the model). four-stroke engines are more demanding on the octane number and purity of gasoline, since clogging of nozzles or carburetors instantly reduces the return.
Use a fuel filter between the tank and the engine. For 8 kW engines, even the microscopic water in gasoline can cause hydraulic shock or corrosion of the plunger pair of TNVD.
Do I need to register a boat if the engine is 8 kW, but in the documents 5 hp?
Registration is required if the engine power exceeds 10 hp (8 kW). If the documents (PS) indicate a power of 5 hp, then formally you are not required to register the engine. However, the GIMS inspector can make measurements. If the actual power will be significantly higher than stated, problems may arise. safest option - to have documents where the power does not exceed the threshold of 10 hp.
Can the 8 kW engine power be increased to 15 hp?
In theory, on some models (especially Chinese ones), this is possible by replacing the carburetor, the petal valve and reconfiguring the ignition. However, the life of such a motor will drop sharply, and the risk of failure of the crankshaft or piston group will increase many times over, and the factory power of 8 kW is the optimal balance of reliability and traction.
What is the speed of an 8 kW boat?
For an inflatable boat with a length of 3.2-3.6 meters with one passenger, the speed will be 25-30 km / h For heavier PVC boats (3.8-4.0 m) with two passengers, the speed on planing will be about 20-22 km / h. The speed depends greatly on the shape of the bottom, loading and the screw selected.