Choosing a powertrain for a boat always starts with a specification study, where engine performance is a key parameter. Many newcomers to water-engines are confused when they see 9.9 in some catalogs and 7.3 in others, not understanding why the numbers differ in visual similarity, which is because different countries and standards use different calculus systems.
The main question that worries the owner of a PVC boat or boat is how these numbers affect the actual speed and thrust of the vessel. Power of the outboard motor It's the ability to overcome the water resistance and get the boat to planing, and understanding the physical nature of these quantities allows you not just to buy a motor, but to find the optimal boat-motor-propeller bundle.
In this article, we will look at the different units of measurement, how to translate one value into another, and why sometimes a motor with lower numbers in your passport can be more effective than a competitor, and how horsepower differs from kilowatts, and how torque plays a role in real water use.
Basic units of measurement: horsepower and kilowatts
Historically, the marine engine industry has been dominated by two major measurement systems. horsepower (hp)which is still the de facto standard for most outboard motor manufacturers, such as Yamaha, Tohatsu or HondaThe second system is an international system that uses kilowatts (kW)It is mandatory for technical documentation and certification in many countries, including the EU.
The difference between these two is purely mathematical, but it's often confusing when comparing performance: one mechanical horsepower is approximately 0.7355 kilowatts. So to get a value in kW, you have to multiply the power in hp by this factor. For example, a popular 5 hp motor in kilowatts would have a value of about 3.68 kW.
Why do manufacturers continue to use horsepower when the world has long since switched to the metric system? The answer lies in marketing and consumer psychology. The number 9.9 looks more impressive and understandable to an experienced boater than 7.3. In addition, many countries have legal restrictions specifically on horsepower, which allows manufacturers to create modifications to engines with limited power to simplify registration.
⚠️ Attention: When buying an imported motor, look for a badge: If the motor is EU certified, it can only be specified with kW, which will require recalculation to understand the real power class.
It is important to understand that rated capacity This is not a constant, but a measure achieved at certain crankshaft speeds. Manufacturers always indicate how many rpms the declared power is removed. Ignoring this parameter can lead to the engine being set up incorrectly.
Torque: The Hidden Power of Your Motor
If the power determines the maximum speed, torque It's responsible for the thrust and the motor's ability to accelerate a heavy boat, measured in Newton meters (Nm) or, in the old system, in kilogram-force-meters. For a water motor, torque is often more important than peak power, especially if you plan to use a fishing boat with a lot of gear or towing water skis.
Two-stroke engines traditionally have higher torque at low and medium revs than four-stroke counterparts of the same power, making them "roarier" at the start and better adapted to harsh conditions. Yamaha CLP or Suzuki Lean BurnThey successfully eliminate this difference.
Torque directly affects the choice of pitch of the propeller. If you set the propeller with too much pitch on the motor with low torque, the engine will not be able to spin to operating speeds. This will lead to detonation, overheating and eventually the piston group will fail. So the balance between power and torque is critical.
How to check the torque by hearing?
If the throttle is suddenly opened, the engine is choking and not gaining momentum, and the boat is not accelerating, the screw pitch is probably too high for the current engine torque. Try reducing the screw pitch by 1-2 inches.
When comparing motors from different brands, always look at the torque chart, if available. The motor may have the same maximum power of 20 hp, but one will give a maximum torque of 3000 rpm and the other will give a maximum torque of 4500 rpm. The first will be better to pull from the spot, the second – more efficient at high speeds.
Table of translation and comparison of specifications
So to make it easier to navigate the different engine specifications, we've developed a composite table that will help you quickly navigate the unit ratios and understand what power class a motor is, and that's relevant for standard mechanical horsepower.
| Power (hp) | Power (kW) | Typical boat class | Recommended screw (step) |
|---|---|---|---|
| 2.5 - 3.5 | 1.8 - 2.6 | Small PVC, inflatable chairs | 5 - 6 inches |
| 5.0 - 6.0 | 3.7 - 4.4 | PVC 280-320, small aluminum | 6 - 7 inches |
| 9.8 - 9.9 | 7.2 - 7.3 | PVC 320-360, boats up to 3.5m | 8 - 9 inches |
| 15.0 - 18.0 | 11.0 - 13.2 | PVC 360-400, boats 3.5-4.5m | 9 - 11 inches |
| 20.0 - 25.0 | 14.7 - 18.4 | Boats 4.0-5.0m, heavy PVC | 10 - 12 inches |
Using this table, you can quickly assess whether the motor is suitable for your boat. Note that the pitch values of the screw are given tentatively and depend on the specific screw manufacturer and the material made. Aluminum screws usually have a smaller pitch than stainless screws.
Power effect on speed and output on planing
The main purpose of the engine is to make the boat plane. planing is a mode of movement in which the vessel glides on the surface of the water, relying on dynamic lift, rather than Archimedian push force. To enter this mode, a certain power density is required, usually between 20 and 40 hp per ton of displacement, depending on the contours of the hull.
The power shortage keeps the boat in displacement mode, and in this state, the water resistance increases dramatically, and even adding gas gives a minimal increase in speed, but it increases fuel consumption enormously. Engine power It should be sufficient to overcome the "hump" of resistance and push the body to the surface.
However, too much power is also dangerous: too powerful a motor can lift the bow of the boat too high, which will lead to loss of controllability and even tipping the boat when a sharp turn is taken, and excessive load on the transom can damage the design of the boat, especially if it is a question of inflatable models of PVC.
The optimum power is 70-80% of the maximum boat manufacturer permits, which is needed to compensate for the weight of passengers, cargo and wind.
The speed of the boat increases disproportionately to the power of the engine. To double the speed, theoretically you need to increase the power by 8 times (since the water resistance increases in a cube from speed), so chasing maximum hp figures in the hope of a record speed often does not make economic sense.
How to choose the right screw under the power of the engine
The propeller is a kind of outboard motor transmission that converts torque into thrust. An incorrectly selected propeller can strangle even the most powerful engine. The basic parameter here is the propeller's pitch, which indicates the theoretical distance the propeller will travel in one revolution in a solid environment.
The task of the water motor is to ensure that, with the throttle fully open and the boat is normally loaded, the engine reaches the upper limit of the recommended range of revolutions (usually 5000-6000 rpm for most suspension units). If the motor does not twist, the pitch of the screw is too large, if it twists, it is small.
- 🚤 Small step: Improves acceleration and traction, but reduces top speed, ideal for heavy boats or towing.
- 🚤 Mid-step: Universal option for mixed operating conditions (fishing, cruising).
- 🚤 Big step: Maximizes speed on light boats, but worsens acceleration and can overload the engine.
Plastic and aluminum screws often deform when hitting an obstacle, which changes their geometry and efficiency. Stainless steel screws hold the pitch better, but they are more expensive and can transfer the impact load to the engine's lower unit in a collision.
Always have a second propeller in increments of 1-2 inches less than the main one, which will help if you plan a long trip with a full load of fuel and equipment.
Certification Nuances and Power Limitations
In Russia and many CIS countries, there are power thresholds that share the requirements for the management and registration of boats. For a long time, the 10 hp limit determined the existence of GIMS license and the need to register a vessel. Motors up to 10 hp inclusive were considered low-power and did not require rights.
However, the legislation is changing, and now the criterion is often not only the power of the engine, but also the length of the vessel. However, manufacturers continue to produce engines with a capacity of exactly 9.9 hp (actually 15-hp with a limiter) to fall into the preferential category for consumers.
⚠️ Attention: Self-forcing of the engine (removal of power limiters) cancels the manufacturer's warranty and can lead to fines from the GIMS if a non-conformity with the marking on the body is detected.
When buying a used motor, be sure to check the hood and lower unit markings. If the numbers do not match or the stickers are overstuffed, this may indicate an interference with the design. Also remember that the power specified in the PSM (Small-Sized Vessel Passport) must match the actual power of the motor.
Frequently Asked Questions (FAQ)
How to convert kilowatts to horsepower without a calculator?
For a quick estimate in mind, you can use a simplified rule: divide the value in kilowatts by 0.74 or multiply by 1.36. For example, 10 kW is about 13.6 hp For accurate calculations, use a factor of 1 kW = 1.35962 hp.
Is it true that a two-stroke engine is more powerful than a four-stroke at the same hp?
No, 10 hp is 10 hp in any system. However, two-stroke motors are usually lighter and have a higher power-to-weight ratio, which subjectively feels like "more power" on light boats. They also have higher torque at the bottom.
Can the engine be more powerful than the boat manufacturer recommends?
It is strongly recommended not to exceed the maximum power specified in the marking plate on the transom of the boat. This is a safety issue: a more powerful motor can destroy the transom, turn the boat over at full speed or put a load on the hull leading to its rupture.
Does the height of the engine installation affect the power sold?
Yes, it's direct. If the motor is set too low, the water resistance to the lower unit and the anti-cavitation stove increases, if it's too high, cavitation occurs, and the screw idles, losing traction, and the optimal height is selected by experiment.