Comparison Mercury 3.3M vs Mercury 2.5M

The maximum operating power of the outboard motor, expressed in horsepower.

Horsepower (hp) has traditionally been used primarily to refer to the power of internal combustion engines, including gasoline engines (see "Engine type"). However, in outboard motors, these units are also used for electric models (see ibid.). This is due to the fact that the majority of gasoline engines are on the market, and boat manufacturers prefer to indicate the maximum recommended engine power in “horses”.

The general patterns when choosing outboard motors in terms of power are as follows. On the one hand, a more powerful unit will allow you to develop more speed and is better suited for a heavy boat (see "Maximum boat weight"). On the other hand, weight, dimensions, cost and fuel/energy consumption also directly depend on power. Therefore, it does not always make sense to chase the maximum performance.

In addition, the choice of motor for maximum power also depends on the characteristics of the craft on which it is planned to be used. It is not worth exceeding the recommended power stated in the specifications — firstly, the boat transom may not be designed for a heavy large-sized unit, and secondly, the boat itself may not be suitable for acceleration to high speeds. There are also more specific recommendations. For example, from the point of view of efficiency and safety, the engine power at the level of 60 – 80% of the ma...ximum specified in the characteristics of the boat is considered optimal. Lower values may be useful if economy and low noise level are important to you, and higher values if high speed and acceleration dynamics are key points.

There is one more specific point associated with this parameter: most often, the characteristics indicate the power output directly to the propeller, however, some manufacturers (mostly east european) can go for a little trick, indicating the power on the main motor shaft. When power is transferred to the screw, losses inevitably occur, so the useful power of the motor in such a case will be less than claimed. Thus, when choosing and comparing, it's ok to clarify what kind of power is meant in the characteristics — on the propeller or on the shaft.

The maximum operating power of the outboard motor, expressed in kilowatts.

The practical value of motor power is described in detail in “Maximum power" is higher. Here we note that the kilowatt (derivative of watt) is just one of the units of power used in fact along with horsepower (hp); 1 HP ≈ 735 W (0.735 kW). Watts are considered the traditional unit for electric motors (see "Engine Type"), but for a number of reasons, outboard motor manufacturers use this designation for gasoline models as well.

The highest shaft speed that the outboard motor is capable of developing.

Theoretically, the speed of rotation of the propeller (or turbine — see "Motor type") depends on the engine speed, and, accordingly, the speed that the boat is capable of developing. However, in addition to this indicator, many other factors also affect the performance of the motor — engine power (see above), gear ratio (see below), propeller design, etc. As a result, situations are quite normal when a more powerful and high-speed motor has lower revolutions than the weaker one. Therefore, this parameter is, in fact, a reference one, and has almost no practical value when choosing. Unless it can be noted that high-speed motors are more susceptible to noise and vibration than low-speed ones; however, this moment can be compensated by the use of various technical tricks.

The design of the exhaust system in a gasoline outboard motor (see “Engine type”), more precisely, the method of exhaust gases used in this system.

— Above the screw. This category includes two types of engines. The simplest option is when exhaust gases are emitted directly into the air. Such systems are extremely simple and cheap, but the exhaust can create a noticeable inconvenience for people in the boat (not only because of the gases, but also because of the rather high noise level); therefore, they are found only in the simplest outboard motors, and even then quite rarely. A more common option is to release exhaust gases into the water above the propeller (most often through the so-called anti-cavitation plate — a flat ledge above the propeller). Such systems are more comfortable than "air" ones, while they are simpler and cheaper than propeller exhaust (see below), although they are still considered less technically advanced.

— Through the screw. In systems of this type, the exhaust is led into the water directly through the propeller hub; in fact, the position of the exhaust pipe coincides with the axis of rotation. This reduces the noise level compared to systems using exhaust over the propeller, and also slightly increases power and traction characteristics. The downside of these advantages is the design complexity and, accordingly, the high cost.

The gear ratio describes how fast the propeller of the outboard motor rotates relative to the speed of rotation of its shaft. For example, a gear ratio of 2 means that for each revolution of the shaft, the screw, in turn, makes two revolutions (that is, it rotates twice as fast). In modern outboard motors, this parameter, in fact, is purely reference, because. the practical characteristics of the unit (power, traction, etc.) depend on many design features and are practically not related to the gear ratio.

The types of gears provided in the design of the outboard motor are, in fact, the direction in which it can move the boat.

— Front. Standard gear for forward movement. Available in all outboard motors without exception, by definition.

— I'm neutral. In this case, neutral gear means the mode of operation of the motor, in which its shaft rotates idly, without transferring rotation to the propeller or water jet. Thanks to this, you can completely remove the thrust without turning off the motor and without lifting its “leg” out of the water. Considering that starting after a shutdown can be a rather troublesome procedure (especially if you have to do this often), and removing a spinning propeller from the water is generally undesirable — having a neutral gear is a very useful feature, and most gasoline engines (see "Engine type") have this mode. But in electric models (see ibid.), stopping and starting do not constitute a problem, so the role of the “neutral” in them is played by turning off the power and completely stopping the motor (and the neutral gear itself is not indicated in the specifications).

— Back (reverse). A mode of operation in which the engine pulls the entire vessel backwards; in propeller motors, it is implemented by rotating the propeller in the opposite direction, in jet engines, by using reverse dampers. The reverse functi...on greatly facilitates both manoeuvring in narrow spaces and emergency braking on the water, so it is found in the vast majority of gasoline engines and almost all electric ones.

Note that electric motors (see “Engine type”) can have several gears of the same type — for example, 5 forward and 3 reverse. In such models, each "gear" is a separate switch position corresponding to a certain engine power. In gasoline engines, power control is carried out smoothly, using a throttle, so they have less than one gear of each type.