Comparison Tohatsu M9.8BS vs HDX T5BMS

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 number of cylinders in a gasoline outboard engine (see "Engine type").

Usually, the optimal number of cylinders is selected by the manufacturer based on the working volume (see below) and the overall layout of the motor. Therefore, from a practical point of view, this parameter can be called secondary. At the same time, it can be a good indicator of the overall level of the engine: basic models have one cylinder, while top models can have 4 or more.

The working volume of a gasoline outboard engine (see "Engine type"). This term usually means the total working volume of the cylinders.

The larger this value, the higher the motor power, usually (see the relevant paragraph). At the same time, with an increase in the working volume, fuel consumption, weight and dimensions of the unit also increase; and power depends not only on this indicator, but also on a number of other factors — ranging from the number of strokes (see "Engine duty cycle") or the presence of turbocharging (see below) and ending with specific design features. Therefore, situations are not excluded when a smaller engine will have more power, and vice versa.

The diameter of a single piston in a gasoline (see "Engine type") outboard motor. In most cases, this parameter is purely reference; situations where data on the piston diameter is really needed are extremely rare — usually during the repair or maintenance of the engine.

The type of fuel tank used by the gasoline outboard motor (see "Engine type").

— Built -in. As the name implies, in such engines the fuel tank is an integral structural element. This eliminates the need for the user to look for a separate fuel tank and build a system for supplying gasoline from the tank to the engine. On the other hand, the tank significantly increases the dimensions and weight of the entire structure, which is especially important for two-wheel tractor control (see below). Therefore, for powerful engines that consume a lot of fuel and require volumetric tanks for its storage, this option is poorly suited — it is typical for relatively modest models whose power does not exceed 25 hp.

— External. This category includes engines that do not have built-in fuel tanks and are designed to supply fuel from a separately located container through a special hose. The tank itself is most often supplied as a kit, but there are exceptions. However, anyway, the absence of a fuel tank directly in the motor housing makes the motor itself lighter, more compact and more mobile (the latter is relevant for models with a two-wheel tractor control system, see below). And for powerful high consumption units that require appropriate tanks, this is generally the only available option — otherwise the motor would turn out to be too heavy and bulky.

The total volume of the fuel tank provided in the design or delivery set of the outboard motor (depending on the type of tank — see "Fuel tank").

The larger the capacity of the fuel tank, the longer the engine will be able to work without refueling, the less often it will be necessary to replenish the fuel supply in the tank. On the other hand, volumetric tanks have appropriate dimensions and weight, especially when filled; the latter is especially critical for motors with built-in tanks (see above).

The total weight of the outboard motor. This parameter is indicated, usually, only for the unit itself, without taking into account the fuel in the tank and the tank itself (if it is external, see "Fuel tank"), as well as additional equipment. Motor weight data can be useful in order to estimate the overall balance of the boat and the change in its payload capacity.