Maximum power
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.Exhaust system
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.
Transom height (deadwood)
The height of the transom of the boat for which the motor is designed. This parameter is also called “leg length” (meaning, of course, the “leg” of the motor itself, and not the boat).
The transom is a part in the stern of the vessel, designed to mount the motor. The length of the motor leg must match the dimensions of the transom, otherwise the propeller will be at an off-design depth, which is fraught with problems in operation and loss of power (regardless of whether it is too deep or too shallow). The height of the transom, usually, is indicated in the documents for the boat itself — it is from these data that one should proceed when choosing a motor according to the length of the leg.
Weight
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.
