Electric outboard VS Petrol outboard motor
Boat electric motors are not inferior to petrol motors because of the number of stereotypes associated with their advantages and disadvantages. We considered the manufacturers of outboard motors, honest assessments of experts and boaters, and our boating experience in testing motors of various types and brands. We also “sifted” the opinions spread on the Internet and the press. The result is information that will help you choose an outboard motor for an inflatable boat more objectively. We do not give preference to any outboards manufacturer for objectivity, and we do not mention the names.
Except for the hand-powered outboard propeller driven by American Thomas Rees, the first genuine electric outboard motor was introduced by the Frenchman Gustave Trouve in 1881. Today, an electric boat outboard motor is a pretty worthy alternative to internal combustion engines in many cases.
If your inflatable motorboat was designed for a relatively powerful outboard motor and you like planning at a reasonably high speed, there are no suitable electric boat motors. But if the inflatable boat is designed for a low-power engine and is used in displacement mode (i.e. you do not expect to perform planning) – perhaps you will consider the electric motor the best option. Moreover, electric boat motors can fit a rowing inflatable boat with a hinged transom and a compact motorboat with a stationary transom.
Disadvantages of an electric outboard motor
Usually, the disadvantages of an electric outboard motor are considered low power (which is associated with low speed) and low autonomy (the time and mileage of swimming depending on the battery’s capacity and the mode of operation). Nevertheless, everything in the world is relative.
Electric outboard motor traction force
Getting acquainted with electric motors, you will notice that the power is mentioned not in horsepowers or kilowatts, which is usual for gasoline motors. Traction force (thrust) usually indicates a characteristic of the power of electric motors. It is generally expressed in kilograms or pounds (lb). Although, strictly speaking, pulling force is measured in kilogram-force or pound-force units. Moreover, both units of measurement are already considered obsolete, and in the International System of Units (SI), force is measured in Newtons (N, N). Often, the traction force of boat electric motors is also indicated in Newtons. However, many sellers and even manufacturers of electric motors convert pounds to kilograms for the convenience of buyers and show the equivalent of horsepower next to the traction force. And this equivalent greatly confuses the potential user: the minimum is about 0.2 hp, and the average is 0.5-0.65 hp. But! First, the conversion of thrust to horsepower (and vice versa) is not entirely correct. Secondly, even if we ignore some errors of “conversion” and proceed from the lowest required power, the number of obtained horsepower is quite enough for trouble-free sailing in an inflatable boat.
Which factors influence traction force
In reality, the traction force of any outboard motor depends on several factors, from the design features of the outboard motor to the weight of the boat and cargo and the propeller’s design. There is no direct dependence on horsepower. Even if we proceed from the nominal maximum power, for example, gasoline engines of different brands for 2.3 hp and 2.6 hp give an equal tractive force of 28 kg. On the other hand, with equal power, different maximum thrust is possible – for example, 2.5 hp motors of different brands with 35 kg and 36 kg traction.
Choosing an electric outboard motor
Given that electric motors are chosen for inflatable boats designed for low-power motors (usually up to 3.3 hp), if desired, it is easy to select an electric outboard motor that will fully satisfy the requirements for the minimum permissible motor power. Recall that the minimum acceptable power is ¼ of the maximum permitted power indicated in the technical characteristics of the inflatable boat, for example, with a maximum allowable power of 3.3 hp. The minimum required is 0.825 hp. There are a lot of suitable electric outboard motors. After all, there are boat motors of relatively reliable power, for example, 3 hp. Moreover, the electric motor and battery mass is less than the most miniature gasoline outboard motor.
Outboard motor power
Often, power in watts is indicated along with the traction force. It is sometimes “converted” into horsepower at the rate of 1 hp. Metric (“European”) to 735.49875W, which is incorrect and useless. What manufacturers indicate as the input power is, in fact, the electrical power consumed by the boat motor. It is almost always much more than the propeller’s output (reactive) power due to the inevitable energy losses in the engine and gearbox. By the way, for outboard gasoline motors, the power on the propeller is also often less than the nominal since sometimes the power on the shaft is indicated.
Current consumption is essential, unlike power in watts, which can be considered poorly indicative. Usually, the maximum hourly consumption is indicated at the top speed with the highest transmission and total power, hence choosing the battery of optimal capacity. A battery with a capacity of 85 Amps / h (Ah) is often considered to be the minimum sufficient. For example, if the maximum consumption of the motor is 30 A / h, then with a fully charged battery of 85 A / h, it is guaranteed to move the boat for at least 2.5 hours at a maximum speed of 5-6.3 km / h. Moreover, with the lowest consumption (smallest gear), the charge of such a motor will be enough for 13.5 hours of sailing at a speed of 1.8 km / h.
Nevertheless, if the battery is discharged before the end of the voyage and you do not want to work with oars at all, the following recommendation is popular. Stock up on a backup battery for an echo sounder, but of a relatively large capacity (not the usual 7A / h, but, for example, 12A / h). It should be enough for both the echo sounder and to reach the coast on the motor.
Electric outboard motor characteristics
The maximum speed that a trolling motor performs depends on the tractive force, the boat’s size and load, and the battery charge level. But in most cases, it will be in the above range of up to 5-6.3 km / h (excluding tailwinds and currents). True, sometimes 8 km / h is achievable, but this is an exception. Gliding is, of course, impossible.
But electric boat outboard motors do not even claim to race records. Relatively low speeds turn into an advantage if, for example, you use a motorboat for trolling fishing. Electric outboard motors are often called trolling boat motors. Since the electric motor, unlike any petrol outboard engine, quickly provides the optimal speed for trolling.
Add to this the absence of exhaust emissions and the quiet operation. The price for a complete set – a motor, a battery and a special charger – is much less than the price of the cheapest gasoline engine. For many, these are powerful arguments in favour of an electric motor.
Choosing a battery for an electric outboard motor
There are essential nuances in choosing a battery. First, electric motors usually have a built-in battery, but most require an external battery. Such boat electric motors are rated for 12 V or 24 voltage battery power as a standard. For 24V, you can use two 12V batteries (but it’s still more troublesome and more expensive). The second nuance is fundamentally essential.
As you know, there are starting and traction batteries. It is better for a boat with an electric motor to use a deep discharge “boat” traction battery. Traction batteries are designed to provide an engine with the current for a sufficiently long period. At the same time, they give a relatively small current and do not require immediate recharging. When fully discharged (“deep”), they can wait 2-3 weeks to recharge. Moreover, they are charged with virtually no loss of original capacity and have longer battery life.
Starter motor batteries (for example, conventional car batteries) are designed to start the gas engines. They are capable of delivering a relatively strong current for a short time. But they are not intended to provide current for a long time, and deep discharge is critical for them. After deep discharge, starting motor batteries require virtually immediate charging (a delay of even a few days is deadly). Moreover, even if recharging immediately, deeply discharged starting batteries lose on average 20% of their capacity.
Disadvantages and recommendations
The disadvantage of deep-discharge batteries is that they are much more expensive and sometimes are much heavier than starting ones of equal capacity. But the difference in weight, considering the carrying capacity of even a compact boat, can be ignored. Also, the price range compensates for the great advantage in the labour resource. For example, one fishing trip with heavy use of the motor usually requires about 100 A / h battery capacity. A 100 A / h starter battery will survive 5-7 fishing trips. Moreover, the swimming time will be reduced from one trip to another. On the other hand, the resource of discharge-charging cycles of a deep discharge boat battery usually starts from 500.
True, such a recommendation is popular: you can use the starter battery, but with a significantly larger capacity than the energy consumption expected when using the boat. For example, if the “estimated” need is 100 A, then use the battery of 200 A / h. It will be discharged not so “deeply,” and it will be possible to recharge without significant loss of capacity. But it doesn’t extend the life of the starter battery enough to feel the financial gain.
Another nuance is that a deep discharge “boat” battery requires a special charger (a device from a starter battery is not suitable). But even with the price of a special charger, getting the correct traction battery eventually pays off.
In addition to the above, when choosing an electric motor outboard for your inflatable boat, consider other essential nuances, especially the minimum engine power required for your boat.
Please read our article: Choosing the Outboard Motor.