Weight of the outboard motor: selection criteria and impact on performance

The choice of outboard motor always begins with the study of technical specifications, where weight occupies one of the key positions, this parameter directly determines not only the ease of transportation and installation, but also the ease of transportation and installation. driving For owners of inflatable PVC boats or lightweight aluminum "kazak" every extra kilogram on the transom can be a decisive factor in the stability and handling.

Many beginners mistakenly believe that the lighter the engine, the better, but in reality, everything is more difficult. Too light a motor If you take a big boat, you may not get enough of a lower unit sink, which will lead to a propeller sucking the air and losing traction. And conversely, the excess mass on the stern of a small boat can completely disrupt the differentiation, making the control dangerous, which is why understanding the physics of the process and knowing exactly what the mass of the unit is the basis from which proper equipment begins.

In this article, we will discuss in detail what makes up the weight of modern outboard motors, how the body material affects the final figure, and why it is sometimes worth sacrificing lightness for reliability, you will learn how to correctly calculate the load on the transom and which models are considered reference in their weight categories.

Factors that form the final mass of the engine

The weight of an outboard motor (outboard motor) is not a random variable, but the result of engineering trade-offs between power, durability and mobility. Traditionally, the bulk of the weight is powerplant and lower unit. In two-stroke models, the bulk of the weight is taken up by the cylinder block and crankcase itself, while in four-strokes, the weight of the oil in the crankcase and the more massive cylinder head with a gas distribution mechanism are added.

A huge impact on the final figures has the material from which it is made. lower unit Aluminum alloys can reduce weight significantly, but can be less resistant to corrosion in salt water than stainless steel or heavier composites. Tohatsu or YamahaThey actively use magnesium alloys and composite materials to reduce weight without losing strength.

⚠️ Attention: When calculating weight, always consider the mass of a remote fuel tank. A full 12 litre canister will add about 9-10 kg to the total mass on the transom, which is critical for light inflatable boats.

Also worth mentioning is the launch system. Electric starter models are always heavier than their manual start counterparts due to the presence of a starter, battery and more powerful ignition coil. The difference can be anywhere from 3 to 7 kg depending on the class of motor. For those who fish alone and often drag the technique with a drag, the choice is between Electric Start and Manual Start It is often a choice between comfort and mobility.

Effect of mass on the differentiation and driving qualities

The right different of the boat is the key to fuel economy and top speed, and if the engine is too heavy for the body, the stern is too submerged, and this creates a nose-dig, where the boat goes with a high angle of attack, increasing the resistance of the water. maximum speed The fuel consumption increases as the engine has to overcome the increased resistance.

On the other hand, if the engine is too light, the boat's nose will be raised too high, which leads to instability, especially in the case of excitement, the boat will become sensitive to lateral wind, and the propeller may occasionally be in the cavitation zone, losing efficiency, in which case motorists will have to use ballast or move passengers and cargo forward, which is not always convenient.

• 🚤 The heavy motor shifts the center of gravity to the transom, improving germination per wave, but reducing speed.
• ⚓ A light motor improves speed in shallow water, but can make the boat roll on the course.
• ⚙️ The optimal weight provides a horizontal position of the bottom when entering planing.

It is important to note that the weight requirements for the engine are different for different types of shells. For glittering bodies, the weight on the transom plays a more critical role than for displacement modes. When choosing a motor, always consult the boat data sheet, where it is indicated. maximum capacity and the recommended engine weight.

Comparison of two-stroke and four-stroke models by weight

The perennial controversy between two-stroke and four-stroke goes on in the matter of mass, and historically, four-stroke engines are much heavier than two-stroke engines of the same power, and it has to do with the design features of the oil crankcase, valves and more complex lubrication system that actually add weight.

But modern technology can bridge this gap, and light alloys and design optimization make the weight difference less tangible, especially in medium- and high-power applications, such as modern 5-6 hp four-stroke engines that can be only 1-2 kg heavier than two-stroke engines, while benefiting from environmental friendliness and efficiency.

Power (hp)	Motor type	Average weight (kg)	Design features
2.5 - 3.5	two-stroke	9.5 - 11.0	Simple carburetor, no oil in the crankcase
2.5 - 4.0	four-stroke	11.5 - 13.5	The presence of an oil pallet, valves
5.0 - 6.0	two-stroke	18.0 - 21.0	Two-cylinder units
5.0 - 6.0	four-stroke	24.0 - 27.0	Single-cylinder with OHV system
9.8 - 10	two-stroke	35.0 - 38.0	Two-cylinder, often with TNVD

As you can see from the table, the difference in small power is not so great as to be the decisive factor in the rejection of a four-stroke engine. However, in a class of 15-20 hp and above, the difference can reach 10-15 kg, which is already significant for a small PVC boat, where each user must decide for himself what is more important: fuel-saving And the quietness of the work, or the ability to easily remove the motor alone.

Weight categories and their purpose

All outboard motors can be divided into several weight categories, each of which dictates its own operating conditions, understanding these categories helps you to quickly navigate the range and not buy excessively heavy equipment for simple tasks.

The first category is ultralight motors weighing up to 10 kg, which are usually engines with a capacity of 2-3 hp, their main advantage is the ability to carry one person to any distance without using a trolley. flyboat up to 3 meters long, used on small water bodies or as spare waters.

⚠️ Warning: Do not install motors weighing more than 15 kg per transom of inflatable boats without rigid decking and reinforced transom.

Second category: medium weight motors (15-30 kg) and most 5-9.9 hp models fall into this category. This is the middle ground for fishing and recreation. They already require effort to carry or use a wheeled cart, but provide enough power to plane with 1-2 passengers. long-leggedThis also adds a few pounds to the total weight.

The heavy category (35 kg and above) includes powerful engines from 15 hp and above, which are almost impossible to operate without a stationary installation on a boat or the use of a powerful lift system (David-kron), weight here ceases to be the main criterion of choice, giving way to reliability, traction and resource.

Specificity of electric motors

Electric outboard motors, which weigh differently than ICEs, deserve special attention. The main weight is not the engine itself, but the traction battery. If the motor itself can weigh only 7-10 kg, then the battery required to ensure autonomy can add another 15-30 kg depending on the capacity and technology (GEL, AGM, LiFePO4).

The advantage of the train is that it can spread weight, and you can put a heavy battery on the bottom of the boat, and you can keep the stability of the boat, while the engine itself will be light, and it changes the way you balance the boat, and the lack of fuel means that the weight of the boat will not change as you go, unlike the gasoline equivalents, where the tank gradually empty.

Modern lithium-ion batteries can significantly reduce the weight of the system. A model LiFePO4 battery of the same capacity will weigh half as much as lead-acid. However, the cost of such solutions remains high. For those who value silence and the absence of exhaust fumes, the trade-off of additional battery weight is often acceptable.

How to reduce the weight of the system with an electric motor?

Use lithium batteries instead of lead batteries. They're more expensive, but they weigh 2-3 times less at the same capacity. You can also use two small batteries instead of one large one, spreading the weight over the sides of the boat for better stability.

Practical tips for transportation and installation

Working with a heavy motor requires safety. Raising the motor to the transom is one of the most traumatic moments. Never try to lift a unit weighing more than 25 kg alone without using mechanical means. Damage to the back or falling on the leg can ruin the entire season.

To facilitate installation, use transom wheels or special lifting devices. Tilt (Colling), make sure the mechanism is working and smeared, and a rusty or jammed recline mechanism will turn a light operation into a heavy metal fight.

• 🛠️ Always use the safety cable when installing the motor on the transom.
• 🤝 Bring in a second person for motors over 20 kg.
• 🔧 Check the condition of the handles regularly, they are often plastic and fragile.

When transporting on a trailer or roof of a car, also consider the weight: the dynamic load on fasteners when driving on the highway is much higher than static. Secure the motor in the transport position using rattle belts, but do not drag plastic elements.

Weight table of popular models (Summary)

For comparison, we'll give you weights from popular models from different manufacturers. Note that the weight is for the standard length of a lower unit (usually S is a short leg).

Model	Power (hp)	Tactfulness	Weight (kg)
Tohatsu M 3.5 B2	3.5	2	9.8
Yamaha F2.5 SMHA	2.5	4	17.0
Honda BF5D	5.0	4	27.0
Suzuki DF9.9B	9.9	4	39.0
Mercury F15	15.0	4	49.0

Analyzing the table, you can see that four-stroke low-power engines (up to 3 hp) can be almost twice as heavy as two-stroke counterparts due to the complexity of the design. However, in the 10+ hp class, the weight difference becomes less critical in terms of percentage to total power.

⚠️ Note: The weight is “dry” for the passport. For two-stroke engines, add the weight of the oil in the mixture (about 0.5-1 kg per tank), for four-stroke engines, the weight of the oil in the crankcase (0.3-0.8 kg).

FAQ: Frequently asked questions

How critical is the difference of 5 kg for an inflatable boat?

For boats up to 3 meters long, the difference of 5 kg can be significant, affecting the position of the transom in the water. For boats 3.5 meters long or more, this difference is almost imperceptible on the go, but important when carrying.

Can I reduce the weight of the motor by myself?

In theory, you can replace some steel parts with titanium or aluminum, but it requires deep engineering knowledge and can disrupt the balancing. It's easier to remove the fuel tank from the engine and transfer it to the boat, which will give you a gain of 1-3 kg per transom.

Which is heavier: an electric starter motor or a manual start?

The electric starter motor is always heavier, and the difference is the weight of the starter, the battery, the power generator and the relay, which averages 3...5 kg to the passport weight.

Does the weight of the motor affect the life of the transo?

Yes, the constant vibration of a heavy motor can destroy the structure of the transom more quickly, especially if it is made of low-quality plywood. Heavy motors require the installation of a transom plate made of aluminum or stainless steel to distribute the load.