Choosing a power unit for a boat is always a search for a compromise between power, economy and, of course, weight. outboard motor weight It's one of the critical parameters that directly affects the boat's load capacity, stability and handling. Many beginners make the mistake of focusing solely on horsepower, forgetting that every extra kilogram on the transom shifts the center of gravity and increases the different at the stern.
In today’s environment, when environmental demands are growing and technology is improving, there is a clear division in the market into lighter two-stroke models and more massive but economical four-stroke counterparts. Yamaha, Tohatsu or HondaThis will help you avoid problems in transportation and operation, and in this article we will take a closer look at how much popular models actually weigh, and provide up-to-date data for comparison.
Don't underestimate the effect of engine mass on a vessel's behavior on water. A heavy motor on a lightweight PVC boat can turn a walk into a test of strength, especially when oncoming waves. The optimal weight ratio of the boat to the engine is approximately 1:4 or 1:5 for the best planing. Below we will cover all aspects related to weight, so that your choice is balanced and justified.
Effect of engine mass on the performance of the vessel
The weight of the outboard motor is not just a number in the passport, it's the physical force that acts on the trawler and the hull of the boat. When the engine is too heavy for a particular hull, the boat starts to peck with its nose or, conversely, overburied with stern. This phenomenon is called a defferent, and it's difficult to fight it even with transom plates or changing the boarding of passengers. two-stroke motors They often outperform their heavier competitors.
In addition, being overweight on the stern reduces the overall stability of the craft. In pinned weather, this may not be as noticeable, but when you go on a wave or perform sharp turns, heavy stern makes the boat more rolling and prone to rolling. For PVC inflatable boats, this is especially critical, since their cylinders have a limited carrying capacity, and every kilogram of equipment takes up the payload for passengers or cargo.
On the other hand, a light engine on a heavy metal or composite boat will also not produce the desired effect, in which case the bow of the boat can bounce upwards, creating huge sailing and impairing visibility. Here, the balance is important: the engine must be powerful and heavy enough to push the hull efficiently, but not so massive as to disturb the balance. Centre of gravity The entire system "boat-motor-crew" should be at the optimal point.
⚠️ Warning: Exceeding the recommended maximum power and engine weight for your boat may cause the transom and loss of buoyancy. Always check the hull plate.
Comparison of two-stroke and four-stroke motors: weight matters
The main battle in the outboard market is between two-stroke and four-stroke technology. four-stroke engines This is due to the design features: the presence of an oil crankcase, a more complex gas distribution mechanism (valves, camshafts), and, as a rule, a more massive cylinder block.
The difference in weight can be enormous. For example, a 5 hp two-stroke engine can weigh about 20 kg, while a four-stroke engine can pull 25-27 kg. As the power increases, this delta increases. A two-stroke 50 hp will be lighter than a four-stroke of the same power by 15-20 kg, which is a significant factor for a small boat. However, manufacturers are constantly working to simplify the design by introducing new aluminum alloys and composite materials.
It's worth noting that today's four-stroke engines are becoming lighter thanks to the use of technology that came from the automotive industry. Suzuki and Tohatsu They're using the forced exhaust cooling system to optimize the shape of the crankcase to reduce mass without losing strength. However, physics is physics: more parts and liquids (oil in the crankcase) mean more weight in the dry state.
- 🏍️ Two-stroke engines are traditionally 15-25% lighter at the same power.
- ⚖️ Four-stroke models require a stronger transom due to the greater mass.
- 📉 The difference in weight becomes less noticeable at power over 100 hp.
When choosing between these two types of engines, it's important to consider not only weight, but also operating conditions. If you plan to take the motor out of the water often for transportation or storage, every kilogram saved will affect your back. two-stroke technology It is more attractive for small boats.
Why are 4-strokes heavier?
The four-stroke engine has a full-fledged oil crankcase lubrication system, a valve mechanism and often a heavier generator, all of which add mass but provide less fuel consumption and quiet operation.
Table of weight of outboard motors by power (2T vs 4T)
For a visual comparison, we have prepared a composite table showing the average weights of popular models of engines of different power, the data are averaged, since the specific values may vary depending on the manufacturer (for example, the average weight of the engine is not the same as the average weights). Yamaha against HondaHowever, the general trend is clear: with increasing power, the difference in weight between engine types changes.
Note that the weight is in the dry state, i.e. without fuel, oil and propeller installed. In real use, you need to add about 3-5 kg to these numbers for a full tank and technical fluids, which is especially important for boats with limited capacity.
| Power (hp) | Average weight 2T (kg) | Average weight 4T (kg) | Difference (kg) |
|---|---|---|---|
| 2.5 - 3.5 | 9.5 - 13.0 | 12.5 - 14.5 | ~ 2.0 |
| 5.0 | 19.0 - 21.0 | 25.0 - 27.0 | ~ 6.0 |
| 9.8 - 10 | 26.0 - 29.0 | 33.0 - 36.0 | ~ 7.0 |
| 15 | 34.0 - 36.0 | 41.0 - 45.0 | ~ 8.0 |
| 30 | 48.0 - 52.0 | 58.0 - 65.0 | ~ 12.0 |
From the table, you can see that at low power (up to 5 hp), the weight difference is not so critical, but it becomes noticeable in the 10-15 hp class, which is most popular for PVC boats with a length of 3.0-3.6 meters. It is here that a boater has to make a difficult decision: sacrifice weight for the sake of the economy of a four-stroke or choose the lightness of a two-stroke.
Overview of the mass of engines of popular brands: Yamaha, Tohatsu, Honda
Other manufacturers approach the issue of weight differently. - Japanese company Tohatsu Historically, it is famous for its compact and lightweight two-stroke engines, which often form the basis for the Mercury and Nissan Marine brands. Their designs often have a vertical gas release system, making the daywell narrower and lighter.
Company Yamaha In their four-stroke series (e.g. the F-series), they use Micro Computer Ignition and compact cylinder heads to minimize size and weight. However, even with the innovations, their four-stroke engines remain the benchmark for reliability, not lightness.
Honda It's a green and technological company that's putting PGM-FI into low-power applications, which adds weight to the nozzles, sensors and ECUs, but it's a big improvement, and Honda's engines are often heavier than their competitors, but they make up for that with smooth rides and low noise.
When buying a motor, pay attention to the length of the deadwell. The long leg (L) is always heavier than the short leg (S) by 2-4 kg due to the increased amount of metal in the deadwell and lower unit.
It's also important to consider that the weight of the motor is not just a block of cylinders, but it's distributed between the top (power head) and the bottom (dadewood with gearbox). MercuryWeight management system is used, which makes it easier to lift the motor behind the bumper, but the total mass remains unchanged.
⚠️ Attention: The weight listed in the catalogs is often "base weight" and the actual weight may vary depending on the remote control system installed, the increased power generator or the type of starter (manual or electric).
How engine weight affects transportation and installation
Transporting a outboard motor is a separate headache for small car owners. If you plan to carry the engine in the trunk of a passenger sedan or hatchback, every kilogram counts. A heavy four-stroke engine with a capacity of 15 hp, weighing 45 kg, alone to lift and carefully stow in the trunk will be extremely difficult, unlike a lighter two-stroke.
Installing the motor on a transom also requires physical effort. The heavier the unit, the greater the risk of dropping it on the leg or damaging the trench board if handled carelessly. For motors weighing more than 30 kg, it is strongly recommended to use trunk trolleys or special lifting mechanisms, especially if you are fishing alone.
Weight also affects the speed of the boat, and a lighter engine allows the boat to go more quickly into planing, because it puts less pressure on the stern, and this is especially true for boats with flat bottoms or low keel speed, where it is important not to overload the transom plate.
☑️ Check before buying the engine
Tips for choosing a motor, taking into account the load capacity of the boat
When choosing an engine, always rely on the specifications of your boat. On the transom of any certified boat there is a metal plate or sticker indicating the maximum power allowed and the maximum weight of the engine. Ignoring this data can lead not only to fines from GIMS, but also to dangerous situations on the water.
If your boat has weight limitations but you need power, consider using lithium batteries instead of lead-acid batteries if you have an electric motor, or simply accept a smaller fuel supply.
For PVC boats up to 3.2 meters long, the optimal choice is often motors with a capacity of 5-6 hp They are light enough to be removed by one person, and provide an acceptable speed. For boats 3.5-4.0 meters already require a power of 9.9-15 hp, and here the question of weight becomes the edge: take a heavy but economical 4-tact or light and voracious 2-tactnik.
- 🎣 For fishing alone on a small boat, choose the lowest possible weight.
- 🚤 For family walks and tourism, the reliability and economy of the 4-tactnik are more important.
- ⛽ Keep in mind that a full tank of gasoline will add 5-10 kg to the total weight on the transom.
The ideal motor is not the most powerful or lightest, but the one that balances the boat optimally and fits your physical transport capabilities.
Frequently Asked Questions (FAQ)
Does the weight of a four-stroke engine increase significantly when filling with oil and gasoline?
Yes, the weight increases significantly. Gasoline is heavier than water (about 0.75 kg/liter), and a four-stroke engine also has oil in the crankcase (from 0.3 to 1.5 liters depending on the model), a full 25 liter tank will add almost 20 kg to the weight of the engine, which is critical for light boats.
Can I put a four-stroke engine on a boat designed for a two-stroke?
Technically, you can do it if the power matches. But a four-stroke motor is longer and heavier. It can upset the boat's balance by shifting the center of gravity backwards, causing the nose to bury in the wave. Also check the strength of the transom.
Which engine is lighter: Tohatsu 9.8 or Yamaha 9.9?
These engines are often structural twins (OEMs) and the weight of their two-stroke versions is almost identical (about 26-27 kg), the difference can be 100-200 grams depending on the year of production and configuration (type of ignition coil, carburetor).
Does the length of the deadwell (S, L, XL) affect the weight of the engine?
The long leg (L - 508 mm) is heavier than the short leg (S - 381 mm) due to the elongated lower unit pipe and deadwell. The difference can be from 2 to 5 kg. For shallow water, always choose a short leg, if the boat draught allows.
Is it true that the new four-stroke engines are as light as the two-stroke ones?
This is an exaggeration. Although the gap is narrowing thanks to new materials, the four-stroke engine still contains more metal parts and liquids. Miracle did not happen: the four-stroke 15 hp still weighs about 40+ kg, while the two-stroke engine is about 34 kg.