The choice of a 9.9 horsepower outboard motor is often driven not only by the desire to save on category B rights, but also by the need for comfort while fishing or cruising. The noise of exhaust and vibration can turn a pleasant outdoor outdoor trip into a test for the nervous system, so the question of which of the available units works the quietest worries many water engines. In this segment, manufacturers use a variety of technologies to reduce acoustic pressure, from multi-stage fuel combustion to complex jamming systems.
However, do not rely solely on the marketing promises of manufacturers who claim that they are the only ones who are not two-stroke The real acoustic picture is made up of many factors: build quality, carburetor tuning, the condition of the lower unit and even the fuel used. Often, an engine that worked silently in a store, after running and several seasons, begins to make a characteristic loud roar that requires intervention.
In this article, we will discuss the design features of low-noise engines, compare the sound pressure levels in decibels of popular brands and give practical tips for reducing the noise of your existing unit. Understanding the physics of the process will help you make the right choice when buying or effectively upgrade your current engine.
Design features of low-noise motors
The main source of noise in outboard motors is the combustion process of the fuel-air mixture and the subsequent emission of exhaust gases. two-stroke This process is accompanied by a characteristic high-frequency sound that travels through both the air and the water, creating a palpable vibration on the hull of the boat. Engineers have been fighting this phenomenon for years, implementing systems. multistage combustion, such as the Yamaha HPDI or Mercury ProOptiMax, which allow more complete combustion of fuel, reducing the volume of the exhaust.
Four-stroke engines are initially quieter than their two-stroke counterparts, thanks to a separate exhaust stroke and a more massive, deaf-hearted crankcase that better quenches vibrations. However, even four-strokes have their own leaders and outsiders in noise levels. A key role here is played by the jamming system: the use of Helmholtz resonators, labyrinth mufflers, and the release of gases through the propeller hub (Propeller Exhaust) significantly changes the acoustic profile.
⚠️ Attention: Not all Silent or Quiet engines are really quiet at all speeds, and often noise reduction is achieved only when idling, whereas at full speed the difference from conventional models becomes minimal.
And the important thing is also the design of lower unit. Tohatsu or SuzukiAnd there are special rubber dampers between the lower unit and the bracket, which allow you to cut off the vibration from the transmission to the transom, and it's the transom vibration that is often perceived by our ears as a hum, even if the motor itself is relatively quiet.
When choosing a motor, pay attention to the presence of a system for the release of gases through the screw hub - this reduces the noise of gurgling exhaust at low speeds, which is critical for trolling.
Comparison of noise levels: 2T vs 4T
The perennial controversy between two-stroke and four-stroke engines in the context of noise has a very specific technical rationale. Two-stroke engines with a capacity of 9.9 hp are traditionally considered more noisy due to the high speed of rotation of the crankshaft and the nature of the exhaust. E-TEC or OptiMax We managed to significantly reduce the gap, approaching the comfort of four-stroke analogues.
Four-stroke engines benefit from a smoother, lower-frequency sound that the human ear perceives as less annoying. In addition, the lack of need to mix oil with gasoline and more stable operation at idle speeds make them favorites for those who appreciate silence. But it is worth remembering that 4T engines are heavier, which can affect the differentiation of the boat and, as a result, the level of noise created when moving water on the board.
For clarity, we give a comparative table of the average values of sound pressure (in dB) at a distance of 50 meters for popular models with a capacity of 9.9 hp The data are averaged according to the results of independent tests and data-sheet specifications.
| Motor model | Type of engine | Noise level (dB) | Features of jamming |
|---|---|---|---|
| Yamaha 9.9 GMHS | four-stroke | ~98-102 | Spinhead release |
| Tohatsu M9.8B | four-stroke | ~96-100 | Multi-stage combustion |
| Suzuki DF9.9A | four-stroke | ~97-101 | Lean Burn System |
| Yamaha 9.9 CMHS (2T) | two-stroke | ~105-110 | Classic exhaust |
As you can see from the table, the range is small, but even 3-5 decibels on hearing feel like a significant decrease in volume. Leaders in the category of “quietest outboard motor 9.9” invariably become four-stroke models from Japanese manufacturers, which give priority to ecology and acoustic comfort.
Rating of the quietest models on the market
If you look at the 9.9 hp outboard motor market, there are a few models that have proven to be the quietest. Tohatsu M9.8BIt is actually an OEM for many other brands, including Mercury, and its four-stroke design is characterized by exceptional smoothness and low vibration.
Second place deserves to be taken. Yamaha 9.9 GMHSJapanese engineers have introduced a multipoint injection system (in more powerful versions) or a carefully tuned carburetor with an effective jamming system, especially a gas release system that directs the flow through the propeller blades, breaking large bubbles into small ones and reducing the noise of gurgling.
- 🌊 Suzuki DF9.9A It is known for its Lean Burn system, which not only saves fuel, but also provides a very clean and quiet combustion of the mixture at medium revs.
- ⚓ Honda BF10 (power slightly higher, but often considered in the same row) - the standard of reliability and silence, although it has a higher cost of maintenance.
- 🔧 Mercury F9.9 - a direct relative of Tohatsu, but with its own throttle setting, which makes its work on singles almost inaudible.
It is important to note that Chinese manufacturers such as Parsun or HideaThey also offer four-stroke versions of the 9.9 hp, which replicate the design of Japanese counterparts, and their noise levels can be comparable to the original ones, but the quality of the muffler and suspension materials often underperforms, which over time leads to rattling.
⚠️ Attention: When buying a motor, be sure to check the tightening of all external bolts and the quality of the installation of the casing. Often, the "Chinese" noise appears due to poor fitting of plastic parts of the casing, which begin to vibrate at resonant frequencies.
Effect of motor condition on noise level
Even the world's quietest motor can cause a deafening roar if left unattended. carburetor Or the injection system directly affects the nature of combustion, the over-enriched mixture leads to cotton in the muffler, and the poor one leads to fever and metal ringing, and regular cleaning of the fuel system is the first step to silence.
The second important factor is the condition of the lower unit and gearbox: worn-out paddle shaft bearings or insufficient lubrication in the gearbox create a characteristic howling sound that is transmitted over long distances through the water. It is also worth checking the propeller: chipped blades or imbalance cause vibration that enhances the overall noise background.
☑️ Diagnostics of a noisy motor
The combustor and piston coarse also increases compression and can cause detonation, which sounds like a ringing knock. For two-stroke engines, proper preparation of the fuel mixture is critical: using poor-quality oil or breaking proportions leads to muffler coking and exhaust resistance, which increases volume.
Secrets of reducing noise yourself
If you're not happy with the silence, but you can't buy a new one, you can reduce the noise yourself. The first and most effective way is to upgrade the exhaust system. Install an additional resonator or replace the standard silencer with a more productive one (if there is one for your model) can give a tangible result.
The second method is vibration insulation, whereby the internal surfaces of the engine casing are glued with vibration-absorbing materials (bitumen or polymeric mastics with an aluminum layer) to reduce the resonance of the plastic, and additional rubber pads can be installed between the motor and the trunk of the boat, if the suspension design allows.
Recommended sequence of actions:1. Remove the engine casing.
2. degrease the internal surfaces of the plastic.
3. paste a vibroisolating material (for example, Vibroplast).
Plug a layer of noise absorber (Splen) over it.
5. to assemble the casing, making sure that there are no gaps.
The third is to replace the propeller. Modern propellers with increased pitch or special geometry of the rear edge can reduce cavitation noise, which is often confused with engine noise, especially in glitter boats, where the noise of water and propeller dominates the sound of the engine.
Can I use car silencers?
Theoretically, yes, but it requires a complex overhaul of the exhaust system and can cause the engine to overheat due to backpressure, and the exhaust above water can be dangerous for people on the boat.
Psychology of perception of noise on water
Interestingly, objective measurements in decibels do not always correlate with subjective comfort. High-frequency sound of a two-stroke motor is perceived as more unpleasant and "piercing" than the low-frequency hum of a four-stroke engine of the same volume, so choosing the quietest motor should focus not only on the numbers, but also on the nature of the sound.
In open water, sound travels differently than on land. Water conducts low frequencies very well, so a powerful bassing roar can be heard by fish at a greater distance than a squealing sound. For trolling fishermen, this is a critical parameter, since motor noise can scare off cautious fish such as a perch or pike.
- 🎣 For fishing, the low-frequency component of noise, which is transmitted through water, is more important.
- 🚤 For walks with family, the lack of high-frequency screeching and vibration is more important.
- 🔇 The overall fatigue from the four-stroke engine is much lower with long transitions.
At speeds above 20 km/h, the noise of the 9.9 hp engine is almost completely covered by aerodynamic noise and water noise, so chasing absolute silence makes sense primarily for trolling and short-range modes.
For maximum fishing silence, choose a four-stroke engine with a propeller hub exhaust system and use an electric trolling motor to approach the catching point.
Final selection and recommendations
To sum up, the title of the quietest 9.9 outboard motor is shared by modern four-stroke models from Tohatsu, Yamaha and Suzuki, the difference between them is minimal and often depends on the specific instance and the quality of the setup. If the budget is limited, you can consider two-stroke engines with the E-TEC system, which are much quieter than classic two-strokes.
Keep in mind that silence is not only a characteristic of the engine, but also the result of proper maintenance and installation. Timely replacement of candles, the use of quality fuel and oil, as well as checking the condition of the lower unit will help to maintain acoustic comfort for many years. Remember that too quiet motor on the water can even be dangerous, as it will not be heard by other boats, so always follow the safety rules.
Ultimately, the best motor is one that works reliably and allows you to enjoy nature rather than fight the roar of the engine. We hope our review has helped you get to the nuances and make the right choice for your boat.
Is it true that a four-stroke engine is always quieter than a two-stroke?
95% of the time, especially at idle and medium revs. However, modern two-stroke direct injection engines can be quieter than older four-stroke models, depending on the specific technology and year of production.
Can the engine be made completely silent?
No, it's impossible. The internal combustion engine always makes noise, and you can only minimize it to an acceptable level with silencers and vibration insulation, but you can't completely eliminate the sound of the internal combustion engine.
Does the octane number of gasoline affect the noise of the engine?
Yes, using gasoline with an octane number below the recommended one can cause a knock that sounds like a metal knock and raises the overall noise level.
Should I buy a used motor for the sake of silence?
Caution: A used motor may have developed bearings or a burnt-out muffler, which will make it very noisy. It is better to buy a new budget engine than an old premium one in poor condition.