Choosing an outboard motor for a boat or boat always starts with performance analysis, where the architecture of the power plant plays a key role. Many aquatic motorists pay attention to the cast and power, forgetting about the fundamental parameter - the ratio of cylinder diameter to the stroke of the piston. It is this geometry that determines the nature of the motor, its propensity to high speeds and, critically, durability in harsh operating conditions.
Long-distance engines, where the piston stroke exceeds the diameter of the cylinder, have historically been considered the benchmark of traction and reliability. But this coin has a flip side, which is often silent manufacturers. crank-and-wheelerThis will help you avoid disappointment when buying equipment that may not be suitable for your tasks.
In this article, we will look at the specific flaws of long-distance construction and why, in today’s world of high-speed gliders and heavy cruise boats, it is not always the best choice.
Fundamental limits on maximum turnover
The main and most obvious disadvantage of a long-distance design is the physical limitation of the maximum speed of rotation of the crankshaft. When the piston stroke is large, the average speed of its movement in the cylinder increases even at relatively low engine speeds, which leads to the fact that the inertial forces become enormous long before the motor reaches its peak performance.
The high speed of the piston causes significant loads on the piston-ring In boating technology, where motors often operate in full-load mode for long periods, this becomes a critical factor. If a short-range "square" engine can safely spin up to 6000-7000 rpm, the long-distance analog will begin to experience problems with lubrication and mechanical strength by as much as 4500-5000 rpm.
The consequence is that you can't take off high-liter power. To get more horses from a long-distance engine, engineers would have to turbocharge it, which is rare in the context of mid-range outboard motors because of complexity and cost, and you end up with a heavy, bulky motor that's physically incapable of developing the high rpm required to operate fast propellers efficiently.
⚠️ Attention: Attempt to force the long-distance engine above passport values by replacing the screw with a more "light" can lead to catastrophic failure - a rod break or a twist of the liners due to oil starvation at high speeds.
When selecting a propeller for a long-distance engine, always choose a step that allows you to reach maximum speeds with a margin of 200-300 units, so as not to drive the engine into the red zone when the boat is fully loaded.
Problems with thermal regime and detonation
The geometry of the combustion chamber in a long-distance engine often resembles a narrow cylinder, which creates imperfect conditions for the spread of the flame front. The fuel mixture burns longer, since the spark plug torch takes longer to reach the distant edges of the chamber. detonationespecially when using low octane fuels or when operating under high load at low revs.
In addition, the ratio of the surface area of the combustion chamber to its volume in such engines is higher than in short-range engines, which means that more heat is released through the walls of the cylinder head and the piston itself. On the one hand, this could be a plus for cooling, but in practice this leads to increased heat losses and decreases. thermal efficiency.
In outboard motor conditions, where the cooling system depends on the intake of water, overheating of individual areas of the piston (especially the center of the bottom, which is farthest from the cooled walls) can be fatal.
How does the shape of the camera affect the detonation?
In the narrow combustion chamber of a long-distance engine, the flame front travels a long way, and if there is a pressure surge from the inertia of the gases, a shock wave occurs, destroying the oil film and causing micro-explosion of the mixture.
To combat this, you have to reduce compression or use more expensive fuels, which directly affects the operating costs of the boat owner, while modern short-speed engines effectively use high-speed combustion to improve efficiency.
Mechanical losses and friction
The increased stroke of the piston automatically means the longer paths that the rubbing pairs take in a single work cycle, which is directly proportional to the mechanical losses in the engine. cylinder-piston And crankshaft bearings, and the bigger the stroke, the higher the speed of the slide and the corresponding friction.
Higher piston speeds require more viscous oils or oils with special additives to maintain the load-bearing capacity of the oil film. However, even special oils cannot fully compensate for the increased resistance, as a result, a significant part of the energy generated by combustion is spent not on rotating the propeller, but on overcoming the internal friction of the unit.
- 📉 Increased wear of piston rings due to work in the zone of maximum temperatures and speeds.
- 🛢️ Increased consumption of engine oil on the burn, especially on two-stroke models of the old series.
- 🌡️ Heating motor oil to higher temperatures, which accelerates its oxidation and degradation.
These disadvantages are especially noticeable on two-stroke outboard motors, where the lubrication is carried out by a mixture. The long-distance construction in the "two-stroke" is guaranteed high oil consumption and increased exhaust smoke, since the speed of the mixture through the crankcase and cylinders is too high for effective lubrication without loss of emission into the exhaust.
Comparison of specifications: Long move vs. Short
To make a sense of the difference, you need to look at comparatives. Long-distance motors are often praised for torque, but it's important to understand how much it costs and how it compares to other parameters. The table below shows the key differences in operation.
| Parameter | Long-drive engine | Short-drive engine |
|---|---|---|
| Piston stroke | More cylinder diameter | Less cylinder diameter |
| Torque. | High at low revs. | Displaced in high-speed zone |
| Maximum speeds | Limited (usually up to 5000-5500) | High (6000-8000+) |
| Mechanical losses | High. | Low. |
| Dimensions (height) | More (in vertical position) | Compact. |
As you can see from the table, Long-distance engines lose in specific powerFor a water-powered motor, this means that to get the same speed of the boat, you'll have to buy a motor with a larger displacement than the short-range one, which is a direct route to overpaying when you buy and higher fuel consumption in the long run.
In addition, the large height (in the case of V-shaped or inline multi-cylinder engines) can cause problems with the layout of the transom and the centering of the boat. The heavy bottom of the engine shifts the center of gravity, which does not always favorably affect the stability of high-speed planing vessels.
Resource impact and maintainability
Paradoxically, the claimed “unkillability” of long-distance motors is often a myth when it comes to modern materials and operating modes. The high thermal and mechanical stresses discussed above lead to accelerated wear and tear. crankshaft Vibrations characteristic of long-distance structures (due to the greater mass of reciprocating parts) also contribute to fatigue destruction of metal.
Because of the specific design (high cylinder block), replacing the piston group may require complete disassembly of the engine or the use of a special tool to dry valves in hard-to-reach places. In short-range motors, access to the units is often more organized ergonomically.
☑️ Signs of wear of a long-distance motor
The lubrication system is particularly important. In long-distance engines, oil is in the crankcase, which experiences significant spikes when the boat rolls. The long stroke of the piston creates the effect of a pump that actively sprays oil, which can lead to oil starvation when working long hours at high speeds or when rolling if the oil intake does not have an effective compensation system.
⚠️ Attention: When operating a long-distance engine in planing, it is critical to monitor the oil level. Due to the high consumption of carbon monoxide and splashing, the level can fall to the critical level faster than the sensor indicates.
Economic inexpediency for planing
The modern marine engine industry is moving towards lighter design and environmental friendliness. Long-distance engines, with their high fuel and oil consumption and the difficulty of achieving environmental standards (Euro-4, Euro-5 and higher), are becoming economically unprofitable, and manufacturers are forced to implement complex exhaust neutralization systems that do not go well with the dirty combustion process in long-distance cylinders.
For the boat owner, it translates into a high cost of ownership. Fuel efficiency. fuel-consumption Long-distance motors tend to be worse, especially in transient modes. If your boat is designed for planing where speed and speed are important, a long-distance motor will choke the vessel's potential by requiring the throttle to be kept open at all times to maintain speed.
The only niche where the downsides of a long-distance engine turn into upsides is heavy displacement vessels, towboats or generator sets operating at constant low revs. But for 90% of recreational boating (cruising, speed fishing, water skiing), such architecture is a thing of the past.
For glittering boats, the priority should be power and weight, so short-range or "square" engines are the only option.
Frequently Asked Questions (FAQ)
Can a long-distance engine be boosted to increase power?
Theoretically, you can install a turbine and change the control program, but in practice it is extremely risky. A long-distance piston group is not designed for increased pressures and temperatures. The risk of a piston or a rod break increases many times, and the life of such a motor will be reduced by many times.
Is it true that long-distance motors are better at pulling the screw?
They have more torque at low rpm, which gives the illusion of better traction. However, because of the low speed limit, they can't unlock the potential of a high-speed propeller, and as a result, the boat can go worse on planing than a more rotating short-speed engine of the same power.
What type of engine to choose for a heavy fishing boat?
For a heavy boat, which is often used full-load and at low speeds (trolling), a long-distance engine may be acceptable because of the thrust at the bottom. But if you plan to move quickly to the catch point sometimes, you better choose a modern four-stroke balancing motor that will provide both thrust and speed.
Does the long run affect the noise of the engine?
Because of the large reciprocating masses and vibrations, long-distance engines often run noisier and have a lower-frequency, humming sound that can be tiring during long-term operation, unlike the more "sounding" short-distance ones.