Every owner of water-motor equipment sooner or later wonders at what turns his engine works most efficiently. Choosing the right mode of operation It has a direct impact on fuel consumption, reciprocating group life and overall swimming comfort, and beginners often mistakenly assume that top speed is always the goal, forgetting that the internal combustion engine has its peak efficiency zones.
Finding a balance between speed and fuel costs is an art that requires understanding the physics of propeller operation. Torque. And the thrust changes nonlinearly depending on the position of the throttle, and in this article we'll look at how to find the middle ground where the motor consumes the minimum and passes the maximum.
It is also important to consider that the notion of optimality can vary depending on the type of vessel you are on and the tasks you are putting in front of you. Economic regime For a plane boat and for a heavy displacement boat, it's going to be very different. Let's take a closer look at the physics of the process and the practical recommendations.
Operating range and concept of cruising speed
First, you need to clearly distinguish the concepts of maximum and cruising speed. Maximum speed It is achieved with a fully open throttle (100% of the gas), but in this mode the engine runs at its limit. Cruising speed This is a mode in which the boat confidently holds the planing, and the engine operates in the most favorable thermal mode.
Most modern four-stroke and two-stroke engines have a working range limited by the maximum speeds specified in the instructions (usually 5000-6000 rpm). However, constant operation at the limit leads to accelerated wear. The optimal range is considered to be approximately 75-85% of the maximum power.
It is in this sector. torque It often reaches its peak, delivering excellent traction at a reasonable flow rate. If your motor is capable of 5,500 rpm, your comfort zone will be in the range of 4,000-4,500 rpm, in which the vibrations are minimal and the lubrication of the knots is most efficient.
⚠️ Attention: Full throttle is only permissible in emergency situations or in short-term testing, and continuous operation at the limit reduces the life of the engine by 30-40%.
Also, the effect of coolant temperature should be considered. At low rpms, the pump may not provide enough pressure to remove heat, especially at high loads. speed-up It is always a compromise between the thrust, the speed of rotation of the pump and the flow rate of the mixture.
The impact of the screw step on working turnover
One of the key factors determining what speeds the engine will keep under load is the pitch of the propeller. Screw pitch This is the theoretical distance that the propeller travels in one full rotation, and if you set the propeller with too much pitch, the motor will not physically be able to spin to working speed, and you will have a choking effect.
When the engine fails to reach the passport speed of 5,000-5,500 times even at full throttle, it is called "spinning" and it is extremely dangerous for the piston group, because it is very dangerous for the piston group. mix-up It flushes the oil film off the cylinder walls, causing bullies, and vice versa, too little a step will cause the motor to "twistle" into the cutoff, which also leads to overheating and destruction of parts.
The engine is selected experimentally, and you need to ensure that when the boat is fully loaded (passengers, fuel, equipment), the engine reaches the upper limit of the recommended range (WOT - Wide Open Throttle). If the passport range is 4500-5500 rpm, then at full gas the tachometer should show 5300-5400.
- 🚀 Small step: Easy acceleration, higher maximum speeds, but lower top speed and higher consumption on cruising modes.
- 🐢 Big step: Higher top speed on light boats, but heavy acceleration and the risk of not getting the speed.
- ⚖️ The best step is: Allows the motor to reach the upper limit of the WOT range at full load.
How does the screw pitch affect the flow rate?
Changing the pitch of the screw by 1 inch changes the engine's speed by about 150-200 units. Reducing the pitch will allow the engine to spin more easily, but to maintain the same speed you will have to open the throttle stronger, which can increase the flow. Increasing the pitch will load the engine, causing it to run at lower rpms at the same speed that often (but not always) leads to savings if the motor is not "pressed."
Table: Revs. fuel consumption
Understanding the flow rate of the boat is key to planning the range of the trip. Fuel consumption increases exponentially, not linearly, and increasing the speed of the boat by half can require an increase in engine power by 4-8 times, which directly hits the wallet.
Below is the average table for a 50 hp petrol four-stroke engine (example data). Notice the jump in flow rates when going to planing and when going to maximum rpm. Economic speed Often is immediately after entering the plane, when the throttle is already covered, but the boat has not yet begun to burrow its nose.
| Mode of work | Turnover (rpm) | Speed (km/h) | Expenditure (L/h) | 1 l (km) mileage |
|---|---|---|---|---|
| Idle move | 900 | 0 | 0.6 | 0 |
| trolling | 1500 | 5 | 1.2 | 4.1 |
| The economy of plane | 3500 | 28 | 9.5 | 2.9 |
| Cruiser | 4500 | 36 | 14.0 | 2.5 |
| Full gas. | 5500 | 42 | 21.0 | 2.0 |
As you can see from the table, maximum efficiency It's not at minimum speed, but at a smooth planing zone, but at long distances, where time is not critical, a speed reduction of 30 to 32 km/h can be very economical.
Reducing speed from 40 km/h to 30 km/h can increase the range on a single tank by up to 25%, despite the fact that you will spend more time on the road.
Role of trim gas in optimizing the regime
The trim gas is a powerful tool for adjusting optimal rotations without interfering with the design of the propeller. By changing the angle of inclination of the diewood, you change the angle of attack of the propeller relative to the surface of the water. Properly exposed trim allows the boat to occupy the ideal position for glides.
If the lower unit is too high (positive trim), the propeller takes in air, cavitation and loss of traction occur. The engine starts to gain momentum ("gaz"), but the speed of the boat drops. engine-damaging If the lower unit is too low, the bow of the boat is buried, the water resistance is high, and the motor cannot develop power.
The adjustment process is this: accelerate the boat to cruising speed. Smoothly raise the trim. You will see that the speed will start to increase, and the engine will remain the same or slightly increase. At some point, the speed will stop increasing, and the bow of the boat will start to pick up. This is a signal that you need to lower the threem a little. Ideal position - when the kilvater stream is smooth, without bubbles, and the boat goes flat.
- 📉 Nedo-trim (nose down): High flow, low speed, splashing into the cockpit.
- 📈 Per-trim (nose up): Unstable course, cavitation, risk of loss of control.
- ✅ Optimal trim: Maximum speed at current speeds and smooth course.
Remember, the optimal trim angle varies with the loading of the boat and the excitement on the water, and you often have to lower your nose to soften the blows, sacrificing speed for safety.
Features of two-stroke and four-stroke engines
The search for optimal speeds depends greatly on the type of engine. Two-stroke engines (2Ts) are traditionally considered more squishy and sharp, they get on plane mode faster, but their fuel consumption curve is more flat — they eat almost as much at medium speed as they do at full speed, and they have a high-speed shifting operating range.
Four-stroke engines (4T) have a more elastic torque at the bottom, they allow you to move comfortably at low speeds, where the two-stroke can even stall or work unstable. Economy mode 4T is in a wider range, making them preferable for quiet tourism and fishing.
⚠️ Attention: Two-stroke engines require strict oil ratio in the fuel. At low revs (trolling), candles can throw if the mixture is too rich in oil. Use special oils for low revs if you plan to go for a long time at a minimum.
Also worth noting is the cooling difference: Four-strokes often have a more efficient thermoregulation system, which allows them to keep their temperature more stable when working for long periods at partial loads, while two-strokes on low gas can become underheated, which leads to increased gas formation.
To prevent coarseness on two-stroke engines after prolonged work at low speeds (trolling), be sure to give the "gas" for 10-15 seconds before turning off. This will help burn out excess deposits on candles and in the combustion chamber.
Overclocking and glossing
And there's a lot of people who think, in the wrong way, that you have to open the gas at 100 percent to get to plane quickly. speed-up No planing out (when the boat growls but sits in the water) is the most inefficient and dangerous mode. The engine is running at maximum, the thrust is huge, but the speed is low.
Proper acceleration technique: smoothly but confidently open the gas to 70-80%. Once the boat jumped on the plane and the speed began to increase, you can add gas to cruising value. If the boat is heavy and does not go to the plane, try to lower the trim gas slightly (literally a few degrees) or redistribute the weight of passengers closer to the center / junction.
Prolonged acceleration of the "stretching" leads to overheating of the exhaust system and silencer, since the exhaust stream at high speeds at low speed of the boat is not sufficiently cooled by off-shore water, which can lead to burnout of the muffler partitions.
☑️ Checking readiness for plane
Frequent errors in operation
So to sum up, it's worth systematizing the typical mistakes that boaters make when they try to find the perfect mode, the first and most common is ignoring the tachometer readings. Absence of instrument The first thing you need to do is to rely on hearing, which is not always accurate.
The second mistake is using an overpowered motor for a small boat, where the owner puts 50 hp on a 3-meter PVC boat and has to run at a minimum of gas all the time to avoid flying into space. carburetor Or the injection system works in an inefficient zone, the candles are overgrown with swelling, and the engine resource is wasted.
The third mistake is to neglect the cleanliness of the bottom, to throw shells or algae dramatically increases resistance, to keep the same speed, you have to add gas to get the engine out of the optimal zone, and regular washing and antifouling coatings are also about fuel economy.
⚠️ Warning: If you notice that you have to open the throttle harder than usual to maintain your cruising speed, check the screw for winding lines and the bottom for fouling. This is the first sign of loss of efficiency.
How often should I check the optimality of the speed?
It is recommended to have a WOT test at the beginning of each season and after each rotor change or significant change in the load of the boat, and check is also necessary if you feel a change in acceleration dynamics.
Does the octane number of fuel affect the speed?
Yes, using fuel with an octane number below the recommended number can cause detonation. The electronics of the motor (or the motor itself) will be forced to work less efficiently, you may not be able to develop full speed without the risk of piston damage.
What to do if the engine is not gaining momentum?
The reasons may be in the "flip" (too big a screw pitch), the restriction of fuel supply (clogged filters, jellyplates), problems with the ignition system or in the technical condition of the engine itself (compression).
Can you save fuel by constantly working at a minimum?
Paradoxically, no, an internal combustion engine has a minimal efficiency threshold, and running at too low revs under load (when the boat is not yet on the plane) consumes more fuel per kilometer than confident plane at medium revs.