Going into the plane is the moment when a heavy boat breaks off the water and starts to slide across the surface, like on its wings. For many boatmasters, it is not just a matter of comfort, but a necessity that saves fuel and travel time, as well as passing shallow areas with less roll.
However, if engine If you pick it up wrong, the boat will dig its nose, consume an inordinate amount of fuel, and not even reach the lowest possible speed, and the right calculation is the balance between the weight of the craft, its geometry and the specifications of the propeller group.
In this article, we will discuss the physical principles, formulas and practical nuances that will help you avoid mistakes when choosing or setting up a motor.
Physics of the process: what is glossing
Gleaning is a mode of movement in which a vessel is held to the surface of the water by dynamic pressure forces arising from the flow of water around the bottom. Unlike displacement mode, where the boat is pushed out by the Archimedean force, here the main work is done. hydrodynamic liftTo get into this mode, you have to overcome what's called the hump of resistance.
The critical parameter is Froud's number. For most small craft, the transition mode begins at a speed of about 15-18 km / h, and confident planing occurs after 20-22 km / h. If the engine is weak, the boat gets stuck in transition mode, creating a huge wave and swaying.
It is important to understand that the geometry of the body plays no less a role than horsepower. Flat bottom It comes out faster on plane, but requires more power to maintain speed, whereas the keeled contours cut the wave better, but they need a more powerful start to lift them.
Key factors affecting the calculation
Before you grab the calculator, you need to collect accurate data, and an error in estimating the weight or ignoring the condition of the housing will lead to the fact that the design power will be insufficient. Total mass It is composed not only of the weight of the boat itself, but also of the weight of the crew, fuel, equipment and engine.
The second important factor is the bottom condition: the seashelled or algae-covered bottom increases friction resistance by 20 to 30 percent, and the transom installation angle is also critical: if the motor is too deep or, conversely, is upended, the efficiency of the screw decreases, and there is not enough power to break away.
⚠️ Warning: Never install the motor more powerful than the boat manufacturer permits, which can cause the transom to collapse and the boat to tip over at full speed due to loss of stability.
The third aspect is the type of propeller: the pitch and diameter of the propeller must match the engine speed and the weight of the vessel, too much strangulation will choke the engine before it reaches its maximum speed, and too little will prevent it from developing the desired traction.
- 🚤 Weight of the boat in gear: includes the body, motor, battery, anchor and all loose objects.
- 👥 Crew: The average weight of a person is taken for 75-80 kg, but it is better to take with a margin.
- ⛽ Fuel reserve: heavy gasoline (1 liter ≈ 0.75 kg), a full tank can add 30-50 kg.
Formulas and methods for calculating power
There are several empirical formulas to give an approximate value of the required power. One of the most common methods is based on the ratio of the total mass of the vessel to the desired speed. To enter the plane, you usually need to ensure a speed of at least 1.5-1.7 of the displacement speed.
The simplified formula for preliminary assessment is as follows: P = (M * V^3) / K, where P is power in hp, M is mass in tons, V is speed in knots, and K is a coefficient that depends on contours (for glittering housings 200-250), but it is easier to use the rule of horsepower per ton.
For a steady planing of lightweight aluminum or PVC flat-bottomed boats, 30 to 40 hp per tonne of displacement is enough. For keeled sea bots, this figure rises to 50 to 60 hp per tonne. This means that a 500 kg boat with a crew will require a minimum of 15 to 20 hp to start, but for a comfortable ride is better than 25 to 30 hp.
When using formulas, always round the resulting value upwards: an engine operating in 80-90% of the maximum power mode has a greater resource than a motor that constantly runs on full gas.
Table of correspondence of mass and power
You can use reference data for quick orientation, which is relevant for standard loading conditions and body condition, and remember that having passengers on the nose can shift the center of gravity and make it difficult to get into mode.
| Total boat mass (kg) | Min. power for plane (hp) | Optimum power (hp) | Expected speed (km/h) |
|---|---|---|---|
| 300 – 400 | 9.9 | 15 – 20 | 35 – 45 |
| 400 – 600 | 20 | 25 – 30 | 40 – 50 |
| 600 – 800 | 30 | 40 – 50 | 45 – 55 |
| 800 – 1000 | 40 | 60 – 75 | 50 – 60 |
The data in the table are for short leg engines with standard gear ratios. When using long leg motors or special gearboxes, the efficiency can vary.
Optimal power is always above the minimum: the horsepower supply ensures safety and saving of engine life.
Practical steps for the selection of a propeller group
The paperwork is just the beginning. The actual test is on the water. The first step is always visual inspection and preparation. Make sure the transom is smooth, deformed, and the motor attachment bracket allows you to adjust the angle of inclination.
Then there's the screw pickup phase, and if the boat doesn't go to the plane, try to reduce the pitch of the propeller, which will allow the engine to gain speed faster, and if the engine is screaming, but the speed is not increasing, the pitch is large or the screw is damaged.
☑️ Checking before the test swim
During test runs, use a tachometer. The engine should reach the maximum recommended by the manufacturer speeds (usually 5000-5500 rpm for two-stroke and 5500-6000 for four-stroke) with a fully open throttle and normal loading.
⚠️ Warning: Prolonged engine operation at speeds above maximum (torsion) leads to cylinder bullying and destruction of the piston group in a matter of minutes.
Common mistakes and their consequences
One of the most common mistakes is ignoring weight distribution, so if the crew is sitting on the nose, the boat burrows into a wave, and glides become impossible even when there is excess power, and the proper landing is closer to the center of gravity or shifting to the stern during acceleration.
Another problem is that you use a motor with an inappropriate lower unit, and if you put a long leg on a short-trans manoeuvre boat (even with a wire) you change the flow dynamics and reduce efficiency, and if you put the motor too high, you get a stove that's above water.
Impact of gasoline quality on power
Using gasoline with an octane number below the recommended or old fuel (over 1 month) reduces the octane number and can cause detonation. The engine automatically (or manually) loses power, and the plane becomes impossible. Always use a fresh AI-92 or AI-95 with the addition of a stabilizer.
Don’t forget aerodynamic drag: Installing a bulky awning transformer or improperly placing the cargo on the trunk can create additional drag, which will “eat” 10-15% of the power.
Specifics for different types of housing
Boats PVC inflatable floor (DND or floorboard) have soft contours, they are easier to go to the plane, but require fine tuning of the pressure in the cylinders, underpumped bottom works like a sail, slowing the boat, they have a lower buoyancy margin, so overloading the engine is dangerous.
Aluminum Kazankas and flat-bottomed crows are very sensitive to excitement, easy to glid, but comfortable, and it's important not to overpower them to keep them in control.
Fibreglass keel boats require the highest specific power, their deep keel creates a lot of resistance at low speeds, but provides excellent seaworthiness, and for such vessels, power calculations should be carried out with a reserve factor of at least 20%.
For PVC boats with a hard floorboard bottom before going to the plane, it is useful to raise your nose a little, moving the cargo or passengers closer to the stern to facilitate separation from the water.
Frequently Asked Questions (FAQ)
Can I go to plane with a lower power engine if I remove passengers?
Yes, reducing the total weight of the vessel reduces the required power, but if the power reserve is critically small, the boat can only go to the plane in ideal weather conditions and with minimal fuel supply.
Does the fuel type (AI-92 vs. AI-95) affect the ability to enter the plane?
Indirectly, yes. If the engine is designed for the AI-95 and the AI-92 is flooded, detonation may occur, and the electronics (or the user) will limit power, which will not allow the necessary rpm to develop for planing.
What if the boat goes to the plane, but immediately breaks off when gas is added?
Most likely, the screw pitch is wrong (too big) or the centering is broken. Also check the angle of inclination of the lower unit: perhaps the engine "drips" the nose too much, and the boat begins to gallop (poroizing).
Do I need to do a run to get out on plane?
Yes, planing is a dynamic process, and you need to add gas smoothly but surely to overcome the transition mode, and a sharp start from the spot (from the nickel) is possible only on very light boats with powerful engines.