planing is the moment when the boat stops plowing water and starts to slide on the surface, which is the state that every boater looks for when buying his first or new one. outboardHowever, it often happens that the acquired engine simply is not able to tear the boat from the water, leaving it in displacement mode at maximum speed.
This is not only disappointing, but also dangerous for technology, since working at limit loads without achieving the planned speed leads to overheating and increased fuel consumption. To avoid buying a "weak" engine or, conversely, overpaying for excess power, you need competent training. engineering.
In this article, we will discuss the physical principles of planing, look at proven formulas and provide tables that will help you choose the perfect power plant for your boat, whether it is lightweight. PVC Or a heavy metal boat.
Physics of the process: what is glossing
planing is a mode of movement in which a vessel is held to the surface of the water by a dynamic support reaction, that is, the velocity of the water on the bottom. Unlike displacement mode, where buoyancy is ensured by Archimedes' law, here the main role is played by the main role of the vessel. speedTo enter this mode, you need to overcome the so-called “hump of resistance”.
At low speeds, the boat behaves like a floater, and as the engine speeds up, the water resistance increases dramatically, the hull starts to burrow its nose, and the stern crouches, and that's the critical point. propeller It's enough to break through that barrier, the boat's nose rises, the area of contact with the water decreases, and the resistance drops -- glides begin.
⚠️ Warning: Attempting to operate the motor for a long time at full speeds in the hump mode (when glides have not yet begun, but the speed is already high) can lead to cavitation and damage to the propeller or gearbox.
The key parameter here is the Frood number, which is a dimensionless value that characterizes the fluid's mode of motion. To go plane, the Frood number must exceed the value of 3. That's why heavy boats require motors with a large margin of power to reach the required speed.
For light inflatable boats, plane often occurs at 15-18 km / h, while heavy aluminum boats need to accelerate to 25-30 km / h.
Factors affecting the required power
Before you grab a calculator, you need to collect the raw data. The simple formula of 1 hp per 25 kg of weight only works in a first approximation and often fails. The real power requirement is affected by many variables that you can't ignore when planning a purchase.
First of all, the total mass of the boat in use is taken into account, and this includes not only the weight of the hull itself, but also the weight of the boat. motorA mistake in the weight calculation, even 10 to 15 percent, can move the boat from the planing zone back into displacement mode.
The geometry of the body is also critical. Flat bottoms require less power to start, but behave worse on the wave. Sawy bottoms (V-shaped) cut the wave, but require more energy to detach from the water. aluminum lighter than fiberglass, but heavier than PVC.
- 🌊 Condition of the water surface: on quiet water ("silent") to go to plane is easier than on the oncoming wave.
- 🎣 Load: The distribution of weight of passengers affects the differentiation; the displacement in the stern helps to "stand on skis."
- 🔧 Type of screw: The pitch and diameter of the propeller directly affect the efficiency of thrust transmission.
The new engine develops the declared power, while the mileage motor can lose up to 10-15% of efficiency due to the load, wear of the piston group or improper setting of the carburetor.
Basic formulas for calculating engine power
There are several empirical formulas for estimating the power required, and the most common formula in a boating environment is to take into account displacement and desired speed, and to give an approximate value in horsepower.
For a rough calculation, you can use the ratio of 1 hp per 20-25 kg of gross weight to go to the planing. However, it will be more accurate to use the formula that takes into account the completeness coefficient of the hull. For lightweight planing boats, a simplified version is often used:
P = (D * V^3) / KWhere P - power in hp, D - displacement in tons, V - speed in knots, K - coefficient (for light boats 150-200, for heavy boats 200-250). It is important to understand that the speed here is taken in knots (1 knot ≈ 1.85 km / h).
Consider an example: a boat with a crew and an engine weighs 400 kg (0.4 tons), we want to go at 30 km / h (about 16 knots), and we put it into a formula with a factor of 180 (average):
P = (0.4 16^3) / 180 = (0.4 4096) / 180 ≈ 9 hpThus, for this configuration, theoretically enough 9-10 hp However, you should always take the engine with a margin of 15-20% to compensate for headwind, wave or additional load.
Always round the resulting design power up to the nearest standard value in the engine line (5, 6, 9.8, 15, 20 hp).
Calculation for PVC boats and inflatable vessels
Inflatable boats have their own specifications. Their bottom is often soft flooring or inflatable tubes, which creates high resistance at launch. In addition, the shape of the cylinders creates additional vortices. PVC boats It requires a lot of attention.
Inflatable vessels, the transom is critical. If it is soft or deformed under load, part of the engine power will go to vibration and cruising the stern, rather than pushing the boat forward. A rigid glued transom is mandatory for motors over 5 hp.
When calculating, consider that inflatable tubes create sailing. Wind will slow the boat more than a rigid hull. So if you plan to walk on large bodies of water, where it is often windy, the calculation formula should be adjusted to increase power.
- 🎈 Boat length: for boats up to 3 meters usually enough 5-8 hp, for 3.2-3.6 m - 9.9-15 hp.
- 👥 Number of persons: Each additional passenger adds about 80-90 kg (human weight + clothing).
- ⚓ Bottom: The presence of hard floorboard (plywood/aluminum) reduces resistance and facilitates access to the plane.
⚠️ Warning: Exceeding the maximum power specified by the PVC boat manufacturer may cause the transom seams to rupture or the boat to tip over due to a sharp different at the stern.
The optimal choice for most 3-meter PVC boats is a 9.8 hp engine. It allows you to plane comfortably with one or two passengers and has an acceptable fuel consumption. For larger boats (3.6 m and above), it is already worth looking towards 15-20 hp.
Why are 9.8 HP engines so popular?
These engines often weigh around 25-27 kg, allowing them to be carried in the trunk of a passenger car without the need for a trailer. In addition, in many regions, engines up to 10 hp (or up to a certain power in kW) do not require vessel registration and entitlement, although GIMS rules may vary.
Features of selection of motor for metal boats
Aluminum and steel boats are much heavier than inflatables. The metal hull has greater inertia and requires significant momentum to accelerate it. Here, the formula "1 hp by 25 kg" often turns into "1 hp by 30-35 kg", especially for hulls with deep keeliness.
When calculating for metal, pay special attention to the height of the transom. If the motor is set too low, the water pipe will be captured by the keel, creating resistance. Too high installation will lead to cavitation and loss of traction at the start, which will prevent you from entering the plane.
For heavy boats, low-speed torque is an important parameter. Two-stroke engines are often preferred over four-stroke engines of similar power precisely because of the faster start, although 4-strokes are more economical and quieter.
| Type of boat | Length (m) | Weight with load (kg) | Min. power (hp) | Optimum power (hp) |
|---|---|---|---|---|
| Aluminum (flat bottom) | 3.0 - 3.5 | 300 - 400 | 8 - 10 | 15 - 20 |
| Aluminum (skewered) | 3.5 - 4.0 | 450 - 600 | 15 | 25 - 30 |
| Glass-plastic (caution) | 4.0 - 5.0 | 800 - 1200 | 30 | 40 - 60 |
| PVC (with floorboard) | 3.2 - 3.6 | 350 - 500 | 9.8 | 15 - 18 |
Using a hydrofoil (interceptor) on a lower unit motor can greatly improve the plane output of a heavy boat, a small device that redirects the flow of water, pressing the stern and helping the boat “take off” at lower power.
☑️ Check of readiness for calculation
The effect of the propeller on the output of plane
Even the most powerful motor will not pull the boat into the plane if the propeller is not selected correctly. The propeller is the transformer of engine power into thrust. The main parameters that affect the result are diameter, pitch and number of blades.
The propeller's pitch determines how far the boat will go in one turn of the propeller in ideal conditions. A big pitch gives a high maximum speed, but "strangles" the engine at the start, not allowing you to gain momentum to enter plane. Small pitch, on the contrary, provides excellent acceleration, but limits the "maximum speed".
For heavy boats or situations where the engine is slightly weak, it is recommended to use propellers with reduced pitch (Low Pro, Special series), which allow the engine to reach maximum speed faster, creating the necessary thrust to break away from the water.
- 🔄 Number of blades: 4-blade screws are better pulled from place and at low revs, but have less efficiency at high speeds.
- 📏 Diameter: increase in diameter increases traction, but may not allow the motor to unwind to nominal value.
- ⚙️ Material: Aluminum screws are lighter and cheaper, steel screws are stronger and have thinner blades, which improves hydrodynamics.
⚠️ Attention: Installation of the screw with an excessively small pitch can lead to the "spread" of the engine when it exceeds the maximum permissible speeds, which threatens the destruction of the piston group.
The correctness of the selected screw is checked by tachometer. At full gas (with a load corresponding to normal operating conditions), the engine must give the speeds specified in the data sheet as the maximum operating range (for example, 5000-6000 rpm). If the revolutions are lower, the step is large, if higher, small.
Carry a spare propeller with a smaller pitch. This is a lifeline if you have to swim with full load or against strong winds, and the regular propeller will no longer cope.
Practical tips for improving glossing
Sometimes the calculations are right, the engine is good, and the boat is not going, and in such cases, practical techniques for improving hydrodynamics and weight distribution will help, and often the problem is not power, but balancing.
The first rule is differentiation. Shifting passengers and cargo into the boat's nose helps to bury the nose, which can be helpful during acceleration, but the very getting on plane often requires a shift of the center of gravity closer to the transom or balancing. Try transplanting the passenger closer to the stern.
The second thing is bottom cleanliness: seashells or algae-covered bottoms increase resistance by 20-30%, regular washing and antifouling (for metal boats) will bring back lost speed.
Also check the tilt of the lower unit (trim). If the motor is too "looking" up, the stream of water hits at an angle and pushes the boat down, pressing it against the water. Put the lower unit down one division, this will change the angle of attack and help the boat to get on the plan easier.
How does the angle of the motor affect the speed?
The right angle (different) allows the hull to be in optimal position relative to the water. Too much lower unit lift leads to cavitation and loss of stop. Too low creates resistance with a ram. Experiment with adjusting the angle in calm water, doing swims for 1-2 minutes with different fixations.
Can old gasoline interfere with the exit to plane?
Yes. Fuel that has been in the tank for more than 2-3 months (especially with ethanol additives) loses its octane number and evaporates. The engine is unstable, does not develop full power. Always use fresh gasoline for test runs.
Does the height of the motor installation on the transom affect?
The anti-cavitation plate should be on the same level as the bottom of the boat or slightly higher (at 1-2 cm for planing boats). If the engine is low, the water flows around the transom, creating a powerful resistance that the motor cannot overcome.