Choosing a power unit for a boat is always a compromise between the speed, efficiency and weight of the boat you want. Many beginners, when they first encounter specifications, confuse horsepower with the actual force that pushes the boat forward. boat-power This is the physical parameter that determines whether your boat will be able to go plane or will only lazily waddle through the water.
Understanding the nature of traction is essential not only when buying a new engine, but also when upgrading an existing kit. Often owners try to squeeze more speed by changing screws, but without understanding the basic physics of the process, this only leads to engine overheating or cavitation. In this article, we will discuss in detail how to translate the usual "horses" into understandable kilograms of force and why for electric motors this parameter is indicated directly.
It should be noted immediately that propeller It depends on a lot of factors: the pitch of the propeller, its diameter, the number of blades and, of course, the engine speed. If you plan to use a boat for trolling or transporting heavy loads, kilograms of traction will be the main criterion for you, not the maximum speed on the tachometer.
Physical nature of traction and unit of measurement
In the technical documentation for gasoline engines, you rarely find a direct indication of thrust in kilograms. Manufacturers operate with power per shaft, measured in horsepower (hp) or kilowatts. However, for the end user, especially in the context of water-motor theme of the CIS, the concept of "thrust in kg" is more tangible. This is the force with which the screw pushes the water back, pushing the boat forward.
The situation is different for electric motors, because their design is simpler and there is no difficult transformation of combustion energy into mechanical motion through a high gear ratio gearbox, manufacturers often specify that the power is not a problem. traction It's easier to choose, but it requires understanding conversion, for example, the popular 55 lbs label doesn't mean the weight of the engine, it indicates the strength of the traction.
⚠️ Warning: Never confuse maximum thrust with cruising thrust. Peak value is achieved short-term and can lead to rapid wear of the knots if used continuously.
It's important to distinguish between static thrust and thrust in motion. Static thrust is measured on the spot (bollard pull) and shows the potential of the motor when starting or towing. The thrust in motion depends on the speed of the incoming flow of water. The faster the boat moves, the less efficient the propeller if its pitch is chosen incorrectly. Efficiency of propeller rarely exceed 60-70%, which means a significant loss of power to heat water and create vortices.
Why does thrust fall at speed?
As the boat speed increases, the angle of attack of the propeller blades changes: water ceases to flow around the blade profile efficiently, cavitation occurs, and the actual thrust in kg can fall by 30-40% of the static value.
Power conversion: from hp to kilograms of force
Because gasoline engines don't have direct thrust markings, boaters use empirical formulas to approximate their calculations. 1 hp is thought to be capable of generating enough thrust to propel a certain displacement. However, direct conversion to kilograms of thrust force (static thrust) gives the following averaged values for standard propellers.
Here is a table showing the approximate matching of the power of a gasoline engine to the thrust it generates. Remember that these values are relevant for medium pitch screws and can vary depending on the manufacturer and engine model.
| Power (hp) | Traction (roughly, kgs) | Recommended displacement (kg) | Type of boat |
|---|---|---|---|
| 2.0 - 2.5 | 35 - 45 | 400 | Single PVC |
| 5.0 | 80 - 95 | 900 | PVC/Aluminum |
| 9.9 | 160 - 180 | 1800 | Cruising PVC |
| 15.0 | 240 - 270 | up to 2,500 | Metal/PVC |
| 30.0 | 480 - 520 | 4500 | Kater/Kazan |
The table shows that boat-power It grows nonlinearly. Doubling the power doesn't double the thrust because of aerodynamic and hydrodynamic losses. For heavy metal boats like the Kazanka or Voronezh, it's critical to have a power reserve, because their hull creates high drag.
To enter the planing, the thrust-to-weight ratio should be at least 25-30 kg of thrust per 100 kg of total weight of a boat with cargo and passengers.
Specificity of electric motors and lbs marking
In the world of electric traction, there are laws of their own. Min Kota, Haswing or WatersnakeFor the Russian user, this creates additional confusion, since the number on the nameplate (for example, 34, 40, 55) seems abstract.
The conversion formula is simple: 1 lb (pound of power) is about 0.453 kg. However, for marketing purposes and for simplification, a factor of 0.45 is often used or even divided in half for rough assessment. For example, a motor marked 55 lbs develops thrust of about 25 kg. This value is enough to confidently drive a 3-3.5 meter inflatable boat with one or two passengers.
- 🔋 30-35 lbs (about 14-16 kg) - suitable for small boats up to 2.7 meters, quiet fishing, no current.
- ⚓ 40-45 lbs (about 18-20 kg) - a universal choice for boats 2.9-3.2 meters, allows you to go against the moderate current.
- 🌊 55 lbs and higher 25+ kg – required for heavy boats, for use on the current, wind or with a full crew.
It is important to understand that electric motor It only gives you maximum thrust when you're fully charged, and as you drain the battery, the voltage drops, and the actual thrust in kilograms can drop by 15 to 20 percent, which is critical when you come back against the wind.
Influence of screw geometry on traction force
Even the most powerful motor will not generate the right thrust if the propeller is not matched correctly. The propeller geometry is the balance between pitch and diameter. The propeller pitch determines how far a boat theoretically will go in one rotor revolution in a solid environment.
To increase traction at low speeds, for example, when trolling or moving in overgrown algae places, screws with a high speed are used. low-stepThey work like a reduced gear in the car: speed drops, but torque and thrust increase. Conversely, to planing you need a screw with a large pitch that will allow the engine to reach maximum speed and develop full power.
⚠️ Attention: Installing a propeller with too little pitch on a powerful engine will cause the engine to "twistle" (rpm above acceptable), which threatens to break the piston group. Too much stride will not allow the engine to spin, causing detonation and overheating.
Three-bladed screws usually provide better speed and efficiency on the clean run, while four-bladed screws give the best. bottom-dragging And they're going to take the boat out to the plane faster, and they're going to sacrifice the maximum speed, and it's going to depend on the task, and for a heavy-tip, anchored fishing, a 4-blade is better.
When buying a propeller, pay attention to the markings, for example, 9.25x10-K. The first number is diameter, the second is step.
Environmental factors that reduce real traction
Theoretical calculations of thrust in kg always assume ideal conditions. In reality, a water motor encounters water resistance that changes every second. Water temperature, salinity and density directly affect the efficiency of the screw. In cold and dense water, thrust will be higher than in warm and fresh water.
A huge factor is the fouling of the bottom of the boat and the propeller itself, and even a thin layer of algae or shells can reduce the damage. Efficiency of the screw The smooth surface of the screw is critical to creating laminar flow, and any bullying, chipping or deformation of the blades turns the screw into a source of turbulence that dampens the traction force.
Also consider the height of the transom. If the motor is set too high, the propeller will capture air (cavitation), and the thrust will instantly fall to zero. If too low, the resistance of the lower unit will increase. The optimal position is when the anti-cavitation plate is in the same plane with the bottom of the boat or 1-2 cm lower.
☑️ Testing for traction loss
Practical advice on increasing efficiency
To get your boat to develop maximum thrust at its current power, you need to take a comprehensive approach to the kit preparation. Start with the distribution of the cargo. Shifting the center of gravity to the bow of the boat increases the wet surface area and resistance, "eating" the thrust. Try to place heavy things (batteries, anchors) closer to the transom or in the center.
Using an anti-cavitation plate on a lower unit motor is a proven way to improve hydrodynamics, and this small device stabilizes the flow of water onto the propeller and prevents the capture of air on the turns. traction stabilizingFuel or energy consumption is reduced by 5-10%.
Regular maintenance is also key. lower unit lubrication, gland inspection and no backlashes in the tiller ensure that all power is transferred to the screw without loss of friction in the control mechanisms. Remember to change the anodes, as lower unit corrosion increases surface roughness.
How does the weight of the boat affect the necessary thrust?
Dependence is direct, but not linear. For displacement mode (slow stroke), thrust must overcome the friction resistance of the hull. To go to plane requires a short-term force, 3-4 times the resistance at cruising speed. If thrust is not enough for "hunch-hunch" (transition mode), the boat will waste a lot of fuel, but not accelerate.
Can I increase the thrust of the electric motor by flashing?
Theoretically, the increase in current on the controller will give you an increase in thrust, but it will lead to overheating of the windings and rapid failure of the battery. The factory settings are already optimized for the limit of the capabilities of a particular model of the motor.
Why does the engine stop when you add gas?
This is a sign that the pitch of the propeller is too big for the current load, the engine can not quickly gain momentum and "choking" the solution is to reduce the pitch of the propeller or reduce the load of the boat.
What kind of traction is needed for trolling?
Trolling doesn't require high maximum traction, it's important to be able to run at minimum speeds (1-2 km/h), and it's more important to have a speed control system than peak kilograms of effort.
Does the diameter of the screw affect the thrust?
A larger screw diameter while keeping the stride allows for larger masses of water to be moved at a slower speed, which increases thrust at low speeds, but a larger diameter requires more space under the lower unit and may not fit a particular motor.