Choosing an outboard motor for an inflatable vessel is always a compromise between speed, payload, fuel consumption and, of course, budget. Engine power It's a key parameter that determines not only your vessel's performance, but also its safety on the water, and a miscalculation can cause the boat to fail to plane or become unmanageable at high speeds.
Before you go to the store, you need to clearly understand that PVC Exceeding the recommended power is not just a violation of the rules of operation, it is a direct risk to life. In this article, we will discuss all the nuances of choice so that you can make an informed decision.
⚠️ Warning: Installing a motor above the manufacturer's permitted boat may cause the transom to break and the vessel to tip over at high speed.
The market today offers a wide range of solutions, from lightweight electric motors to powerful four-stroke units, and it is important not to get confused in marketing tricks and choose exactly what you need for your tasks.
Major factors affecting the power selection
The first and most important factor is passport-powerThis figure is put on a plate (plate) on the transom and indicates the maximum power that the hull is able to withstand without the risk of deformation or destruction, and this parameter cannot be ignored categorically, since the inflatable sides and the transom plate have their physical strength limits.
But it's not always right to rely on the maximum value. loading capacity If you plan to go out on your own with minimal gear, you don't have to take the engine to the limit of the boat's ability, but instead, for family walks or fishing with lots of equipment, the thrust supply will be critical to the plane.
Power reserve of 10-15% of the minimum required for planing will help you feel more confident on the wave and with a full load of the boat.
Also consider the type of water body: Large areas with wind and excitement require a more powerful unit to maintain control and speed, and quiet lakes and slow-flowing rivers can be managed with a less powerful but more economical option.
Calculation of power for planing
The main purpose of the selection of the engine is to provide a confident exit to the gellingThis is a mode of propulsion in which the boat slides over the surface of the water, resting only on the lower part of the hull, which dramatically reduces drag and fuel consumption. For PVC boats, the critical threshold is usually about 20-22 km / h.
There is a simplified calculation formula: to go to plane on 1 ton displacement requires about 25-30 hp However, in the case of inflatable boats calculation is more difficult, since it is important to overcome the inertia of the soft bottom and the sailing of cylinders. Usually for boats up to 3 meters is enough 5-8 hp, for 3.2-3.6 meters - 9.9-15 hp, and for boats over 3.8 meters already requires 18-20 hp and above.
If the engine is weak, the boat will “dig” water with its nose, creating a huge wave and consuming a lot of fuel, but never going into economical sliding mode.
☑️ Checking the planing readiness
Do not forget that with the passage of time and increasing the fouling of the bottom of the algae, the need for power may increase slightly, so a small reserve will never hurt.
Comparison of two-stroke and four-stroke engines
When choosing power, you can not ignore the type of engine, since they have different power. torque-curveTwo-stroke (2T) engines are traditionally considered to be faster and lighter, they gain speed faster, which helps to more easily "pull" a heavy boat on the glise, but they are noisier, vibrating and require a fuel mixture.
Four-stroke (4T) engines are heavier and tend to have less maximum power in one PVC boat range, but their thrust is smoother and more predictable. They're quieter, more economical and more environmentally friendly. If you care about comfort and you don't want to chase top speed. four-stroke It may be the best choice even with a slightly lower passport capacity.
The difference in weight can be anywhere from 5 to 15 kg, which is a significant factor for an inflatable boat that often has to be carried by hand. A powerful but heavy motor may require the use of a transom wheel or the help of a partner.
Effects of weight on differentiation
A heavy four-stroke engine can flip a lightweight PVC boat in a sharp maneuver or pull ashore if not careful. Lightweight two-strokes are safer for small models in this regard.
It is also worth noting that two-stroke engines are better at carrying full-speed over a long time, whereas four-stroke engines may require more frequent maintenance under extreme loads.
Table of correspondence of boat length and engine power
To make the choice easier, we suggest you look at the average data, remember that these values are advisory in nature and you should always check the instructions for a particular model of the boat.
| Boat length (m) | Type of bottom | Recommended power (hp) | Maximum power (hp) | Speed on glisse (km/h) |
|---|---|---|---|---|
| 2.4 - 2.9 | Plywood/NDM | 3.5 - 5 | 6 - 8 | 22 - 28 |
| 3.0 - 3.2 | NDMA | 8 - 9.9 | 10 - 12 | 30 - 35 |
| 3.4 - 3.6 | NDMA | 15 - 18 | 20 - 25 | 38 - 45 |
| 3.8 - 4.0 | NDNDM/Aluminum | 20 - 25 | 30 - 40 | 45 - 55 |
As you can see from the table, as the length increases, the power requirements increase nonlinearly, especially when you go over the 3.2 meter mark, where it is desirable to have a motor of at least 9.9 hp for comfortable movement of two people with a load.
⚠️ Attention: Using a motor below the minimum required for planing will lead to increased fuel consumption and accelerated engine wear due to overloading.
When choosing, always focus on the maximum load of the boat you plan. If you often take your dog, child and a full set of fishing gear, choose the engine closer to the upper limit of the table.
The effect of the screw on the efficiency of power use
Many people forget that motor power is only half the equation. propellerA well-chosen propeller unlocks the potential of the engine and compensates for the lack or excess of thrust. The screw's pitch is measured in inches and indicates the theoretical movement of the propeller in one revolution in a solid medium.
If the boat does not go to the plane with a standard propeller, but the engine does not reach its maximum speed (the engine "chokes"), then the pitch of the screw is too large for this boat-motor cargo bundle. In this case, you need to install a screw with a smaller pitch. Conversely, if the engine roars at full speed, but the speed does not grow, the screw pitch is too small.
For example, switching from a 9-step propeller to a 8-step propeller can add 2-3 km/h speed and confidence to the boat at the start, effectively turning a 9-horsepower engine into a more tractionary unit.
Also worth paying attention to is the material of the screw. Aluminum screws are lighter and cheaper, but when hitting an underwater obstacle, they can wrinkle, losing efficiency. Stainless steel screws are stronger and hold the pitch better, but they are more expensive and can damage the lower unit on a strong impact if a cut stud is not installed.
Electric motors: an alternative to gasoline power
In recent years electric motors They are becoming more popular as an alternative to traditional ICEs, measured in thrust (pounds) or watts. For comparison, an electric motor with a thrust of 30-34 lbs (pounds) roughly matches a gasoline engine of 1-1.5 hp at low speeds, but it is not able to bring a heavy boat to the plane.
Electric motors are ideal for trolling, fishing in engine-free areas or as an additional running engine. They are silent, do not require mixture and complex maintenance. However, for active travel over long distances or plane of their power (even top models in 80-100 lbs) is still not enough for most medium-sized PVC boats.
If you prioritize silence and the smell of gasoline, and you're willing to put up with 5-7 km/h, electric is a great choice, but if you need to get to the catch point quickly or overcome headwinds, you can't do without a gasoline unit.
An electric motor won't replace a gasoline motor if you want to planing and long-distance crossings. These are tools for different tasks.
It is important to consider the weight of batteries, which can be 10-20 kg, which significantly affects the total weight of equipment, especially in small boats.
Safety and legal aspects of operation
The power selection is also dictated by the legislation. In Russia, for example, to control a small vessel with an engine of more than 8 kW (about 10.88 hp) requires the presence of rights of category "M" and registration of a boat in the GIMS. Motors with a capacity of up to 10 hp (inclusive) allow you to swim without a license and registration (at a boat length of up to 20 meters), which makes them extremely popular.
Therefore, often owners buy boats with the maximum allowed power transc 15-18 hp, but install the engine 9.9 hp This gives a margin of traction for confident planing with the load and the ability, if necessary, to "steam" the engine up to 10 + hp (although the latter is a violation and removes the warranty).
⚠️ Warning: Operating an unregistered vessel with an engine above 10.88 hp (8 kW) will result in fines and delays. Always check the engine documents.
Also, don't forget personal protective equipment, because even at low speed, falling from a boat can cause injuries, and a screw blade blow is fatal. safety-line (checks) torn when the driver falls is a mandatory safety requirement.
To sum up, the ideal PVC outboard motor is a balance between your physical capabilities (motor weight), the requirements of the law and the desired comfort on the water. Don't chase for maximum power unless there is an urgent need for it, but also do not save where it comes to safety of access to the plane.
Frequently Asked Questions (FAQ)
Can I put a motor more powerful than it says on the transom of the boat?
The transom plate and inflatable tubes are designed for a certain load, exceeding the power will lead to deformation of the transom, loss of leakage of the seams and a high risk of overturning the boat with a sharp set of speed.
Is a 5 hp engine enough for a boat 3.2 meters long?
The 5 hp engine will allow the boat to move 3.2 m, but the plane is possible with only one person and a minimum load. When fully loaded, the boat will go in displacement mode at a speed of about 10-12 km / h, which will significantly increase fuel consumption.
Which is better for fishing: 2T or 4T motor?
For fishing, where silence and no vibration are important, a 4T (four-stroke) motor is preferable, it works quietly at idle speeds and does not require oil mixing, but the 2T (two-stroke) is lighter and cheaper, which is also important for mobile anglers.
How do you know if the engine is not enough?
If the throttle is not accelerated when opening to a full boat, the nose is heavily buried in the water, and the engine does not gain maximum speed (audible from the sound), then the power is not enough or the screw's pitch is not selected correctly. The boat must "jump" on the wave and level out.
Do I need to register a boat with a 9.9 hp engine?
No, if the total power of the engines does not exceed 8 kW (about 10.88 hp) and the length of the vessel is less than 20 meters, registration with the GIMS and rights are not required. The 9.9 hp engine formally falls into this category, but during the inspection, questions may arise if the engine is clearly redesigned (expanded).