Choosing a boat is always a search for a balance between compactness in transportation and power on the water. Inflatable boat for an outboard motor 20 hp. This is perhaps the most popular segment in the small fishing and recreational class, and it is this engine power that is considered the “golden mean”, allowing you to comfortably move with two or three passengers and a full set of equipment without losing mobility.
Many beginners mistakenly believe that it is enough to simply buy a motor of the required power, without thinking about the specifications of the body. PVC It's a complex engineering structure where every element, from the density of the bottom to the shape of the cylinders, affects the behavior of the vessel, and the wrong choice can lead to dangerous situations on the water or rapid wear and tear of expensive equipment.
In this article, we will discuss in detail all the nuances that must be considered when buying or operating such a tandem. keel-structure more important than the inflatable floor, how to correctly calculate the displacement and what mistakes are most often made by owners of small craft.
The main rule, which is forgotten by 80% of buyers: the power of the engine should correspond not only to the length, but also to the carrying capacity of the boat. For the engine 20 hp minimum length of the body should be 320-340 cm, and the carrying capacity - at least 450-500 kg. Ignoring these parameters leads to overloading of the transom and loss of stability on the wave.
Criteria for the choice of the housing and the effect of power on the design
The first thing to look at is the geometry of the cylinders and the total length of the craft. For a 20 horsepower engine, the optimal length is from 3.2 to 3.6 meters. Shorter models, even if the transom formally withstands this power, will behave unstablely on the planing, and long versions (more than 4 meters) will require a more powerful motor to effectively enter the mode.
Diameter of cylinders For the 20 hp engine, it is recommended to choose models with a cylinder diameter of at least 42-45 cm. Narrow cylinders can lead to the fact that the boat will burrow too deep with its nose when accelerating, creating a digging effect, which is dangerous when a head wave is hit.
Also important is the material from which it is made. flyboatFor such loads, the density of PVC tissue should be at least 750-850 g / sq.m., and in critical zones (bottom, stringers) - up to 1100 g / sq.m. The thin material will quickly become unusable from constant vibration and impacts on water at high speeds.
Pay attention to the number of compartments. exploitation This is only possible with at least three independent inflatable compartments, which ensures that if one cylinder is punctured, the vessel will remain afloat and allow it to reach shore.
Underbody design: RIB, floorboard or NDND?
Bottom type is perhaps the most important technical parameter that determines performance. There are three main options for a 20 hp engine, each with its own advantages and disadvantages, depending on where and how you plan to use the vessel.
The most popular solution is NDMA This design provides excellent seaworthiness, because the soft bottom cushions the shocks against the wave. However, it is worth remembering that with prolonged sun exposure or heavy heating, the pressure in the bottom can increase, which will lead to deformation of the body ("bubble"). For the engine 20 hp NDNDM is an excellent choice for rivers with moderate chop and lakes.
The second option is hard. flat-floor These boats take longer to assemble, but they provide the perfect hull rigidity, which is critical for sliding. The hard bottom doesn't deform under load, allowing the motor to push the vessel efficiently. The downside is more weight and build time.
Secrets of assembling the solder floor
When assembling a hard floorboard boat, always start laying sheets from nose to transom, which will prevent the formation of a “hump” at the rear, which can make it difficult to install the motor and create additional resistance to water.
Type three: rib It's the most expensive, but also the most reliable option. The rigid V-shaped body cuts through the wave, providing a soft and dry run even in severe excitement. For a 20 hp engine, RIBs 3.2-3.6 meters long are the benchmark for comfort and safety.
When choosing, it is also necessary to consider the presence of quilt The Kielson gives the body a longitudinal rigidity, preventing the deflection under the weight of passengers and the motor, which is especially important for inflatable floors.
transom board: requirements for strength and installation of the motor
The tran takes over all the engine's thrust and vibration loads. A 20 hp engine (about 40-50 kg) requires a transom board reinforced with fiberglass or aluminum linings. Standard plywood, unprotected, will quickly gain moisture and dissect.
The most important parameter is angleTran must provide the correct angle of inclination of the engine's lower unit relative to the bottom. If the engine is too upward or, conversely, too submerged, this will lead to loss of speed, increased fuel consumption and the risk of air capture by the propeller ("cavitation").
- ⚠️ Warning: Never use transom wheels to permanently store the boat in an inflated state with the engine installed. The transom design is not designed to place a point load at one point for a long time.
- ✅ Check the height of the transom. The standard is 381 mm (15 inches) or 508 mm (20 inches) The mismatch between the height of the transom and the lower unit of the motor will require the use of spacers, which is not always safe.
- 🔧 Power: A 20 hp engine prefers a transom plate (or the possibility of installing it) that distributes the load from the lower unit over a large area of the cylinder.
Motor fastenings must be reliable. Use only regular rods that come with the engine, and regularly check their puff, Vibration at high revs can weaken even well-twisted bolts.
Lubricate the threads of the transom bolts and engine rods with graphite lubricant before each fishing, which will prevent the metal from boiling and allow you to quickly remove the motor if necessary, even if it was filled with water.
Loading capacity calculation and weight distribution
Many people ignore rated carrying capacity as a recommendation, not a rule, but for a 20 hp inflatable boat, overloading is a direct route to loss of handling, and the displacement of the vessel must exceed the total weight of passengers, equipment, fuel and the engine by at least 20 to 30 percent.
Consider a typical situation: two adult men (180 kg), equipment and fishing boxes (50 kg), a motor with a tank (50 kg), totaling 280 kg. If the boat has a carrying capacity of 350 kg, then the reserve is only 70 kg. This is not enough for safe stay on the water, especially if water or a wave comes on board.
The table below shows approximate data on the correspondence of the length of the boat and the permissible load for the engine 20 hp:
| Boat length (cm) | Diameter of the cylinder (cm) | Recommended maximum load (kg) | Optimum loading (kg) | Type of bottom |
|---|---|---|---|---|
| 320 | 42 | 400 | 300-320 | NDNDM/Pyol |
| 340 | 45 | 500 | 380-420 | NDNDM/Pyol |
| 360 | 45-50 | 600 | 450-500 | NDNDM/RIB |
| 380+ | 50+ | 750+ | 600+ | RIB/Pyol |
Weight distribution is also critical. The main cargo (passengers, anchors, heavy bags) should be located closer to the center of gravity, which is usually in the middle of the cockpit or slightly closer to the nose. Shifting the cargo to the transom will raise the boat's nose, worsening visibility and handling. Shifting to the nose, on the contrary, can lead to splashing of the cockpit.
60/40 rule: For better performance, 60 percent of the weight must be on the front 2/3 of the boat's length, and only 40 percent on the back, which will ensure the optimal angle of attack when going into planing.
outboard motor 20 hp: two-stroke or four-stroke?
Selection of engine flyboat Two-stroke engines (2T) are lighter, cheaper and easier to maintain, weighing between 36 and 42 kg, allowing them to be mounted on boats with less payload capacity, and are less sensitive to fuel quality and easier to start in cold weather.
Four-stroke (4T) engines are heavier (about 45-55 kg), but they are quieter, more economical and more environmentally friendly. They do not require mixing gasoline with oil, which makes it easier to refuel. However, for boats up to 340 cm long, the 4T weight can become critical, shifting the center of gravity to the transom and "heavier" the stern.
For a 20 hp engine, the choice is often in favor of the 2T, if mobility is the priority and the ability to easily pull the boat ashore by hand. If the boat is stored on water or has a sufficiently large load capacity (more than 500 kg), then the 4T will be a more comfortable solution for long walks.
It's important to remember that gear ratio, which is typically 2.08:1 for motors of this power, means that the crankshaft turns just under two turns per turn, allowing for a high-pitch propeller to achieve high speed.
Safety and preparation for water access
Operating a boat with a powerful engine requires strict safety rules. Inflatable boats are more vulnerable to puncture than rigid hulls, so inspection of the water area before descent is mandatory. Sharp stones, snags and broken glass are the main enemies of PVC.
Always carry a refit, a pump (preferably an electric one with a battery for quick pumping) and a spare paddle. The engine can stall at any time, and the ability to row is your salvation.
☑️ Checklist before exit
⚠️ Warning: Never leave a working motor unattended, even for a few seconds. A sudden jerk of a gas cable or an accidental shift of gear can cause people to fall overboard or damage the boat to the dock.
Control the temperature of the squirrel board. When you work at high speeds for long periods of time, vibration and heat can weaken the adhesive seams in the engine mounting area. Periodically stop and visually assess the state of the stern.
Frequently Asked Questions (FAQ)
Can I put a 20 hp engine on a boat that says "max. 15 hp"?
Exceeding the recommended power of the engine leads to the destruction of the transom, loss of stability and overturning of the boat, and it is also a violation of the rules for registration of small craft and insurance conditions.
Which screw is better to choose for a PVC boat 340 cm and a 20 hp engine?
The optimal choice would be a propeller with a pitch of 9-10 inches and a diameter corresponding to the engine's lower unit. For heavy boats or full load, it is better to take step 9", for light and high-speed - 10" or 11". It is important that the engine goes to full speed (about 5000-5500 rpm) at full load.
How often should I lift a boat on a hot day?
In hot weather, the pressure in the cylinders increases. If the boat is in the sun, the pressure may exceed the norm, which will lead to rupture of the seams. It is recommended to slightly (0.01-0.02 atm) underpump the boat before going out into the sun or periodically put excessive pressure, controlling it with a pressure gauge.
Do I need registration for such a boat?
In Russia (according to the current rules of GIMS) registration is subject to small vessels weighing more than 200 kg in running order OR with an engine capacity of more than 8 kW (about 10.8 hp). Since 20 hp significantly exceed this threshold, registration and the availability of rights category "M" is mandatory.