The desire to own a swimming vehicle often comes up against high prices of factory units, especially when it comes to power plants. Many water enthusiasts wonder whether it is possible to create a reliable and slender vehicle. boat-engine Experience shows that the competent layout of available units allows you to assemble a unit that is not inferior in performance to low-power budget models.
Creating such a device requires not only technical skills, but also a deep understanding of hydrodynamics and the principles of the internal combustion engine. Unlike factory analogues, homemade design allows you to flexibly select components for specific tasks, whether it is a quiet course for fishing or maximum traction to overcome currents. propellerTo avoid overheating and cavitation.
In this article, we will discuss key design steps, from the selection of a donor engine to the final configuration of the fuel system, how to turn a regular gasoline or motorcycle engine into a full-fledged power plant for a PVC boat or metal boat. Safety and tightness of the units will be our priorities at each stage of assembly.
Choosing a Powertrain: The Heart of Your System
The first and most important step is to choose a donor engine, and its power, weight and dimensions will determine the overall efficiency of the entire structure. petrol-saw Or trimmers with an engine capacity of 1.5 to 2.5 cubic inches, and they have high revs, which requires a high-quality gearbox, but they are incredibly light and easy to maintain.
If you're targeting a heavier vessel that requires 3 to 5 hp, you should look at two-stroke engines from mopeds or older motorcycles. Two-stroke cycles are preferred because of their simplicity and ability to work at any angle of roll, which is critical for a boat. Four-stroke engines require a strictly vertical position to lubricate the crankcase, which complicates the design of the suspension.
When choosing a donor, be sure to look for an integrated ignition system. Old contact systems require regular cleaning and adjustment, while electronic ignition (CDI) provides a stable spark even when moisture is injected. It is critical that the engine has forced air cooling, as water cooling will require a complex water intake system and heat exchanger.
- 🚀 Power: For boats up to 3 meters long, 2-3 hp is enough, for 4-5 meters, 5 hp and above will be required.
- ⚖️ Weight: Try not to exceed the weight of 10-12 kg for transom motors, so as not to overload the stern of the boat.
- 🔧 Availability of spare parts: Choose popular models (for example, Husqvarna, Stihl, Minarelli) so that the piston or carburetor can be quickly found in case of breakdown.
⚠️ Attention: Never use engines with a volume of more than 150 cubic meters without a major upgrade of the control system and enhanced lower unit, as vibration can destroy the transom of a light boat.
lower unit design and transmission system
lower unit is a design that transfers torque from the engine to the propeller. Self-made motors most commonly use a rigid coupling through a shaft or belt gear. Gross gear is more reliable, but requires precise alignment and the presence of glands to prevent water from entering. Belt gear is easier to manufacture, but is prone to slipping at high loads and requires protection from water.
To transfer the rotation to the screw, a gearbox is needed, since internal combustion engines develop maximum power at high revs (3000-6000 rpm), and the screw needs 2000-3000 rpm to work effectively. As a gearbox, ready-made units from motoblocks, Bulgarians or even meat grinders are often used, although the latter wear out quickly. The ideal solution is an angular gearbox from the engine. chainsaw Or a specialized marine gearbox.
The sealing of a rotating shaft is the most difficult part of the engineering task, and it uses cuffs that are installed in the body of the lower unit. It is important to provide a bearing lubrication system, because working in an aqueous environment is aggressive. The body of the lower unit is usually made of a duraluminium pipe or profile that is resistant to corrosion.
Why can't we use direct transmission?
In direct transmission, the propeller will rotate at the same speed as the crankshaft (5000+ rpm) and this will lead to cavitation (boiling of water), huge noise and zero thrust. The screw will simply grind the water without pushing the boat forward.
When designing a lower unit, you have to consider the length of the lower unit. For inflatable boats, the distance from the lower unit slice to the anti-cavitational plate must be at least 35-40 cm so that the screw is fully submerged in water, but does not touch the bottom in shallow water. Too long lower unit increases sailing and the risk of hitting underwater obstacles.
- 🛠️ Shaft material: Use only stainless steel or power tool shafts, regular steel will rust in one season.
- 🌊 Defense: Be sure to provide a casing above the screw to protect the legs of passengers from injuries.
- 🔗 Connection: All threaded connections must be treated with a thread fixer, as the vibration will quickly unwind the usual nuts.
Manufacture of the propeller and calculation of the step
The propeller is the element that directly converts engine power into thrust. At home, it is almost impossible to make a screw of complex geometry, so craftsmen often use ready-made screws from outboard motors-"wheelchairs" or cut them out of sheet metal. The simplest option is a screw of aluminum sheet thickness of 2-3 mm, cut into a pattern and bent at a certain angle.
The key parameters of the screw are diameter and pitch. The screw pitch is the distance that the screw will travel in one full revolution in a solid environment. For low-power engines (2-3 hp), a 15-20 cm diameter screw with 10-15 cm increments is considered optimal. Increasing the pitch increases the maximum speed, but reduces thrust at low revs, which can lead to the fact that the motor will not be able to “put” the boat to planing.
To test the propeller, you can do a test in a water tank, and you can start the engine for a short time and look at the flow patterns of the water, and if you have a lot of bubbles and foams, the pitch is too big or the screw is bent incorrectly.
The screw's attachment to the shaft must be reliable. Often a cone connection to the veneer, like in factory engines, or a simple threaded stud with a nut and a puck, is used. It is important that the screw can turn when it hits a hard object, otherwise you can break the shaft or damage the gearbox. Some craftsmen use friction clutches or cut-off slatches to protect the transmission.
⚠️ Attention: Balancing a homemade propeller is imperative! Uneven distribution of mass will cause a violent vibration that will destroy the lower unit bearings and can damage the boat's transom.
Fuel system assembly and management
The fuel system of a homemade motor is usually based on a flowing principle, the tank is located above the carburetor, which ensures the supply of fuel without a pump. However, for two-stroke engines running on a mixture of gasoline and oil, it is important to use hoses that are resistant to the aggressive effects of the mixture. A conventional fuel hose can swell and block the supply of fuel.
The motor controls include rev adjustment (gas) and, in more complex models, gear shifting. The simplest versions use a gas handle mounted on a tiller, which is connected by a rope to the carburetor throttle. The stop button, which closes the ignition circuit by mass, or mechanically overlapping the fuel supply, is used to stop the engine.
The carburetor is the brain of the engine. In homemade structures, there are often problems with adjusting the mixture due to the wrong position of the carburetor (for example, upside down), in such cases, carburetors with adapters or rotary cameras that allow you to work in any position, quality fuel filtration through the sump and a mesh filter in front of the carburetor will prolong the life of the engine.
☑️ Fuel system inspection
For ease of control, the handle is made from a metal or wooden pipe, it holds a gas trigger and a turnbar, the rotating mechanism must be tight enough that the motor does not prowl in excitement, but allows you to confidently change course. Often use a rotary fist from a bicycle or scooter as the basis for the steering mechanism.
Mounting the motor to the boat (transe suspension)
A homemade motor must not only work, but also be securely attached to the boat. A standard transom is 25-50 mm thick, and the engine mounting assembly must match that. A simple rod is made from two corners and screws with lambs, but for powerful motors it is better to use an eccentric clamping mechanism or a screw press, which provides rigid fixation without backlashes.
The most important element of the suspension is the ability to adjust the inclination angle of the lower unit (different). The correct angle of inclination determines whether the boat will go with the nose down or, conversely, bully it. Adjustment is carried out by moving the mounting point of the motor or using adjustable stops. Incorrect differentiation can lead to the burying of the boat's nose in a wave or loss of controllability.
To protect the transom of the inflatable boat from abrasion and metal pressure, a soft pad of rubber or dense fabric must be installed between the strubbine and the transucin. The metal parts of the fastener must be painted or coated with an anti-corrosion composition, since salt water quickly destroys even galvanizing.
| Parameter | Light engine (up to 3 hp) | Medium engine (3-5 hp) | Heavy motor (5+ hp) |
|---|---|---|---|
| Type of attachment | Lamb scabs | Screw clip | Reinforced clamping with rubber pads |
| lower unit material | Aluminum pipe | duraluminium | Stainless Steel/Dural |
| Motor weight | 4-7 kg | 8-12 kg | 12-18 kg |
| Boat speed | 8-12 km/h | 15-20 km/h | 20-25 km/h |
First start and break-in of a homemade motor
The first run of the assembled unit should be carried out in a safe environment, ideally in a barrel of water or on the shore, securely fixing the engine, which will allow you to check the operation of the cooling system (if any), the absence of vibrations and the tightness of the fuel hoses.
The process of running a two-stroke engine is mandatory. In the first hours of operation (3-5 tanks), you should not give full gas. You need to alternate modes of operation, allowing the engine to warm up and the parts to wash away. Watch the temperature of the engine body and gearbox - excessive heating indicates problems with lubrication or overload.
The success of a homemade motor depends not on the power of the engine, on the quality of the gearbox assembly and the tightness of the lower unit. Water is the main enemy of mechanics.
In the process of operation, constantly monitor the condition of the fasteners. Vibration is a natural companion of the ICE, and it can weaken even well-tightened connections. Regular inspection, lubrication of moving parts and cleaning the screw from algae will extend the life of your creation for many seasons.
What kind of oil and gasoline mix should I use for the scaling?
For the two-stroke engine, it is recommended to use a mixture in a ratio of 1:25 (40 ml of oil per 1 liter of gasoline). This will provide a rich lubrication of rubbing vapors. After running (about 5-10 hours of operation), you can switch to the standard ratio of 1:50, specified by the oil manufacturer.
Can I use an electric motor instead of an internal combustion engine?
Yes, it's even easier. You'll need a traction battery (12V or 24V), a controller and an electric motor (often using screwdriver engines or specialized motor-wheels). Electric motors are quieter, more environmentally friendly and require less maintenance, but have a limited range.
What to do if the engine is stalled at high speeds?
Most likely, the fuel supply is disrupted (carburetor jetty is clogged, there is no ventilation in the tank) or spark formation is impaired (high-voltage wire failure, soak on a candle), and the cause may be air sucking through the carburetor gaskets.
Do I need to register a homemade motor in GIMS?
According to the legislation of the Russian Federation, small vessels with an engine of more than 8 kW (about 10.8 hp) are subject to registration. If your homemade engine is weaker than this threshold, registration is not required, but the presence of documents confirming power (or lack thereof) may raise questions during the inspection, so it is better to have drawings or calculations.
How to protect the engine from corrosion?
Use oxide film aluminum alloys or stainless steel. All surfaces are regularly coated with corrosion sprays (such as WD-40 or specialty marine coatings). After each outing, wash the engine with fresh water, especially if you have swam in a salty body of water.