Own water transport is always freedom of movement and new opportunities for fishing or recreation, but the cost of regular transportation is not enough. boat-engine Many small boat owners are asking themselves, can we adapt a household gasoline tool to run on water? The answer lies in the design of trimmers, whose internal combustion engines are almost identical in design to the low-power engines used in water technology.
Creating a homemade device requires not only technical skills, but also a deep understanding of hydrodynamics and mechanics, since screw-piece It works in an aggressive aquatic environment, which is very different from air, and in this article, we will go through all the stages of reworking, from the selection of a donor to the final break-in, and also weigh the risks associated with the operation of homemade units on the water.
Choosing a donor: what trimmer is suitable for alteration
The basis of the future water jet or screw engine will be a gasoline trimmer, and its choice should be approached as critically as possible, since it is the specifications of the "donor" that determine the final traction and reliability of the entire design. direct-transmission It's very undesirable to use a torque shaft that's rigidly connected to the engine, which makes it easier to dock with a lower unit. The use of curved bar and cable-driven trimmers is highly undesirable, because at high water loads, the cable heats up and tears quickly, and power losses can reach up to 30%.
When choosing power, you should focus on the engine volume in the range from 1.5 to 2.5 horsepower, since less powerful models simply will not be able to bring a boat with a passenger to planing or confidently move against the current. It is important to pay attention to the cooling system: most trimmers have air cooling, which requires the revision of the casing or the installation of additional blowing, since in a closed space of lower unit the engine can quickly overheat.
- 🔧 Engine capacity: preferable models from 33 cc and above to ensure sufficient torque.
- 🔧 Shaft type: A rigid straight shaft is a prerequisite for reliable operation under load.
- 🔧 Cylinder position: The horizontal arrangement is often more convenient for layout, but the vertical is also applicable with the correct gearbox.
- 🔧 Availability of spare parts: pop-ups Stihl, Husqvarna or Karcher It is easier to maintain in case of breakdown.
Do not forget about the weight of the unit, because too heavy motor on the transom is small. flyboat Lightweight models from China often win in price, but lose in life bearings, which experience increased loads when working in water due to the lack of lubrication provided by the manufacturer for dry conditions.
Why are two-stroke engines better for boats?
Two-stroke trimmer motors are often preferred over four-stroke trimmers for small boats, as they are lighter, more compact and can run at a slope, which is critical when changing the boat's trimmer, and are easier to maintain and cheaper to repair.
Design of lower unit and Reduction Part
The most challenging part of the project is to create a lower unit pipe, which runs inside a shaft that transmits rotation from the engine to the propeller. This part usually uses a 20-25 mm steel or dural pipe that must be perfectly straight to avoid shaft beating and breaking. bearing-assembliesInside the pipe, an intermediate shaft is installed, which is connected to the engine through a homemade or adapted gearbox.
The gearbox part is necessary to change the direction of rotation by 90 degrees and transfer the force to the propeller. As a donor gearbox often used corner grinders (Bulgars) or gearboxes from old drills, but their tightness leaves much to be desired and requires careful refinement of the ossils and lubricantsresistant to washing with water.
⚠️ Attention: Pressurization of the gearbox is a critical moment. Even a microscopic crack in the body or a poor quality oment will lead to water getting inside, which will cause the gears to corrosion and jamming the engine at the most inopportune moment. Use only water-resistant lubricants of high quality.
The engine shaft and the lower unit shaft are often coupled with a clutch that allows the units to be separated for transportation or maintenance, which simplifies the design by making it modular: the engine stays on top, and the lower unit is attached to the transom. The attachment of the entire structure to the boat must be reliable, adjustable. screw-deepening depending on the load of the boat.
| Component | Material/Source | Function | Requirements |
|---|---|---|---|
| lower unit | Steel pipe 25 mm | Shaft protection | Anti-corrosion coating |
| Val Val | Steel 45 or stainless steel | Transmission of rotation | High twisting strength |
| gearbox | USM (Bulgarka) 900 W | Changing angle | Tightness, lubrication |
| Firming | Aluminum/Steel | Fixation to transom | Altitude adjustment |
It is important to calculate the gear ratio, since standard trimmer engines develop high revs, which for the screw can be excessive, which will lead to cavitation and loss of efficiency. propeller paired with a gearbox will allow you to use the engine power as efficiently as possible.
Use graphite lubricant for lower unit threaded joints – it will prevent parts from boiling and make it easier to disassemble after the season of operation.
Manufacture of the propeller DIY
The propeller is the heart of the water jet system, and its geometry directly affects the speed and efficiency of the boat. For homemade trimmer-based engines, the most common propellers are those with a diameter of 120 to 160 mm and a pitch that depends on the power of the engine. You can make a screw from sheet stainless steel or durable aluminum, cutting the blades according to a pattern and giving them the desired angle of attack.
The fabrication process begins with the marking of the workpiece, which must be perfectly round to avoid imbalance and vibration during operation, the blades are formed by hot or cold forging, or simply bent if a sufficiently plastic material is used, after which the edges are carefully grinded to reduce water resistance.
- 🌀 Diameter: is selected based on power (for 2 hp, it is optimally 130-140 mm).
- 🌀 Screw pitch: The distance that the propeller will travel in one revolution in a solid medium (usually 60-90 mm).
- 🌀 Number of blades: Two-bladed easier to manufacture, three-bladed work softer and quieter.
- 🌀 Material: Stainless steel is stronger but heavier; aluminum is easier to repair when hit.
The screw attachment to the shaft is through a veneer joint or a cone fit with a nut, which must be lawn so that the vibration does not lead to its spontaneous unwinding. Losing the screw on the go is a common problem of homemade structures, so the reliability of the mounting unit should be a priority.
⚠️ Warning: Never start a motor with a damaged propeller. Even a small crack or chip on the blade will cause a strong vibration that can destroy lower unit or damage the engine bearings in minutes.
To improve efficiency, you can experiment with the shape of the blades, making them more bent or changing the angle of attack, but this requires test runs and speed measurements. Often ready-made propellers from motorblocks or old outboard motors are better than artisanal products, and they are worth considering as an alternative.
☑️ Screening of the screw before launch
Cooling system and fuel autonomy
Because trimmer engines are designed to work in the air, installing them in a closed space of a lower unit or under the casing risks overheating. Standard airflow may not be enough, especially in hot weather or when operating at full speed, so forced ventilation or refrigeration of the cooling system to water is often required.
Water cooling is a more efficient but difficult option, and involves installing a pump on a lower unit pump that pumps water from the side of the engine through the engine cooling shirt, drilling channels in the cylinder block or making an external heat exchanger, which requires high skill and access to the lathe.
If the redesign of the cooling system is not possible, you should ensure a powerful flow of air to the cylinder, making special air intakes and exit holes in the casing, and also helps to install a deflector that directs the flow of air from the flywheel directly to the cylinder ribs, which allows you to keep the temperature within acceptable limits.
As for fuel, trimmers work on a mixture of AI-92 or AI-95 gasoline with two-stroke oil. Mixing proportions (usually 1:25 or 1:40) must be strictly observed, since lack of oil will lead to bullies of the piston group, and excess will lead to the formation of a dip and loss of power. The fuel tank can be used regular from the trimmer, placing it in a protected place, or connect a remote tank to increase autonomy.
| Parameter | Air cooling | Water cooling | Hybrid |
|---|---|---|---|
| Complexity | Low. | High. | Medium |
| Efficiency | Medium | High. | High. |
| Risk of corrosion | Low. | Highly | Medium. |
| Noise | High. | Low. | Medium |
For long walks, it makes sense to arrange the flow of fuel from the tank above the carburetor, which will ensure stable operation of the engine even when the boat rolls, using transparent fuel hoses will allow you to visually control the flow of the mixture and notice the suffocation of the system in time.
How to make water cooling easier?
The easiest way is to use an external heat exchanger (radiator) from a car or motorcycle, installing it on a lower unit and passing off-board water through it with a small pump.
Installation of the motor on the boat and safety
Installation of a homemade motor on a boat requires careful preparation of the transom, which must withstand not only the weight of the unit, but also the dynamic loads that arise when moving along the wave. PVC It is better to use removable mounts that do not damage the cylinders and allow you to quickly remove the engine when approaching the shore.
The attachment should allow for adjustment of the depth of the propeller. Too high will lead to air capture and cavitation, and too low will lead to increased water resistance and the risk of impact on underwater obstacles. The optimal position is considered when the anti-cavitation plate is 2-3 cm below the bottom of the boat when moving.
⚠️ Warning: Before starting, be sure to check the reliability of all bolt joints and stoppers. The vibration from the engine running on water is much stronger than when running a trimmer in the grass, and can weaken the fastener in minutes.
When operating a homemade motor, you should not forget about the rules of safety on the water. Life jackets, anchors and a serviceable pump are a mandatory minimum. Also, you should remember that homemade motors are not certified and their use may be limited by the rules for registration of small craft in your region.
The engine should be run in a gentle mode: the first 3-5 hours of operation should be avoided full speed, giving the parts to get lost. During the break-in, you must constantly monitor the engine temperature and the state of lubrication in the gearbox, adding or replacing it if necessary.
Safety and reliability of fasteners are more important than maximum speed - a homemade motor should first of all guarantee the return to shore.
Advantages and Disadvantages of Homemade Designs
The main advantage of the trimmer motor is that it's cheap and it's available. If you already have a trimmer, the cost of building the engine is just the cost of materials for a lower unit and a screw, which is much less than the cost of even the simplest Chinese outboard motor.
In addition, such engines have high repairability: in case of a breakdown, the engine can be quickly removed, replaced with a new one or repaired in any garden appliance repair shop. The mobility of the design is also a plus - the engine is easily disassembled and placed in the trunk of a passenger car.
- ✅ Savings: The cost of creating 3-5 times lower than the purchase of a finished engine.
- ✅ Repairability: Spare parts are available in garden appliance stores.
- ✅ Mobility: Easy to transport and store.
- ❌ Resource: In intensive operation on water, the resource is less than that of specialized motors.
- ❌ Noise: The lack of a silencer makes the engine quite noisy.
However, there are drawbacks: the lack of reverse (reversing) on most models, which makes maneuvering difficult, and a higher noise level compared to four-stroke outboard motors.
However, for single fishermen, those who like to quietly hunt in small ponds or owners of small boats, this motor will be an excellent budget solution, it gives independence from the electricity grid and allows you to overcome distances that are not available for rowing.
To reduce noise, you can make a homemade muffler from metal pipe and metal shavings, but watch out, this will not reduce engine power too much.
Frequently Asked Questions (FAQ)
How much does it cost to make a trimmer motor yourself?
If the trimmer is already there, then the materials (pipe, shaft, bearings, metal for the screw) will take from 3 to 7 thousand rubles. If you buy a trimmer specifically, the budget will increase to 10-15 thousand rubles, which is still cheaper than a new outboard motor.
Can I put an electric trimmer on a boat?
Theoretically, it's possible, but it's not practical. Electric trimmers have low power and short battery life. For a boat, it's better to use specialized electric motors with traction batteries that will provide several hours of travel.
Do I need to register a boat with a homemade engine?
According to the legislation of the Russian Federation, registration is subject to small vessels with an engine of more than 8 kW (about 10 hp). Since trimmer engines usually have a power of up to 3 hp, registration and control rights are most often not required, but the rules can change, so it is worth checking with GIMS.
What is the maximum speed of a boat with such a motor?
Single inflatable boat with a trimmer-based engine with a capacity of 2 hp can reach a speed of about 10-12 km / h With two passengers, the speed will decrease to 6-8 km / h. For planing such power, as a rule, is not enough.
How often should you change the oil in the gearbox of a homemade motor?
In homemade gearboxes, especially if non-specialized units are used (for example, from a Bulgarian), it is recommended to change the oil after each release to the water or once a month per season, since the risk of water ingress and emulsification of the lubricant is very high.