The creation of a homemade outboard motor from a trimmer power unit is a popular solution among owners of PVC inflatable boats and small metal vessels seeking savings and autonomy. petrol-engine from the lawn mower allows you to get a lightweight, repairable and inexpensive power plant capable of speeds up to 10-15 km / h The main idea is to adapt the rotational moment of the trimmer shaft to work with the propeller through the transmission mechanism system.
The success of the project directly depends on the correct selection of components, in particular, the gearbox and the screw, since the regular trimmer bar usually has a straight shaft, not designed to work in water at an angle. You will have to independently manufacture or modify the lower unit, ensure the tightness of the nodes and organize an effective cooling Despite the apparent complexity, many fishermen successfully assemble such units using available spare parts from motorcycles.
It is important to understand that a homemade motor requires careful balancing and regular maintenance, since the operating conditions on the water are much tougher than when cutting grass. Two-stroke enginesThe most common trimmers require a high-quality mixture of oil and gasoline, and vibrations can quickly destroy poor-quality connections. In this article, we will discuss all the steps in creating a reliable water jet or screw propulsion system.
Selection and preparation of the power unit
The first step is to assess the technical condition of the trimmer itself. For outboard motors, models with an engine capacity of 1.5 to 2.5 are optimally suited. cubic centimeters (cm3), as they provide sufficient power for the movement of a boat with one or two passengers, two-stroke engines are preferred because of their simplicity, lower weight and the ability to operate at any angle of inclination, which is critical when driving on a wave.
You need to dismantle the bar with the cutting element and inspect the condition centre-sleeves If the shaft is straight (hard), it can be used directly by transferring torque to the gearbox, and if the rope (flexible shaft) is installed inside the bar, it will have to be replaced with a rigid steel axis, since the cable cannot withstand the loads of the propeller and may twist or burst.
⚠️ Warning: Never use a flexible trimmer cable to transfer power to the propeller, which will cause instant shaft collapse and potentially injury. Only a hard shaft!
When preparing the engine, you should replace fuel hoses with better quality, resistant to gasoline, and check the condition. carburetorOften, regular jelly-cutters have to be reconfigured, as the motor will run at constant high revs, rather than in the ragged mode, as in mowing grass, and it is also recommended to install a remote fuel tank to increase the autonomy of swimming.
Selection and refinement of the reducer
The key element of the design is the gearbox, which should reduce the number of revolutions and change the direction of rotation by 90 degrees. Trimmer engines develop from 8000 to 12,000 revolutions per minute, while the optimal speed of rotation of the propeller is 2000-3000 rpm Without the gearbox, the screw will work in cavitation mode, without creating thrust, but only foaming water.
The most affordable solution is to use an angle gearbox from an old trimmer, a Bulgarian or a chainsaw. However, regular gearboxes often have a gear ratio of 1:1 or close to it, which is not enough. gearbox from the motor units or independently assemble the belt gear, which allows you to smoothly adjust the gear ratio by changing the diameter of the pulleys.
Calculation of gear ratio
To get the optimal thrust, you need to divide the maximum engine speeds by the desired propeller speeds. For example, 10,000 / 2500 = 4.
When installing the gearbox, it is important to ensure that the shafts are connected securely. Either slash joints or screw-fixed couplings are used. The shaft exiting the gearbox must be made from the shaft. stainless steel or have a high-quality anti-corrosion coating, as constant contact with water is inevitable even when using lower unit.
Manufacture of lower unit and Cooling Systems
lower unit is a pipe that runs the shaft to the propeller. It often uses a 20-25 mm steel or dural pipe to make it. A gearbox is installed at the bottom of the pipe, and a motor attachment at the top. The design must be as tight as possible so that water does not get to the bearings and the engine.
The focus is on cooling, two-stroke trimmer engines are usually cooled by incoming airflow, and when installed in lower unit, the airflow is blocked, which leads to overheating. shell with an air intake at the front or water cooling, if the design of the gearbox allows water to be pumped through the shirt.
To seal the rotating shaft, we use coils (cuffs), we need to pick up a lubricant under the diameter of the shaft and press it into the casing of a gearbox or a lower unit, lubricating the bearings through oils or using a consistent stain. waterproofIt's a place that's laid down when it's assembled.
Calculation and installation of propeller
The propeller is the final element that converts engine power to thrust. For low-power motors (2-3 hp), the optimal diameter of the propeller is 150-200 mm. The screw pitch must be adjusted so that the engine does not overload, but does not reach maximum speeds without loading. Too much step will lead to the fact that the motor will not be able to spin the screw.
The screws can be made independently from a sheet dural 2-3 mm thick or purchased ready-made models with a landing hole under the shaft. The blades must have the correct aerodynamic shape and be balanced. The imbalance will cause a strong vibration that will quickly destroy. bearing and the engine mounts.
The screw attaches to the shaft through a veneer or a cone connection to the nut, and the nut is necessarily fixed by a self-contrary element or a splint so that the vibration does not lead to its unscrewing and loss of the screw. Under water this will cause the boat to stop.
| Parameter | Optimal value | Impact on the move |
|---|---|---|
| Rotor diameter | 160-190 mm | Increasing the diameter increases traction, but requires more power |
| Screw pitch | 80-120 mm | A large step increases speed, a small step increases thrust. |
| Number of blades | 2-3 pcs. | Three blades work smoother, two more efficiently at low power |
transom assembly and fastening
Once all the components are ready, the final assembly is made, the engine is attached to the lower unit through a transition plate, it is important to ensure that the structure is rigid so that the engine does not "walk" when it is running.
☑️ Pre-launch checks
The scooter should securely fix the motor on the transom of the PVC boat. Since homemade structures are often heavier than serial counterparts, the center of gravity may shift. It is recommended to provide for the possibility of adjusting the height of the propeller immersion to avoid cavitation or, conversely, excessive water resistance.
Control is either by turning the entire motor behind the humming wheel or, in more complex versions, through a system of cables. The rummel must be comfortable and be able to lock in the "gas" position. Often a chainsaw trigger is used, fixed to the handle.
⚠️ Note: When mounting the motor on an inflatable boat, be sure to use a protective lining on the balloon so that the vibration and friction of the metal do not damage the PVC fabric.
First launch and break-in
The first start is better done on the shore, fixing the engine in a bucket of water to check the operation of the screw and the absence of leaks. Start the engine and let it work at idle speeds. Check the temperature of the gearbox and engine body, they should not heat up to a state where you can not touch your hand.
It takes a few hours to run a homemade engine, and you don't have to put a full load on it, you have to wipe the rubbing parts, check the glands under load, make sure you're stable. If the engine stalls when the valve is opened abruptly, you may need to adjust the jelly.
Successful break-in is completed when the motor stably keeps its revs under load, and the temperature of the knots does not exceed 70-80 degrees Celsius.
During the test swims, watch the behavior of the boat. If the stern is too burrowed or, conversely, try changing the angle of inclination of the lower unit or the position of the motor on the transom.
Maintenance and safety
Homemade motor requires more frequent maintenance than factory. After each release, be sure to rinse lower unit with fresh water to wash off the salt and silt. Regularly check the tightening of fasteners, as vibration is the main enemy of homemade structures.
Once a season, you need to disassemble the gearbox, change the lubricant and check the condition of the coils. If an emulsion (white coating) appears in the lubricant, then the oal is leaking water and requires urgent replacement, ignoring this feature will lead to corrosion of the bearings and jamming of the shaft.
Keep in mind safety. A homemade engine rarely has an emergency jamming system, so steer it carefully. Always carry a spare set of candles, tools and fuel. Remember, you're floating on a device that has not passed factory safety tests.
⚠️ It is strictly forbidden to operate a homemade motor at night or in conditions of heavy fog without navigation lights and communication means.
Frequently Asked Questions (FAQ)
What is the maximum speed of a boat with a trimmer engine?
With an engine of 2-3 hp and a correctly selected propeller, an inflatable boat with one passenger can reach a speed of 5-7 km / h. With two passengers, the speed will decrease to 3-4 km / h. This is enough for quiet movement along the reservoir, but not for planing.
Can I use an electric motor from a trimmer?
Theoretically, you can, but you'll need a powerful battery and controller. Electric trimmers typically have low torque at low revs, making them inefficient for a screw without a sophisticated reduction system. Gasoline options are more popular because of autonomy.
Do I need to register such a motor in the GIMS?
According to the current legislation of the Russian Federation, boats with engines of more than 8 kW (about 10.8 hp) are subject to registration. Trimmer motors usually have a power of up to 3 hp, so they do not need to be registered if the total weight of the equipped vessel does not exceed 200 kg.
Why does the trimmer motor stop under load?
It's probably a fuel mix imbalance or an air filter that's clogged, and it could be a screw pitch that's too big, because the engine just choked, because it doesn't have the power to turn it, and try to reduce the diameter or pitch of the screw.