Trimmer instead of outboard motor: myths, reality and risks

The idea of using a garden tool to move on water comes to many anglers and summer residents, especially when budgets are tight and the old engine is out of order. On various forums and video platforms, you can find videos where craftsmen successfully scour the water on inflatable boats equipped with converted gasoline trimmers. This concept seems attractive for its cheapness and availability of spare parts, but behind the simplicity there are a number of engineering nuances.

Before making such a decision, you need to understand the fundamental difference between boat-engine and petrol-trimmerThe outboard motor is designed to operate under constant load conditions, has water cooling and a specific transmission system. chainsawInstalling such a unit on a transom requires a deep understanding of mechanics and readiness for experiments that do not always end in success the first time.

In this article, we will discuss in detail the design features, the necessary improvements and, most importantly, risk of turning the shaft in lower unit when overheatingIt's a major technical problem with homemade devices, and you'll learn why some models are better at it than others, and whether it is worth the effort, given the security and resource of the host.

Design differences and working principle

The main difference is in the lubrication and cooling system. two-stroke The outboard motor is lubricated with oil mixed with gasoline and cooled by off-board water circulating through the shirt. In the trimmer, air cooling is forced, by impeller on a flywheel. When mounted on the boat, the air flow is blocked by the jacket or water, which leads to instantaneous cooling. heat-up cylinder and piston group.

The second critical point is the direction of rotation. Many boat propellers are designed for a particular direction of thrust, and the trimmer shaft often has a direct transmission without a gearbox that changes the rotational vector by 90 degrees, as is done in classic lower unit. To implement the idea of a trimmer on the boat, enthusiasts use homemade lower units, where the engine is attached to the top, and the torque is transmitted through a flexible shaft or a chain gear to the screw.

⚠️ Attention: Using the trimmer as an outboard motor without upgrading the cooling system will result in a piston slack and engine wedge within 10-15 minutes of operation.

There are specialty attachment motors that are structurally similar to trimmers, but they have water-cooled gearboxes, and if you're planning to redesign a garden tool, you're going to have to take the power and transfer the torque yourself. cycling-star Or pulleys from car generators.

Required improvements and alterations

Just mounting a trimmer to a transom won't make the boat move efficiently. The first step is to build a reliable one. bracketStandard mountings of garden tools are not designed to focus on water and vibration loads that arise when the propeller operates in a dense aqueous environment.

The most difficult part is the organization. coolingSome craftsmen cut holes in the casing to get air in, others try to organize a forced blowout, but the most effective, albeit time-consuming way, is to redesign the exhaust system and air suction so that hot gases do not heat the crankcase. carburetor It is more efficient because the water medium creates more resistance than the air.

It's important to consider gear ratio. The garden trimmer is very high, but it has a low torque at low revs. The boat rotor, on the contrary, needs to be able to do it. tractionSo it's mandatory to install a gearbox or use a pulley system of different diameters. Without that, the screw will operate in cavitation mode without creating an efficient jet.

• 🛠️ Making a transition plate of stainless steel or aluminum for fastening to a transom.
• ⚙️ Selection and installation of gearbox (for example, from a Bulgarian or drill, if the speed allows) or belt transmission.
• ❄️ Organization of a channel for air intake or water cooling (in complex versions).
• 🔧 Replacement of the standard screw with a propeller with a pitch suitable for small capacities.

Choosing the right trimmer model

Not every garden tool is suitable for this kind of operation. The key is the engine size and the type of drive shaft. For a boat, the best models are those with an engine capacity of 33 cubic cm or higher. Smaller volumes simply won't pull even a lightweight inflatable boat with one person.

The second important aspect is the design of the shaft. In cheap models, the shaft is often a flexible cable in the pipe, which, when under pressure on water, can twist or collapse due to friction. hard-shaft These models usually have a direct torque transmission and a more reliable gearbox design at the bottom of the bar.

Model/Type	Engine capacity (cc. cm)	Power (hp)	Shaft type	Fitness
Budget trimmer	25-30	0.8 - 1.0	Flexible cable	Low (risk of breakdown)
Professional motorbike	40-50	1.5 - 2.0	Hard shaft	High (requires further development)
Chainsaw (remaking)	45-55	2.0 - 2.5	Hard shaft	Medium (hard to fasten)
Special motor for the boat	35-45	1.2 - 1.8	Hard shaft	Optimal (ready-made) solution

When choosing a donor, pay attention to the availability of parts. Chinese brands may offer a low price, but finding a piston group or carburetor in six months can be a challenge. Stihl, Husqvarna or their qualitative analogues, where the unification of details is high.

lower unit installation and assembly process

The building of a homemade motor starts with the creation of a lower unit, which is a pipe that runs inside the shaft, the top of the pipe is attached to the engine, the bottom ends with a screw, and it is important to ensure that the top is tight so that water does not fall on the engine, while leaving access for maintenance.

The anchorage to the transom is through a strubbine or a welded plate. This is critically important for the ability to tilt the lower unit. If the screw is too deep, resistance will increase; if it is too small, air suction and cavitation will begin. The angle adjustment is usually done through a hinge connection with a locking bolt.

For connection of the engine with the shaft is often used couplingThe rigid connection can cause the engine crankshaft bearings to quickly collapse due to vibration of the boat hull, and protection of the propeller must be provided, since open blades on a small boat pose a danger to the swimmer and the operator himself.

• 📏 Precise alignment of the shaft relative to the lower unit to minimize the beats.
• 🔩 Use stainless bolts and nuts, as regular steel rusts during the season.
• 🛡️ Installation of a protective casing on the screw for safety.
• ⛽ Organization of convenient access to the fuel tank and the possibility of refueling on the go.

⚠️ Attention: When assembling lower unit, use only quality closed type bearings (2RS or ZZ), otherwise the water will quickly wash the grease and jam the shaft.

Operation and maintenance of the homemade unit

Operating a trimmer motor requires constant temperature monitoring. Unlike a regular outboard motor, there are no overheating sensors. The operator must carefully monitor the sound of the engine and immediately release the gas at the first sign of power loss or sound change.

After each foray into the water, you need to flush the system with fresh water, especially if you walked on a salty pond or sandy water. Abrasive particles trapped in the lower unit pipe act as sandpaper on the shaft and glands. Regular lubrication of bearings and checking the tension of belts or chains are also mandatory.

Fuel mixture should be prepared strictly in the proportion specified by the manufacturer of the trimmer, but taking into account the increased load. It is often recommended to slightly increase the proportion of oil (for example, make a mixture of 1:40 instead of 1:50) to improve the lubrication of rubbing vapors, although this can lead to an increase in the soak on the candle.

Store such a motor should be in a dry room removed from the boat. Prolonged stay in the assembled state on the transom can lead to corrosion of fasteners and deformation of thin-walled pipes of the lower unit under its own weight.

Advantages and disadvantages of the solution

The main advantage of this design is priceThe cost of an old trimmer or a Chinese spit is much less than even the cheapest Chinese outboard motor, and the repairability of such units is high: there are parts in any service for repairing garden equipment, which is more common than boat workshops.

But the disadvantages are significant. noise. Garden engines don't have water mufflers, and the sound is scary. The second downside is that there's no reverse. You have to either turn the whole of the lower unit around or use special machinery to make it difficult to build. The third downside is that you have a low lifespan at constant loads.

Hidden engine life

The resource of the garden trimmer when operating in the outboard motor mode is reduced by about 40-50% due to constant high loads and less efficient cooling compared to air flow when moving the mower.

Two-stroke trimmer engines emit a mixture of gasoline and oil directly into the atmosphere (or into the water if the exhaust is directed down), which creates an oil film and an unpleasant smell that can interfere with fishing.

Security and legal aspects

Using improvised machinery on the water always carries risks, and if the lower unit breaks down or the propeller breaks off at speed, the consequences can be fatal for the people in the boat. Firming It should be designed with a margin of safety, and the screw must be protected by a ring.

From the point of view of GIMS, registration of a homemade engine can raise questions: if the engine power exceeds 5 hp (and often during alterations the power indicates passport, but in fact it can be higher), registration is formally required. However, in practice it is difficult to prove the power of the engine without a badge and documents, but it is not worth the risk.

⚠️ Attention: Never use homemade motors at night, in heavy fog or in places with heavy traffic, the lack of navigation lights and the unpredictability of technology make you invisible and vulnerable.

Always carry spare oars. A homemade motor is always a lottery, and it can fail at any time. The ability to row is a must for the owner of such equipment.

Final comparison with factory analogues

If you compare the effort spent on building a trimmer engine with buying a budget factory engine, the difference becomes obvious. Factory models, even the simplest ones, have a warranty, reverse, emergency stop system and a well-functioning cooling system.

Trimmers only make sense in two cases: if you already have an unnecessary powerful unit lying in the garage, or if you are an enthusiast, for whom the process of creating a mechanism is more important than the result, in other cases, the savings can be imaginary due to constant repairs and improvements.

Ultimately, the choice is yours: engineering is not in a hurry, and who knows, maybe your design will be the prototype of a new budget outboard motor, and the key is to approach it wisely, with safety and with good judgment.

Can I put an electric trimmer on a boat?

Theoretically, it's possible, but it's almost meaningless: electric trimmers have low power and short battery life; for a boat, it's better to look at specialized electric motors with traction batteries that are more efficient and safer.

Which screw is better to choose for a homemade motor?

The best solution is to use a finished screw from low-power outboard motors (2-3 hp) with a pitch corresponding to your revolutions. It is difficult to experiment with the shape of the blades yourself without a wind tunnel.

Do I need to register such a boat in the GIMS?

Boats up to 3 meters long and weighing up to 200 kg (including the engine) are not subject to registration if the engine power does not exceed 8 kW (about 10 hp).

Why does the trimmer stop when loaded in water?

It's probably the carburetor, the mix's standard settings are poor for water loads, you need to adjust your jellybox or clean up your fuel system, and you can overheat because of bad cooling.

How many kilometers per hour does a boat with a trimmer engine develop?

On a light inflatable boat with one person, you can reach a speed of 5-8 km / h. This is enough for a calm movement along the reservoir, but not for glides or overcoming a strong head current.