In the conditions of constant growth of prices for fuel and lubricants, many owners of small boats are thinking about finding alternative energy sources for their swimming facilities. outboard motor on gas This is not a garage-workshop myth, but a very real technical possibility, which has been actively discussed in the circles of water engines for many years: the conversion of an internal combustion engine to liquefied hydrocarbon gas (LPG) or natural gas (CNG) promises tremendous economic benefits, but at the same time poses serious questions for an enthusiastic engineer about safety and reliability.
The challenge lies in the environment itself: water does not forgive mistakes, and the combination of flammable gas, sparks and hot parts requires flawless technical execution. Unlike cars where the HBO system (gas equipment) is brought to automaticity, on the water you often remain one-on-one with the equipment, and any leak or failure in the gearbox can lead to fatal consequences. propane-butane or methane, while getting a cleaner exhaust and increased spark plugs.
Before you can get to the cylinder and gearbox, you need to soberly assess your strength and knowledge. This article is not a call to action, but is a deep technical analysis of the situation, based on the physics of the ICE and practical experience of conversion. mixingfeatures gas-evaporation at low temperatures and how they behave motor-power If you're ready to experiment, the information below will be your starting point.
Economic feasibility and payback of conversion
The main driver of interest in the gasification of outboard motors is the cost of a kilometer of mileage or a motor hour. The price difference between AI-92 / AI-95 gasoline and propane butane can reach a double value, which, when using a boat actively (for example, in commercial fishing or tourism), gives a tangible financial freedom. gas-cylinder The 4th generation (GBO) and its proper installation can be significant, and it's not just the cost of the kit that needs to be considered, but the cost of reworking the intake system.
When you look at the economy, you can't ignore the power loss. Gas has a lower energy intensity than liquid fuels, and you get less heat when you burn the same mass of the mixture, and that means that to achieve the same speed, you have to keep the throttle open more often, using more fuel in liters, although the monetary value will be preserved. octane propane is higher than gasoline (about 105-110), which theoretically allows to increase the degree of compression, but on standard engines this advantage is difficult to realize without flashing the ECU.
The payback period depends on the hours of the season. For an owner who goes out a couple of times a month on weekends, the installation of HBO may never pay off, remaining only an interesting technical project. Savings can reach 40-50% of total operating costsThis makes the project highly liquid.
And it's also worth mentioning the engine life, because it burns cleaner, it doesn't flush the oil film off the cylinder walls (when you set it right), and it doesn't burn on candles, and it prolongs life. catalyst However, the lack of a lubricating effect of the gas can lead to accelerated wear of the valve seats if they are not made of heat-resistant alloys. For two-stroke engines where oil is mixed with fuel, the conversion to gas is technically extremely difficult and requires a complete replacement of the lubrication system.
Technical features of installation of LPG on the outboard motor
The process of converting an outboard motor is very different from the automotive one because of the spatial constraints and operating conditions. Yamaha, Tohatsu or HondaThe placement of a gas evaporator, electromagnetic valves and injectors requires engineering ingenuity. The gearbox must be installed above the level of possible water intake, but also have access to a coolant (antifreeze) to evaporate the gas, which in low-capacity air cooling becomes a problem.
Particular attention should be paid to the fuel supply system. The liquid phase of the gas is fed from the cylinder to the gearbox, where it evaporates. In the cold or in intensive gas extraction, the gearbox can freeze, which will lead to a drop in pressure and stop the engine. In automotive systems, this problem is solved by antifreeze from the engine cooling system, but on air-cooled outboard motors (up to 5-6 hp) there is simply no coolant. You have to use electric gearbox heaters, which increases the energy consumption from the generator or battery.
☑️ Testing of engine readiness for HBO
Engine control systems (EMS) also require adaptation, and modern four-stroke motors are controlled by complex algorithms. fuel-cardOr use a gas controller that adjusts the signals of regular gasoline injectors. In simple carburetor versions, the task is simplified to install a gas mixer (dispenser) instead of a carburetor diffuser, but this loses the accuracy of dosing and the ability to work on gasoline without manual reconfiguration.
The safety of the cylinder is a separate topic: the cylinder must be securely secured, protected from mechanical damage and, critically, have an emergency pressure relief system if heated. On the boat, space is limited, and the placement of the cylinder inside the cockpit is prohibited by safety regulations. Usually, the balloon is placed in a special smeared-out handle, which requires the laying of a high-strength copper or steel fuel line through the entire hull of the vessel.
Comparison of specifications: Gasoline vs Propane-Bhutan vs Methane
The choice of gas determines not only the economy but also the technical implementation of the project. Two main options are currently being considered for outboard motors: liquefied petroleum gas (propane-butane) and compressed natural gas (methane), each with its own physical properties that dictate operating conditions.
Propane-butane mixture (LPG) is stored in cylinders at relatively low pressure (about 16 atmospheres at 20°C), which allows the use of cylinders of cylindrical or toroidal shape. This fuel is heavier than air, so when leaked it is pushed down, creating an explosive concentration in the holds and lowlands of the boat. Methane (CNG) is lighter than air and escapes upwards when leaked, which is safer in confined spaces, but requires storage under pressure of 200 atmospheres, which involves the use of heavy and expensive composite or steel cylinders.
The effect of octane number on the operation of the motor
The high octane number of gas (105-110 units) allows highly compressed engines to operate without knocking. However, if the engine is originally designed for 92nd gasoline, the gas will not give a gain in power without changing the angle of ignition. Moreover, the gas burns slower than gasoline, so that effective combustion requires earlier ignition.
Temperature is also key: propane evaporates poorly at temperatures below -10...-15°C, making its use in winter navigation (if any) problematic without special heaters. Methane lacks this disadvantage, as it is always in a gaseous state, but its fueling infrastructure is much weaker, especially in remote coastal areas.
Below is a comparative table of the main fuel specifications to better understand the differences:
| Parameter | Gasoline AI-92 | Propane-Bhutan (LPG) | Methane (CNG) |
|---|---|---|---|
| Aggregate state | Fluid | Pressure liquid | High-pressure gas |
| Pressure in the tank. | Atmospheric | ~1.6 MPa | ~20 MPa |
| Octane number | 92-95 | 105-110 | 110-120 |
| Flamming temperature | ~ -40°C | ~ -100°C | -180°C (boiling) |
| Fuel weight (1 litre) | ~0.75 kg | ~0.54 kg | ~0.7 kg (calculated) |
When choosing a fuel type for your project, it is important to consider not only the price, but also the availability of refueling stations in your region, and if you walk on remote rivers in Siberia or the Far East, having a refueling infrastructure can be a decisive factor outweighing the economic benefits.
Safety and operational risks
Safety is an aspect that cannot be ignored for a second: the gas is odorless (the odorant is added artificially, but it may not be heard on the water due to wind and wave splashing), it is explosive at certain concentrations. The slightest leakiness of the connection in the fuel ramp or gearbox can lead to the accumulation of gas in the hold. Since propane is heavier than air, it will accumulate at the lowest points of the body, creating a “gas cushion”.
⚠️ Attention: It is strictly forbidden to place gas cylinders and equipment in unventilated knobs without direct outlet of the off-shore air. Gas accumulation in a closed volume in the presence of a spark (for example, from a starter or generator) is guaranteed to lead to an explosion that can tear the boat apart.
Vibration and impact loads are another enemy of gas equipment on a boat. Unlike a car, a boat is subjected to constant shocks to the wave, sharp rolls and trimmings. The cylinder anchorages, tubes and hoses must have a multiple margin of safety and be protected from rubbing. Use of automotive components "as is" without additional protection and damping is unacceptable. Any crack in the metal or rubbing of the hose on the water can be fatal.
Install a gas analyzer in the hold of the boat, and this inexpensive device can alert you to the buildup of gas long before the concentration becomes explosive.
The fire hazard of refueling is also increasing, you need to fill the boat with gas with extreme caution, following all the rules of static electricity and grounding. The sparks when you disconnect the fueling gun on a swing boat is a real threat. Also, in the event of a fire on the water, you do not have the opportunity to simply "drive" or abandon the car; you are surrounded by water, but trapped inside the burning object.
The legal aspect is also not to be discounted: In many countries, including the Russian Federation, the conversion of a vehicle (and a boat with an engine over a certain power is considered a vehicle) requires registration of changes in the design and obtaining certificates of conformity. Operation of a boat with an uncertified LPG can lead to problems during inspection, GIMS checks and, most importantly, in the event of an insured event, the insurance company has the right to refuse payment, citing violations of operating rules.
Effect of gas on engine life and technical fluids
The switch to gas changes the thermodynamic cycle of the engine. The gas burns at higher temperatures than gasoline, which leads to increased heat stress of the parts of the cylinder-piston group (CPG). If the engine cooling system does not cope with heat removal (for example, overgrown with shells or a defective thermostat), the risk of burnout of the piston or valve increases, especially for motors operating at their limit (full gas).
On the other hand, the gas does not condense into an oil crankcase, does not dilute oil or create soda. This allows you to change the oil less often, and the engine itself works smoother and quieter. The lack of sulfur compounds in the gas reduces the acidity of the oil, which positively affects the life of bearings and crankshaft liners. However, as mentioned earlier, the dryness of the gas can lead to the saddles of the valves in the cylinder head if they do not have a hardening surface (character for older engines, not adapted for gasoline).
The gas prolongs the life of the oil and reduces the gas formation, but increases the heat load on the CPG, requiring a proper cooling system.
Gas spark plugs live much longer, electrode ignition is minimal, spark breakdown is more stable due to the high resistance of the gas mixture, and this is especially true for two-stroke engines, where candles often throw oil, although converting 2T motors to gas is technically extremely difficult because of the need to supply oil separately.
For four-stroke motors, such as popular motors Mercury FourStroke or Suzuki DFBut it's important to understand that if the engine already wears the CPG, switching to gas can reveal hidden defects, because the gas is more demanding to compress. Low compression on the gas will lead to unstable operation and cotton in the intake manifold.
Practical advice on installation and maintenance
If you're determined to do a gas-powered boat, start with the components you want. Don't skimp on gearboxes and valves, take proven brands that have proven themselves in harsh environments. 4th generation HBOTs (e.g., GABTs) Lovato, BRC, Digitronic2nd generation mechanical systems ("dispensers") are too rough and will not provide stable operation on different modes.
Installation of the wiring should be done using marine fittings. All joints should be soldered and protected by thermal shrinkage with adhesive layer. Use corrugated hoses to protect the harnesses from moisture and ultraviolet. Gas tubes (copper or steel) run along the sides, eliminating their contact with sharp edges and moving parts. Be sure to install a mechanical gas cutter on the cylinder so that you can block the supply manually at any time.
⚠️ Attention: Never solder or weld near gas lines, even if the cylinder is turned off, the residual gas in the tubes can ignite, all work on the gas part is carried out only after the pressure is completely ejected and the system is aired.
The maintenance of such a system requires regularity. Before each season, you need to check the tightness of all connections with a soap solution or a gas analyzer. The filters of gas injectors and gearboxes need to be changed more often than on the car, since the quality of gas at different gas stations can vary greatly, and the condensation in the cylinder is a frequent occurrence. Also check the tension of the belts (if the gearbox is given from them, which is rare) or the operation of the electric fans of the cooling gearbox.
How to deal with condensation in a gas cylinder?
There is always a liquid (oily condensate) in the cylinder that needs to be drained, and on a boat this is done through a special valve (usually on a multivalve of a cylinder), which should be done outdoors, away from the sources of fire, and extremely carefully so as not to get burned by gas vapors.
The system must be set up on the go, using diagnostic equipment. The lambda probe will help to adjust the mixture in real time. Don't adjust the engine "by ear" or by the color of the candles - these are the methods of the past century that do not guarantee safety and efficiency. Modern gas injector allows you to accurately dose the fuel, but requires competent calibration of injection cards.
Conclusion: Is the game worth the effort?
The gas-powered boat is a project for enthusiasts, engineers and those who spend hundreds of hours on the water. For the average summerman who rides on weekends, sheepskin is not worth it: equipment costs, installation complexity and legal risks will offset the savings on fuel. However, for commercial fleets, rental bases and long-distance expeditions, it is a real way to reduce and increase autonomy.
The rule is safety over economy. If you're not confident, you'd better stay on gasoline or diesel. Gas doesn't forgive negligence. But if you do it smartly, with all the rules and with quality components, you'll have a quiet, clean and economical engine that will keep you busy for years to come.
Remember, technology is moving forward, and perhaps in a few years, we'll have factory-made gas boat solutions that don't have all the "childhood diseases" of home-made artisanal designs, but today you can pioneer this niche with caution and common sense.
☑️ Final check before the first launch
Frequently Asked Questions (FAQ)
Can a two-stroke outboard motor be converted to gas?
Technically, it's possible, but it's very difficult and impractical. The main problem is lubrication. 2T engines have oil mixed with gasoline. The gas doesn't contain oil, so you'll have to completely remodel the lubrication system into a separate oil pump (like 2T injection pumps) or add oil to the gas artificially, which is difficult to implement. In addition, two-stroke engines are less efficient and the savings will be minimal.
Will the engine power drop after the installation of the LPG?
Yes, the power drop is inevitable, and it's about 5 to 10 percent on aspirated engines without adjusting ignition, because of the lower cylinder filling ratio (gas takes up part of the volume) and the lower combustion rate. You can use ignition variators to compensate, but you can't completely recover the loss.
Do I need to register the installation of BOO on the boat in GIMS?
Yes, any change in the design of a small craft that affects safety (and gas equipment certainly does) requires changes to the boat's documents, you will need to undergo a technical inspection and obtain the appropriate certificates for installed equipment, otherwise there may be fines and problems with operation.
What is the range of a gas cylinder with a volume of 50 liters?
The range depends on the engine power and operating mode. On average, a 50-liter cylinder (fueled by 80%, i.e. ~40 liters) is equivalent to about 20-25 liters of gasoline. For a 9.9 hp engine in economical mode (about 4-5 hp load), this will last for 15-20 motor hours or about 80-100 km of travel. The exact calculation depends on the efficiency of the particular gearbox and settings.
Is gas dangerous in hot weather when the balloon is in the sun?
The gas cylinders are equipped with a safety valve that releases gas when pressure increases (usually above 25-30 atmospheres). If the cylinder is installed in a ventilated outlet runduct, the gas will escape into the atmosphere without creating an explosive concentration inside the boat, the main thing is the correct installation and the presence of an emergency overboard system.