Outboards - 2 Stroke or 4 Stroke?

There are two types of outboards available, 2 stroke and 4 stroke. Each has its good and bad points, depending on what you want the engine to do. Team Giira has put together a handy guide to help you decide which is the best outboard motor for your boat..


2 Stroke Outboards:
2 stroke engines are lighter and faster so when speed and acceleration are required, this would be the engine of choice. They are also more affordable and have a higher resale value. As 2 stroke engines have been around for almost 80 years, parts are readily available, making repairs easy. Also, the design on these engines is less complex than a 4 stroke, meaning problems are less likely to arise.

There are some disadvantages to the 2 stroke engine. 2 stroke outboards are not as environmentally friendly as 4 strokes as they produce far more pollution. Their engines are lubricated by the oil that is mixed into the gasoline, meaning the exhaust is smoky and contains unburned oil, which in turn pollutes the water.

With new regulations, outboard manufacturers are looking to find new ways of producing the same power with less pollution. Manufacturers of 2 stroke outboards include Evinrude, Yamaha, Nissan and Mercury.


4 Stroke Outboards:
4 stroke engines are quieter, smoother and more economical. Heavier with slower acceleration, they are normally chosen for lakes and rivers. With less pollution and smoke they are also kinder to the environment. You are more likely to use a 4 stroke engine if you are interested in a smoother and quieter ride.

4 stroke motors have more complex engines which could lead to more repairs. When a repair is required it is usually more expensive and parts are limited. 4 stroke outboards are generally heavier than 2 stroke outboards, so if you own a smaller boat it may not be able to hold the weight of a 4 stroke engine.

Because 4 stroke engines are newer, they are continually being improved to make purchase and repair easier. Manufacturers of 4 stroke outboards include Johnson, Honda, Mercury, Nissan, Suzuki, Tohatsu and Yamaha.


Environmentally Friendly Outboards:
If you're looking for a new outboard, it's worthwhile considering one of the newer, environmentally friendly models. As well as producing lower emissions, there are a number of other advantages such as better fuel economy, quieter running and reduced operating costs.

The Mechanical Differences Between 2 and 4 Stroke Outboards:
The terms "2 stroke" and "4 stroke" refer to the number of times the piston moves during one firing cycle of the engine.


In the 4 stroke engine, the four movements, or strokes, are:
1. Intake stroke: With the intake valve open, the piston moves down allowing a mixture of air and fuel to enter the combustion chamber.
2. Compression stroke: The piston moves up the cylinder with the valves closed, so that the air-fuel mixture is compressed.
3. Combustion stroke: With the valves still closed, the fuel is ignited by the spark plug. The explosion pushes the piston down again.
4. Exhaust stroke: The exhaust valve opens and the piston moves up to push the exhaust gasses out of the chamber so the process can be repeated.


In a 2 stroke engine, there are only two movements of the piston for each combustion, one up and one down:
1. Combustion stroke: Combustion of fuel drives the piston downwards. As the piston moves down, it exposes an exhaust vent and fuel inlet in the wall of the cylinder allowing exhaust gasses out, and new air and fuel is sucked in.
2. Compression stroke: As the piston moves back up, it blocks of the exhaust vent and fuel inlet and thus compresses the air/fuel mixture. The spark plug ignites the air and fuel causing the piston to move down again.

The fuel inlet and the exhaust vent in the wall of the cylinder eliminate the need for valves and the mechanism for opening and closing them which is why 2 stroke engines are much simpler in design.

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Anatomy of a Sail Boat - Part VIII : Outboard Motor (OBM)

To a thouroughbred sailor, the words 'motor' and 'sailing' in the same breath would be a big 'No-No'...but the Outboard Motor is nowadays a much required part of a sailboat nowadays, with most modern designs having one and many older ones accomodating one.


Why only sailboats, outboard motors are a much preferred option compared to stern-driven and shaft-driven engines on many smaller yachts and boats due to their ease in maintenance and flexibility in configuration.

Most of the world's best automobile and bike manufacturers lead the way in marine motors too, except for a few specialist like Mercury, Evinrude, Force etc.

When it comes to sailing, an outboard motor has become a must nowadays due to reasons such as safety and convenience. Also, due to pleasure being an important factor in sailing nowadays, at times sailors prefer to be motored back to shore if the winds are dead or conditions are not favoring them.

OUTBOARD MOTORS

Designed to be mounted at the stern (rear) of the craft, outboard motors for a boat are developed as a self-contained unit with engine and all related drive-train assembly, and a propeller. They are the most common motorized method of propelling small watercraft.

As well as providing propulsion, outboards provide steering control, as they are designed to pivot over their mountings and thus control the orientation of the propeller. The transmission leg in the water also acts as a rudder even when the propeller is not providing power.


When boats are out of service or being drawn through shallow waters, outboard motors can also be tipped forward over their mounts to elevate the propeller and transmission shaft out of the water to avoid accumulation of seaweed or hitting underwater hazards such as rocks.

HISTORY

One of the first practical outboard motors were designed and developed by Norwegian-American inventor Ole Evinrude in 1909.

Earlier, most of the outboard motors (OBMs) have used 2-stroke cycles with carburettor engines due to their simplicity, reliability, low-cost, high power-to-weight ratios (weight is obviously an important element to be considered at sea). The lightweight also helped as too much weight at the stern of the boats tends to impede their handling.

But due to the high specific fuel consumption and high emissions associated with traditional carburated two-stroke engines has led to the development of more sophisticated engines.

MODERN OBMs

Lately, manufacturers have marketed four-stroke cycle engines, at times with computer controls, fuel injection, variable camshaft timing, multiple valves per cylinder, and supercharging. Many others have made variations and modifications focused on developing computer controlled direct-injected two stroke engines. Both these designs can reduce fuel consumption and emissions markedly, especially at low engine speeds, albeit at the expense of greater cost and complexity.


Marine engines also benefit from the ability to draw an unlimited supply of cooling water from the environment; this eliminates the need for radiators and cooling fans, lowering complexity and weight.

A CONVENIENT OPTION

Most small OBMs are truly self-contained, with integral fuel tanks and controls mounted on the body of the motor and steered by a "tiller" directly connected to the motor. Such small motors are light and provide sufficient power to move a small dinghy at around 15 km/h. They are highly portable, able to be removed by simply loosening their mounting clamps on brackets.

At the same time, manufacturers have produced large outboard designs, with sufficient power to be used on boats as long as 9 metres or more. Manufacturers have also produced electric outboards. These are used for specialised applications, notably trolling for bass in the US, where their quietness and zero emissions outweigh the range deficiencies shared with electric cars. Diesel outboards are also available, but their weight and cost make them impractical for most purposes.

A few outboard motors have been produced with pump-jet propulsion replacing the conventional propeller. Like other types of pump-jet propulsion systems, they offer virtually no cavitation, giving the boat more maneuverability and the ability to operate in very shallow water. However, the low efficiency compared to propellers has seen them restricted to specialized applications.

OBM USE IN INDIA

In India, most of the manufacturers such as Yamaha, Suzuki, Evinrude have opened their shop since some time and have a good and reliable service network. At the current market maturity, OBMs are the preferred option due to their simplicity and costs. With the spurt in demand, most of the manufacturers are getting-in their international teams to train their Indian tie-ups with guidelines on optimum usage and maintenance.

At the same time, which OBM to go in for is always a debatable propostion. Obviously, purpose should be the key criteria for making a choice, but at times water conditions such as salinity and muddiness.

At GIIRA Yachts, our panel of advisors are able to provide the best option for your use, so do mail-in incase you have any technical queries or want to decide on the best OBM option for your purpose.
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