single or twin boat engine? 1 or 2 Outboards? What is better twin outboards or single?

There are many fiberglass boats from 23 to 26 feet and rigid inflatable boats (RIBs) from 28 to 32 feet, which can be equipped with one or two outboard motors. And when you are going to buy one of these boats, a question arises. Which is better? Is the boat with one outboard motor or with two?

Each option has advantages and disadvantages, so that the choice will depend on the buyer and the boat's intended use.

In this video, I will analyze the reliability and safety in navigation, stress, maneuverability, acceleration, weight, cost, maintenance, fuel consumption, maximum speed, and boat trim.

To decide which of the two options is better, you have to look at the pros and cons offered by each configuration.

So let's get down to it.
When deciding between one and two motors, it's important to remember that if you opt for a twin outboard motor configuration, it's not recommended to install two motors with half of the power of the single outboard motor. Opting for a configuration with a minimum of 25% or 30% more power is advisable to obtain similar performance. If the boat with a single engine configuration has 300hp, installing two 200hp engines instead of two 150hp engines is advisable.

Reliability and Safety in Navigation

The reliability of outboard motors has greatly improved. But it's clear that if you have two and one breaks down; there will always be another one to get you back to port.

One of the most recurrent reasons for outboard motor failures comes from problems in the fuel supply system. These failures are caused, in most cases, by the clogging filters and pipes. Another cause of breakdowns is the appearance of pores in hoses that allow air to enter the fuel circuit. Such problems will render both outboard motors unusable if they have the same fuel tank and fuel supply lines. Therefore, it is highly recommended to have separate fuel tanks and separate fuel supply lines for each outboard.

Less Stress

Twin motors installed on a boat are subjected to less stress because each one only has to supply half of the thrust needed to move the boat.

At high speeds, a single-engine boat may be operating at its maximum. At the same speed, twin-engine boats will run more smoothly and comfortably. The boat can travel longer distances at a higher maximum speed without overloading the engines.

Maneuverability and Boat Handling

The main advantage of twin outboards is maneuverability at low speeds.
 
Having two engines means having two thrust vectors that allow you to turn the boat by using the throttle controls, being able to make tight turns or spinning on its axis without having to make the frequent forward and reverse changes that you have to make with a single engine. The maneuverability provided by twin engines is proportional to the separation between them; the further apart they are the better maneuverability they will provide to the boat. Nowadays there are outboard motors that can be driven by joystick that allow fantastic boat control.

On boats with a single outboard motor, the "paddlewheel effect" influences the maneuvering. When the boat is stationary, and the propeller is started or sailing at low speed and accelerating rapidly, the propeller will move the stern of the boat in its direction of rotation. The propellers rotate in opposite directions on boats with two outboard motors, compensating the "paddlewheel effect."

Hole Shot

Acceleration depends on several factors, including the hull of the boat. But generally speaking, at equal power, a boat with twin outboards should accelerate faster than a single-engine configuration. This is because there are two propellers, so more blade area is available. On the other hand, there is more frictional resistance, which is not as important during acceleration.

Weight

The single-engine configuration is lighter than the twin-engine configuration.
For example, we compare 300-hp single-engine and 400-hp twin-engine configurations. We can see, for example, that a Yamaha F300 weights 562 lbs (254 kg), while each Yamaha F200 (i4) engine weights 516 lbs (234kg), making a total of 1032lbs (468kg). So the weight difference is 470 lbs (214kg).

In real life, many twin installations offer power figures that far exceed those of a single outboard. The weight difference between a twin-outboard of 150 hp and a twin-outboard of 200 hp is minimal. However, the additional 100 horsepower provides a much better top speed than a single 300.

Cost

Two motors are more expensive than one. The prices vary greatly depending on the brand, model, and power. A 300hp engine costs around 31000$, and a 150hp engine around 20000$, which multiplied by two would be 40000$, 9000$ more. But it would be advisable to install a pair of 200hp engines, with a price of around 24000$. The total cost would be 48000$, 17000$ more. In addition, we must add the cost of all systems and accessories of the outboard motor that are doubled.

Maintenance

In this section, the boats with one outboard motor win, as it is logical to have two engines the maintenance is doubled, changing oil, filters, etc. It must be done on each outboard, thus doubling the maintenance cost.

Fuel Consumption

Generally, the single-outboard configuration will consume less fuel than the twin-engine configuration. As a guideline, at about 25 knots, the consumption of twin engines will be about 10% to 15% higher. As speed decreases, this difference in consumption also decreases.

Top speed

Theoretically, a single-outboard boat should reach higher top speeds than a twin-outboard configuration at equal power. Everything previously mentioned about engine weight, hydrodynamic resistance, and transmission mechanical losses should be considered.

However, in reality, the twin-engine setup is slightly more powerful, as instead of installing a 300-hp engine, it is normal to install two 175 or 200hp outboards instead of two 150-hp ones. That extra power results in a higher top speed.

Boat trim

If the motors have power trim, we can individually adjust each motor's trim angle and correct heeling brought on by the wind or uneven weight distribution by slightly lifting the motor of the submerged side. However, using trim tabs in these situations will always be better.

What is better twin outboards or single?

With this information, you have to assess which option best suits your type of sailing. However, I believe the configuration with two motors is the best if you can afford the higher cost involved.