Fuel saving on a boat

Author: David Medialdea

Factors influencing fuel consumption on boats

Skipper habits
Engine Set
Transmission and propeller

Fuel economization starts from the beginning of the project of building a boat. Finding a good compromise of use will be determined by the choice of different options offered by the market, from the type of hull, engine-inverter type and type of propeller. Gasoline engines typically consume between 15%-20% more than a diesel engine, and the average price difference per liter is usually around 10% higher (gasoline). This data makes us discard the gasoline engine and focus on the diesel engine.

Skipper habits

An experienced skipper will have responsibility over consumption. A boat on water can be moved with the strength of one hand, but if we want to increase speed, the force required multiplies exponentially. The cause is the drag that occurs when the bow has to “open” the water, causing a bow wave proportional to the length and boat speed, finishing another stern wave generated by going forward. Therefore, wrong speed can cause a waste of fuel. The skipper must also have knowledge about the characteristics of the engine to know which system is more efficient for that specific engine, or what is equal to the time when the engine will deliver the maximum torque. Load and weight distribution also influences consumption if we estimate an “X” amount of consumption in liters of fuel, always take an extra rational amount for safety.
Control of water tanks also helps to reduce consumption. An overloaded vessel will have an unnecessarily high consumption. A constant speed, without sudden changes, will be key to maintaining a moderate consumption. The latest generation engines provide relevant data to make piloting more efficient, referring to instantaneous fuel consumption, doubling the speed means increasing consumption to more than double.


Engine set

A suitable engine for a boat is definitely the best argument to the economization of fuel consumption. If an engine is too small for the boat, it will force us to always take it to limits such as 100% of its power, and an excessively large engine is also inefficient. Picking a suitable engine depends on a few factors to consider such as the type of use that we will give it, and also importantly the power to weight ratio when it comes to a boat that will be used for sporting purposes. Without doubt, when the time to equip out boat with an engine reaches, factors that will influence the choice will be those such as: space, price, weight…but once these decisive factors have been picked, the most important will be the engines power.

When an engine for a boat is calculated, it is usually based on a standard boat with standard conditions. But in reality, since the hulk is not always at its optimum clean status, the weight of the boat changes, and we may also vary the weight by adding accessories such as additional fuel tanks, water tanks, endless meters of anchor chain or extra batteries. Therefore, from the moment the ship leaves the shipyard until we finally adapt it to our type of navigation, it undergoes a few changes, and usually this will always come with an increase in weight, which should lead to us thinking in choosing a more powerful engine than the strict standard. This option may be given to us in the shipyard or when we decide to refurbish. A more powerful engine will also be of good help for when the weather conditions are adverse, since with extra power we will have a better chance to get around and this will also give us a greater peace of mind.



What is the saving that is provided from a slightly oversized engine?
It is easy to understand that an engine operating at a high engine speed is consuming the maximum and an engine operating at a lower rpm will have a more sustained use. We can confirm this statement by comparing power curves. Most repowered engines usually increase engine power source from 25%-50% higher.

Transmission and propeller

A bad combination of the inverter and the propeller will also be a reason for an increased consumption. It is inevitable to make the analogy with a car, in a car we have a gearbox that allows us to change speed according to the conditions that we are presented with; weather it’s a slope, downhill or a change in weight load. What we are doing is changing the car’s gear ratio. In the case of the boat, the transmission is always fixed in 2:1, some rising for job applications reaching 6:1. On a boat we have a constant, we are always navigating on a slope, not considering currents and winds, therefore it is important to make a good estimate of the propeller and it is essential to have the most accurate data possible. To calculate the propeller we will always try to find the point where they match the maximum speed of the boat with the maximum torque of the engine, which is the time where the engine will be most efficient. A heavily loaded or too lightly loaded boat blade is wasting fuel. There are variable pitch propellers in the market, but we won’t go into further details.

hélice de barco de alto paso de 4 palas SRX


Finally, maintenance is vital if we are going to have a sustained consumption. A dirty hull will increase the friction against water, a dirty or damaged propeller prevents effective operation, forcing the skipper to take the throttle further than needed. Engine maintenance also influences consumption, diesel that has been in the tank for too long and has lost properties, a clogged air filter, injectors that aren’t spraying correctly, an incorrect oil level will also consume in excess, on top of the fact that you could cause serious damage to the propellant. Anyways, it is also necessary to maintain or disconnect accessories that our engine may have if they aren’t being used, like for example extra generators, bilge pumps, or any other accessory that isn’t essential for navigation.  


In conclusion, we could say that if a skipper is responsible and committed, if we have a slightly higher engine power than the standard, a propeller that is calculated to maximize engine performance and a precise maintenance, all of these are key for sustained consumption and also environmentally friendly.


Perhaps you’re interested on:

– Repowering a boat, step by step
– How to select the most suitable revolutions for our engine


This post is also available in: Spanish

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