By Thomas P.

The lower the fuel consumption is the happier the captain gets.

Saving fuel is an issue that «burns» all of us, especially when on our transom one or more high-horsepower outboard engines are mounted.

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It is very well known that the most economical performances in modern outboard engines are observed when they operate between 3000 and 4000 rpm corresponding to cruising speeds of about 20-35 knots.
At this range of rpm besides, we mostly operate our engine or engines since at this range we use to travel especially when we have to cover long distances.
So we will try to explain the ways to reduce the fuel consumption at low and medium rpm focusing of course on the factors that we can change.

We should always keep in mind that the following important parameters will help us to improve our fuel mileage:

Prop selection

One of the major factors that affect the overall performance of our boat-engine combination and its fuel consumption is our propeller itself.

By matching the right propeller is sure that we can improve our boat’s fuel economy. But we never forget that the propeller which will eventually choose should allow the engine to reach at least until the middle of the maximum rpm range recommended by the manufacturer.

  • Prefer a propeller of larger diameter

Propellers with larger diameter are generally getting better fuel economy at low and medium rpm than the relevant ones with smaller diameter, a fact due to their less slippage.
When we are switching from a smaller diameter propeller to a larger one we just have to reduce its pitch in order to retain the same engine’s rpm.

Indicatively it is mentioned, always relying on our test data, that a larger diameter propeller can easily achieve 30% lower fuel consumption than a smaller diameter one. A typical example is the much lower fuel consumptions of the Eco Enertia compared to the Tempest Plus, the Mirage Plus or the Revolution propeller.

  • Prefer a propeller with the longer possible pitch

Switching from a shorter pitch propeller to a longer pitch of the same brand it is almost sure that we will achieve greater fuel economy at the midrange rpm. Suffice off course our engine’s rpm at wot to be near at the middle of its maximum rpm range.
Let’s suppose for example that the maximum rpm range of our engine is between 5700-6300 rpm and the propeller we have matched is the Revolution 19" which allows our engine to reach 6300 rpm.

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In this case we can switch to a Revolution 21", especially if we want to cover a large distance, thereby increasing our cruising speed at the midrange rpm while reducing the fuel consumption. Of course our engine’s rpm will drop but they will still remain near the middle of its maximum rpm range.
The reduction of the fuel consumption at medium rpm is noticeable when we increase the pitch of our propeller by 2 inches.

But beware! We must not fall into the trap and increase the propeller pitch further due to the even greater fuel economy, sacrificing the boat’s performance at low rpm, as well as the proper operation of our engine, the rpm of which will drop very low.

  • Prefer a 3-blade propeller instead of a 4-blade one, in the same diameter

The 3-blade propellers provide usually better fuel consumption than the 4-blades of the same diameter; except of course when we are carrying heavy loads.

The reason is the additional drag generated by the extra blade of a 4-blade propeller.

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Engine mounting height

We all know that there is no ideal engine mounting height for each use as the height of the engine on the transom depends on many factors interacting together.

In general the engine is better to be mounted as high as possible, but on that point in which the boat performance at low rpm will not be suffered.
Raising our engine to the highest possible position, the immersion of the lower unit is reduced. In other words there is less gearcase in the water which means better fuel economy due to its reduced drag.

At this point it is important to emphasize the value of using the power lift, which enables us always to have the ideal engine height so as to achieve the maximum fuel economy in relation to the way we use our boat and the conditions we operate it in.

It is noteworthy that the proper engine mounting height in combination with the proper propeller can give us up to 50% lower fuel consumption!

Proper engine’s trim

The proper adjustment of the engine’s trim during our ride determines the boat’s attitude, the angle i.e. of our bow related to the surface water, which affects significantly, among other things, the fuel consumption.

Knowing in advance in which rpm range of the engine we have the greatest economy while underway, we only have to push the throttle lever until the engine reaches the desired rpm while adjusting the trim in the proper position.

But what is the proper position of the engine’s trim?

Certainly an experienced captain is able to determine the proper position of the trim just observing where the hull cuts the water looking the spray that exits from its either side, watching the wake of the stern and listening to the sound of the engine.
If we don’t belong to the above category then it would be wise to experiment several times with the engine trim while observing the indications of the tachometer, the Gps and the flow scan so as to achieve the best combination of speed and fuel consumption.

When the conditions and our loads permit, our engine shall be trimmed neither too far down nor too far up. We are trimming out the engine so as to bring the A / V plate parallel to the water surface and our propeller running alongside it. Usually the best trim position is a little bit higher than the point referred as above, where we achieve to reduce the exposure of the lower unit in the water while by raising the bow, at the same time, we try to keep the most possible part of the hull out of the water minimizing the amount of its wetted surface.

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All the referred above lead to the most efficient running angle, minimizing the drag and improving the fuel economy maximizing thus our mileage.

However, be careful! Don’t trim up too much!
If you have trimmed out your engine too much the propeller will tend to lose its bite and aerate leading to negative results in both the speed and the fuel consumption.

Weight and its balancing on board

The heavy load and the unbalanced weight distribution are the enemy of the center of gravity and the right attitude of a boat.
The weight and the weight distribution on board affect more than any other factors the fuel consumption. This is why we have to be particularly careful of the carrying loads and the placement points.

The extra load increases the wetted surfaces of the hull resulting to greater drag and fuel consumption. The heavier the load is, the more the fuel burn gets.

It is a fact that most of us usually take aboard much more load than we really need.

So remove everything you don’t need and load only what is necessary on each trip. No need for example to carry extra refrigerators or spare anchors with many meters of chain when we will not need them. No need to travel with fuel and water tanks full when not travelling long distances. This of course does not mean that we are risking our safety for the sake of lower fuel consumption.

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But even more important than the carrying loads is where they are located on board. By storing additional weights in the wrong places we change the boat’s center of gravity destroying the right relationship of center of gravity-engine mounting height thus virtually cancelling the operation of trim.

The small and all stepped boats are more sensitive to centre of gravity’s changes which affect greatly the attitude, downgrading thus the quality of the ride and increasing the fuel consumption especially in bad weather.

So, take care of the right boat’s attitude and save fuel by storing the heavier objects around the center of gravity and on the aft third of your deck, placing the less heavy towards the bow. Keep the bow light so as to be able «to listen» the trim settings and stay high and dry during riding.

The overload and the unbalancing weight on board kill the ride.

Maintaining a clean hull

We know very well that a clean and polished hull is more slippery, goes faster and maximizes the fuel economy.

Many times we have noticed sudden reduction of our engine’s rpm, decrease of the speed and increase of fuel burn due to the additional drag created by a fouled bottom.
It is very important therefore to maintain our hull’s gelcoat in excellent condition and free of fouling in order to get the maximum performance without wasting fuel unreasonably.

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Apart from the above mentioned factors there are many others which play an important role in fuel efficiency such as:

  • The way of driving

By handling the throttle smoothly, avoiding sudden accelerations and aggressive driving, maintaining steady course and constant speed is sure to improve fuel economy.

  • Maintaining our engine in excellent condition
  • Choosing to travel in good weather
  • Proper plan of our course and the choice of the departure time

Avoid traveling with bad weather conditions because the strong headwinds and rough seas create additional resistance while forcing us to constantly deviate from our course and fluctuate our speed. If it is possible choose to travel having the sea on the quarter achieving greater economy.

We will never get the optimum fuel economy if we don’t know our boat very well, if we don’t handle it with the right way or if it is not properly rigged.
Think about how many hundreds of miles we cover every season and calculate how much money we can save by implementing the above ways to reduce fuel consumption.

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...keep Ribbing!

 

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