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Operational measures

This section discusses fuel efficiency measures that can be taken without investment in new capital equipment.
It is important to note that this does not imply that the measures are cost-free - in every case there is some penalty to be paid for energy efficiency, either in terms of higher operational costs or longer periods at sea. The crucial issue is whether the penalty incurred is offset by savings in fuel. Unfortunately, it is impossible to generalize about the validity of energy efficiency measures - this will vary considerably from vessel to vessel and fishery to fishery. It is up to the vessel owners/operators to evaluate whether these measures are applicable in their particular situation.


Slowing down

Speed is the singular most important factor to influence fuel consumption. Its effect is so significant that, although they may be well known by many vessel operators, the underlying principles are worth repeating once again. As a vessel is pushed through the water by the propeller, a certain amount of energy is expended in making surface waves alongside and behind the boat. The effort expended in creating these waves is known as the wave-making resistance. As the vessel's speed increases, the amount of effort spent making waves increases very rapidly - disproportionately to the increase in speed. To double the speed of a vessel, it is necessary to burn much more than double the amount of fuel. At higher vessel speeds, not only is more fuel lost to counteract wave resistance, but also the engine itself may not be operating at its most efficient, particularly at engine speeds approaching the maximum number of revolutions per minute (RPM). These two effects combine to give a relatively poor fuel consumption rate at higher speeds and, conversely, significant fuel savings through speed reduction.

The choice of operating speed (particularly while in transit) is usually under direct control of the skipper. Fuel savings that can be made by slowing down require no additional direct costs. Vessel speed during fishing may be constrained by other parameters such as optimum trawling or trolling speeds and may not be so freely altered.

Saving fuel through speed reduction requires two principle conditions:

So what can be saved by slowing down? The actual savings made by slowing down are almost impossible to predict due to the many factors involved. As engine speed is reduced from the maximum RPM:

Engine performance

Diesel engines. The amount of fuel that a diesel engine consumes to make each horsepower changes slightly according to the engine speed. A normally aspirated diesel engine (one which does not have a turbocharger) tends to use more fuel per horsepower of output at lower engine speed, as illustrated in Figure 2. At a lower RPM the engine may actually be working less efficiently.

Figure 2: Typical fuel consumption curve for a normally aspirated diesel engine

A turbocharged diesel engine that is fitted with a small compressor to force more air into the engine has slightly different characteristics. This type of engine may work more efficiently at slightly lower speeds, but efficiency may drop rapidly as the speed is further decreased. The example graph in Figure 3 shows the engine working most efficiently at about 80 percent of the maximum RPM. Note that, in both of these figures, the scale of change in fuel efficiency is actually very small - in the order of a few percent for a 20 percent reduction in the engine's RPM.

The characteristics of the fuel consumption curve vary from engine to engine, especially among smaller-capacity motors, but as a rule of thumb:

Figure 3: Typical fuel consumption curve for a turbocharged diesel engine

Temperature. Diesel engines are also sensitive to fuel temperature changes. During a long voyage, the fuel in the tank of a trawler slowly heats up because of the temperature of the fuel entering the tank via the return. This results in a small loss of power, about 1 percent per 6°C (10°F) above 65°C (150°F). The effect is more noticeable on vessels operating in tropical climates.

Outboard motors. A conventional gasoline 2-stroke outboard motor may have some particularly unexpected fuel consumption characteristics. The amount of fuel used to generate each horsepower of output increases rapidly as the load is reduced (Aegisson and Endal, 1992). This is due to a breakdown in the flow of fuel mixture and exhaust gases in the engine, resulting in significantly less efficient combustion. It is important to note that as with the normally aspirated diesel engine, an outboard still burns less fuel per hour at lower speeds, but will do so inefficiently - the amount of power produced is disproportionately smaller than the savings in fuel. There is still some benefit from operating at reduced engine speeds, but this is less than might be expected.

Kerosene powered outboard motors are even less suited to fuel savings through a reduction in engine speed. As the throttle opening is reduced, the motor draws proportionately more petrol than kerosene, the cost of which will further diminish savings from reduced fuel consumption per hour. Although fuel can be saved by operating 2-stroke outboard motors at reduced throttle openings, it should be noted that:

This, however, leaves the vessel operator with a reduced power margin to use when speed is necessary for safety reasons (e.g. to avoid bad weather) or when the penalty price paid for increased fuel consumption is likely to be compensated by better market prices for the catch.

Hull resistance. As mentioned above, the resistance of the hull in the water increases rapidly as speed increases, principally due to the rapid build-up of wave-making resistance. The change in resistance of the hull is much more significant than the change in efficiency of the engine. Figure 4 shows how the typical power requirement of a small fishing vessel varies with speed. At faster speeds, note that:

The exact form of the power/speed diagram will vary from vessel to vessel, but Figure 4 presents a reasonable approximation of a general form for a vessel with an inboard diesel engine. An outboard powered vessel will require approximately 50 percent more power, primarily on account of the low efficiency of outboard motor propellers. It is important to realize that the fuel consumption of both a diesel engine and a petrol outboard motor is approximately proportional to the rated power output, and high horsepower requirement equates directly to high fuel consumption.

Combined effects. When considering the combined effects of speed reduction on the fuel consumption of a fishing vessel, it is very important to remember that the change in the engine's fuel consumption per hour is not of real interest. Almost all fishing operations require the vessel to travel from a port or landing site to a known fishing ground. Therefore, the important factor the quantity of fuel used to travel a fixed distance, or the fuel consumption per nautical mile (nm). The fuel consumption per nautical mile shows, not only how engine performance changes with speed, but also propeller and hull interactions that are not evident from per-hour fuel consumption data.

Figure 4: Power/speed diagram

For small changes in speed, an approximation of the change in fuel consumption per nautical mile can be made using the following equation:

Table 1

Fuel consumption of a 10 m trawler (free-running)

Speed (kt)

Reduction in speed

Reduction in fuel consumption in (litres/nm)










Source: Aegisson and Endal, 1992.

As a worked example, a vessel running at 9 knots (kt) uses 19 litres of fuel per hour. The fuel consumption per nautical mile is therefore:

If the vessel speed were reduced to 8.5 kt, the new fuel consumption is estimated using the equation above:

That is to say that a 6 percent reduction in speed (from 9 to 8.5 kt) results in a fuel savings of approximately 11 percent. The above method is only valid for a quick estimate, as it may conceal several propeller and hull interactions that affect fuel consumption. These are best revealed by performing simple measured trials with the fishing boat in question (see Annex 3, A guide to optimum speed). Trials with speed reduction of free-running trawlers (Aegisson and Endal, 1992; Hollin and Windh, 1984) show that fuel savings can be considerably larger than those indicated by the equation above.

Figures 5 and 6 show typical fuel consumption curves taken from trial data. Figure 5 also illustrates the very large difference in fuel economy between gasoline outboard motor power and inboard diesel power (this is discussed further in the section Engines). The data for the outboard motor propulsion indicate that a 1kt reduction in speed from 9 to 8 kt (11 percent) results in fuel savings of about 25 percent.

Figure 5: Comparative fuel consumption curves for a 13 m canoe

Figure 6: Fuel consumption curve for a 13.1 m purse seiner

The exact magnitude of the fuel savings is closely linked to the original speed of the vessel. The maximum speed of a displacement hull (measured in knots) is about 2.43 x Öwaterline length (measured in metres) after which it starts to plane and pass over, rather than through, the water. The nearer the vessel is to this maximum displacement speed, the larger the gain to be made from slowing down.

Towards an optimum speed. Saving fuel by reducing speed is all very well but, as stated in the introduction to this section, nothing is gained without penalty. In this case the cost to the vessel operator is time, and a difficult decision has to be made as to whether it is worth slowing down. A reduced speed could imply less time for fishing, less free time between fishing trips or even lower market prices owing to late arrival.

Considering only the resistance of a vessel in the water, maximum operating speeds can be recommended as follows:

These guidelines result in the maximum operating speeds recommended in Table 2.

Table 2 may serve as a first estimate in the selection of a reasonable operating speed, but this is not necessarily the optimum speed. The estimation of an optimum speed requires the vessel operator to strike a balance between savings made from slowing down and the costs incurred by spending either more time at sea or less time fishing. Clearly, if late arrival at the port or landing station means that the market will be closed and the catch unsellable, it is worth travelling as fast as possible to ensure a market. Similarly, if the market is always open and prices do not fluctuate, then it may well be worth saving fuel and returning home at a slower rate. The question is, how much slower?

Table 2

Recommended maximum operating speeds

Waterline length (m)

Maximum operating speed (kt)


Long thin vessels

Short fat vessels




























An important part of this decision is determined by an evaluation of the skipper's time. Such an evaluation will be, at best, a subjective judgement according to individual priorities. How much would a skipper gain by arriving an hour earlier and how much would be lost by arriving an hour later? These gains and losses may not always be quantifiable. For example, the crew will want to spend time with their families between fishing trips, yet this has no definite value and cannot be readily identified as a cost, should it be lost through late arrival.

It is very important to recognize that the individuals involved in the management and operation of a fishing vessel have different valuations of time. Decision-making is easier if the owner of the vessel is also the skipper. However, when the owner is not on board, a conflict of interests may arise, which does not encourage fuel savings.

For example, the skipper (who makes the decision on board to go slower or not) may be tired and want to return home as early as possible. The vessel's owner, on the other hand, may have already secured a market for the catch and be more interested in reducing operating costs (including fuel) rather than bringing the vessel back to port hastily. The crucial issue is how the person who makes the decision about vessel speed is involved in the cost sharing of the vessel. If the fuel costs are always paid from the owner's revenue, the crew of the vessel may not be motivated to go at a slower rate for the sake of fuel economy.

Based on Lundgren (1985), a quantitative method for estimating optimum speed is laid out in Annex 3. Although the determination of an optimum speed is dependent on the uncertain process of estimating the skipper's valuation of time, the method outlines relatively straightforward measures that can easily identify speeds at which the vessel should not travel, regardless of the human aspects of the decision.

Engine maintenance

Careful initial running-in and regular maintenance are extremely important for ensuring the reliability as well as the performance (including fuel consumption) of any engine. This applies equally to inboard and outboard marine engines. Every engine manufacturer recommends service intervals and these should be adhered to rigorously, especially for basic services such oil changes and filter and separator replacement.

Summary Table 1

Slowing down



3No incremental direct costs

8 Requires restraint to reduce speed

3Fuel savings can be very significant

8 Crew and owner may have different interests

3Very easy to put into effect

8 Less convenient


8 If speed is reduced through the installation of a smaller engine, safety margin may be reduced

The consequences of not adhering to running-in and maintenance guidelines may lead to an irrecoverable decline in the performance of an engine. This is best illustrated by an example: a study regarding energy efficiency in small-scale fisheries in India (Aegisson and Endal, 1992) tested two identical engines on the same canoe. One of the engines had been very poorly maintained, and it consumed twice as much fuel but achieved only 85 percent of the speed as the other.

Summary Table 2

Hull condition



3 Fuel savings can be significant

8 Vessel must be taken out of service to improve hull condition

3 Relatively easy to put into effect vessels

8 Requires dry-docking of larger (expensive)

3 Use of antifouling paint protects wooden-hulled vessels from marine borers

8 Paint and labour costs can be significant

The requirement for careful preventative maintenance is all the more acute in areas with low-quality fuel. This can lead to high carbon deposits, low engine temperatures and a significant loss of power. With diesel engines, the high sulphur content in low-quality fuel requires the early substitution of injectors. The first sign of the need for substituting injectors is increased fuel consumption (or a drop in power) and black exhaust smoke. The following list outlines the potential causes of heavy exhaust smoke in diesel engines (Gilbert, 1983):

Summary Table 3

Fishing operations



3 Fuel savings can be significant

8 May require considerable investment to increase vessel autonomy


8 Often very difficult to change operational routines in an established fishery


8 Both new operational routines and increased navigational awareness require training and knowledge


Frictional resistance, or skin friction, is the second most significant form of resistance following wave-making resistance. In simple terms it is a measure of the energy expended as the water passes over the wet surface of the hull. Like wave-making resistance, its effect is felt most on faster vessels or vessels that travel longer distances between the port and fishing grounds. It is possible to reduce frictional resistance by operating at slower speeds.

Unlike wave-making resistance, however, frictional resistance is partially controllable by the vessel operator because it depends on the smoothness of the underwater surface of the hull. The more attention paid to the surface finish of the vessel during construction and maintenance, the less energy will be wasted overcoming skin friction. This applies equally to fishing vessels of all sizes.

Constructing a vessel with a very smooth underwater surface, as well as the maintenance of such a surface, is not necessarily easy to achieve. Both of these require increased expenditure on labour costs, materials and (in the case of larger vessels) dock or slipway time.

There are some general pointers that can assist a vessel operator in deciding how much time and money is worth spending on achieving and maintaining a smooth finish. It is both difficult and expensive to improve a severely degraded hull finish - if the vessel was originally launched with a very rough hull it will require a lot of effort to improve this at a later date.

The actual benefit resulting from efforts to improve hull condition depends on the operational pattern. A slow-speed vessel, such as a trawler, operating very near to port does not benefit greatly from an improved hull condition. In one test (Billington, 1985), fouling was found to reduce the free-running speed of a trawler by just under 3 kt. At the same time, it had no noticeable effect on trawling speed or fuel consumption during fishing. In this case the vessel operated very close to its home port, and the significant expenditure made to keep the hull in smooth condition did not prove worthwhile.

Any vessel that travels significant distances to the fishing ground or is involved in a fishing method that requires steaming, such as trolling, should stand to benefit from maintenance of the hull condition.

The amount of effort spent on hull maintenance should be commensurate with:

All of these are dependent on the local conditions and the fishery. However, the nature of the flow of water around the hull makes the condition of the forward part of the hull and the propeller more important in reducing skin friction. As a guide (Towsin et al., 1981):

In United States naval trials (Woods Hole Oceanographic Institute, n.d.), the fouling that had accumulated over 7.5 months on the propeller, alone, was found to result in a 10 percent increase in fuel consumption in order to maintain a given speed.

The causes of increased skin friction can be placed in two categories:

Summary Table 4

Sail-assisted propulsion



3Fuel savings can be significant

8 To be most effective the vessel needs to be designed and constructed from the outset with sails in mind. It is often very difficult to retrofit sails to an existing motorized fishing vessel.

3Can improve vessel comfort

8 Requires crew to have knowledge of or be trained in the use of sails

3Improves vessel safety

8 Sails are an additional maintenance item 8 Sail can require substantial additional crew effort, and it is invariably easier to motor.


The loss of speed or the increase in fuel consumption owing to the growth of marine weed and small molluscs on the hull is a more significant problem for fishing vessel operators than hull roughness. The rate of weed and mollusc growth depends on:

Estimates indicate that fouling can contribute to an increase in fuel consumption of up to 7 percent after only one month, and 44 percent after six months (Swedish International Development Authority/FAO, 1986b), but can be reduced significantly through the use of antifouling paints. A Ghanaian canoe, for example, was found to halve its fuel consumption and increase its service speed by 30 percent after the removal of accumulated marine growth (Beare in FAO, 1989a).

A small fishing vessel that is either beach-landed or hauled out of the water frequently (between every fishing trip) is not likely to benefit from the use of antifouling paints. Under these conditions, the rate of weed and mollusc growth is low, as the hull surface is dry for extended periods. In addition, antifouling paint is by nature soft and not particularly resistant, so in the case of a beach landing craft, significant amounts of paint would be lost during launching and landing.

Antifouling paint releases a small amount of toxin into the water that inhibits the growth of weed and molluscs. There are several different types of antifouling products, ranging from cheaper, harder paints to more effective and more expensive hydrolysing or self-polishing paints. All types of antifouling paint have a limited effective life (typically about one year), after which they need to be replaced because they no longer have a toxic property and weeds start to grow quickly. Self-polishing antifouling paints become smoother over time and can offer reasonable protection from fouling for up to two years, but the paint system is expensive to apply and requires complete removal below the waterline of all previous paint. Self-polishing antifouling paints can result in fuel savings of up to 10 percent (Hollin and Windh, 1984), but are only likely to be viable for vessels that travel long distances to their fishing grounds and that are hauled out or dry-docked about once a year.

In small-scale fisheries, the use of antifouling paint is uncommon, but through its use can result in significant savings, or at least minimized losses. There are a few alternatives used in small-scale fisheries that present a cheap and often effective solution to the problem:

Paint mixed with weed killer. The underwater surfaces of a small vessel can be covered with paint that has been mixed with a small quantity of agricultural weed killer. No special paint is necessary and the weed killer is often cheap and readily available. The major disadvantage of this technique is that the release of the toxin is not controlled. During the first days of immersion, release is rapid but the effectiveness of the antifouling product reduces quickly thereafter. Any antifouling paint must be used with care - it is a toxin and may have negative effects on other marine growth, particularly edible molluscs and seaweeds, in the area where fishing vessels are anchored.

Shark liver oil and lime. In some fishing communities where antifouling paint is unavailable or expensive, an indigenous solution to the problem of fouling has been developed based on a thick paint made from shark liver oil and lime. Oil is extracted from the livers of sharks and rays by a process of cooking and partial decay. This pungent smelling liquid is then applied either directly to the interior wooden surfaces of the vessel (to protect against insects that eat wood or against caulking) or mixed with lime and then applied to the exterior underwater surfaces of the vessel. The mixture is reasonably effective in limiting marine growth, and discourages marine wood borers. The major advantage of the technique is that it is very cheap, often not requiring the purchase of any products. However, when applied to the underwater surfaces of a vessel, it remains soft and is not very durable, therefore requiring reapplication about once a month to remain effective. It should be noted that, in many tropical coastal communities, lime is made from the controlled burning of coral heads collected from nearby reefs. This activity is not only destructive to local habitat and fisheries but is also illegal in many countries.


The concept of deterioration of the condition of the hull with age is most applicable to steel vessels. Although wooden vessels, and even to a certain extent glass fibre vessels, experience an increase in hull roughness with age (primarily owing to physical damage and the build-up of deteriorated paint), the effect is more significant with steel which is also subject to corrosion.

Following are the principal causes of hull roughness.

On larger steel vessels the increase in power requirement to maintain speed can be approximated at about 1 percent per year, although the rate of increase in hull roughness usually slows with vessel age. Therefore, after ten years a steel vessel requires approximately 10 percent more power (and 10 percent more fuel) to maintain the same service speed as when it was launched.

This loss is, to a certain extent, inevitable but can be minimized by careful hull maintenance and, in the case of steel vessels, regular replacement of sacrificial anodes and anticorrosive paint.



The operational pattern of a fishing vessel has a direct influence on the fuel efficiency. Larger fishing vessels, with an autonomy of several days or more at sea, tend to limit the length of fishing trips to the time necessary to fill the available hold space. In smaller-scale fisheries the tendency is to restrict the length of a fishing trip to a single day, often owing to the lack of storage facilities on board or long established routines. In many such cases, effective fuel savings could be made by staying longer at the fishing grounds, particularly if a considerable part of the day is spent travelling to and from the fishery. For example, if trips could be made in two days instead of one, the catch over those two days would be made at the cost of the fuel for one return journey rather than two. This would effectively cut the cost of the fuel expended on travelling to and from the fishing grounds, per kilogram of fish caught, by up to 50 percent.

There are, however, often serious obstacles that make increasing individual vessel autonomy very difficult, especially the first step of extending fishing trips to more than one day's duration:

Fishing technology

Within a given fishery the type of fishing gear in use is often a predetermined choice, dictated by the target fish species, physical conditions (bottom type, currents), weather conditions and vessel type. The combination of these factors often means that only one gear type is applicable in that particular fishery.

However, in a trawl fishery, particularly a coastal smaller-scale fishery, it is occasionally possible to use pair trawlers rather than the classic single-vessel otter trawl. Pair trawling can result in a reduction in fleet fuel costs by 25 to 35 percent per tonne of fish (Aegisson and Endal, 1992) compared with otter trawling.


The use of satellite navigators and echo sounders is becoming more widespread in small-scale fisheries as the technology has become not only cheaper but also more portable (especially satellite navigators). Navigational aids of this type can contribute to fuel savings of up to 10 percent (Hollin and Windh, 1984), depending on the type of fishery and the difficulty in locating small, focused hot spots. Not only can the equipment assist the vessel skipper in easily relocating fishing grounds (thereby reducing fuel wastage), but it can also identify new grounds and contribute to increased navigational safety.

Both satellite navigators and echo sounders require a reasonable navigational ability and are most effectively used with maritime charts.


The use of sail as auxiliary propulsion can result in very large fuel savings (up to 80 percent with small vessels on longer journeys) but the applicability of sail is however by no means universal. Very specific circumstances are required for motor sailing to be a viable technology, in terms of weather conditions, the design of the fishing vessel as well as crew attitude and knowledge.

Sailing puts additional requirements on the vessel with respect to stability and deck layout, and sails are usually only a viable technology for use on vessels that have been specifically designed for sailing. Smaller fishing vessels may require the addition of further ballast or an external ballast keel to improve both stability and sailing performance across or towards the wind. On any fishing vessel, sails are an impediment to the workability of the vessel, and the mast and rigging occupy what could have otherwise been open deck space.

Sailing is a skill in itself and, to be effective, the crew must be both proficient and willing - there is often a considerable amount of hard work involved in the setting of sails, particularly on larger vessels. A simple fact of life is that it is invariably easier for the crew to forget about sailing and just motor.

However, sails can result in large fuel savings, depending on wind strength, wind direction relative to the course to or from the fishing grounds and the length of the journey. Typically, indicative values are in the order of 5 percent (for variable conditions) to 80 percent (for a small vessel on a long journey, with a constant wind at 90° to the course). These figures are, however, very dependent on the sailing ability of crew, the shape of the vessel's hull and the condition and design of the sail(s). There are several very different designs of sailing rigs, which have evolved in fisheries around the world. It is important that the design of a sailing rig for a fishing vessel be kept simple, safe and workable.

On smaller vessels, it is preferable to use a single sail rig that can be easily and efficiently reduced in area. As a secondary form of propulsion, sails contribute to a big increase in vessel safety, particularly if the vessel is capable of navigating under sail alone in case of engine failure.

Figure 7: Increase in power requirement owing to hull roughness

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