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Technical measures

This section deals with fuel efficiency measures that require investment in new equipment or the modification of existing equipment. Many of the technical ideas outlined are best considered when a vessel owner is either contemplating the construction of a new vessel or overhauling an existing vessel. Wherever possible, some indication is given of the cost of technical alternatives along with the fuel savings that could be expected through their application. Very little attempt has been made to enter into detail regarding the financial aspects of the costs and savings. This is principally owing to the extreme variation in costs in the geographical areas where this guide is applicable.


The propeller is the most significant single technical item on a fishing vessel. Its design and specification has a direct influence on fuel efficiency. Poor propeller design is the most frequent single contributor to fuel inefficiency. In this section some of the basic concepts of propeller design and installation are presented and a very quick and easy method for checking, approximately, the appropriateness of an installed propeller is discussed in Annex 4. It is important to appreciate throughout this section that propeller design is not straightforward, particularly in the case of trawlers, where technical specification must be entrusted to a qualified and experienced professional. Such assistance may be available through either local representatives of propeller and engine manufacturers or, in some cases, the technical services of government fisheries extension programmes.

What does the propeller do? This may appear to be a rather obvious question - a propeller turns the power delivered by the engine into thrust to drive the vessel through the water. In propeller design, it is important to ensure that it drives the vessel efficiently.

Photo 1: The start of erosion resulting from cavitation near the leading edge of the forward face of the blade - J. WILSON

Factors affecting propeller efficiency

Diameter. The diameter of a propeller is the most important single factor in determining propeller efficiency. A propeller works by pushing water out astern of the vessel, with the result that the vessel moves forward. In terms of efficiency, it is better to push out astern a large amount of water relatively slowly, than push out a small amount of water very quickly in order to achieve the same forward thrust. Hence the diameter of the propeller should always be as large as can be fitted to the vessel (allowing for adequate clearances between the blades and the hull) so that as much water as possible passes through the propeller.

A well-documented case study (Berg, 1982) of the retrofitting of a larger-diameter propeller to an existing fishing vessel demonstrated a 30 percent reduction in fuel consumption at cruising speed, and a 27 percent increase in bollard pull (maximum towing force). In this case, the propeller and gearbox were replaced and a propeller of 50 percent larger diameter installed - this operation was only possible because the vessel had originally been constructed with a very large aperture (the space that accommodates the propeller).

Shaft speed (RPM). The larger the diameter of the propeller, the slower the shaft speed RPM that is required to absorb the same power. Therefore, for an efficient propeller, not only should the diameter be as large as possible but, as a result, the shaft speed needs to be slow. This usually necessitates the use of a reduction gearbox between the engine and the propeller shaft. However, it must be remembered that a large propeller and high reduction gearbox is invariably more expensive than a smaller propeller and simpler gearbox.

Cavitation. Cavitation is a problem resulting from a poorly designed propeller, and although it does not directly affect fuel efficiency, it does indicate that the selection of the installed propeller was not correct and, in the long run, the effects of cavitation will lead to increased fuel consumption.

Cavitation occurs when the pressure on the forward face of the propeller blade becomes so low that vapour bubbles form and the water boils. As the vapour bubbles pass over the blade face away from the lowest pressure areas, they collapse and condense back into water. Typically bubbles form near to the leading edge of the forward face of the propeller blade, and collapse near to the trailing edge with the effect often being more acute near the blade tips. The collapsing of the vapour bubbles might appear trivial, but is in reality a very violent event, resulting in erosion and pitting of the surface of the propeller blade, and even cracking of the blade material. Strangely enough, cavitation is often associated with low fuel consumption, as the propeller is unable to absorb the power of the engine, and the engine runs underloaded.

The only solution to cavitation is a change of propeller. One with more blades, a higher blade area ratio or a larger diameter should be considered.

Number of blades. In general, at a given shaft speed (RPM), the fewer blades a propeller has, the better. However the trade-off is that, with fewer blades, each one carries more load. This can lead to a lot of vibration (particularly with a two-bladed propeller) and contribute to cavitation. When the diameter of the propeller is limited by the size of the aperture, it may often be better to keep shaft speed low and absorb the power through the use of more blades.

Blade area. A propeller with narrow blades (of low blade area ratio, see Figure 8) is more efficient than one with broad blades. However, propellers with low blade area ratios are more prone to cavitation as the thrust that the propeller is delivering is distributed over a smaller blade surface area. Cavitation considerations invariably require that the chosen blade area ratio is higher than the most efficient value.

Figure 8: Blade area ratios

Blade section. The thickness of a propeller blade has little effect on efficiency, within the norms required to maintain sufficient blade strength. However, like the blade area ratio, the section thickness can affect cavitation - thicker propellers induce larger suction and are more prone to cavitation.

Boss. The size of the propeller boss directly affects propeller efficiency. This is particularly significant when considering the installation of a controllable pitch propeller, which has a significantly larger boss than a fixed pitch equivalent. Typically, the drop in propeller efficiency owing to the larger boss size of a controllable pitch propeller is about 2 percent.

A loss in efficiency of about the same magnitude is associated with the large bosses of many outboard motor propellers, through which the exhaust gases are discharged.

Figure 9: Blade rake

Rake. The rake of a propeller blade has no direct effect on propeller efficiency, but the interaction effects between propeller and hull are significant. Often the shape of the aperture in the hull is such that the more the propeller blade is raked aft, the larger the propeller diameter that can be fitted, and rake becomes very beneficial. More rake, however, requires a stronger, heavier propeller, which is more expensive to manufacture.

Clearances and the propeller aperture. The distances between the propeller and the hull affect how efficiently the propeller operates within the flow of water around the hull, and the amount of vibration caused by the propeller. Table 3 shows recommended clearances.

Table 3

Clearances, three-bladed propeller


(% of propeller diameter)

1 Minimum clearance between tip and hull1


2 Minimum clearance between tip and keel


3 Minimum distance from deadwood to propeller1 at 35% of propeller diameter


4 Maximum distance from propeller to rudder at 35% of propeller diameter


5 Maximum bare shaft length

4 x shaft diameter

1 These clearances are closely associated with the number of blades and can be estimated by ¬ = 0.23 - (0.02 x n), and ® = 0.33 - (0.02 x n) where n = the number of blades on the propeller.

In general, the larger the clearances the better. However, if the aperture size is limited, larger clearances also imply a smaller propeller diameter, which is very detrimental to efficiency. During the design stage, the inclusion of large clearances have the effect of raising the counter and may force more obtuse waterlines just forward of the propeller. Both of these increase the resistance of the hull in the water. A small aperture requires the installation of a small-diameter propeller, which may not be able to absorb all of the engine's power efficiently, thus resulting in inefficient performance, engine damage or poor towing capacity. An intermediate solution to a small aperture can be found, for example by:

In general:

In the design and installation of trawler propellers, the tip-to-hull clearance can be as little as 8 to 10 percent of propeller diameter. The penalty of increased vibration being compensated for by the higher thrust and efficiency of a larger diameter propeller.

The absolute minimum tip-to-hull clearance should never be less than 50 mm on any vessel.

Photo 2: Filling the propeller aperture with fashion pieces, particularly forward of the propeller, reduces efficiency and increases vibration - J. WILSON

Blade condition. Poor condition of propeller blades owing to damage, fouling, corrosion or erosion reduces propeller efficiency. The extent to which blade surface condition influences efficiency depends on speed and propeller loading - highly loaded propellers are more sensitive to surface condition.

Roughness and damage. The efficiency of a propeller is most influenced by surface roughness and damage towards the outer regions of the blade, particularly on the leading edge of the forward (low-pressure) face, where roughness provokes early cavitation. Cavitation then results in the erosion of the blade material and more severe blade roughening. On larger propellers, roughness can account for an increase in fuel consumption of up to 4 percent after 12 months of service.

Damage to the trailing edges of the blades, in particular bending, affects the lifting characteristics of the blade section and results in either under or overloading at the designed shaft speed. This will have a serious effect on both fuel efficiency and, in the case of diesel power, engine condition. Outboard powered vessels operating in shallow waters or beach landing are particularly susceptible to fuel inefficiency owing to damaged propellers.

Fouling. The effects of weed and mollusc growth on propeller efficiency is much more important than roughness. The extent depends on whether the weed remains attached to the propeller when it is in service - if cavitation is present, fouling is usually removed from the critical outer areas. United States naval trials found that weed growth on the propeller alone accounted for an increase in fuel consumption of 10 percent after 7.5 months.

Photo 3: Too little clearance between deadwood and propeller - J. WILSON

The maintenance and cleaning of propeller blades can provide significant benefits from a relatively small amount of effort. The surface area of the propeller is very small relative to the hull, and proportionately greater savings can be made (or rather losses can be avoided) per person-hour of effort through proper maintenance of propeller blades.

Larger propellers require periodic surface recondi-tioning and polishing, particularly if either cavitation, corrosion or damage has been significant. This must be carefully carried out by skilled personnel to avoid further damage.

Devices. Peripheral devices such as fins, ducts and nozzles may have beneficial effects on propeller efficiency, but their value very much depends on the inefficiency of the current propeller and its unsuitability to its working application. It should be noted that fins, ducts and nozzles require special design, are potentially expensive to install and can be prone to damage. Their application is specific (the case of the nozzle is further discussed on p. 20.)

Propeller design - have you got the correct propeller?

The first step in assessing whether an installed propeller is suited to the vessel and engine is observation. Does the vessel perform as well as others of similar power and design? If the answer is no, it is important not to jump to the conclusion that the propeller is incorrectly specified. Other factors must also be considered, such as the condition of the underwater surfaces of the hull. When was the vessel last cleaned and painted? What is the condition of the propeller - is it clean, undamaged and smooth? What is the power of the engine and what condition is it in - should it deliver the same amount of power?

The propeller may be incorrectly specified if:

Therefore, a preliminary check is advisable before consulting a propeller designer or naval architect for further assistance. A simple method for making a first estimate of what the basic parameters of a propeller should be is outlined in Annex 4. It should be noted that this method is an abridged version of a more detailed method and is not intended as a design tool.

Engine overloading. Overloading of the engine through the installation of a propeller with too much pitch is the most common source of fuel inefficiency. Overloading can also result from the use of a propeller with too large a diameter, but this is less common. With inboard diesel engines, a sure sign of an overloaded engine is a lot of black smoke in the exhaust before reaching the designed RPM. Overloading can result in burnt valves, a cracked cylinder head, broken piston rings and a short engine life. It is important to remember that, with a diesel engine, it is the load and not the revs that determines fuel consumption. Therefore, continuous overloaded operation results in an unnecessarily high fuel consumption and increased maintenance costs.

Engine underloading. Engine underloading from the installation of a propeller with too small a diameter or of insufficient pitch affects vessel performance. It can also result in engine damage if it is allowed to rev above its specified maximum RPM. Engine underloading is likely to be accompanied by a low fuel consumption and, often, cavitation.

If the preliminary check indicates that a change should be made to the propeller, it is worth remembering that some small changes to the pitch can be made without the expense of buying a new propeller. The repitching of a propeller is a specialized task, however, and the propeller will need to be sent to a manufacturer for reshaping.

Outboard motors. The choice of propellers for outboard motors is generally more restricted and, correspondingly, there is less scope for errors! In many cases an outboard motor may only be offered for sale with one particular propeller, especially in areas such as in fishing communities in developing countries where the engines have only one application. However, it may on occasion be necessary to order a new propeller, should the original one be damaged, and it is worth checking to see if it is suited to the vessel. The important question is similar to that for inboard engines - does the engine reach its designed RPM under full load? If it does not, then a lower-pitched propeller should be considered, and if the engine has a tendency to over-rev then a higher pitched propeller should be considered.

The required pitch can be estimated from Figure 18 in Annex 4, following the same principles as those that apply to an inboard installation. If the estimate indicates that the pitch of the installed propeller is correct, a propeller with a different diameter (but the same pitch) should be tried.

Trawlers. The design of trawler propellers requires special attention, as the propeller has to perform under two completely different operating conditions - towing and "free running".

With a fixed-pitch propeller it is impossible for the propeller to be operating at optimum design conditions while both free running and towing. The propeller designer must strike a compromise based on the time the vessel spends operating in the two situations. For vessels working a great distance from their home port, the benefits to be gained from designing a propeller with increased towing power (and therefore catching capacity in the case of a trawler) may well be outweighed by the increased cost of fuel for the transit journey, and the design will err towards a higher-pitched propeller. A day boat operating relatively close to its home port would inevitably have a propeller optimized for towing.

Photo 4 : Very little clearance between hull and blade tip - J. WILSON

The installation of a controllable-pitch propeller can enable the propeller to operate efficiently while both towing and free running, but its operation requires both skill and knowledge. In general, the use of controllable-pitch propellers is not recommended in fisheries where the correct setting of the pitch cannot be guaranteed, since the setting of an incorrect pitch can easily result in significantly increased fuel consumption.

However, if a controllable-pitch propeller is well designed and correctly operated, it can result in fuel savings of up to 15 percent compared with a fixed-pitch propeller operating in a nozzle.

Photos 5 and 6: A poor installation - note damage to blade tips, very fouled hull surface and poor use of the space in the propeller aperture - J. WILSON

Nozzle. A nozzle is a short duct enclosing the propeller. Under certain circumstances, it can significantly improve the efficiency of a propulsion system. The duct is close fitting to the propeller, slightly tapered with an aerofoil cross-section.

A nozzle works to improve the efficiency of the propulsion system in two distinct ways:

  • First, the duct helps to improve the efficiency of the propeller itself. As the propeller blades turn in the water, they generate high-pressure areas behind each blade and low-pressure areas in front, and it is this pressure differential that provides the force to drive the vessel through the water. However, losses occur at the tip of each blade as water escapes from the high-pressure side of the blade to the low-pressure side, resulting in little benefit in terms of pushing the vessel forward. The presence of the close-fitting duct around the propeller reduces these losses by restricting water flow at the propeller tips.
  • Photo 7: Propeller nozzle - KORT PROPULSION CO. LTD

    Figure 10: Clearances - J. WILSON

    When to use a nozzle. The installation of a propeller nozzle can result in significant fuel savings or increased towing power, but not in all situations.

    Figure 11: Propeller in nozze

    As indicated above, a nozzle has the most significant effect at slow vessel speeds and therefore is more applicable to trawlers and draggers rather than other types of fishing vessels. Even with trawlers and draggers, the beneficial effects of nozzle installation are only felt while actually fishing - it is likely that free running speed will be reduced.

    The calculation illustrated in Figure 12 can assist in a first technical assessment to determine whether or not the installation of a nozzle is beneficial. This is only intended as a rough guide and, if it appears beneficial to install a nozzle, the services of a naval architect or propeller manufacturer should be sought to examine the case in more detail.

    In the Figure, the vessel speed would be taken as the dominant working condition (in the case of a trawler, the trawling speed and not the free-running speed). The propeller RPM is calculated from the full power RPM of the engine, divided by the gearbox ratio:

    Figure 12: Assessing the benefits of a nozzle (single-screw vessels)

    Propeller RPM =

    Engin RPM


    Gearbox reduction

    The shaft horsepower (SHP) is taken as the maximum continuous rated power output of the engine, measured in horsepower (HP).

    For a trawler equipped with a 440 horsepower engine (at 1 900 RPM) and a 5:1 reduction gearbox, and that has a normal trawling speed of 3 knots, the following equation is used to calculate the horizontal position across the graph in Figure 12:

    The vertical position is determined by the trawling speed, 3 knots. The point of intersection is clearly in the benefit area and it may be worth while considering the installation of a nozzle on technical grounds. The next step would be to seek the advice of a naval architect or propeller manufacturer.

    Summary Table 5

    Propeller nozzle installation (on trawler)



    Increase in tow force

    Usually a slight reduction in maximum free-running speed

    Protection for the propeller

    Larger turning circle

    Vibration may be reduced

    Manoeuvrability astern reduced

    Increased catching power or fuel savings

    Increased rudder load


    Expensive installation


    May require new propeller


    May require new rudder or rudder modifications

    Source: Smith, Lapp and Sedat, 1985.

    What difference can a nozzle make? A nozzle that has been correctly chosen and installed can result in an increase in towing force of about 25 to 30 percent (calculated from Smith, Lapp and Sedat, 1985), depending on the inefficiency of the original installation. On a trawler, this gain can be used in one of three ways:

    However, it must be remembered that nozzles are not suitable for all fishing vessels. In general, only trawlers see a real benefit from the installation of a nozzle. The penalties associated with nozzle installation include:

    Nozzles may have limited application as a retrofitted device. If the vessel was designed to have an open propeller, there is often insufficient space within the existing aperture to accommodate a nozzle that can enclose a propeller capable of absorbing the engine's power.


    Two aspects of hull design directly affect the fuel efficiency of a small fishing vessel. The underwater form of the hull at the stern, in particular the area around and just forward of the propeller aperture, affects how efficiently the propeller operates in the wake of the hull. The overall hull form, in particular the slenderness of the hull, affects the vessel's resistance and, therefore, its power requirement and fuel consumption.

    Photo 8: This deadwood will need a lot of fairing yet - J. WILSON

    Water flow into the propeller

    The section The propeller covers some details regarding the design of the propeller and the appropriate clearances between the propeller and the hull. However, to achieve a reasonably efficient installation, some attention must be paid to the shape of the hull around the propeller aperture.

    In an ideal installation, the propeller would operate in a flow of smooth, undisturbed water. In practice, this is impossible to achieve owing to the unavoidable presence of the structure supporting the bearing and propeller shaft (the deadwood, propeller post, skeg, strut or outboard motor leg) just ahead of the propeller. The disturbance caused by the structure can be minimized by:

    Photo 8 shows a poorly faired deadwood, which would impair propeller efficiency and result in increased propeller vibration, especially with a two- or four-bladed propeller. In Photo 9, the back edge of the deadwood has been smoothed off and the propeller will operate in a better, more even flow. Ideally, the smoothing should start at about 1.3 times the propeller diameter, forward of the back edge of the deadwood.

    Photo 9: Good fairing forward of the propeller - J. WILSON

    Hull form

    In most instances, the hull form is either a fixed parameter (i.e. the vessel already exists and the modification of the overall shape would be prohibitively expensive) or is determined by a qualified naval architect following a detailed design process.

    However, in general a long thin vessel is more easily driven than a short fat vessel. The form of the power/speed curve (shown in Figure 4, p. 7) varies depending on hull shape. With a short fat vessel, the curve is steeper and the maximum reasonable speed (beyond which fuel consumption becomes excessive) is around 15 percent slower than that for a long thin vessel. Recommended maximum operating speeds are given in Table 2 (Gilbert, 1983).

    A finely shaped, thin bow with a narrow angle of entry can help to reduce wave resistance. However, such a design has limited carrying capacity for the length of vessel and may not be economically feasible, in spite of better fuel efficiency.

    The shape of the stern of the vessel also influences resistance and tight surface curvatures, and sharp shoulders should be avoided to minimize flow separation (when the water passing the hull fails to follow the hull's form, thereby creating small eddies and increased resistance). In principle, the surface of the hull should not be at an angle greater than 15 to 20° relative to the centre line (Schneekluth, 1988), but adherence to this guide angle is often impossible, especially in fatter vessels, with a fuller form. The most critical areas of the stern for high curvature and steep angles are the zone just below the counter and the area just forward of the top of the propeller aperture. Where adherence to the guide angle is impossible, it is better to exceed the angle by a great amount over a short distance than to exceed the angle by a small amount over a long distance.

    For slow-speed vessels (most fishing vessels), a flat transom stern presents higher resistance characteristics than a cruiser or elliptical stern. However, the transom stern creates significantly more deck space as well as internal storage capacity, and it has therefore become a common feature in the design of most small vessels.


    The fuel economy of a fishing vessel is always based on the size and type of engine installed. If the particular engine installed is inefficient and poorly specified, for example, no matter how much the operator slows down, the vessel will always be fuel-inefficient. In many cases, there is no alternative to the type of engine that could be installed - larger offshore vessels and trawlers invariably have inboard diesel propulsion, principally based on the grounds of fuel and propulsive efficiency as well as reliability and safety.

    This section is intended to assist in the preliminary specification of an engine for a small fishing boat, in order to achieve fuel efficiency. Circumstances in which a choice must be made between available technologies are emphasized, as in the case of outboard motor-powered vessels.

    How big?

    The section Engine operation discusses the fuel savings that could be achieved by travellling at a slower speed. An important issue raised is that, while a vessel is operating at reduced speeds (achieved by throttling back), its engine is actually being underused. It would have been better from the outset for the owner to have purchased a smaller engine that could be operated at 80 percent of maximum continuous rating (MCR) (approximately the most efficient service engine speed) in order to achieve the same reduced vessel speed. The purchase and installation of a smaller engine should not only reduce capital costs and fuel consumption, but also reduce maintenance costs.

    Based on previous work by Gulbrandsen (in FAO, 1988), for small fishing vessels (up to 11 m) involved in passive fishing methods such as gillnetting and handling, the following recommendation is made:

    Figure 13: Fairing of deadwood or skeg

    This should correctly specify the size of an inboard diesel engine which, while operating at 80 percent of MCR, should achieve a service speed of about v = 2.16 x ÖL, where v is the vessel speed in knots and L the waterline length in metres.

    For example, a 9.6 m fishing vessel with a waterline length of 8 m and an in-service displacement of 3.5 tonnes should have a diesel engine of no more than 21 HP (= 6 x 3.5). This engine should give the vessel a service speed of about 6.1 kt (= 2.16 x Ö8) at 80 percent of MCR.

    Under tropical conditions, a diesel engine produces marginally less power and the maximum installable power could be increased by up to 10 percent, and up to 6.6 HP per tonne displacement.

    If the vessel is to be equipped with an outboard motor, a larger engine is necessary because of the outboard motor's smaller and less efficient propeller:

    The installed power requirements for larger craft involved in active fishing methods, is more dependent on the type of fishing method used, the amount or size of fishing gear and the amount of time spent travelling to the fishing grounds.

    The specification of the engine size of a small fishing vessel can be relatively straight forward, based purely on technical grounds. However, there are always compromises that have to be made and other factors must be taken into account that may indicate a larger engine than that specified above, including:

    Choice of engine type

    Operators of smaller inshore fishing vessels may be faced with a bewildering choice of installing propulsion units in a new vessel or replacing an existing power unit that has reached the end of its useful life. Following are the principle factors influencing the type of engine chosen.

    Fuel consumption. The nature of inboard diesel engine installations and gasoline outboard motors makes their fuel consumption characteristics fundamentally different. A gasoline engine consumes about 2.4 times as much fuel per horsepower per hour than a diesel engine. To make the matter worse, as indicated above, the smaller propeller size (and lower efficiency) of an outboard motor means that 50 percent more horsepower than that of its equivalent inboard engine is required to achieve the same service speed. The amount of fuel consumed per year by an outboard motor-powered vessel could easily be up to 3.5 times the amount of fuel consumed by a diesel-powered vessel with the same performance. In many countries, diesel is significantly cheaper than gasoline and, in financial terms, the difference in the two fuel bills may be even greater.

    Capital cost and credit availability. The purchase and installation cost of a diesel inboard engine is considerably higher than that of an outboard engine. In situations where savings are limited and credit is unavailable, an outboard may be the only affordable engine type and it may be impossible to consider the choice of more fuel-efficient technologies, in spite of lower operating costs. Recently, however, Chinese manufactured marine diesel engines have started to appear in small-scale fisheries and are available at around 30 to 50 percent of the cost of alternative engines from Japan or Europe. Even if such a price reduction is achieved at the expense of quality and durability, the cheaper engine may still prove a legitimate choice in situations of capital scarcity and high interest rates.

    Taxes, duties and subsidies. Local and national policies often favour particular technologies, either by subsidizing particular fuels (such as the case of kerosene in southern India or premixed outboard motor fuel in Senegal), or by offering reduced import duties on particular types of engines.

    Amount of use. In the long term, an inboard diesel engine may be cheaper to own and run than an outboard engine, not only on account of its greater fuel efficiency but also because of its longer operating life. However, if the engine is only used for a few hours per year, it may still be better to consider an outboard engine. It is not possible to generalize when considering the minimum hours of use per year that are necessary to justify the choice of a diesel engine, as it depends on local taxes and duties, the type of vessel, the cost of fuel and maintenance, etc. Studies made to date indicate that, if use is above 250 to 350 hours per year, an inboard diesel engine is probably justified on financial grounds. However, it is worth noting that engine use in some countries would need to reach 650 hours per year before diesel would be an appropriate technical choice.

    Availability of parts and technical skills. The choice of operable technologies is often quite limited. For a particular engine to be a feasible option, not only must it be physically available locally, but so must spare parts and maintenance skills.

    Vessel structural strength. If a vessel operator is considering the installation of an inboard diesel engine in a boat that has previously been powered by an outboard motor, the boat will inevitably have to be strengthened and/or modified in order to cope with engine and shaft installation and the increased vibrations. Not every vessel, particularly beach-landing canoes, can be easily adapted to inboard engine installations.

    Diesel inboard engines. There are few alternative technologies within the range of diesel engines that are suitable for installation in small fishing vessels. Smaller diesel engines are normally aspirated, principally on account of simplicity and cost, whereas larger engines may be turbocharged to maximize efficiency and save weight. Table 4 summarizes the key characteristics of diesel engine installations.

    Table 4

    Diesel inboard engine



    3 Allows efficient propeller installation

    8 High purchase cost (2-4 times that of an equivalent outboard motor)

    3Fuel efficient

    8 Complicated and costly installation

    3 Diesel fuel usually both available and cheap

    8 Low-quality fuel can lead to increased maintenance costs

    3Known technology

    8 Weight


    8 Requires a strong, structurally sound vessel


    8 Fixed installations are not suited to beach landing

    Typical fuel consumption: 0.25 litres/HP/hour Effective1 fuel consumption of other engines compared with a diesel inboard engine:

    1 The effective fuel consumption includes an allowance for the difference in the propeller efficiency of each installation. Data in this column indicate the actual amount of fuel consumed by a power unit of the same performance.

    Turbocharged diesel engines. A turbocharged diesel engine is fitted with a small compressor unit that is driven by the exhaust gases and forces air into the engine and increases the power output. A turbocharged diesel engine should be lighter and about 15 percent more fuel efficient than a normally aspirated diesel engine of the same power, consuming about 0.21 litres/HP/hour.

    An important point is that in order to maintain fuel efficiency, the turbocharger must be driven hard. If it is anticipated that the engine will spend a lot of time operating at intermediate loads, then a normally aspirated engine would be a better choice.

    Outboard engines. Outboard motors originated as recreational engines for occasional use, mostly at high speed. Very few models specifically designed for slower, heavier vessels are available, which is a significant contributing factor to their fuel inefficiency.

    All outboard engines have the great advantage of easy and quick installation, and those below about 45 HP can also be easily dismounted for safekeeping when not in use. The structural modifications necessary to mount an outboard engine are relatively simple and do not require advanced skills.

    There are several types of outboard motors available on the market, the most popular is the standard 2-stroke gasoline engine, which burns a mixture of gasoline and 2-stroke lubricating oil. However, newer technologies in the outboard motors include 4-stroke engines and direct fuel injection engines, both of which have improved fuel efficiency.

    Gasoline 2-stroke outboards. The gasoline 2-stroke outboard engine has found widespread application in small-scale fisheries, particularly in developing countries, often as a result of fisheries department motorization programmes and proactive support from the engine manufacturers. The engines are relatively cheap and, both, parts and technical maintenance skill are often readily available, locally.

    Gasoline 4-stroke outboards. Gasoline 4-stroke outboard engines are relative newcomers to small-scale fisheries and, although they were initially only available through one major manufacturer, they are becoming more commonplace in response to environmental emissions regulations. Regular maintenance is not technically demanding but it may still be difficult to find locally skilled mechanics to perform overhauls.

    Gasoline 4-stroke outboards have the significant benefit of running on unmixed fuel and have a much better fuel economy than the equivalent2-stroke. At maximum speeds, fuel consumption is about 60 percent of that of the equivalent 2-stroke, falling to about 45 percent at service speeds. Four-stroke engines are both slightly heavier and more expensive than the 2-stroke equivalents and are best applied when fishing under power (such as trolling) and in fisheries where vessels must cover significant distances to reach fishing grounds.

    Diesel outboards. Diesel outboard engines are not widespread in small-scale fisheries, primarily on account of the high cost of purchase and maintenance difficulties. However, the technology is now reasonably well proven and the engines are particularly fuel-efficient. Diesel outboard engines are best for fisheries that require high engine hours and that are also very well served technically. One set of field trials estimated that a diesel outboard would only be a viable alternative to a gasoline 2-stroke model of similar performance if the engine was in use for about 600 hours per year or more.

    Kerosene outboards. Kerosene outboard engines are based on standard gasoline 2-stroke engines that have been modified in order to run on kerosene. The engine still requires the usual gasoline/oil mixture for starting and stopping and is, therefore, a bi-fuel motor. Kerosene outboards are suitable only in countries where there is a significant subsidy on the price of kerosene, for example in India. Their operation requires careful attention, particularly while starting and stopping, and their useful life is inevitably very short.

    Longtail engines. The longtail motor is an interesting local solution to the problem of propulsion for small fishing vessels. The propulsion unit consists of a long, often exposed, propeller shaft which is attached directly to the crank shaft of a small stationary or automotive engine. The engine is then mounted on the transom of the fishing vessel in a pivoting bridle, and the propeller and shaft are immersed in the water at an angle. The longtail is based on the local availability of very cheap stationary engines or marinized automotive engines and the technology is a simple but ingenious and cheap way of putting these engines to use in a fishing vessel. Only relatively small motors (up to 20 HP) are appropriate on longtail installations being used in a seaway, as they can be difficult or dangerous to handle. On some calm inland waterways, however, their use by skillful operators with engines of up to 100 HP is common for the transport of passengers and produce.

    Many longtail installations are of local design and manufacture and little quantitative information exists regarding their performance. A diesel-powered unit would probably consume about 0.25 litres/HP/hour, but the high-revving propeller (usually directly driven from the crankshaft and no gearbox is fitted) would be very inefficient - its effective fuel consumption would be similar to that of a gasoline 4-stroke outboard.

    Table 5

    Gasoline 2-stroke outboard engine




    8 Fuel-inefficient

    3Can run on low-quality fuel

    8 Short useful life (2 years)

    3Good performance with fast acceleration

    8 Requires 2-stroke oil as part of fuel (expensive)

    3Known technology

    8 Low-quality oil can lead to unreliability and increased maintenance costs

    3Light weight (1.3-1.8 kg/HP)

    8 Significant exhaust emissions

    Typical fuel consumption: 0.55 litres/HP/hour Effective1 fuel consumption of other engines compared with a gasoline 2-stroke outboard engine:

    1 The effective fuel consumption includes an allowance for the difference in the propeller efficiency of each installation. Data in this column indicate the actual amount of fuel consumed by a power unit of the same performance.

    Direct fuel-injection (DFI) petrol outboards. Direct fuel injection is a relatively new engine technology that has so far been applied to road vehicles and outboard engines. It can be applied to both 2- and 4-stroke engines and is based on a technology similar to that used in diesel engines, where fuel is injected under high pressure directly into the combustion chamber of the engine. Two manufacturers offer DFI engines and claim fuel savings averaging around 40 percent, but reaching up to 80 percent compared with the fuel consumption of an equivalent standard 2-stroke engine, as well as lower exhaust emissions. At present, only larger engines are in production (the smallest DFI engine available is 135 HP). However, within the next few years, smaller engines with DFI technology may be released and could easily find application in small-scale fisheries. The high-pressure injection system, which is a central part of DFI technology, will probably be sensitive to fuel purity and quality.

    Table 6

    Gasoline 4-stroke outboard engine

    Advantages Disadvantages


    3More economical

    8 About 35% more expensive than the 2-stroke equivalent

    3Lower exhaust emissions

    8 About 15% heavier than 2-stroke equivalent

    3Reasonable performance

    8 Newer technology

    3Longer life (3-6 years)

    8 Requires greater maintenance skills


    8 Requires good quality fuel



    3Does not require premixed fuel or 2-stroke oil


    Typical fuel consumption: 0.33 litres/HP/hour Effective1 fuel consumption of other engines compared with a gasoline 4-stroke outboard engine:

    1 The effective fuel consumption includes an allowance for the difference in the propeller efficiency of each installation. Data in this column indicate the actual amount of fuel consumed by a power unit of the same performance.

    Engine installations

    The installation of an engine in fishing vessels is often a forgotten factor in fuel efficiency. If the engine is poorly installed, it will operate below its designed fuel efficiency level.

    Outboard motor mounting. Care must be taken when installing an outboard engine in order to ensure the correct immersion of the propeller. For a relatively slow-speed craft such as a fishing vessel, the anti-ventilation plate (the horizontal plate just above the propeller) should be about 2.5 to 5 cm below the bottom of the transom.

    The mounting of outboard engines in large traditional fishing canoes often dictates the use of a side mounting rather than a centre-line mounting in a well or on a small transom, owing to cost and structural considerations. When deciding the feasibility of the additional cost of centre-line installation, a vessel operator should be aware that side mounting not only results in a veering tendency but also reduces maximum speed by up to 0.5 kt. This is equivalent to a loss of about 4 HP or 2 litres of fuel per hour on this type of canoe.

    Inboard engine shaft angle. As discussed earlier, a steep shaft angle can allow for the installation of a larger propeller diameter. However, as the angle becomes steeper, the propeller starts to push down rather than forwards and fuel is wasted. The maximum recommended shaft angle is about 15°.

    The choice of a steeper shaft angle also introduces significant variable loading to the propeller blades. This is due to the fact that, as the propeller blades rotate upwards, they are receding from the onrushing water, and as they rotate downwards, they are moving against the slip stream, resulting in variable angles of attack, vibration and early cavitation.

    Table 7

    Diesel outboard engine



    3Very economical

    8 About 2.5-3 times the price of a 2-stroke equivalent

    3Cheap commonplace fuel

    8 At least twice the weight of a 2-stroke equivalent

    3Very good speed maintenance under load

    8 Slower acceleration

    3Does not require premixed fuel or 2-stroke oil

    8 Few manufacturers, not widespread


    8 Requires greater maintenance skills


    8 Requires good-quality clean fuel


    8 Limited user serviceability


    8 Air-cooled models are noisy

    Typical fuel consumption: 0.25 litres/HP/hour Effective1 fuel consumption of other engines compared with a diesel outboard engine:

    1 The effective fuel consumption includes an allowance for the difference in the propeller efficiency of each installation. Data in this column indicate the actual amount of fuel consumed by a power unit of the same performance.

    Exhaust and air flows

    Any engine, whether installed in an engine room in a large craft or in an engine box in a smaller vessel, must not only have access to fresh air for combustion, but the ventilation should be adequate so that the exhaust gases can easily escape. A restricted flow of exhaust gases and fresh air can easily cost the operator 10 percent more in fuel consumption.

    Air intake. An adequate air flow into the engine room or engine box is important not only to supply air to the engine for combustion but also to prevent overheating of the engine room or engine box. This is particularly important with the installation of air-cooled engines, where the flow of air is the only mechanism by which the heat of the engine is dissipated.

    Table 8

    Kerosene outboard engine



    3Burns fuel that can be very cheap

    8 Shorter life than gasoline engine

    3Similar price to 2-stroke equivalent

    8 Kerosene must be 40-50% the price of gasoline to make the engine viable


    8 Subsidized kerosene often in short supply


    8 High wear, more carbonization, very short service life


    8 Requires gasoline/2-stroke oil mixture for low speeds, starting and stopping


    8 Speed reduction can result in increased fuel costs


    8 Requires good quality kerosene

    Typical fuel consumption: 0.5 litres/HP/hour Effective1 fuel consumption of other engines compared with a kerosene outboard engine:

    1 The effective fuel consumption includes an allowance for the difference in the propeller efficiency of each installation. Data in this column indicate the actual amount of fuel consumed by a power unit of the same performance.

    As a guide, the cross-sectional area of the air intake into the engine room or engine box should be at least 8 cm2 per horsepower for a water-cooled engine (i.e. a 40 HP engine requires an air intake of at least 40 x 8 = 320 cm2). An air-cooled engine requires a larger air intake, the minimum dimensions of which are usually stipulated by the manufacturer. In any engine room or engine box, the air intake should supply cool, fresh air low down in the engine room, while hot air should be ventilated from the top of the engine room or box.

    If a diesel engine is starved of air, the exhaust tends to show black smoke. Care must be taken, as this could also be a sign of other mechanical problems (see the section Engine maintenance).

    Air outlet. Some of the air that enters the engine room or box leaves via the engine exhaust, but ventilation must be allowed so as to avoid the build-up of heat in the engine room or box, itself. Hot air should be taken out from the top of the engine room or box, where the air temperature is highest. The cross-section of the air outlet should be approximately the same as that of the air inlet, around 8 cm2 per horsepower for a water-cooled engine.

    Engine exhaust. The exhaust pipe should be as straight as possible, and sharp 90° bends should be avoided, as each bend can reduce the air flow by 25 percent. The diameter of the exhaust pipe should be stipulated by the engine manufacturer. If it is too small or contains too many sharp bends, backpressure builds up in the system, resulting in loss of power and, in more extreme cases, white smoke in the exhaust.

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