Propulsion research by model testing

Jahrbuch der Schiffbautechnischen Gesellschaft 77 (1983)

Per cent Final fuel cost as per cent saving of original basis ship cost over basis

Typical ship built 1970-175 Basis 100

Reduce speed about 15% e.g. 15 to 12.75 knots or 27 to 23 knots 24.4 75.6 Increase deadweight by saving weight and/or increasing block co-efficient 2 74.1

Improvement in lines and proportions 8 hull design

68.2 Improved shell finish, use of self-polishing anti-fouling 6 hull smoothness 64.1

Slow revving propeller 60 rpm vs 110 rpm 12 propeller design

56.4

Reaction fins or nozzle 5 53.6

Improved fuel consumption of main engine, e.g. 175g to 210 g/kWh 16 engine design 45.0 Use of shaft-driven alternator in lieu of one diesel alternator, 2 44.1 machinery improvements

Reduction in electrical load and heat demands 1 _43.7

Better utilisation of waste heat 2 42.8

Icz

laboratorium voor

Scheepshydromechanica

Propulsion research by model testing

1 Introduction

In the development of more economical shipping most major shipbuilding nations have carried out extensive research programmes to improve the fuel efficiency both of newbuildings as well as of

existing ships.

Their achievements in developing more fuel efficient ships have to be regarded as a constant progression in which feedback of performance results from innovative features applied in new designs is very important.

The five main areas of possible improvement for fuel efficient ship design can be classed as follows: Hull design,

Hull smoothness,

Propeller design and hull interaction,

Main engine design,

Auxiliary engineering systems design.

There is little likelihood of achieving the maximum possible improvement in all of these areas on any one ship, since operational requirements for each type of ship call for compromises.

As an example, however, Table 1 published by the British Ship Research Association [1] may serve

to indicate the attainable improvements in fuel costs per tonne-mile for a typical large present-day newbuilding as compared with a similar ship built in 1970-1975.

From Table 1 it will be clear that the main area of propeller design and hull interaction can con-tribute significantly to the total improvement in fuel cost.

The research carried out in this area is very much dependent on the results of model testing and some findings with respect to this aspect of propulsion research will be dealt with in more detail in

this paper, both for newbuildings and for existing ships.

25% "t= 20% 5 c-9 15% 5%

Fig. 1. Attainable efficiency improvements by lowering RPM and increasing propeller diameter for the power range of 10,000 kW-14.000 kW

92 Jahrbuch der Schiffbautechnischen Gesellschaft 77 (1983) 2 Low RPM-large diameter propellers 2.1 Conventional propellers for newbuildings

A well known statement about propellers is that it is very difficult to design a really bad propeller but equally difficult to design a really good one. Without too much difficulty it should be possible to design a propeller with an efficiency within about 2% of the best possible for particular design

con-ditions.

It is safe to suppose, therefore, that it will be very unlikely to improve the efficiency of the con-ventional propeller much further anymore and that the efficiency of such a propeller will remain principally dependent on the magnitude of the propeller load.

Increasing the propulsive efficiency can thus most effectively be achieved by decreasing the pro-peller load, which means decreasing the thrust per unit propro-peller disk area in the range of normal operation by adopting as large propeller diameters as possible and as low revolutions as tolerable within present technological constraints.

With the fairly certain expectation of high fuel prices in the future it is evident to assume ships of the coming generations to have much lower power requirements than existing ships of same size and proportions, so that the propeller RPM may be relatively decreased and the diameter accordingly increased for optimal efficiency.

With the installation of geared drives, also for show speed engines, it has now become possible to deliver the required torque for the lower rotational speed.

The savings obtained through the above approach are such that some generalized results from research carried out at MARIN for proprietary projects was considered to be of interest for

presenta-tion in non-dimensional form in Fig. 1.

For finding the effect of lower propeller revolutions on propulsive efficiency, the propulsive effi-ciency can be analysed by treating it as being composed of the propeller effieffi-ciency in open water, the hull efficiency and the relative rotative efficiency. Decrease of the propeller revolutions at a simul-taneous increase of the propeller diameter without modification of the aft body hull form will result

in general in:

0% 70% 50% RPM

E

An increase in propeller efficiency in open water.

A decrease of the hull efficiency, mainly due to a reduction of the effective wake. Little or no change of the relative rotative efficiency.

The gain in propeller efficiency for given power levels can be calculated from systematic propeller series diagrams such as the Wageningen B-Series and the linearized results for shaft powers in the range of 10,000 to 14,000 kW have been plotted in Fig. 1 on base of RPM in percents of a basic RPM which was assumed to be around the lowest speeds offered at present by manufacturers of direct

drive engines in the power range considered.

The effects of increasing the propeller diameter with unmodified aft body shape on the hull effi-ciency are such, that of the percentage change obtainable in open water effieffi-ciency in general at the larger diameters only 60 to 70 percent can be actually realised in the propulsive efficiency as can be seen in Fig. 1 and it will be clear that real savings in fuel consumption are substantially smaller than one would normally expect from propeller open water efficiency calculations.

The large reduction in hull efficiency is worth trying to be remedied by adopting a suitable change in the stern hull form to obtain a higher wake and a uniform flow into the propeller disk. Several stern forms have been proposed and examined and sofar semitunnel sterns are considered to result in the best characteristics from this point of view [2].

The tunnel form also allows propeller tip-hull clearances to be selected in the order of 0.15 D instead of the normal 0.25 D values, which may be of advantage if there are draft restrictions.

If a tunnel is not suitable highly skewed propellers could be applied to arrive at the same effect, since also for these propellers the tip clearances can be reduced to about 0.15 D to insure the same

level of propeller hull excitation as with the conventional propeller at 0.25 D clearance. The propulsive

efficiency improvements due to the 10% larger allowable propeller diameters through skewed pro-pellers or tunnels will amount to about 5 percent.

From the above it will be evident that a really successful LRPM design can only be derived through

extensive model testing of various hull form and propeller diameter variations for a specific project. Except for the need to predict the propulsive efficiency components accurately, there are several other problems to be solved by model testing in connection with the application of large slowly turning propellers such as the cavitation on the propeller, the pressures on the hull above the pro-peller, the shaft thrust, torque and bending moment variations for the bearing, gearing and coupling calculations, etc.

2.2 Retrofit propellers for existing ships sailing at

operating conditions different from the original design

If the decision has been made not to operate the ship at full power anymore, an improvement in propeller efficiency can be achieved by adopting a propeller of reduced blade surface area and by incorporating a small increase in diameter to absorb the reduced service power at optimum conditions

for the lower operating point in the original power-RPM curve.

Much greater improvements in propulsive efficiency can be obtained if the original power-RPM curve is modified by choosing the lowest possible RPM at the reduced service power at which the resulting torque is still within the torque limitations of the existing transmission system.

In this case the low RPM large diameter propeller concept can be adopted if a larger increase in

propeller diameter is permissible.

It will be necessary then to change the reduction gear ratio to ensure that the turbine runs at its optimum rate of revolutions corresponding to the lower power output.

Further the propeller efficiency can be increased by reducing the number of blades for the replace-ment propeller.

The above considerations have been guide lines for the development of the worlds present largest propellers in operation, four-bladed retrofit units for a series of 300,000 TDW tankers by Lips [3] of 10.5 m diameter and for a series of 357,000 TDW Tankers by Stone Manganese [4] of 11.0 m diameter, which have been extensively model tested at MARIN's laboratories.

These vessels are currently sailing at 70% of their installed power, the 300,000 tonners originally had six-bladed propellers of 9.2 m diameter, the 357,000 tonners five-bladed propellers of 9.5 m diameter.

94 lahrbuch der Schiffbautechnischen Gesellschaft 77 (1983)

From the model tests it was found that the propeller open-water efficiencies of the new designs are some 12% to 14% better than those of the original propellers under the present operating

condi-tions.

The large diameter for the speed-adapted propellers had a strong influence on propeller-hull inter-action and the tests showed that the hull efficiency was reduced due to the large diameter of the new propellers. However, as a final result, the total propulsive efficiency was improved by 10% for the 300,000 tonners and by 7% to 8% for the 357,000 tonners, which values have been verified on the

vessels in the meantime.

2.3 Propellers with vane wheel

A most interesting application of the principle of decreasing the thrust per unit propeller disk area by adopting a larger diameter and accelerating a larger mass of water to a lower mean outtake velocity from the propeller plane is the idea published by Grim at the annual meeting of this society in 1966 15].

This idea is simply to place behind the driven propeller of practically conventional design a second freely rotating vane wheel or "Leitrad" on the same shaft with a diameter larger than that of the propeller. The blades of this vane wheel are designed in such a way that the vane wheel is absorbing energy from the propeller race at its inner radii, which it transmits immediately in thrust at its outer radii outside the race of the propeller. In this way the required impulse to produce the total thrust of the propeller plus vane wheel is transmitted through a larger mass of water and hence at a lower required power than for the single conventional propeller of same thrust.

Experiences with this type of propulsor have been published recently by Grim [6] from which it is clear that from full scale trials with a research vessel carried out with a conventional propeller and with a propeller with vane wheel as shown in Fig. 2 it can be concluded that the latter system is 9 %

more efficient at constant speed of the vessel than the conventional propeller.

Grim even states that if optimization of the propeller and vane wheel could have been carried out in time, the power saving would have been between 10% and 12%.

This is a remarkable achievement, since the diameter of the vane wheel is only 15% larger than that of the original conventional propeller.

For a LRPM large diameter propeller of the same diameter as of the vane wheel turning at about a 20% lower RPM than the original propeller, the above power saving would have been about 8% as

can easily be deduced from Fig. 1.

The high performance of the propeller and vane wheel combination in the authors opinion can not be attributed to the large diameter concept only and part of the power saving must certainly have to be attributed to a contra-propeller effect resulting from the action of the vane wheel, leaving less rotational energy in the propeller race than with a LRPM large diameter propeller.

The advantages of the propeller and vane wheel combination over a LRPM propeller are evident, the combination can operate at normal shaft RPM so that for larger ships reduction gears are not necessary. Since the tip speeds of the vane wheel will be somewhere in between 12 to 18 m/sec as compared to around 30 m/sec for the LRPM propeller, cavitation and hull pressure fluctuations will be no problem for the propeller and vane wheel combination, while smaller tip-hull clearances can be accepted than with the LRPM propeller.

Further research on this propulsor for applications on larger ships will be of much interest to the

shipping industry.

3 Contra-propellers

The contra-propeller effect mentioned above was already first made successful use of in 1865 by Arthur Rigg in Great Britain by attaching fixed deflection blades to the rudder post abaft a screw propeller to remove the rotational velocity form the outflow jet for producing an increment in thrust. As a result of continued experimentation on the contra-propeller a considerable number of patents were issued to cover developments and modifications. Among these the most appropriate to mention here is the German Patent 214203, issued to Wagner for a contra-propeller without a shroud ring, described in 1912 in a paper presented at this society [7].

In Fig. 3 this contra-propeller arrangement is illustrated for a single screw ship.

According to Wagner the following improvements in required power based on actual full scale testing could be obtained with contra-propellers:

Twin screw ships 10%-15% Single screw ships 8%-12%

As can be seen from Pig. 3 the contra-propeller consisted of several guide vanes placed star-like with respect to one another. As the laterally protruding guide vanes were easily damaged, and, more-over, singing occurred, this construction was later abandoned.

A similar decrease of the rotation losses may be obtained by fitting guide vanes in front of the screw in such a way that a rotation is given to the water contrary to the direction of turning of the screw.

In the beginning with this arrangement also several guide vanes were used, just as in the contra-propeller, but this was later abondoned as well and in the rare cases after the 1920's in which guide vanes were fitted ahead of the screw, they consisted simply of a specially shaped contra-stern post.

Another spin-off of the contra-propeller experiments is the adoption of a contra-rudder post and/or a contra-rudder to reduce the rotation losses.

Guide vanes, contra-propellers and contra-rudders were extensively tested at MARIN in the 1930's and 1940's and in 1946 Van Lammeren [8] concluded the following:

"The usefulness of guide vanes and contra-propellers in any individual case must be judged on its merits. The effect of these devices is in general not large owing to the comparatively small part of the screw race which they can influence, and depends upon the interaction of screw, rudder and after body of the ship. The improvement in the propulsive coefficient which may be obtained with a contra-rudder depends entirely on the interaction between screw, rudder and after body. In general, however, it may be said that with a good combination of screw, rudder and after body, and with the right clearance, hardly any improvement in propulsion is to be expected from such a special rudder".

96 Jahrbuch der Schiffbautechnischen Gesellschaft 77 (1983)

Evidently sinceWagner's observations on the attainable improvements in 1912 the design and inter-action of screw, rudder and after body of the ship and the choice of clearances had improved so much that the contra-propeller and its derivatives lost practically all interest after about 1950, when the cost of manufacturing of these devices became prohibitive in the time of low energy costs.

Recently, however, the conditions for the adoptation of the contra-propeller concepts have become more favourable again, because energy costs have become exorbitant, clearances of the propeller have become too ample from a propulsion point of view because of fear of vibration excita-tion and the propeller-hull interacexcita-tion has deteriorated considerably from the adopexcita-tion of very high block coefficients, low L/B ratios and high BIT ratios.

Since the potential maximum power savings to be expected from eliminating completely the rota-tional energy losses behind a propeller operating in open water have been theoretically determined to be in the order of 10% it is no surprise to find all the old patents reappearing in hardly concealed disguise under the present economic conditions, as is illustrated below.

3.1 Reaction Fins for high block ships

Recently Mitsubishi Heavy Industries have developed a device called -Reaction Fin" [9], which is to introduce a swirling flow forward of the propeller to counteract the vortex set up by the screw itself, and this is achieved by placing fins in an essentially radial pattern in the stern aperture. The principle of this device is stated to be similar to that of a contra-propeller. To gain maximum effect the incidence angles of the fins must be optimised by tank testing. For strength reasons the tips of the

fins are linked by a ring as is illustrated by Fig. 4_

The hull form in the after body influences the effectiveness of the Reaction Fin to a marked degree and it is accepted that they are best suited to high block coefficient ships.

Sea trials were carried out on three large bullccarriers [9], both with and without a reaction fin on one ship, with a fin on a second ship and without a fin on a third vessel, sister ship of the second.

Their results show a reasonable agreement between tank predictions and measured power savings, namely around 7 %-8% in the ballast condition.

The tests and trials would appear to show that for suitable hull forms there are savings to be made by adding reaction fins to the propeller arrangement. The sea trials as mentioned were carried out in the ballast condition, and the agreement with predictions has led Mitsubishi to claim potential savings of 4 %-6 % in loaded operations, based on their model tests in that condition.

Improvements in noise and vibration are also claimed from acceleration measurements on one of the vessels. Manoeuvrability is said to be little affected by the addition of the fins.

At model size, no significant effect on the extent of cavitation is recorded, nor on propeller induced vibration.

Since from the 10% maximum potential saving in energy from the total elimination of rotational losses, the contra-propeller action of the ship rudder alone is already contributing 3 %-4%, it will be clear from the above that part of the 4%-8% power savings obtained with the Reaction Fin must have to be attributed to an improvement in the flow on the after body and in the propeller-hull

inter-action.

3.2 Additional Thrusting Fin fitted laterally to the rudder

One of the shortcommings of the contra-rudder is that it takes care of rotational components of velocity only in a relatively narrow vertical region, close on either side of the propeller axis. It is logic then to fit one or more curved vanes or stubhydrofoils, projecting laterally from a streamline or contra-rudder and this idea was first put forward in 1952 [10].

It is intended that these vanes or hydrofoils produce thrust by utilizing the rotational components in the outflow jet to port and starboard of the propeller hub. In this respect the scheme is similar to the contra-propeller except that the horizontal vanes do not have to be mounted on a fixed part of the ship structure.

The idea of using lateral foils on the rudder has been revived recently by Ishikawajima-Harima Heavy Industries, who carried out extensive design work and model tests on a so-called Additional Thrusting Fin (IHI A.T. Fin) consisting of two fins to be fixed horizontally on the rudder horn of a

VLCC, which is claimed to recover as much as 4 %-5 % energy losses [11].

At MARIN this year some proprietary projects contained model propulsion tests with and without lateral foils on the rudder and although these were not very successful in the beginning a recent set of carefully designed fairly large foils with end plates showed efficiency improvements which even slightly surpassed the 4 %-5% mentioned above. More research will be carried out to confirm these

findings.

Anyhow, this idea seems to merit attention and may well work for certain types of existing ships

also, because of its obvious simplicity of application.

3.3 Pram-type aft body with asymmetric shaft gondola

An extreme form of contra-stern frame for a single screw ship was applied in the small passenger ship "GUYANE" of 900 tons displacement built in France in 1950, which incorporated the ideas as published and patented by P. Carlotti [12].

This ship was extensively model tested at MARIN's deep water basin with two aft body configura-tions in combination with the same fore body. The two aft bodies were designed as close as possible in displacement, length, construction waterline area etc. but the first aft body was of conventional section shape and contour, while the second aft body was of pram type with a shaft gondola as indicated in Fig. 5.

Fig. 6. Asymmetric shaft gondola Fig. 5. Pram type aft body with asymmetric shaft gondola

98 Jahrbuch der Schiffbautechnischen Gesellschaft 77 (1983)

The interesting feature of this ship with respect to the theme of this paper is the asymmetric shape of the gondola to improve the water flow into the propeller, which will be clear from Fig. 6.

The results of comparative tests carried out by Troost at MARIN with these two aft body versions have been reported in detail on pages 19 and 20 of [12].

The resistance of the pram type aft body with gondola was 4.5% lower than that of the original aft body, but the total required power was even 15% lower than for the conventional original design.

From a propulsion point of view the asymmetric gondola was a success on the "GUYANE". Unfortunately the asymmetric gondola caused the ship to shear out to one side during an engine failure and this motion could not be corrected anymore with the rudder action alone, which made the Carlotti form unpopular. It disappeared completely from the design tables.

In case of a possible revival of interest in this hull form it will be necessary to investigate in detail not only its powering aspects but also its manoeuvring characteristics in case of engine failure as will be evident from the above.

3.4 Asymmetric aft bodies for single screw vessels

The most extreme form of contra-stern is an asymmetric aft body as shown in Fig. 7, which was first suggested and patented by E. Nonnecke in the mid 1960's.

It was Nonnecke's idea to improve the flow into the propeller by suppressing the flow separation on the hull above the propeller on port side by swelling out the lines to this side, while introducing at the same time a preswirl in the flow into the propeller by forcing the flow above the shaft line from starboard to port side and below the shaft line from port to starboard side by off-setting the bulbous stern in that area to starboard. Extensive model tests carried out in Hamburg [13] showed that this approach was very successful. Both the suppression of the flow separation on the hull and the reduction of the rotational energy losses through the preswirl of flow into the propeller have resulted in a total power saving in the order of 5 %-7 % for all the models tested.

This design idea has progressed in the meantime beyond the model testing stage and the first vessel incorporating an asymmetric aft body, the "THEA-S" of 6,118 tons gross, showed on trials that the fuel reduction in practice would be as much as 8% [14].

Calculations based on wake distribution measurements further showed that the variations of forces and moments and the pressure fluctuations on the hull are considerably reduced by the adoption of an asymmetric aft body.

The asymmetric aft body is an interesting development, which may well have good prospects.

4 Ducted propeller arrangements

To come back now on the approach of reducing the propeller load and thereby improving the propulsive performance of ships, an effective means is to adopt ducts around the propeller, since in this way the total thrust is shared between the propeller and the duct.

The ducted propeller is effective especially when the propeller load is comparatively large, how-ever, severe erosion has been found inside most ducts, which has withheld many owners from

apply-ing this arrangement in their newbuildapply-ings.

This is very much a pity because extensive research carried out at MARIN on systematically varied ducted propeller series and for numerous proprietary projects has shown that savings as indi-cated in Fig. 8 are quite feasible. These savings are presented as a function of propeller loading Bp = Ns VA2.5 and have been confirmed by several full-scale trials measurements carried out

by MARIN personnel.

Thus, for todays bulkcarrier and tanker newbuildings savings in the order of 5 %-10% in propul-sive power are realistic targets to be attained.

In contrast to the low RPM-large diameter propeller, where part of the large improvement in pro-peller efficiency was offset by a reduction in the hull efficiency, with the ducted propro-peller arrange-ment both are improved as compared to the conventional propeller values if the duct is considered to constitute a part of the propulsor in the analysis of the propulsive components.

In view of the erosion problems, as an alternative approach a duct installed just forward of the propeller can be attractive and especially for retrofits this is found to be most effective.

Recently, several applications of the latter arrangement as indicated in Fig. 9 have been success-fully introduced by the industry and proved to be efficient fuel savers both as retrofits and for

new-buildings.

The first successful application in the market has been Mitsui's Integrated Duct Propeller MIDP-system, which was extensively tested at the depressurized towing tank of MARIN on very large models prior to the first installation as a retrofit on the 250,000 TDW tanker "ESSO Copenhagen"

[15].

The MIDP is an annular duct located immediately in front of the propeller. Its shape is non-symmetric and specially designed to suit the hull form of each ship. It is a simple energy-saving propulsion system, which, in many proprietary model test programs carried out at the depressurized towing tank, has been shown to be capable of producing required power savings in the order of 5%-12%. Subsequent sea trials have shown 5 %-10% power savings in full load and ballast condi-tions and other benefits such as a reduction of propeller cavitation and hull vibracondi-tions as well as improved manoeuvrability.

These improvements were found to be attributable mainly to the unloading of the propeller by the higher intake velocity into the propeller due to the duct action, which results in a higher propeller efficiency. The higher propeller efficiency together with the effect of the thrust generated by the

15

00

1.0 60 80 Bp

Fig. 8. Reduction of Ps due to application of a duct around the propeller as a function of propeller loading B_p= N

sD

100 Jahrbuch der Schiffbautechnischen Gesellschaft 77 (1983)

Fig. 9. Ducts ahead of the propeller

duct in all cases tested was only partially offset by a lower hull efficiency, while the relative rotative efficiency remained practically constant for both versions, that is, with and without duct in front of

the propeller.

These findings are valid for the case that one considers the duct to be part of the hull and to act as a flow accelerator in front of the propeller.

A most surprising and unexpected result of the tests, however, was to find the bare hull resistance to be reduced in most cases by fitting the duct to the hull during the resistance tests, which means that the duct drag is more than compensated by viscous drag reductions in the aft body of the model through the homogenising effect of the duct on the stern water flow and through suppression of flow

separation.

This viscous drag reduction may be noticeably influenced by scale effects in the flow around the aft body of full ship models, even the very large ones tested in the depressurized towing tank, so that particular attention has to be given to those cases where a large decrease of model resistance is noted in order not to overestimate this effect in full scale.

Generally, a duct installed ahead of the propeller is less effective than a conventional duct around the propeller. The two versions are only comparable when fitted to types of vessels having unfavour-able viscous and form resistance characteristics, that can be improved by fitting the duct ahead of the propeller in the area of the waterline endings.

In such vessels the improvements in hull resistance normally nearly compensate the losses due to separating the duct from the propeller.

It will be evident from the above that the fuel saving potentials of this device are very much dependent on the quality of the flow in the aft body and that it is most suitable for high block ships.

These potentials have now been recognized by many owners and more than 80 Integrated Duct

Propeller Systems have already been put into service as retrofits as well as for newbuildings.

5 Overlapping propeller arrangements

All the previously mentioned fuel saving design options have been substantiated by feedback from

full scale performance records.

It is considered of interest, however, to mention in this paper also one energy saving propulsion arrangement with great potentialities, which has not been realised in a full-scale application sofar.

In the above different approaches of decreasing the propeller load were discussed for the case of single screw applications. By accepting twin screw propulsion the propeller load can be reduced con-siderably also, however, often the added resistance due to shaft appendages and the reduction in hull efficiency due to lower effective wakes more than offset the increase in propeller efficiency due to the lower propeller loading, so that no power saving is obtained as compared to the single screw

application.

To eliminate the negative effects of appendage resistance and low effective wake as much as possible, the overlapping twin-screw arrangement first proposed by Pien [16] has shown to be very

This arrangement in the authors opinion has been overlooked undeservedly in the past, when it was only proposed as an alternative for a fast twin screw containership [17].

For this ship the model tests carried out at MARIN indicated power savings for the overlapping versus the conventional twin screw version in the order of 7%.

More recently the results obtained at MARIN with the overlapping twin screw alternative to a

single screw 425,000 TDW Tanker were reported [18].

For this ship the obtained power saving with the overlapping twin screw arrangement amounted to

16%, when compared to the original conventional single screw version.

From the above mentioned model tests with overlapping twin screw arrangements carried out at the laboratories of MARIN it can be concluded, that the hull excitation level appears to be somewhat higher than that of a conventional twin screw arrangement, but it is clearly lower than for a single screw arrangement at the same total power. The cavitation properties were considered acceptable but sufficient care must be taken to determine the optimum phasing of the propellers with regard to each other in the overlapping arrangement to avoid unfavourable interaction of cavitating tip vortices.

The overlapping propeller arrangement has about the same hull efficiency compared with the single screw, but the open-water propeller efficiency is significantly higher, due to dividing the loading between two propellers.

From the tests it was clear that outward turning propellers are vital for a good performance of this arrangement and that propeller phasing had no appreciable influence on the efficiency.

Finally, the tests showed that the obtained power savings are fairly constant over a wide range of

speeds and draughts.

For the overlapping propeller arrangements tested at MARIN it was found that with different degrees of overlapping the maximum degree of overlapping does not come off more favourably than a medium degree of overlapping, although the appendages are becoming smaller and one would expect both propellers to cover more frictional wake. This latter observation, however, is true only for the forward propeller which shows a higher wake coefficient with increasing degrees of overlapping. For the aft propellers at the same time, however, the influx velocity is so strongly increased by the action of the forward propeller, that its average wake coefficient decreases with increasing degrees of over-lapping and thereby also reduces the overall efficiency of the overover-lapping propeller arrangement

above a certain degree of overlapping.

The optimum value of the degree of overlapping is obtained with a centre distance of the shafts within the range of 65 %-80% of the propeller diameter.

For a tanker or bulkcarrier in ballast condition the overlapping twin screw arrangement has definite advantages since at same power and RPM the diameter is about 15% smaller than for the single screw version, so that the draught aft can be reduced by about 15% as compared to the minimum draught aft with the single screw version. This can result in additional power savings at light ballast operation and it is clear that the overlapping twin screw arrangement is much more superior in ballast than a low RPM large diameter propeller configuration, where often the necessity arises to increase the draught aft in this condition, which causes required power increases.

Conversely the original single screw diameter might be maintained for the overlapping twin screw arrangement and the RPM can then be reduced which will result in additional power savings.

It will therefore be clear that by combining the principles of lowering RPM and splitting up the thrust over two propellers in an overlapping twin screw arrangement with same diameter as for the

single screw version high improvements can be obtained.

For present day power levels the above examples may not have much value anymore. It is more interesting therefore to mention the results of a recent excercise carried out at MARIN for a 38,000 TDW single screw products tanker with a design shaft power of 8160 kW at 124 RPM for which the experience described above was used in calculating the improvements to be expected by adopting an overlapping twin screw arrangement as an alternative to the single screw version as built. These calculations showed, that overlapping propellers at same power and RPM as the single screw version would be 5.5% more efficient and that another 10.5% improvement can be obtained by designing the overlapping propellers for 90 RPM at the same diameter used for the single screw and for the lower final power now required to attain the originally contracted speed. The final results of these calculations thus showed that the overlapping propellers may require around 16% less power

102 Jahrbuch der Schiffbautechnischen Gesellschaft 77 (1983)

0 0.5 1 13 2 3

Fig. 10. Overlapping twin screw arrangement

The overlapping propeller arrangement and local aft body modifications necessary to optimize the flow into the propellers are shown in Fig. 10.

Finally, the results of the above calculations indicate that the high improvements obtained by combining the principles of low RPM-large diameter and overlapping-twin screw could well exceed

the savings attainable now with a low RPM-large diameter single screw arrangement for the same ship.

It is necessary. however, to further investigate this possibility by actual comparative model tests.

6 Conclusion

In this paper several well established and one possible future way to reduce the required power for newbuildings and existing ships have been discussed. A summary of the potential power savings to be expected from the various arrangements described in the paper can be found in Table 2.

From this Table it will be evident that for newbuildings the low RPM-large diameter and the over-lapping twin screw arrangement solutions will both yield larger power savings than the contra-pro-peller derivatives and the ducted procontra-pro-peller arrangements. The adoption of an overlapping twin screw arrangement in combination with the use of low RPM-large diameter propellers is expected to show the highest power saving potentials and it is strongly recommended to further investigate this alterna-tive to the conventional single screw arrangement especially for ships having to operate frequently at light ballast conditions. Other hybrid solutions may prove to be successful also, but it should be emphasized, that it is doubtful if the complicated physical interactions between the effects of each of the combined energy saving devices can be properly allowed for by an accumulation of separate incre-ments. Comparative model propulsion tests will be necessary for each individual case to properly

quantify the power savings.

For existing ships operating at below design speeds the highest power savings in general can be obtained by retrofitting a redesigned propeller for the lower power and RPM conditions. Ducts ahead of the propeller and contra-propeller derivatives both forward and aft of the propeller can be effective power saving devices if the existing propeller is maintained in case propellers of large diameter cannot be accommodated.

Finally it should be mentioned that Table 2 does not pretend to show an absolute ranking of the various energy saving arrangements, but that it should merely be considered to indicate the range of

possible savings with each individual device.

In deciding which kind of device or measure to choose, it will be necessary to carry out a compara-tive evaluation on a common basis, i.e. through propulsion tests with the actual model of the hull form under consideration.

References

1 Fuel Focus. Lloyd's List, 19th January, 1983.

/ Port Weller's success with semi-tunnel sterns. The Motor Ship, June 1981, pp.99. 3 The new wave in propellers. Marine Engineering Log, Jan. 1981, pp. 65.

4 "Bid to cut fuel costs of ULCC's", Lloyd's List, 9th March 1982. 5 Grim, 0.: Propeller und Leitrad. Jahrbuch STG 60 (1966) S.211.

6 Grim, O.:. Propeller und Leitrad auf dem Forschungsschiff GAUSS", ErgebnisSe und Erfahrungen. Jahrbuch

STG 76 (1982) S.411-422.

7 Wagner, R.: Praktische Ergebnisse mit Gegenpropellern. Jahrbuch STG 13 (1912) S.420.

8 Van Lammeren, W. P. A.; Troost,, L.; Koning, J. G.:, Resistance, Propulsion and Steering of Ships. ,Stam-Haarlem-Holland, 1948.

9 Katsuyoshi Takekuma et al.: Development of Reaction Fin as a Device for Improvement of Propulsive. Perform-ance of High Block Coefficient Ships. Soc. Nay. Architects of Japan, Nov. 1981.

10 World Shipbuilding, 1952, No. 2, pp. 32. 11 IHI Bulletin, Vol. 16, No. 182, May 1982.

12 Carlotti, P.: A note on new forms for ships' sterns.. Transactions of the Institute of Naval Architects, Vol. 91,, 1951, pp. 1

13 Collatz, G.: Treibstoffeinsparung durch asymmetrische Hinterschiffsformen 'fur Einschrauben-Schiffe. HANSA, 1982, Nr.1/2.

14 Breakthrough for ship model basin with new aft design. Lloyd's List, 28th Sept. 1982.

15 Narita, H.; Yagi, H.; Johnson, H. D.; Breves, L. R.: Development and Full-Scale Experiences of' a Novel Inte grated Duct Propeller.. Transactions SNAME 89 (1981) 319.

16 Pien, P. C.; Strom-Tejsen, T.: A proposed new stern arrangement. DTNSRDC Report 24101, 1967..

17 Kerlen, H.; Esveldt, J.; Wereldsma, R.: Propulsions-, Kavitations- und Vibrationsverhalten von iiberlappenden, Propellern fiir em n Containerschiff. Jahrbuch STG 64 (1970) 301..

18 Muntjewerf, J. J.; Oosterveld, M. W. C.: Energy saving propulsion arrangements. Presented' at Lloyd's List Work-shop "Why burn money" at the SHIP-ASIA Conference in Hong Kong, Oct. 1981.

Propulsion research by model testing

Summary. Results of research through model propulsion tests and where possible relevant full-scale correlation: data on the efficiency improvements attainable from the following energy saving propulsion arrangements are reviewed in this paper:

Low RPM-large diameter' propellers,. inclusive the effects of ,skew' and stern tunnels, Propeller retrofits,

Propellers with vane wheel, Contra-propeller derivatives,

Asymmetric shaft gondolas and aft bodies, Ducted propellers and ducts ahead of the propeller, = Overlapping twin screws.

It is concluded that for newbuildings the low RPM-large diameter and the overlapping twin screw arrangement will both yield larger power savings than the contra-propeller derivatives and the ducted propeller arrangements. The adoption of an overlapping twin screw arrangement in combination with the use of low RPM-large diameter propel.

Table 2

Energy saving propulsion ;arrangements Savings in required power per cents of power with conventional single propeller

Low RPM large diameter 5-18

Skew or stern tunnel

1 5

Propeller retrofit 2-10.

Propeller with vane wheel 9-12

Reaction fins

Additional thrusting fins

4 8,

2 5

Asymmetric gondola 5-101

Asymmetric aft body

5 8.

Ducted propeller 5-12

Duct ahead of propeller 5-10

Overlapping twin screw 5-16

Overlapping twin screw + LRPM 10-21 ?

104 Jahrbuch der Schiffbautechnischen Gesellschaft 77 (1983)

lers is expected to show the highest power saving potentials especially for ships having to operate frequently at light ballast conditions.

For existing ships operating at below design speeds the highest power savings in general can be obtained by retrofitting a redesigned propeller for the lower power and RPM conditions, but if the existing propeller is main-tained ducts ahead of the propeller and contra-propeller derivatives both forward and aft of the propeller can be effective power saving devices also.

In deciding which kind of device or measure to choose, it will be necessary to carry out a comparative evaluation on a common basis through model propulsion tests.