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The SMARTUG project is a Research project that the CINTRANAVAL Group is developing in cooperation with the firms SCHOTTEL-GmbH and INDUSTRIAS FERRI.

The goals of this project are to contribute to the sector with ideas that could improve the current state-of-the-art in the tug-boats design in the following aspects:

1. To reduce the environmental footprint of the tug-boat during her operation.

2. To increase the safety of the crew (and personnel) working onboard this kind of vessels.

3. To increase the safety of the vessel, herself, against possible damage.

4. To increase the energy efficiency of the tug-boat.

5. To increase the comfort of the people that have to live onboard the vessel.

6. o increase the performance of the vessel, compared with other vessels of the same size, thereby increasing the safe operating assistance provided to other vessels.

7.      To reduce the operation costs of the vessel, reducing the overhauling costs.

8.      To keep the construction cost at the same level as a conventional vessel of the same size and performance.

For this purpose, the market trends were analyzed and some European Owners considered as relevant in the sector were contacted in order to know their needs and concerns.

An analysis of their responses indicated that the tug-boat that best fits the objectives of this study would be a tug-boat designed for harbour operations, with deep sea towage capability. The most interesting features would include Escort capability, high manoeuvrability, an external fire-fighting system, a reduced crew, and a Bollard Pull of approximately 80 t.

Another important feature would be to have a standard operation time of about 1500 h/year. Consequently, an Operative Profile has been determined, in terms of Total Required Propulsion Power, as shown in the following figure.

Fig 0.1

This document will be focused on the analysis of different propulsion configurations for a tug-boat of the above mentioned particulars and in accordance to the mentioned operative profile.

For the different propulsion alternatives, the Rate of Propulsion Engines will be analyzed, as well as the TBO (Time Between Overhauls). Additionally, the fuel consumption of the different options will be analyzed.


Despite there is not a Propulsion Configuration that unanimously can be considered as the preferred by the Owners, there is a slight majority towards the ASD (Azimuthal Stern Drive) between the Owners that participated in this study. Therefore this configuration has been chosen as the base for this study.

In a first step, the following configurations have been considered:

  1. Diesel-mechanical system of direct transmission.
  2. Hybrid system with PTI
  3. Diesel-Electric system with two Gen-Sets, shared load


The Diesel-mechanical system of direct transmission is formed by two azimuthal Propulsion Units, located at the vessel stern area, that are driven, each one,  by a Diesel Engine, through a shaft line, usually with cardan shafts.

This system has an advantage, when compared with other systems, that is the direct transmission of the power from the Diesel Engine to the Propulsion Unit. In this way, despite there is some friction in the shaft bearings and the cardan shafts, it can be assumed that there is not power loss between the Power Take Off of the Diesel Engine and the Power Take In of the Propulsion Unit.

Another advantage of this system is the simplicity/economy in the installation of this configuration, as no other equipment, with the implied extra cost, is required.

It is an advantage, as well, when compared with other systems, the simplicity in the operation of the system, as the power that is required by the Propulsion Unit is directly developed by the Diesel Engine, without the necessity of any additional Power Management System (PMS).

However, the main disadvantages of the system come also from its simplicity. As the power is directly transmitted without any flexible management system, when the required power is low, each Diesel Engine has to develop exactly the power required any time by the Propulsion Unit, not more, not less, and therefore the Diesel Engines work very often far away from the optimum rates.

The operation scheme of this system can be represented by the following figure, that show the developed power of each Diesel Engine against the required power of each Propulsion Unit.

Fig 1.1.1

 If we apply the Operative Profile that was obtained from the analysis of the answer from the Owners, this operation scheme imply that both Diesel Engines are working during the 100% of the time the vessel is under operation. Additionally, in the following figure the operation scheme of both Diesel Engines is shown in terms of vessel operation time. In this figure it can be observed that both Diesel Engines will be most of the time working far away of the optimum rate.

Fig 1.1.2


The second disadvantage of this system comes also from this simplicity and the rigidity of the system. When an External Fire-Fighting system is required, it is necessary to install either a controllable Pitch Propeller system or a slipping clutch, in order to control the power that is transmitted to the Propulsion Units when the Diesel Engines are powering the FIFI pumps. These elements are expensive and with high overhauling cost.


Before we start to analyze this configurations we have to clarify that, despite we are defining this system as hybrid, it is not strictly an hybrid system as we are not considering the use of batteries.

At the current state-of-the-art the cost of the batteries and the subsequent control system for powering a tug-boat of the particulars we are considering in this study is too high for considering it as reasonable. As an order of magnitude, we will indicate that the cost of the batteries for powering the tug-boat with enough power for free sailing (about 15% of the total power) during just half an hour can reach about 20% of the cost of the tug-boat.

For this reason, we considered much more interesting a system in which each Propulsion Unit is powered by a Diesel Engine of about 4/5 of the Total Power. The system will be boosted by an Electric Motor in order to complement it up to the maximum power of the Propulsion Unit. The Electric Motors would be powered by two Gen-Sets of sufficient power for feeding them, as well as the tug-boat general electric plant, with the necessary PMS.

In this way, when the required propulsion power is low, the tug-boat may work on the Electric Motors, starting the Diesel Engines only when the required power exceeds a determinate value.

This system has been developed by the firm SCHOTTEL, in their models SRP4000PTI, that integrate an Electric Motor in the Propulsion Unit, making unnecessary any additional space or complexity in the shaft line or any additional gear.

In the following figure, it can be seen the operation scheme of the propulsion Diesel Engines and the Gen-Sets. In this figure a constant electric load due to the general electric plant of the tug-boat has been considered.

Fig 1.2.1

As it can be seen in the figure, the tug-boat would be powered by the Electric Motors (and therefore by the Gen-Sets), up to a propulsion power of 20%, when the Diesel Engines would be started and one Gen-Set would be feeding only the general electric plant.

From this point, and up to 80% of the Propulsion Power, the Diesel Engines would power the Propulsion Units. Then, at this 80% and above, the system would be boosted by the Electric Motors.

When we apply this system ot the Operative Profile, we obtain the following figure in which it can be seen the operation time of each Diesel Engine or Gen-Set at each rate.

Fig 1.2.2

The first conclusion that can be extracted from this figure is that the very low rates (below 30%), that were the most frequent rates in the configuration of 1.1, disappeared for the propulsion Diesel Engines, improving the efficiency and overhauls for these engines.

The second conclusion is that the propulsion Diesel Engines will be stopped during 66% of the operation time. This will increase very significantly the TBO, reducing consequently the overhaul cost. In fact, we can indicate that, if for the configuration indicated in 1.1, the propulsion Diesel Engines are working 1500 h per year, with this configuration these engines will be working 500 hours per year.

Regarding the Gen-Sets, form the figure 1.2.1 it seems that one of the Gen-Set is working the 100% of the Operation Time, while the other on is stopped the 82%of the operation time. However, as both Gen-Sets are identical and interchangeable, the total time each Gen-Set will be working will be 59% of the Operation Time, that means that they will work 885 h per year.

Finally, as an advantage when compared with the base configuration indicated in 1.1, a FIFI system can be installed, driven by the propulsion Diesel Engines. With this configuration, neither CPP nor slipping clutch is required as the vessel control during this Operation Mode can be done by the Electric Motors, running separately from the Diesel Engines.


The third system we analysed in this study is formed by two Propulsion Units that are powered, each one, by one Electric Motor.

For feeding the Electric Motors, two Gen-Sets, of sufficient capacity for supporting also the general electric plant of the tug-boat, are foreseen. They will be running in function of the required power for the propulsion. The PMS will share the load between both Gen-Sets in the way that, while the required power is low, only one of them will be running. When the load on this Gen-Set is 100% of its capacity, the second one is started and the load is shared between both of them. The following figure shows this operation scheme.

Figure 1.3.1.

If we apply the Operative Profile, we can obtain the following figure:

Figure 1.3.2

The consequence that can be obtained from this figure is that, despite the TBO is significantly increased as one of the Gen-Sets is stopped 93% of the time, the very low rates are not avoided. Consequently, despite this is an improvement when compared with the configuration indicated in 1.1, the most convenient system is the “hybrid” one indicated in 1.2.


The configurations indicated in 1 are a summary of those studied within the frame of the SMARTUG project regarding the propulsive configuration. Other configurations were also studied but, due to the similarity to those indicated in 1, they were not included in this document.

As a result of what has been explained in the previous point, it is proposed, as an improvement of the state-of-the-art in the design of harbour tug-boats, the configuration of the Propulsion System in a battery-less hybrid system, as the indicated in the point 1.2 above.

From this point, the fuel consumption for the base configuration (Diesel Mechanical system of direct transmission) and the proposed configuration (Hybrid system with PTI), for different options in the market, in order to know whether there is an improvement in this regard, or not.

The first conclusion is that this matter is strongly dependant on the specific fuel consumption curve of the engines that are applied to the system, and therefore no general conclusions can be obtained regarding the consumption, but case-by-case analysis has to be performed.

However, and in order to give an idea of the possibilities of the system, we analyzed the fuel consumption of a commercial 2600 kW Diesel Engine for the propulsion configuration of 1.1 Diesel Mechanical system of Direct Transmission.

We chose this Engine in order to assure, with the SRP4000 Propulsion Unit, the minimum Bollard Pull of 80 t that we indicated in the introduction.

The propulsion Diesel Engine that we considered has the specific fuel consumption curve that is shown in the following figure.

Figure 2.1.

In order to have a more complete analysis of the different systems in terms of fuel consumption, for this configuration a 150 kW Gen-Set is considered for supporting the load of the General Electric Plant of the tug-boat. This Gen-Set is assumed to have a specific fuel consumption of 215 g/kWh and a continuous work has been assumed during the whole operation time of the tug-boat.

With all these considerations, and applying the operative profile indicated in 0, the fuel consumption of the tug-boat would be about 316 l/h, that, for the 1500 h/year Operation Time, will drive to a total fuel consumption of 474 m3 per year.

In order to compare the proposed system in terms of fuel consumption, we chose a commercial propulsion Diesel Engine of 2060 kW, with the specific fuel consumption curve shown in the following figure.

Figure 2.2.

The propulsion system will be boosted by 540 kW Electric Motors, powered by two Gen-Sets of 600 kW, that will feed the electric motors and the general electric plant of the tug-boat. These Gen-Sets would have the specific fuel consumption curve indicated in the following figure.


Figure 2.3.

In these conditions and with the operative profile indicated above, the total fuel consumption of the tug-boat would be 265 l/h, that, for the 1500 h per year indicated in 1 imply a total fuel consumption of 398 m3 per year.

This implies that the proposed system has a fuel saving of 76 m3 per year that is a 16% saving, compared to the base ones.

We can therefore conclude that the proposed system is an improvement in terms of overhauling cost and in fuel consumption cost, improving the environmental footprint of the tug-boat.


As part of the SMARTUG project, which is intended to improve tug-boat design, safety, comfort and provide a reduced environmental footprint, an analysis of the propulsion configuration has been carried out for an 80 TBP harbour ASD tug-boat.

The Propulsion Configurations that have been analyzed were the following:

  1. Diesel Mechanical system of Direct Transmission
  2. Hybrid system with PTI
  3. Diesel Electric system with two Gen-Sets, shared load.

It has been concluded that the most interesting configuration, emphasising engine working rates and overhauls, is the second one due to the following reasons:

  1. The working rate of the engines is closer to the optimum one, avoiding the very low rates.
  2. The working time of the engines is reduced, increasing the Time Between Overhauls.

Additionally, the fuel consumption of the different configurations has been analysed.

In this regard, a strong dependence on the specific fuel consumption curve of the commercially available engines has been detected, and therefore no general conclusions can be obtained. It would be necessary to perform a specific study on a case-by-case basis.

However, in order to give an idea of the order of magnitude, the fuel savings can reach 16% for the second configuration compared with the base one.

For all these reasons, the recommended configuration in the framework of the SMARTUG project is the hybrid system with PTI, due to the advantages implied in terms of operational cost and environmental footprint.