ADVANCED PROPULSION SYSTEM FOR FISHING VESSELS
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Key words: Propeller, design, performance, composite materials, tests
Abstract: this project’s main objective is to increase fishing sector competitiveness by the use of composite materials in the propeller manufacturing reaching great advantages in the propulsion. These advantages are energy saving together with weight and noise reductions.
During the project there was a need in bringing together both the composite materials science and design and analysis techniques as well as the development and manufacturing of a prototype and feasibility, durability and service behaviour studies.
Results obtained from its lightness (60% reduction in weight), lower friction coefficient, reduced noise and vibrations as well as important energy saving show that a brilliant future awaits composite materials selection for the improvement of currently existing propulsion systems.
Nowadays fishing industry appears as one of the most critical activity areas. International stressed competence and the lack of natural resources are causing the need in emphasising more correctly fishing sector enterprises technological R+D plans with the aim of improving competitiveness with respect to its equivalents within corresponding sector.
Thus, naval construction should efficiently incorporate those technological advances from production technologies field, advances materials and their processes to place different fishing arts well positioned.
Although CADEM had contacted the naval sector before, it is at the end of the 80ies when a complete Naval-energy project starts: ECOBUQUE PROGRAMME
This programme objective is the development of a Energy Saving in Boat Service where due to the lack of actions driven to maintenance, energy saving could be very high. Moreover, a boat could be defined as an autonomic enterprise in all sense. A boat is full equipped with all its industrial facilities.
This project’s main objective is, as mentioned above, to increase fishing sector competitiveness in the market by the introduction of advanced technologies which allow offering increased characteristics Fishing vessels with respect to currently existing boats. In this case, innovation is based in the fishing vessels propulsion system and, more specifically, in the propeller. This project development highlights some important factors as:
· Weight reduction by using composite materials
· Crew security and comfort
· Energy savings
· Boat manoeuvrability
·Reduction of Propulsion system noise and vibration
Marine propeller has an efficiency close to 50%. That means that 50% of the propulsion engine is not used for boat impulsion but dissipated by friction between the propeller blades and the water. Any slight improvement in the marine propeller efficiency gives a high benefit to the boat an its exploitation.
Composite material manufactured propeller will have an even and dull surface that will be maintained along its service life (cavitation will be null). It will also present any other advantages as:
· Lower friction coefficient
· Evident energy saving
· Null audible noise emission
· Lower weight
This project has been carried out in 18 months divided in the following phases
2 PROPELLER DESIGN
A boat propeller design is conditioned by the propulsion system characteristics and by the boat geometry. In this project, a 5-blade composite propeller with a 1049mm-propeller pitch has been set on a fishing vessel known as "Gure Ama Martina" of 21 meters long equipped with a 480HP engine.
3 PROPELLER CONCEPTUAL DESIGN IN COMPOSITE MATERIALS
Propeller conceptual design in composite materials has been based in the following considerations obtaining an important reduction in weigh and inertia.
Geometric propeller shape conservation (blades active face) in accordance to the geometry and propulsion system of the selected boat
Design configuration of the currently manufactured propellers in composite materials both for aeronautic and naval sector
Failure mode of the manufactured propeller in composite materials during a previous project in this research field
Using of a production method that allows an ulterior process repeatability and the propeller performance in a marine environment especially corrosive, external agents impact, cavitation, etc.
In all cases found in other sectors, manufacturing the cube and blades separately and then joining them mechanically makes propellers. Cube is normally metallic whereas blades are made of composite materials.
There are two tendencies concerning manufacturing: manufacturing by preimpregnates and manufacturing by dry tissue and resin transfer moulding (RTM). First process allows having a higher control on the fibrer content and distribution so that specified performance and process reproducibility are guaranteed. Second process is better for a mass production but requires high investments in moulds. After a deep evaluation of both alternatives preimpregnates technology has been chosen.
Previous studies on propellers lead to consider that cube-blades intermediate zone is especially sensitive to design and position of materials. Considering very difficult that loads could be transferred from blades to cube in an integral construction of the assembly.
Propeller conceptual design in composite materials includes two well different parts:
- Metallic cube. It has the internal cone incorporated for the shaft housing and the corresponding keyslot. The cube has several blades incorporated from where the main blades of the propeller, made in composite materials, come out.
- Blades. They are made completely in composite materials. They include an external coating that forms the wet surface and makes the function of giving the required geometric shape and transferring pressure loads originated on the surface to the torsion box that, at the same time, transfers structural loads to cube blades.
Joint between blades and metallic cube is made by a screwed assembly that works under torsion and compression. Each blade has in its lower area a hole that fits in metallic blades that, following the blade geometry, come from the metallic cube. Once the blades assembly is carried out they are screwed and gaps are filled with structural adhesive.
Design hypothesis used and detailed analysis of the propeller behaviour in service have allowed to define a design concept that combines light advanced materials selection, the use of a manufacturing process susceptible of being repetitive and obtaining an structure able of supporting working loads that may appear during the propeller service life. Thus, result seems to be satisfactory taking into account the product is a prototype.
Currently manufactured propeller is about 216 Kg weight and the one proposed in this project supposes just a 38% in weigh. Considerable energy saving and noise reduction are reached.
Although mechanical characteristics have been determined in a simplified way, only a more precise analysis (by advanced numeric methods) could allow to obtain adequate information proceed with a design optimization.
Concerning to behaviour in service, only a test in real working conditions on the prototype will be able to determine the accuracy of the design in parameters such as corrosion resistance, impact resistance, cavitation resistance, etc.
3 FINITE ELEMENT ANALYSIS
As mentioned above, when a product is going to be subjected to different kind of stresses it is interesting, once the design is determined, to make a simulation to see how its mechanical behaviour is.
The new propeller has two parts, the blades and the core. Due to the complexity of the propeller geometry and with the aim of simplifying its modelization, the study of its behaviour is undertaken in two different phases; In the first phase and concerning 5 blades behaviour will be theoretically the same, just one blade is analysed. In the second phase results obtained in the first phase are applied to the core in each of its blade junction.
In order to define the section of the faces subjected to pressure as well as their thickness in 3-D CATIA programme has been used. For that purpose, Classification society specified standards have been followed resulting in a very laborious work to obtain good curves that allow defining acceptable surfaces.
Elements used in the analysis to form the net were thin shells. The net is based on the surface. 3061 elements of 4 nodes each were generated.
Crucial moment in propeller finite element analysis came when it was necessary to apply stresses that propeller blade should stand.
Propeller designer brought initial data: maximum propeller impulse, 4415 Kg and maximum torque absorbed by the propeller, 764 Kg. By the hydrodynamic theory it was known that stresses that act on the blade have a parabolic distribution for values between 0,2R and R until a maximum of 0,7R. R value is de distance of each point of the propeller surface to the propeller axis which in our case matches the Z axis of the models.
With the aim of simulating the real behaviour of the blade an approach has been done consisting on calculating the group of loads applied to the external nodes of the propeller blades whose resultant force is the pushing force and whose resultant moment is the maximum torque absorbed by the propeller. Furthermore, these loads must follow a parabolic distribution as a function of the radius and the maximum value must be 0.7R. As mentioned before, the design has been done on only one blade to simplify the model and thus the values of impulse force and the maximum absorbed moment has been divided.
The core has 6220 "bricks" elements of 8 nodes. As mentioned before, the loads applied on the cube are the reaction forces obtained in the simulation of the blade behaviour. These forces are obtained for only one blade. As the core is simulated with the 5 blades, the forces are rotated to have all them loaded.
In addition to the before imposed forces centrifugal force must also be considered generated due to propeller rotation. The rotation speed considered estimating this force has been 450 r.p.m.
The calculations, as usual, have been done with the hypothesis of a quasi-static state of loads, not considering the possible dynamic peaks that can occur during normal navigation. The safety margin over the maximum expected stresses provided by the blade design indicates, however, that the blade can withstand the dynamic stresses easily. This must be certified with a series of tests during normal navigation with the new propeller.
It is necessary to undertake a serial of tests in order to check out the technical accuracy of the design, once the prototype is manufactured
Propeller benchmark static test
The push test is undertaken in each one of the blades, through an axial load application equivalent to the boat suffered effort divided by the total number of blades. It results to be 9.526 N. The water opposition to the propeller rotation is also analysed by means of a punctual load in the transversal direction and located at 0,7 R, where the maximum load is located in the parabolic distribution of the efforts generated by the water. The load is 3.836 N. Blade tests are undertaken with the propeller assembled in the gear shaft of a boat.
Figure - Propeller benchmark static test
In situ propeller test
Once the results obtained in the previous test are successful, the propeller is assembled in the boat in order to measure the Power Absorption, noises and vibrations. The measures were undertaken by the Oceanic and Naval Systems Department of the Escuela Técnica Superior de Ingenieros Navales from the Universidad Politénica de Madrid.
.- Power absorption by the shaft in normal performance. As a result of this study the possible energy savings will be determined, as it was one of the main aims of the project.
Figure- Power test in the boat
.- Measurements of the noise and vibration generated by the propeller during the normal performance of the boat . This is an important factor due to the fishing characteristic of the boat, the lower the noise generated by the propulsion system is, the more amount of fishes that will be captured.
Although the tests indicated that the composite propeller generates a slightly superior amount of noise than a conventional one, both rotating in the same conditions, it must be said that the composite propeller, once the engine is rotating at 1.450 r.p.m, shows an energy absorption of 411 CV while the conventional one, once the engine is rotating at 1.750 r.p.m, shows an energy absorption of 390 CV. The lower the revolutions are, the more power absorption the composite propeller shows.
So it was quiet clear that the comparison had to be undertaken at the same level of power absorption, and in this case, the composite propeller is less noisy than the conventional one.
Likewise, with the engine performing the maximum power, the vibrations felt by the ship crew were less when using the composite propeller than when using the conventional one.
From the power absorption measurements at different revolutions, it can be said that composite propeller, with the same initial pitch than the conventional one, suffered a blade deformation due to the push and torque absorption, causing a 15% increase of the propeller effective pitch. As the conventional propeller seems to be optimal for this boat and engine, once the power measurements in the tail shaft report confirms it, it is concluded that the composite propeller, with the effective pitch achieved by the blades deformation, it was not optimal and therefore its hydrodynamic efficiency was lower and the excessive pitch caused turbulence due to the inadequate angle of attack with regard to incoming water flow.
These turbulences may also cause an increase of the underwater noise.
The blades elastic deformation mentioned formerly caused this composite propeller not to be optimal for this boat and engine. Thus, propulsion efficiency was lower and, further more, the engine did not reach its nominal revolutions so there were 300 left and it was not working under the correct effect of the turbo-blower nor the post-cooling of the combustion air was not the correct one. Feasible conclusions can not be made in this sense.
Further more, above mentioned finishing factors, blade thickness, etc., that are also mentioned in the power measurement report conclusions, prejudice making a comparison between the composite propeller and the conventional propeller.
From the undertaken tests it can be said that conceptual design of a composite propeller seems to be adequate, as well as materials used. From a mechanical point of view the mooring area can be successfully considered as it has well supported all loads it has been under.
Weight reduction in the composite propeller has been 62% againts the traditional in bronze.
It has been observed that blades tested in bench and in situ suffer a high elastic deformation which means that once load is release initial position is recovered. However, this makes propeller pitch changes as engine power is increasing so propeller working is not the optimum and its efficiency becomes lower.
It is very important to considered, from the composite propeller design step, a pitch reduced by 15% so its working is comparable to the conventional propeller and, if possible, with the same thickness and surface finishing than the conventional.
For that all, this prototype design may be considered as a research project where subsequent development steps will be necessary until obtaining an optimum and competitive product in the market.