|By John Miele||
|December 27, 2012 01:42 AM EST||
The following paper was presented by France Helices’ president a while back. It discusses the difference between cavitation and electrolysis and the impact of both on propellers and propulsion. These two phenomena are often confused and it is important to learn to recognize the difference between them, so that the cause of problems can be determined and adequate fixes can be made. We have translated this version from the original French version. We hope you find it interesting.
I have often had the opportunity to see during my career that there was some blurring in the judgment of some boating professionals, when it comes to establishing with certainty whether the damage caused to marine propellers originate from cavitation or electrolysis. I also often was shocked to hear the theories issued, shamelessly defying the laws of fluid mechanics and the basic rules of the metallurgy of copper alloys.
This confusion is explained, in part, by the fact that the effects produced by one or the other of the phenomena are often quite similar , judging from results, despite the fact that their causes are different or, which is often the case, they show identical features when the result is noticed.
- Erosion of metal
- Loss of performance
I have therefore found it useful to write this paper in order to enable the persons concerned to distinguish between these phenomena and to demystify the effects, providing the science while limiting the most complex calculations.
The visible aspect of the phenomena the photographs below show a real difference:
The two above photographs show canker, or destruction, from electrolysis scattered on the blade and hub, while the bottom shows a circular path that fits perfectly the profile section of the blade at the base of it.
The reality demonstrates that electrolysis cankers are still scattered randomly while cavitation always follows the same path as that of fluid, it ‘ is a circular route to one or more given rays.
It is no less correct that electrolysis-caused cankers can, when they deteriorate the profile of the blade, cause the extension of the blade section, where they are an additional phenomenon of cavitation.
The phenomenon of cavitation differs primarily in three distinct forms.
- Cavitation at the base of the blade usually in areas of strong blade load radius 0.6/0.8 r
- The cavitation of tip radius 0.6/1 r
- Cavitation at the base of the blades on the top surface to the radius 0.2/0.5 r
The generally stronger blade cavitation is usually due to excessive angle of attack of blade. It is bound to the foundry methods which, to avoid overflow, the angular section of the blade blade requires the manufacturer to increase the angle step at the level of the blade attachment.This method, which is to avoid recovery of the blades in the hub to facilitate the manufacture, is doubly harmful in terms of cavitation.
The Cavitation at the Base of the Blades
In fact, the lack of material associated with shortened blade section must be offset by an increase in the thickness, in order to guarantee the mechanical rigidity of the blade and to create the thrust produced by the blade. The disproportionate increase in the thickness of the section causes strike-slip of the fluid on the back of the blade beyond a certain speed section. Increasing the angle step accelerates the phenomenon by bringing closer to the beginning, the leading edge cavitation, which has the effect of digging a trench, initiating blade failure.
The angle at the base of the blade is lower than theoretical, the working face is therefore in cavitation erosion, with the risk of losing a blade after only a few hours of operation the middle of blade cavitation. This cavitation, in the majority of cases, is rooted in propeller exaggerated in one direction or the other.
If the pitch is too high, cavitation occurs on the back of the blade. Instead if cavitation is located on the working of the blade face this means that the pitch is too low in the considered section.
The non-compliance with section profiles can also lead to a cavitating phenomenon. The manufacture should be defined for each blade size. The first link in the manufacturing chain, modelling, is of crucial importance if it is a manually-machined propeller (see standard ISO 484/2)
This cavitation inevitably occurs when the speed exceeds the limit of 40 m/s, and varies according to the angular position of the blade.
The example below reproduces the tests done in a propeller cavitation tunnel, 5-blade, and shows the appearance and disappearance of the cavitation depending on the position of the propeller blade.
The onset of cavitation is also related to the relative position of the blade relative to the appendages of the keel, as the base of the shaft or keel can mask partially or totally the blade to the passage in front of these appendages. The most striking example is that of the single-engined trawler or caged propeller past the stern, often as wide as the propeller blade itself in its upper part.
The Map of the Wake
This map is used to determine phase of study of what will be the impact of rear forms of the ship, and the appendices to hull, on the rate of water supply to the propeller blade, and then at all points of the dial in which the propeller moves.
The Coefficient of Wake
As the blade profile is frozen, it is possible to calculate a coefficient. This coefficient called w – or wake factor, used to determine the average speed of water supply to the propeller blade. The result is that the average speed of water supply to the propeller blade is not the speed of the vessel.
We therefore write this speed in the form:
Va = V ** (1-w)
- Va = speed of water supply to the blade
- V = vessel speed
- w = coefficient of wake
The determination of the coefficient of wake is either:
- By estimate according to the position of the propeller and the type of ship and in this case the keel block coefficient is fundamental to closer to realistic values
- By theoretical calculation such as the table below
In this wake map, that represents the ship’s wake at a speed of 14 knots, it measures the significant variation in the speed of water supply to the blade. w ,wake coefficient, varies from.001 when the blade is set at 220 ° to 0.85 when the blade is at 0 °.
Wake affects the rate of water supply to the blade, as well as the pressure on the blade, with a resulting non-negligible impact on the variation of the output thrust. The propeller shaft stops receiving the thrust of the propeller.
In some cases it is the use of the propeller which is the cause of cavitation. The photograph below shows a propeller, perfectly calculated, that cavitates and shows traces of removal of metal on the top surface of the leading edge.
The propeller above, mounted on a passenger ferry, suffered forceful accelerations from a breakdown, from 0 to maximum speed in less than a second – the resulting breakdown is immediate – the propeller is eroded in less than 2 hours of operation, when the rise in engine rpm and propeller workload vary, depending on resistance to the hull of the vessel, and a perfectly calculated, from the point of view of dimension, propeller. The propeller, in diameter and in blade surface, may run the risk of cavitating due to a too forceful acceleration, either at a specific operating point, especially in the case of planning hulls when planning.
The cavitation photo shows stretched erosion, while the electrolysis photo shows a misshapen canker.
On cavitation, a rough look showing contiguous round cavities, all highlighting the original color of the bare metal.
Electrolysis shows different colors, indicating that the material is attacked. The alloy takes green and brown colors, quite smooth and uniform at the bottom of the etched surface.
The Means to Avoid Cavitation
Cavitation is a phenomenon seen in with heavily loaded propellers, that beyond a certain number of critical turns, there is a gradual break of the stream of water and a drop of the thrust.
What keeps the ship from reaching the calculated performance?
The signs are noticeable before arriving at this stage, such as:
- Blade erosion
Significant variation of the wake factor may be the cause.
This is why the calculation is of crucial importance in the phase of study, in order to prevent this type of inconvenience.
Cavitation occurs when the pressure δp≥ po-pv
po and pv are absolute pressure
the most commonly used method is to determine the cavitating σ criterion
The criterion can be calculated or taken from a chart, or σ is simply determined from a formula, such as: σ = 380.94 Va ^-1.802
This coefficient allows a reasonable approach to the problem, but in case of high risk, you should recalculate the total cavitating criterion of
rays of the blade when the blade is in the position. The more useful position for the calculation is in the vertical position.
Coefficient σx (where x is the radius of the section)
Hydrostatic pressure value for each section of blade σx
Written σx = (Pa + Pg (h – xR) / p/2 (Va² + (2 pi() * Xr)²)
or h = depth of immersion of the Xr range when the blade is in a vertical position in general the criteria used is σ0.7r, where p is more loaded.
If the cross is located under the red line there is no risk of cavitation.
Iif it lies between 2 lines, the risk is inevitable.
If it is above the pink line, the propeller is completely cavitating.
The drawing of the blade must be modified to reject the bubbles of steam formed by cavitation, to as far as possible away from the trailing edge, and to use this feature as a benefit by the appropriate blade drawing.
In this case, and in contrast to a laminar flow propeller in which the circumferential speed is less than 40 m/s, it will be possible to work the propeller to a circumferential speed of close to 100 m/s. In this case we could call these supercavitating propellers, while, when the blades are partly in the air (surface propellers) they will be named superventilating.
The Quality of Materials
The mechanical resistance of materials is essential in the fight against the erosive effects of cavitation. The material must be tough and more mechanical strength is required to withstand the explosions generated by cavitation.
Thus, aluminum bronze for the mechanical resistance is 630N/mm² is preferable, available rather than the bronze-type brass manganese alloy, where mechanical resistance is rarely greater than 540/mm².
The Quality of Machining
In the case of propellers under strong pressure, the attention to profiles and surface roughness are a necessity to control as far as possible the effects of erosion. The best way is the machining of numerical control of the propeller.
To ensure the accuracy of the profiles created by manual polishing, they must minimally correspond to the ISO 2632 /DIS class or a nominal value RA (um) 0.4.
The phenomenon of electrolysis can attack the blade at any location.
It depends on the flow of current and of the relative position of parts attacked over this flow. It is not uncommon to see other pieces are attacked, such as shafts or hull valves.
Similarly it is not rare to observe in the case of hulls with double shafts that only one side is attacked. We will return later on what causes these attacks, but in this area, there is also confusion: Do not, for example, confused galvanic corrosion with the electrolytic corrosion.
Galvanic corrosion is related to the joining of different metals in a conducting medium (seawater is one). It often results from of sulfation to the equipment in contact with seawater.
Electrolytic corrosion is generated by an external current often associated with an electrical battery or a terrestrial power source connected or not on board. This explains why, in the case of the electrolytic corrosion, two similar metals may corrode by the phenomenon if stacked and if one of the metals is connected to a power source of reversed polarity.
In this case, the propeller and shaft will destroy themselves, and so removed metal will settle on the hull valve. Less noble metal will deteriorate faster than a noble metal, but in the end even the best will eventually deteriorate.
Corrosion occurs between materials, and depends fundamentally on the nobility of the materials with each other. If the gap is large, there is more risk. The table below, expressed in volts, shows differences between materials.
The values indicated are those generally accepted, and they can vary depending on factors such as the temperature or salinity.
Stainless steels become more active in the case of scratches or concretions.
To reduce the risk, avoid during machining sharp angles and strikes from tools or scratches due to handling.
Corrosion by Deoxygenation
This corrosion generally applies in parts of the propeller shaft in contact with inert materials, such as nitrile hydrolube rings. The absence of circulation of water between the propeller shaft and the ring in contact with the shaft part causes a furrow of electrolysis perfectly marrying the support on the ring.
This phenomenon occurs when the shaft line remained motionless for an indefinite period. In order to avoid this problem, the shaft line must be turned, at minimum, once a week. It should be noted that the phenomenon may occur even in the case of use of the finest materials.
Removal of Electrolytic Phenomena in the Project Phase
It is useful to determine the possible causes of electrolysis and delivering solutions which are suitable. In the use phase, maintenance followed as recommended, as well as observation of parts which can be attacked, the careful observation of some preventive measures will ensure the longevity of the propulsion systems.
One should consider the risk:
- Choose suitable materials, for example, in case of a small ship in fiberglass or of a tanker in steel
- The navigation area
- The water temperature and salinity, which can vary significantly.
- Updates to the possible mass of electrical appliances.
- The fact that the engine probes are bipolar or monopolar (back by the mass of the engine)
In the Use Phase
To ensure the monitoring of the quality of protection anodes (the total weight of the anodes attached under the shell is equal to minimum to 1% of the weight of the deposited material that must be protected in theory) during maintenance disassembly, proceed to the foots of hidden parts (cones of propeller shafts, mounted cable glands, worn hydrolube rings).
Many manufacturers deliver kit material ready to install for each shaft diameter.(following an installation type produced by CIS – GMBH)
Use protection zinc whose purity is 99.99%. Many manufacturers use zinc recovery or zinc polluted by the proximity of other materials in foundries.
Ensure before you put the vessel in the water that the anodes are not inadvertently painted. Brush hull valves, especially when they are brass. If these are pink in color, you should replace them. These parts must always present a yellow gold. Any silver and pink spots show the parts suffered an electrolytic attack and that they have lost a minimum 50% of mechanical strength.
Do not paint propeller shafts or propeller. Though they may look good, the risk is great to see after a few days at sea, the paint beginning to bubble; The bubble in which will allow seawater to enter and stagnate and cause an electrolytic attack that may cause the breakage of a propeller shaft of large diameter over a few weeks.
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