What you need to know about the propeller

How does a propeller work? A propeller converts rotation engine shaft at point blank range - the force pushing the ship forward. When the propeller rotates, a vacuum is created on the surfaces of its blades facing forward - in the direction of the vessel's movement (sucking in), and on the back-facing (forcing) & mdash; increased water pressure. As a result of the pressure difference on the blades, a force Y arises (it is called lifting). Having decomposed the force into components — one directed towards the movement of the vessel, and the second perpendicular to it, we get the force P, which creates the thrust of the propeller, and the force T, which forms the torque that is overcome by the engine.

  The thrust depends to a large extent on the angle of attack a of the blade profile. The optimal value for high-speed boat propellers is 4—8°. If a is greater than the optimal value, then the engine power is unproductively spent on overcoming a large torque, but if the angle of attack is small, the lift force and, consequently, the stop P will be small, the engine power will be underused.

  In the diagram illustrating the nature of the interaction of the blade and water, a can be represented as the angle between the direction of the velocity vector of the flow W incident on the blade and the injection surface. The flow velocity vector W is formed by the geometric addition of the velocity vectors of the translational movement Va of the propeller together with the vessel and the rotation velocity Vr, i.e. the velocity of the blade in a plane perpendicular to the axis of the propeller.

  The helical surface of the blade. The figure shows the forces and velocities acting in any one specific cross section of the blade, located at some specific radius r of the propeller. The circumferential rotation speed V depends on the radius on which the section is located (Vr = 2x p x rx n, where n is the rotational speed of the screw, rev / s), while the translational speed of the screw Va remains constant for any section of the blade. Thus, the larger r, i.e., the closer the area under consideration is to the end of the blade, the greater the circumferential speed Vr, and hence the total speed W.

  Since the side Va in the triangle of the speeds under consideration remains constant, as the blade section moves away from the center, it is necessary to turn the blades at a large angle to the propeller axis so that a retains the optimal value, i.e., remains the same for all sections. Thus, a helical surface with a constant pitch H is obtained. Recall that the pitch of the propeller is the movement of any point of the blade along the axis in one complete revolution of the propeller.

  The figure helps to represent the complex helical surface of the blade. During the operation of the propeller, the blade, as it were, slides along the guide squares, which have a different base length at each radius, but the same height - the pitch H, and rises in one revolution by the value H. The product of the pitch and the rotational speed (Hn) is the theoretical speed of the propeller along the axis.

  Vessel speed, propeller speed and slip. When moving, the ship's hull drags water with it, creating a tail stream, so the actual speed of the propeller meeting water Va is always somewhat less than the actual speed of the ship V. the difference is small - only 2 - 5%, since their body slides through the water and almost does not "pull"; her behind her. For boats traveling at an average speed, this difference is 5 - 8%, and for low-speed displacement deep-sitting boats it reaches 15 - 20%. Let us now compare the theoretical speed of the screw Hn with the speed of its actual movement Va relative to the water flow.

  The difference Hn - Va, called slip, determines the work on the propeller mouth at an angle of attack a to the water flow with speed W. The ratio of slip to the theoretical propeller speed in percent is called relative slip:
s = (Hn-Va)/Hn.

  The slip reaches its maximum value (100%) when the propeller is operating on a ship moored to the shore. The propellers of light racing boats at full speed have the smallest slip (8 & mdash; 15%); for propellers of planing pleasure boats and boats, slip reaches 15 —25%, for heavy displacement boats 20—40%, and for sailing yachts with an auxiliary engine, 50 - 70%.

  Light or heavy propeller. The diameter and pitch of the propeller are the most important parameters that determine the degree of use of engine power, and therefore the ability to achieve the highest speed of the vessel.

  Each engine has its own so-called external characteristic - the dependence of the power removed from the shaft on the crankshaft speed with the carburetor throttle fully open. Such a characteristic for the outboard motor "Whirlwind", for example, is shown in the figure (curve 1). Maximum power of 21.5 liters, s. engine develops at 5000 rpm.

  The power that is absorbed by the propeller on a given boat, depending on the engine speed, is shown in the same figure not by one, but by three curves - propeller characteristics 2, 3 and 4, each of which corresponds to a specific propeller, i.e. a propeller of a certain pitch and diameter.

  With an increase in both the pitch and the diameter of the propeller above the optimal values, the blades capture and throw back too much water: the emphasis increases, but at the same time the required torque on the propeller shaft also increases. The screw characteristic 2 of such a screw intersects with the external characteristic of the engine 1 at point A. This means that the engine has already reached the limit - the maximum torque value and is not able to turn the propeller at a high speed, i.e., does not develop the rated speed and corresponding rated power. In this case, the position of point A shows that the engine gives only 12 hp. With. power instead of 22 liters. With. Such a propeller is called hydrodynamically heavy.

  On the contrary, if the pitch or diameter of the screw is small (curve 4), both the stop and the required torque will be smaller, so the engine will not only easily develop, but also exceed the nominal crankshaft speed. Its mode of operation will be characterized by point C. And in this case, the engine power will not be fully used, and operation at too high speeds is associated with dangerously high wear of parts. At the same time, it must be emphasized that since the thrust of the propeller is small, the vessel will not reach the maximum possible speed. Such a screw is called hydrodynamically light.

  A propeller that allows for a particular combination of vessel and engine to fully use the power of the latter is called matched. For the example under consideration, such a matched screw has characteristic 3, which intersects with the external characteristic of the engine at point B, corresponding to its maximum power.

  The figure illustrates the importance of the correct selection of the propeller on the example of the motorboat "Crimea" with outboard motor “Whirlwind” on board it develops a speed of 37 km / h. With a full load of 4 people, the speed of the boat is reduced to 22 km/h. When replacing the screw with another with a pitch of 264 mm, the speed with a full load increases to 32 km / h. The best results are achieved with a propeller having a pitch ratio H / D = 1.0 (pitch and diameter are 240 mm): the maximum speed increases to 40 — 42 km / h, the speed with full load — up to 38 km/h. It is easy to draw a conclusion about the significant fuel savings that can be obtained with a reduced pitch propeller. If with a standard propeller at a load of 400 kg 400 g of fuel is consumed for each kilometer traveled, then when a propeller with a pitch of 240 mm is installed, the fuel consumption will be 237 g / km.

  It should be noted that there are an infinite number of matched propellers for a particular combination of vessel and motor. Indeed, a propeller with a slightly larger diameter but slightly smaller pitch will load the engine just as much as a propeller with a smaller diameter and larger pitch. There is a rule: when replacing a propeller matched with the hull and engine with another propeller with close values of D and H (the difference is not more than 10%), it is required that the sum of these values for the old and new propellers be equal.

  However, out of this set of matched screws, only one screw, with specific values of D and H, will have the highest efficiency. Such a screw is called optimal. The purpose of propeller calculation is precisely to find the optimal diameter and pitch.

  Efficiency. The efficiency of the propeller is estimated by its efficiency, i.e. the ratio of the useful power used to the power consumed by the engine.

  Without going into details, we note that the efficiency of a non-cavitating propeller mainly depends on the relative slip of the propeller, which in turn is determined by the ratio of power, speed, diameter and rotational speed.

  The maximum value of the propeller efficiency can reach 70 ~ 80%, however, in practice it is quite difficult to choose the optimal values of the main parameters on which the efficiency depends: diameter and speed. Therefore, on small vessels, the efficiency of real propellers can be much lower, being only 45%.

  The propeller achieves maximum efficiency with a relative slip of 10 - 30%. With an increase in slip, the efficiency drops rapidly: when the propeller is operating in the mooring mode, it becomes equal to zero. In the same way, efficiency decreases to zero when, due to high revolutions with a small pitch, the screw stop is zero.

  However, one should also take into account the mutual influence of the housing and the propeller. During operation, the propeller captures and throws significant masses of water into the stern, as a result of which the speed of the flow around the stern of the hull increases, and the pressure drops. This is accompanied by the phenomenon of suction, i.e., the appearance of an additional force of water resistance to the movement of the vessel in comparison with that which it experiences during towing. Therefore, the screw must develop a stop that exceeds the body resistance by a certain value Pe = R / (1-t) kg. Here t — suction coefficient, the value of which depends on the speed of the vessel and the contours of the hull in the area of the propeller. On planing boats and motorboats, on which the propeller is located under a relatively flat bottom and does not have a sternpost in front of it, at speeds over 30 km / h t = 0.02 — 0.03. On low-speed (10 — 25 km / h) boats and boats, on which the propeller is installed behind the sternpost, t \u003d 0.06 — 0.15.

  In turn, the hull of the vessel, forming a passing stream, reduces the speed of the flow of water flowing onto the propeller. This takes into account the co-flow factor w: Va = V (1—w) m/s. The values of w are easy to determine from the data above.

  The overall propulsion efficiency of the complex ship—engine—propeller is calculated by the formula:

Here hp is the propeller efficiency; hk — body influence coefficient; hm — Efficiency of shafting and reverse gear.

  The influence coefficient of the hull often turns out to be more than one (1.1 - 1.15), and the losses in the shafting are estimated at 0.9 —0.95.

  Propeller diameter and pitch. Propeller elements for a specific vessel can only be calculated by having a curve of water resistance to the movement of a given vessel, an external characteristic of the engine and design diagrams obtained from the results of model tests of propellers with certain parameters and shape blades. For a preliminary determination of the diameter and pitch of the screw, there are simplified formulas, which make no sense here, because. it is proposed to use more accurate methods for calculating the optimal propeller. These methods are based on the approximation (approximate representation) of graphic diagrams by analytical dependencies, which allows you to perform fairly accurate calculations on a computer and even on microcalculators.

  The diameter of propellers, obtained both from an approximate formula and with the help of exact calculations, is usually increased by about 5% in order to obtain a deliberately heavy propeller and achieve its consistency with the engine during subsequent tests of the ship. For "relief" its screw is gradually cut in diameter until the rated engine speed is obtained at the design speed.

  However, for propellers of small boats, this may not be done. The reason is simple: the loading of pleasure craft varies widely, and the screw, a little "heavy" or "lightweight" at one value of the ship's displacement, it will become consistent with a different load.

  Cavitation and features of the geometry of propellers of small vessels. High speeds of motorboats and boats and the rotational speed of the propellers cause cavitation - boiling of water and the formation of vapor bubbles in the rarefaction area on the suction side of the blade. In the initial stage of cavitation, these bubbles are small and practically do not affect the operation of the propeller. However, when these bubbles burst, huge local pressures are created, causing the surface of the blade to crumble. When the cavitating propeller is operated for a long time, such erosion damage can be so significant that the efficiency of the propeller will decrease.

  With a further increase in speed, the second stage of cavitation occurs. A solid cavity - a cavity, captures the entire blade and can even be closed outside of it. The stop developed by the propeller falls due to a sharp increase in drag and distortion of the shape of the blades.

  Prop cavitation can be detected by the fact that the speed of the boat stops increasing, despite a further increase in the frequency of rotation. At the same time, the propeller makes a specific noise, vibration is transmitted to the hull, the boat moves in jumps.

  The moment of cavitation onset depends not only on the rotational speed, but also on a number of other parameters. So, the smaller the area of the blades, the greater the thickness of their profile, and the closer to the waterline the screw is located, the lower the speed, i.e., the earlier cavitation occurs. The appearance of cavitation is also facilitated by a large angle of inclination of the propeller shaft, defects in the blades — bending, poor surface quality.

  The emphasis developed by the propeller is practically independent of the area of the blades. On the contrary, with an increase in this area, the friction against the water increases and the engine power is additionally consumed to overcome this friction. On the other hand, it must be taken into account that with the same emphasis on wide blades, the vacuum on the suction side is less than on narrow ones. Therefore, a wide-bladed propeller is needed where cavitation is possible (i.e., on speedboats and at high propeller shaft speeds).

  As a propeller characteristic, the working, or straightened, area of the blades is taken. When calculating it, the width of the blade is taken, measured on the discharge surface along the length of the arc of a circle at a given radius drawn from the center of the propeller. The characteristic of the propeller usually indicates not the straightened area of the blades A itself, but its ratio to the area Ad of a solid disk of the same diameter as the propeller, i.e. A / Ad. On factory made screws, the disc ratio is stamped on the hub.

  For screws operating in the pre-cavitation mode, the disk ratio is taken within 0.3 - 0.6. For heavily loaded propellers on high-speed boats with powerful high-speed engines, A / Ad increases to 0.6 - 1.1. A large disk ratio is also necessary in the manufacture of screws from materials with low strength, such as silumin or fiberglass. In this case, it is preferable to make the blades wider than to increase their thickness.

  The axis of the propeller on a planing boat is located relatively close to the surface of the water, so it is not uncommon for air to be sucked into the propeller blades (surface aeration) or the entire propeller to be exposed when traveling on a wave. In these cases, the propeller stop drops sharply, and the engine speed may exceed the maximum allowable. To reduce the influence of aeration, the pitch of the propeller is made variable along the radius - starting from the blade section by r = (0.63—0.7) R towards the hub, the pitch is reduced by 15~20%.

  Propellers of boats usually have a high rotational speed, therefore, due to high centrifugal speeds, water flows along the blades in the radial direction, which negatively affects the efficiency of the propeller. To reduce this effect, the blades are given a significant slope into the stern — from 10 to 15°; .

  In most cases, the blades of the propellers are given a slight saber shape - the line of the middle sections of the blade is curvilinear with a bulge directed along the rotation of the propeller. Such propellers, due to the smoother entry of the blades into the water, are characterized by less vibration of the blades, are less susceptible to cavitation and have increased strength of the leading edges.

  The segmental plano-convex profile is the most widespread among propellers of small vessels. The propeller blades of high-speed motor boats and boats, designed for speeds over 40 km / h, have to be made as thin as possible in order to prevent cavitation. To improve efficiency in these cases, a convex-concave profile ("lune") is advisable. The arrow of the profile concavity is taken equal to about 2% of the section chord, and the relative thickness of the segmental profile (the ratio of the thickness t to the chord b at the calculated radius of the screw, equal to 0.6R) is usually taken within t / b = 0.04 — 0.10.

  A two-blade propeller has a higher efficiency than a three-blade propeller, however, with a large disk ratio, it is very difficult to provide the necessary strength of the blade of such a propeller. Therefore, three-bladed propellers are most widely used on small vessels. Propellers with two blades are used on racing boats, where the propeller is lightly loaded, and on sailing and motor yachts, where the engine plays a supporting role. In the latter case, it is important to be able to install the screw in a vertical position in the hydrodynamic wake of the sternpost to reduce its resistance when sailing.

  Four and five blade propellers are used very rarely, mainly on large motor yachts to reduce noise and vibration of the hull.

  The propeller works best when its axis is horizontal. At the screw installed with an inclination and in connection with this streamlined "oblique" flow, the efficiency will always be lower; this drop in efficiency affects when the angle of inclination of the propeller shaft to the horizon is more than 10°.