Helical surface control.
Propeller blades that are bent upon impact, for example on the bottom, must be immediately straightened, otherwise the operation of the propeller will be accompanied by strong vibration transmitted to the boat’s hull, and its speed may significantly decrease.
To check the blade, make pitch squares similar to the one shown in rice. 222(the pitch must be known or previously measured on a working blade).
Step squares are cut (first in the form of templates from tin or cardboard) for four to six screw radii r equal, for example, to 20, 40, 60 and 80% of the largest radius R.
The base of each pattern must be 2 l r , i.e. 6.28 of a given radius, and the height is a step N.
Arcs with corresponding radii are drawn on a flat board and a propeller is installed in the center with the discharge surface down. By bending the cut square along an arc of the appropriate radiusr,they bring him under the blade.
Having marked the width of the blade and the position of its axis on the template, cut off unnecessary parts at the ends of the template and transfer the markings to a sheet of metal 1-1.5 mm thick. This will be the test step square, which, naturally, should also be bent exactly along an arc of a controlled radiusr.
The screw should be installed on the board in such a way that it can be rotated (Fig. 223). A tight fit of the discharge surface across the entire width of the blade to the pitch square will indicate its correct shape.
Pedometer square
You can quickly and accurately determine the pitch of the screw using a pedometer square (Fig. 224), made of transparent plexiglass. Each inclined line on the ruler corresponds to the pitch of the propeller at a certain radius (for example, 90 mm) of the blade. Screw pitch in centimeters (Fig. 224, a) indicated at the end of the slanted lines. Slanted lines should be clearly visible. They are drawn with a sharp tool and painted with black paint.
The square is used as follows: from the center of the propeller axis on the flat discharge surface of the blade, a radius equal to the base of the square (in our case, 90 mm) is laid out, and a line is drawn perpendicular to the radius. The square is placed on the drawn line and looked through it at the cut of the hub. The pitch of the screw will be determined by the inclined line that will be parallel to the cut of the hub (in our example N≈ 400 mm).
The principle of constructing a square is clear from rice. 224, b. A radius of 90 mm is laid out horizontally, and various values of the screw pitch divided by 2l are laid out vertically. You can choose a different radius, depending on the size of the screw.
Right or left?
Depending on the direction of rotation of the propeller shaft, when viewed from the stern, right-hand (clockwise) and left-hand rotation screws are used. Two simple rules will help you distinguish them.
1. Place the propeller on the table and look at the end of the blade facing you. If the right edge of the blade is higher, the propeller is right-handed. (Fig. 225, b), if higher left - left (Fig. 225, A) . In this case, you will be convinced that it does not matter how the screw lies: with the front (nose) or the rear end of the hub on the table.
2, Place the propeller on the ground and try to place your foot on the propeller blade without lifting your heel off the ground. If the sole right leg fits tightly on the surface of the blade, your propeller is right-handed, if left-handed, then left-handed.
The fact that with a twin-engine installation it is desirable to have propellers in the opposite direction of rotation is well known to all boaters (the issue of the influence of the direction of rotation of propellers on speed and controllability has been discussed more than once on the pages of “KiYa”). It is known that athletes in races sometimes turn one of two motors, which have the same direction of rotation of the propeller, into reverse and, thanks to this, get an increase in speed of several kilometers per hour, and most importantly, achieve better stability on the course (naturally, with this motor it is necessary replace the propeller so that in reverse it creates forward thrust).
Long-term operation of, for example, the “Whirlwind” in reverse is undesirable, since the design of the propeller shaft supports is not designed to constantly accept the propeller thrust in reverse. Therefore, sometimes different types of motors are installed on motorboats: in addition to the “Whirlwind” or “Neptune” (with right-hand rotation of the propeller), they install “Privet-22” - the only domestic motor with a left-hand propeller.
By making a few simple parts, you can adapt the Whirlwind gearbox to work with a left-hand rotation propeller: this will make it possible to use the same type of outboard motors for a twin-engine installation, which is advisable from the point of view of ease of operation and repair.
In the design of the left-hand rotation gearbox I made, I had to abandon reverse gear: to ensure maneuverability, it is quite enough to have reverse gear on one of the two motors, and idling Every engine has one.
To install the bearings, it is necessary to make a new cup 3 (it is best to make it from stainless steel). Using a round file or emery stone, a hole is cut out on the side surface of the glass for passage of the reverse thrust.
Bushing 4 is machined from bronze. Four grooves with a width of 1.5 and a depth of 1 mm are sawed along its entire length along the inner hole with a hacksaw to lubricate the bearings and gear 5. The seal of the gearbox housing on the screw side is ensured by installing two oil seals 1. The reverse gear 5 must be machined on a mandrel with a diameter of 30 ±0 .02 mm with surface cleanliness of class 7-8.
Forward gear 7 needs to be modified according to the dimensions indicated in the sketch. For this purpose, I recommend choosing a gear that has already been used, with teeth worn on one side and the coupling protrusions. A ring 6 is pressed into the groove of a gear with a diameter of 38 mm, which serves to reduce the stroke of the coupling 10.
When assembling the propeller shaft assembly into cup 3, first cuffs 1 are pressed in, then ball bearings 7000103 lubricated with grease and (with a tight fit) bronze bushing 4 are installed. When installing the cup together with shaft 10 in the gearbox housing, it is necessary to find such a position so that the reverse rod moves easily , and the cams of the clutch 11 engaged with the cams of the gear 5. The gap in the meshing of the gears is adjusted using rings installed between the gear and the end of the cup 3.
I have been using the Vikhr-M with a converted gearbox for four years now on the Kazaik-2M and using a propeller from the Privet-22 engine (diameter 235 and pitch 285 mm). I didn’t specifically measure the speed of the boat, but I will say that here on the Volga in Cheboksary, my “Kazanka” is the fastest among boats with two outboard motors.
After two seasons of operation, I had to change the ball bearings 7000103, which, constantly bearing the thrust of the propeller, received more wear. It might make sense to use angular contact bearings.
With the same screw you can achieve maximum speed and maximum load capacity?
No. To achieve high speeds, a pitch or diameter is used that is unsuitable for the load capacity - where the operating conditions are completely different. If you want to get by with just one screw, then decide what is most important, and choose the screw based on that.
3 or 4 blades?
For most boats, 3-blade propellers are recommended. These propellers provide good acceleration and operation at main speed.
A three-blade propeller has less resistance and allows (theoretically) to develop greater speed. The four-bladed one has a larger thrust; the speed with this propeller in modes from low speed to 2/3 should be higher.
4-blade propellers are recommended for heavier boats and boats with hulls high efficiency equipped with more powerful engines. Compared to 3 blades, they work better during acceleration and have less vibration during high speeds.
For my boat there is a 13" and 14" diameter propeller. Is a smaller diameter with a larger pitch the same thing?
The pitch cannot replace the diameter. The diameter is directly related to the engine power, RPM, and speed that your requirements indicate. If operating conditions require a 13" diameter, then installing 12" will reduce its efficiency.
Is it necessary to use high heat to install or remove a screw?
Heat should never be used when installing a screw, and therefore should rarely be required for removal. If it is not possible to remove the screw using a soft hammer, gentle heating with a blowtorch may help. Do not use a welding torch as the fast, harsh heat will change the structure of the bronze, creating internal stresses that can lead to splitting of the hub.
What is the advantage of using a second propeller - left rotation?
Two propellers operating in the same direction on boats (ships) will create a reaction torque. In other words, the two right propellers will tilt the boat to the left.
Two counter-rotating propellers on identical engines will eliminate this reaction torque, because the left propeller will balance the right one. This will result in better straight-line motion and control at high speed.
Aluminum or stainless steel?
Most boats are equipped with aluminum propellers. Aluminum screws are relatively inexpensive, easy to repair, and can last for many years under normal conditions.
Stainless steel more expensive, but much more durable and durable than aluminum.
Why are different propellers used with motors of the same power?
This is due to differences in the engine's reduction ratios. The motor is designed so that the propeller shaft turns more slowly than the crankshaft. This is usually expressed as a ratio, such as 12:21 or 14:28. In the first example, the crankshaft ratio will be 12, and the propeller shaft ratio will be 21. This means that the propeller shaft will only turn 57% of the rpm of the crankshaft. The lower the gear ratio, the larger the propeller pitch, and vice versa can be used.
Compensation of propeller torque.
The rudder (wheel) must be positioned relative to the rotation of the propeller. If the engine has a right-hand rotating propeller, the rudder (wheel) should be on the right or starboard side. This side usually tends to rise as a result of the reaction torque, and the driver's weight compensates for this.
What is the role of the rubber shock absorber in the propeller hub?
It is not intended to protect the blade from impact, as is sometimes believed. This device protects the gears of the gearbox, softening the impact of the impact on the screw. Its main purpose is to prevent excessive wear or breakage of the engine reduction gears that may occur due to the shock that occurs during the gearing process.
The rubber shock absorber in my propeller seems to be slipping. Is it possible?
This possibility exists in principle, but it does not happen very often. Inspect the propeller; if the blades are visibly bent or distorted, then you are likely experiencing cavitation - cavitation is often perceived as hub slippage. The bushing can be replaced if necessary, or the blades can be rebuilt to proper precision to eliminate cavitation.
Cavitation- this is the phenomenon of the formation in a liquid of small and almost empty cavities (cavities), which expand to large sizes, and then quickly collapse, producing a sharp noise. Cavitation occurs in pumps, propellers, impellers (hydraulic turbines) and in the vascular tissues of plants. When cavities collapse, a lot of energy is released, which can cause major damage. Cavitation can destroy almost any substance. The consequences caused by the destruction of cavities lead to great wear components and can significantly reduce the life of the propeller.
Cavitation, (not to be confused with ventilation), is the boiling of water due to an extreme reduction in pressure at the tip of a propeller blade. Many propellers partially cavitate during normal operation but excessive cavitation can cause physical damage to the surface of the propeller blade due to rupture of microscopic bubbles on the blade. There can be numerous causes of cavitation, such as improperly shaped propellers, incorrect installation, due to physical damage to the cutting edge, etc...
Regarding plastic screws.
To date, no screws have better properties than screws made of metals. A good screw should have a long service life and be repairable. So far, available plastics are inferior in all these parameters.
Is it possible to get by with one standard propeller that comes with the motor (boat)?
A specially selected propeller will work with greater efficiency than the standard universal one that is equipped with the boat. It is optimal to have at least two propellers, or even better, three, from which you can always choose the one needed for the various loads of the boat.
§ 46. Factors affecting controllability.
1. The influence of the propeller.
The control of a ship largely depends not only on the rudder, but also on the design of the propeller, its speed of rotation and the contours of the stern of the ship.
Propellers are made of cast iron, steel and bronze. The best propellers for boats should be considered bronze propellers, as they are lightweight, easy to polish, and resistant to corrosion in water. Screws are characterized by diameter, pitch and efficiency.
The diameter of the propeller is the diameter of the circle described by the extreme points of the blades.
The pitch of the screw is the distance along the axis of the screw that any point on the screw moves in one full revolution.
Rice. 103. Formation of screw threads
The efficiency coefficient (efficiency) of a propeller is determined by the ratio of the power developed by the propeller to the power expended on its rotation.
The operation of a propeller is based on hydrodynamic force created by vacuum on one surface and pressure on the other surface of the blade.
Modern ship propulsors are still very imperfect. Thus, propellers, on average, spend about half of the power given to them by the engine uselessly, for example, on screw-like twisting of water particles in the jet.
On boats, two-, three-, and less often four-blade propellers are used. On fishing boats, propellers with rotating blades or so-called adjustable pitch propellers are sometimes installed, which allow you to smoothly change the speed or direction of the vessel with constant one-way rotation of the propeller shaft. This eliminates the need to reverse the engine.
Screws vary in the direction of their rotation. A propeller rotating clockwise (when viewed from the stern to the bow) is called a right-hand rotation propeller, counterclockwise is called a left-hand rotation screw. When moving forward under the stern valance of the ship's hull in front and behind the rudder, a passing (Fig. 103) flow of water is formed and forces arise that act on the rudder and affect the maneuverability of the vessel. The speed of the passing flow is greater, the fuller and blunter the contours of the stern.
The vacuum on the convex side of the blade, called the suction side, draws water toward the propeller, and the pressure on the flat side, called the discharge side, pushes water away from the propeller. The speed of the jet being thrown out is approximately twice that of the jet being sucked in. The reaction of the thrown water is perceived by the blades, which transmit it to the ship through the hub and propeller shaft. This force that sets the ship in motion is called thrust.
In a stream of water thrown by a propeller, particles do not move in a straight line, but in a helical manner. The passing current seems to be pulled behind the ship and its size depends on the shape of the stern part of the boat. The flow slightly changes the pressure on the rudder, which is moved away from the center plane of the vessel.
The combined effect of all flows has a noticeable effect on the controllability of the vessel; it depends on the position of the steering wheel, the magnitude and change in speed, the shape of the hull, the design and operating mode of the propeller. Therefore, each vessel has its own individual characteristics of the action of the propeller on the rudder, which the navigator must carefully study in practice (Table 4).
Table 4
The influence of the interaction of the starboard rudder propeller on the behavior of the vessel.
|
Position of the vessel relative to the water |
Position steering wheel |
Propeller operating mode |
Screw operating direction |
Result |
|
1.Motionless |
Directly |
Only included |
Forward |
The bow will roll to the left (the stern will be thrown to the right) |
|
2.Moves forward |
Right |
Steady |
Forward |
The bow is thrown to the right (the stern is thrown to the left) |
|
3.Moves forward |
Straight or left |
Steady |
Forward |
The bow of the ship will roll towards the rudder deflection |
|
4.Motionless |
Directly |
Only included |
Back |
The stern is thrown to the left. The nose will roll to the right |
|
5.Moves backwards |
Left or right |
Steady |
Back |
Individually for each vessel. Usually the stern goes towards the shifted rudder |
|
6.Moves forward |
Directly |
Only included |
Back |
The bow of the ship will roll to the right, the stern to the left |
A left-hand rotation screw, other conditions being equal, will give results opposite to those shown in the table.
If a right-handed propeller is installed on the vessel, the vessel will turn better to the right; the circulation diameter to the right will be smaller than to the left.
When going astern, the ship's maneuverability is usually worse. A ship with a right-handed propeller in reverse turns better to turn its stern to the left than to the right. Therefore, when moving forward on a ship with a starboard propeller, they tend to approach the berth with the left side, since in this case, with a change of speed to the rear, the stern will be pressed against the wall.
Some motor yachts and boats are equipped with two motors, each with its own shaft and propeller. In this case, the screws usually rotate in different sides. They can be installed either with outward rotation, that is, in the upper part the blades go from the middle to the side, or with inward rotation, when the blades in the upper part go from the side to the middle. One or another direction of rotation of the screws, as well as the inclination of the axes of the screws and shafts to the horizontal and diametrical planes have great importance regarding agility.
The maneuverability of a screw vessel largely depends on the number of screws and their design. As a rule, the more propellers a ship has, the better its maneuverability. The design of propellers can be different. On river fleet vessels, predominantly four-blade fixed-pitch propellers are installed, which, depending on the direction of rotation, are divided into right-hand (Fig. 25) and left-hand rotation (pitch) propellers. The right-hand rotation screw of a forward-moving vessel rotates clockwise, the left-hand rotation screw rotates counterclockwise when viewed from the stern to the bow of the vessel.
Rice. 25. Right rotation propeller
The efficiency of a propeller largely depends on the conditions in which it operates, and above all on the degree of its immersion in water. The bareness of the propeller or the excessive proximity of the propulsion-steering complex to the water surface significantly worsens the propulsion and controllability of the vessel, and the inertial characteristics deviate significantly from the nominal ones (path length and acceleration time increase, the braking process worsens). Therefore, to ensure good maneuverability of screw vessels, they should not be allowed to sail with a large trim to the bow or empty (without the necessary ballasting).
A working propeller makes two movements simultaneously:
moves translationally along the axis of the propeller shaft, giving the vessel forward or backward motion, and rotates around the same axis, shifting the stern laterally.
Let's consider the nature of the water flow from a working propeller. If it operates in forward motion, it forms a stream of water behind the stern of the vessel, twisted in the direction of its rotation and directed at the rudder blade (Fig. 26, a). The water pressure on the rudder blade in this case depends on the speed of the ship and the speed of the propeller: the higher the speed of rotation of the propeller, the stronger its influence on the rudder and, consequently, on the controllability of the ship. When a vessel moves forward, a passing flow is formed behind its stern, directed in the direction of the vessel's movement and at a certain angle to the stern of the hull, which also affects controllability in a certain way.
When the propeller is operating in reverse, the swirling stream of water is directed from the propeller towards the bow (Fig. 26, b) and puts pressure not on the rudder blade, but on the hull of the aft part of the vessel, causing the stern to deflect in the direction of rotation of the propeller. Moreover, the higher the frequency
rotation of the propeller, the stronger its influence on the lateral displacement of the stern of the vessel.
When the propeller operates in forward or reverse motion, several forces are generated, the main of which are: driving force, lateral forces on the propeller blades, the force of the jet thrown onto the rudder blade or hull, the force of a passing or counter flow from the propeller, as well as the forces of water resistance to the movement of the vessel.
Controllability of single-rotor vessels. Let's consider the influence of the propeller on the controllability of the vessel in forward motion (Fig. 27). Let us assume that a single-screw ship with a right-handed propeller is drifting, having neither translational nor rotational motion, and the propeller is set to forward with the rudder positioned straight. At the moment the propeller is turned into forward motion, its blades begin to experience water resistance (the reaction forces of the propeller are hydrostatic), directed in the direction opposite to the rotation of the blades.
Due to the difference in water pressure along the depth of the propeller, the hydrostatic force Da (Fig. 27, a) acting on blade III is greater than the force d] acting on blade I, which is closer to the water surface. The difference between the forces Da and di causes a displacement of the stern in the direction of the action of the force Da, i.e. to the right. Hydrostatic forces Da and D4 are directed vertically in opposite directions and do not affect the ship in the horizontal plane. Despite the fact that the initial period, i.e. the moment the propeller is turned on, is very short in time, the navigator must take into account the phenomenon of the stern yaw in the direction of rotation of the propeller.
After the propeller develops
Rice. 27. Schemes of forces arising when the propeller operates in forward motion
At a given rotation speed, in addition to hydrostatic forces, hydrodynamic forces of the jet are generated, which is thrown onto the rudder blade (Fig. 27, b). The steady forward operation mode of the propeller is characterized by the fact that blades I and III throw the jets away from the rudder blade without exerting pressure on it, and blades II and IV throw a stream of water onto the rudder. In this case, the hydrodynamic force RF is significantly greater than P due to the difference in water pressure along the depth of location of blades II and IV, as well as due to air suction in the upper position of the propeller blade.
With steady rotation of the propeller, the reaction forces of water acting on the propeller blades and the jet thrown onto the rudder blade are stabilized, and behind the stern of the vessel a passing flow with force B is formed, which is decomposed into components b\ and bch (Fig. 27, c) . The speed of the passing flow increases with increasing ship speed and reaches its maximum value at a steady full speed of the ship. In this case, the largest lateral component b\ of the forward force
flow acts on the aft part of the ship's hull in the direction opposite to the rotation of the propeller (i.e., with a right-handed propeller - in left side).
Thus, during steady forward motion, a vessel with a right-handed propeller is exposed to the sum of three lateral forces: hydrostatic force D (the reaction force of water acting on the propeller blades), hydrodynamic force P (the force of the jet thrown onto the rudder blade) and the lateral component forces of the associated flow bi, and (2P+Sbi)>SD.
As a result of this, the stern of the ship deviates in the direction of the sum of the forces P and L\, i.e., with a right-hand rotation propeller, to the left, and with a left-hand rotation propeller, to the right. The deflection of the stern causes the bow of the ship to deflect in the opposite direction, i.e., the ship tends to arbitrarily change course with a right-handed propeller - to the right, and with a left-handed propeller - to the left.
These phenomena must be taken into account in the practice of steering a single-rotor vessel and remember that the agility of such vessels at forward speed in the direction of rotation of the propeller is much better than in the opposite direction. The circulation diameter of single-screw ships with right-hand rotation of the propeller to the right along the course is significantly smaller than to the left, and for ships with left-hand rotation of the propeller it is the opposite.
Let's consider the effect of a right-hand rotation screw on reverse when operating. When the propeller is put into operation in reverse, its blades experience the action of hydrostatic forces, the sum of which is directed to the left, since Oz>0[ (Fig. 28, a). Having developed speed, the propeller creates a spiral-shaped flow of water directed under the hull and to the aft part of the hull, and does not affect the rudder. In this case, the hydrodynamic force P acts. acting on the ship's hull from the jet thrown by blade IV is greater than the hydrodynamic force Pr from the jet thrown by blade II
(Fig. 28, b), due to the fact that the force P4 acts on the body almost perpendicularly, and force R-g- at a slight angle to the body. As a result of this, the stern of the vessel is deflected in the direction of rotation of the propeller.
When moving in reverse, a passing flow does not arise and the vessel is exposed only to the sum of two groups of lateral forces: the reaction forces of the water and the forces of the jet attacking the hull, directed in one direction, as well as the forces of the oncoming flow. In this regard, the operation of the propeller in reverse has a strong impact on controllability, which is why some vessels in reverse become uncontrollable.
In navigation practice, it is necessary to take into account that when operating in reverse, single-screw ships with a first-rotation propeller throw the stern towards the left side, and with a left-hand rotation propeller - towards the starboard side, and the turning moment of the propeller is, as a rule, greater than the turning moment of the rudder.
To avoid loss of controllability of the vessel, it is recommended not to set a high speed of rotation of the propeller in reverse and, if necessary, switch it to forward speed with a short-term increase in speed.
