On the methods of measuring the speed of a ship. Measured lines. Navigation. First steps. Vessel speed Principles for measuring ship speed
05/12/2016
In order to become navigator as a professional, you need to read a lot of nautical articles authored by scientists. In this article, using a simple language not loaded with complex terminology, we will try to find out - what speeds take into account the navigator.
When we talk about the speed of a ship, we are looking at two quantities. One of them - is the movement of the ship on the water. Direct connection between the propeller, the ship's hull and the aquatic environment. The second one is movement of a ship in relation to the world space. This is the path, the segment that we have passed in a certain time. The fact is that the World Ocean and the entire water shell of the Earth is not static. It is free in its movement, although it is subject to physical laws. The system of world waters, their interaction, creates the movement of water masses, and a sea vessel, along with any straw, participates in this movement on a colossal scale. Also, don't forget about wind, which also affects the speed of the vessel. About everything in more detail.
STW—Speed Through the Water— Vessel's speed through the water
SOG— Speed Over Ground — Vessel speed over ground
Knot— Knot — a unit of measure for the speed of a vessel. Nautical mile per hour.
So, we are on watch, we go from point A to point B. Full speed, the propeller threshes the water, our ship, swaying on the waves, cuts the water with the stem. - this is the water in which our ship, its hull and propulsion are immersed. With the positive work of this system, the vessel, as a physical body, moves in the aquatic environment, receiving support. Compare this to a swimmer who rows methodically from one side to the other in a pool. His body moves through the water, which is limited by the walls of the pool, does not have a current that would affect the swimmer. Using only his physical strength, he overcomes the distance, passing through the water.
Let's get back to our ship. Since it is in the system of world currents, then all this water mass moves in a certain direction, carrying the ship along with it. If we stop our ship, STW will be 0 . But we will move around the globe along with water, moving from one point to another. Let's move the ship again. Put on the navigation map location. spotted time. Inflicted new location. measured distance traveled, divided by time that we have pinpointed. We got the speed of the ship relative to the ground - SOG. Abstractly consider our ship as a physical point that moves around the planet at a certain speed. 
Let's remember our swimmer. After the pool, we invited him to swim in the river. At first he tried not to row, and was carried downstream. The speed of movement relative to coastal objects has become equal to the speed of the current. He started paddling upstream. To get back to where he started, he had to swim faster than the current. With respect to the water, he swam quickly ( STW) like in a swimming pool. But relative to coastal objects, his body did not move so quickly. The current of the river "ate" him SOG. And conversely, if he swam downstream, it helped him move.
lag- a device for measuring the speed of a vessel on water (it can be of different types, more details These are the simplest and most primitive examples. To fully understand the picture, the navigator should learn the basics vector geometry, namely, the addition and subtraction of vectors.
In modern navigation, we have at our disposal an instrument satellite observation — GPS, which continuously gives location ship, respectively, calculating SOG, which undoubtedly helps the navigator during work.
Next, on SOG can significantly affect , creating wind drift . Especially, it affects ships with great windage yu, such as container ships, RO-RO, passenger ships, large tankers in ballast displacement and others. For example, in a strong headwind SOG will decrease, and vice versa, with a fair direction, the wind will “help” the ship overcome the resistance of the water.
We hope this introductory article Navigation. First steps. Vessel speed." will help you in understanding science Navigation .
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Navigation. First steps. Vessel speed. (c) NavLib
By the lag. Orientation accuracy largely depends on reliable information about the speed of the vessel. When swimming on lakes and reservoirs, the average speed relative to the bottom can be determined from the log.
Legs are of various designs. The turntable logs, working on the principle of a hydrometric turntable, are stationary and move forward as needed from the bottom of the vessel. Hydrodynamic logs are two tubes that measure the pressure of outboard water during movement and parking. The greater the speed, the greater the pressure in one of the tubes. The difference in pressure can be used to judge the speed of the vessel. In general, logs are complex electromechanical devices.
The river flow, acting on the log, allows you to determine from it only the speed of the vessel relative to calm water, but not relative to the coast. In addition, uneven currents and the movement of the vessel in the turns of the channel distort the log readings.
Along the length of the ship's hull. The speed of the vessel relative to the bottom can be determined by one of the following methods. On the bow and stern, two planes of superstructures are selected, perpendicular to the diametrical plane of the vessel, or two objects that create leading sighting planes. Two observers stand in the bow and stern sighting planes H and K(Fig. 78). Observers choose a fixed object P, located on the coast or water. At the moment of arrival of the object in the nasal sighting plane, the observer H gives a signal by which the observer TO notices the time. When the item arrives P aft sighting plane observer TO. also makes a timestamp. The distance between the sighting planes / and time is used to calculate the speed.
Time marks can be made by a third observer on the bridge, according to the signs of observers H And TO at the time of arrival of the object P in sight planes.
Rice. 78. To the definition of speed
movement of the ship along the length of its hull
Less accurately, the speed is calculated when sighting an object P on one ship object, when the leading sighting plane is absent or when the object of sight is on the beam of the stem and stern of the ship.
With the help of subject direction finding. The essence of this simple and reliable
method is as follows. In the diametrical plane of a vessel moving in a straight course, between points a and b (Fig. 79) measure the distance l called the basis. Being at points a and b , observers at the same moments measure the angles a1 a2 a3 B1 B2 B3, etc. between the basis and the direction to the object P.
When processing the obtained measurements, an arbitrary line is drawn on a sheet of paper, on which a point is placed that determines the direction-finding object. From this point, under the measured angles a1, b1, etc., bearing lines of arbitrary length are drawn. Noticing the length of the base on the ruler on any scale, they fit it between the bearing lines, parallel to the course, until it touches them with the corresponding marks. Thus, the position of the ship's hull is determined at the moments of bearing finding. The distance traveled by the vessel during direction finding, taking into account the accepted scale, is taken directly from the diagram.
Two DFs are enough to build a scheme, but the result is more reliable with several DFs.
The direction finding of an object is carried out using a compass or other goniometric instrument. In the absence of them, a tablet is used, which can be a sheet of plywood, thick cardboard, a piece of a wide board or a deck table.
A tablet with a sheet of paper is placed above the sighting point. A line is drawn on the sheet, coinciding with the basis line. The direction finder is a wooden block with a smooth edge.
The observer at the moment of direction finding, directing the cut of the bar to the object, draws a pencil line and marks it with the measurement number. The corners from the tablet are removed using a protractor.
Rice. 79. To determine the speed of the vessel using the direction finding from it
Direction finding is carried out as follows. The observers, having checked their watches, disperse to their places. At the same moments, for example, after 15 or 20 s, they take the bearing of the same object. Direction finding can take place on the signals of a third observer. By determining the distance and time traveled, it is easy to calculate the speed.
The proposed method is applicable to determine the maneuvering qualities of the vessel: inertial path, circulation, etc.
By the relative speed of approach of ships. Knowing the distances between oncoming or overtaken vessels, as well as the speed of the oncoming or overtaken vessel, you can determine the speed of your vessel or, conversely, calculate the speed of the oncoming or overtaken train from your own speed. |
Denote: S - distance between ships, v1- the speed of our ship, v2 is the speed of the oncoming or overtaken ship, t- approach time. Then
In this formula, the plus sign "+" is taken for the case of a meeting of ships, and the minus sign (-) is taken for overtaking.
When overtaking ships, the relative speed of approach is equal to the difference in speeds, and when meeting, the sum of the speeds of both ships. In other words, in the first case, the overtaken vessel seems to stand still, and the overtaking one moves at a speed equal to the difference in their speeds. In the second case, one of the ships seems to be standing still, while the other is moving at a speed equal to the sum of the speeds of both ships.
During navigation, the above formula has limited application and can only be used in special cases. Therefore, the determination of the speed, as well as the time and distance traveled by ships during meetings and overtaking, can be made according to the universal nomogram of D.K. Zemlyanovsky (Fig. 80). It is easy to use, applicable in ship conditions and allows you to quickly solve any problem without intermediate calculations, provided that the ships are moving on the same or parallel courses.
The nomogram has three scales, and each of them, for convenience, has a double dimension. The rule of using a nomogram is clear from its key. For example, between a ship moving at a speed of 20 km/h and a pushed train at the time of signaling a divergence, the distance is 2.5 km. It is required to determine the speed of the train if the approach time is 300 s.
To determine the speed of the pusher, a ruler (pencil, sheet of paper, thread) is applied on the upper scale to the 300 s mark (see Fig. 80), and on the middle scale to the 2.5 km mark. The answer is read on the lower scale - 30 km / h. This is the joint approach speed, hence the speed of the pusher is 10 km/h.
As you know, in shipboard conditions when navigating inland waterways, it is often not possible to perform even simple arithmetic races.
Rice. 80. Nomogram for determining the speed of the vessel, the time and distance passed by the vessels when meeting and overtaking
four. Therefore, the nomogram can be used to solve problems about time and distance when meeting and overtaking ships.
We will show the methods of calculation according to the nomogram using examples. Boatmasters should not strive to obtain too precise values, such as tenths of a meter and a second. For large distances, it is quite acceptable to round the resulting values up to hundreds of meters, for small ones - up to ten or up to a meter.
Example l. The speed of two oncoming dry cargo ships: going down - 23 km / h, going up - 15 km / h. The distance between ships is 1.5 km. It is necessary to determine the time and distance traveled by the ships before the meeting.
Solution. The sum of the speeds of the ships will be 38 km/h. We find on the lower scale a point with a mark of 38 km and apply a ruler to it. We apply the other end of the ruler to the 1500 m mark on the distance scale, and read the answer on the upper scale - 140 s.
The speed of the boat moving from above is 23 km/h. We apply the ruler on the lower scale to the mark of 23 km, and the other end of the ruler to the mark of 140 s, we read the answer on the distance scale - 900 m. Then the path traversed from below by the moving ship is 600 m.
Example 2. A train with a length of 150 m, going up at a speed of 8 km / h, from a distance of 300 m, giving a go-ahead, begins to overtake a cargo ship with a length of 50 m, which is moving at a speed of 14 km / h. Calculate total overtaking time and distance.
Solution, The total distance, i.e., taking into account the lengths of the ship and the composition, is 500 m (300 + 150 4 "50 = 500 m). The speed difference is 5 km/h.
To determine the time, we apply one end of the ruler on the left scale to the mark of 6 km / h, and the middle of the ruler to the mark of 500 m on the distance scale. The answer is read on the upper scale - 320 s. The total distance traveled by the overtaking ship from the start of the go-ahead is equal to the product of its speed and the overtaking time. According to the nomogram, this is determined by the already known method. We apply the end of the ruler to the mark of 14 km / h, and the right end to the time mark of 320 s. We read the answer on the middle scale - 1250 m.
As can be seen from the above examples, with the help of a nomogram, you can easily and simply solve any problems of passing and overtaking ships, being directly on the ship.
With the help of the radar. To determine the speed of movement, radars are the most widely used among the technical means. The radar screen has fixed range circles (RCDs) that can be used to determine distances. Some radars have moving range circles (VRMs) that make it even more convenient to measure distances. By measuring the distance traveled on an object with the help of a radar and noticing the time, the speed of movement is calculated.
According to the navigation map or according to the directory. IN In this case, the distance traveled is determined from a map or reference book, and the time is determined from the clock. By dividing the length of the passed section by the time, the speed of movement is calculated. This method is most common when sailing on river boats.
The speed of the vessel in the process of high-speed tests is found in various ways.
It is widespread to determine the speed of a vessel on special measuring lines equipped with coastal secant (transverse) alignments, the distance between which is precisely known. On the measured line, the speed of the vessel is determined by the time it takes for the vessel to pass a known distance between the alignments. This method is one of the most accurate ways to measure the speed of a ship.
Cable measuring lines, which are some kind of the mentioned measuring lines with transverse sections, also have a known application. On the cable measuring line, the vessel passes over the electric cables laid at the bottom of the fairway across the direction of the vessel's movement. Electric current is passed through cables, the distance between which must be precisely known. Special electronic equipment installed on the ship records the moment the ship passes over the cable.
Recently, various radio navigation systems, in particular phase systems, have begun to be widely used to measure the speed of a vessel.
The ship's speed can also be measured with relatively less accuracy using the ship's own radar station, which successively at short intervals measures the distance to a specific object that reflects radio waves well.
Measuring the speed of a ship using a fan of bearings of two objects or using other navigational methods, for example, using lighthouses, the distance between which is known, does not have sufficient accuracy.
All of the above and many other methods, including the main method for determining the speed of a vessel on a measuring line, have one common drawback, which is that the speed of the vessel is found relative to the coast, and not to the water. At the same time, the influence of wind or tidal currents, which is difficult to accurately assess, is superimposed on the measurements. Meanwhile, when conducting speed tests and for further use of the data obtained, it is necessary to know the speed of the vessel relative to the water surrounding it, i.e., in the absence of a current. Therefore, the conditions and place of testing are chosen in such a way that the influence of the current is the least or is directed, if possible, along the measuring section. In these cases, the runs of the vessel in the measuring sections are made in mutually opposite directions and in a certain sequence.
Despite some difficulty, determining the speed of a vessel on a measured line or using radio navigation aids should always be preferred to measuring speed using regular ship and special logs or hydrometric vanes due to the low accuracy of the latter, although they measure the speed of the vessel directly relative to the water.
For high-speed tests, measuring lines should be used, located near the place of construction or basing of the vessel, which will save time and fuel required to approach the measuring line. In addition, due to fuel consumption when moving to a remote measuring line, it is difficult to provide a given value of the ship's displacement.
The depth of water in the area of the measured line, i.e. its measuring section and on the approach to it (on both sides), as well as in the area of the ship turning to the opposite course, should be sufficient to exclude the effect of shallow water on the water resistance to the movement of the ship , and hence its speed.
It is known that the system of waves created by the vessel when it moves in shallow water differs from the wave system in deep water and depends on the regime characterized by the so-called Froude number in shallow water.
Where σ is the ship's speed, m/s; g - free fall acceleration, m/s2; H - fairway depth, m.
A change in the nature of wave formation leads to an increase or decrease in the resistance to the movement of the vessel and, therefore, affects its speed.
At the same time, a counter flow of water develops, increasing the speed of the flow around the hull and, consequently, the frictional resistance of the vessel. The complete exclusion of the influence of shallow water requires large depths of the measured line, which are not always possible to provide (Table 1).
Table 1. Values of the minimum depth of the measured line, m 
As a result, when determining the minimum required depths, one usually proceeds from a loss of speed due to the influence of shallow water, which is 0.1% of the measured value. To comply with these conditions, the value Frh≥0.5 should be taken for the wave resistance, and for the friction resistance 
It is on the basis of this approach that the test rules developed by the 12th International Conference of Experimental Pools recommend that the minimum allowable depth on the measuring line be greater than that calculated by the formulas 
where B and T are the width and draft of the ship, respectively. A similar method is recommended by the domestic normal OH-792-68, however, the formulas are written in the form
The measuring line, if possible, should be located in an area protected from prevailing winds and sea waves. Finally, a prerequisite is the presence of sufficient space at both ends of the measuring line, which is necessary for the free maneuvering of the vessel after the end of the run on the measuring section, turning to the opposite course and acceleration after the turn.
Permissible deviations of the water depth at the approaches to the measuring section of the measuring line should not exceed ± 5%.
The ship's run line on the gauge line should be at least two to three miles from coastal hazards. Failure to comply with this condition creates a risk that the vessel at high speeds, even in the case of correct maneuvering, may run aground when the rudder is jammed.
It is not always possible to satisfy all the requirements listed above, so the number of full-fledged measuring lines is very limited.
In table. 2 shows some data characterizing the measured lines of a number of foreign states. As can be seen from the table, the length of the measuring sections of these lines is different, and the depths of many of them are insufficient for testing relatively high-speed vessels.
| Measure lines | Length of the measuring section, mile | The true course of the vessel, hail | Depth of the measured line during the strongest low tides, m |
| England | |||
| skelmorley Gao Loh Abs-Head Polperro Portland The mouth of the river Secrets Plymouth |
1 1 1 1,15 1,43 1 1 |
0 and 180 156 and 335 111 and 191 86 and 226 134 and 314 161 and 341 93 and 273 |
65-75 30-40 44-52 31-37 31 20 20-28 |
| Denmark | |||
| O. Bornholm | 1 | - | 70-80 |
| France | |||
| Porquerolle Thaya: 1st section 2nd 3rd Croix-Trevignon |
3,50 2,36 4,70 5,6 |
48 and 228 48 and 228 48 and 228 120 and 300 |
70-80 70-80 70-80 40 |
| USA | |||
| rockland | 1 | 0 and 180 | - |
On fig. Figure 3 shows a diagram of a measuring line near Rockland (USA), where a large number of high-speed ship tests, including research ones, were carried out. This line satisfies most of the requirements listed above, but it is not protected from the westerly winds and the waves they cause. The length of the measuring section is equal to one nautical mile (1852 m), the length of each accelerating section is three nautical miles. The measuring line is equipped with two coastal transverse (secant) sections perpendicular to the measuring section. One of the cross sections is equipped with three signs (shields), the other - with two.
Rice. 3. Scheme of a measuring line in Rockland (USA). Δ - leading sign.
In addition, milestones are placed along the run line for orientation of the navigator, indicating the boundaries of the accelerating and measuring sections.
Many measuring lines are equipped with so-called leading alignments, on the line of which the measuring section is located. At present, the presence of a leading alignment is not considered mandatory, although there is still an opinion that it is necessary in cases where there is a current in the area of the measured line that does not coincide with the direction of the measured line. However, this opinion is wrong: simple geometric constructions show that in this case, when the vessel is steered along the leading alignment in the same way as the compass, the vessel travels a distance greater than the distance between the alignment lines. That is why the requirement is put forward that the direction of the flow coincides with the direction of the measured line or, in any case, makes an angle with it that does not exceed 15-20 °.
Leading signs (Fig. 4) of measured lines are shields that are installed at such a height that they can be clearly seen from the sea. Usually, the front shield, i.e., the shield located closer to the measuring section of the measuring line, is installed somewhat lower than the rear one in such a way that at the moment the ship passes the alignment, the shields overlap each other, making up almost one whole in the vertical direction. In the middle of the shields, vertical brightly colored stripes are applied, which should also be clearly visible from the sea.
Rice. 4. Leading marks of the measuring line.
Rice. 5. Linear sensitivity of alignments.
1 - front alignment mark; 2 - rear alignment mark.
Nevertheless, an observer on a ship crossing the transverse alignments of the measurement line at right angles cannot practically absolutely accurately determine the moment when the alignment line passes, i.e., the moment when the middle stripes of the shields are on the same vertical straight line, as if constituting a continuation of each other. friend.
The magnitude of the error in determining the moment of complete coverage of the middle bands of the alignment shields depends on the so-called linear sensitivity of the alignment (Fig. 5).
The resolving power of a normal eye is one minute of arc. Let's put on the line of the ship's run along the measured line (Fig. 5) the segment A1A2, corresponding to one arc minute. In the interval A1A2, the angle between the two signs is less than one minute, and, therefore, any point in this interval can serve as a mark for the beginning of the speed measurement. The value OA1=OA2 is called the linear sensitivity of the target and is further denoted by the letter W.
To find an expression for W, we use the relation
tgα=tg(β-γ). (1.2)
converted to the form
After substituting the values tg β and tg γ into expression (1.3) and simple transformations, we have
The first term on the right side of expression (1.4) can be neglected, since it will be of a higher order of smallness compared to the next two. Then equation (1.4) takes the form
dW = tg αDc (Dc + d), (1.5)
where
Replacing the tangent of the angle with an arc and the angle with the value of the resolution of the eye, as well as entering the illumination factor of the target a "(for daylight α"=2 and for night light α"=3.5), we obtain the value of the linear sensitivity of the target (in meters)
Where
Dc - distance from the front sign of the secant alignment to the undercarriage of the measuring line, m; ao - angle of resolving power of the eye; d - distance between leading signs, m.
Here are the sensitivity values of the secant sections of one of the foreign measuring lines:
If we take the sensitivity of a pair of alignments equal to half of the possible absolute error, then the relative error in the length of the measured section of the line (targets 2-3) will be equal to 0.4%.
As can be seen from formula (1.6), in order to reduce the error in determining the distance between the sections and, consequently, increase the sensitivity of the sections, it is necessary that the ratio Dc: d be as small as possible. In practice, however, this ratio is usually not less than three.
In order to evaluate the influence of the error in timing, as well as the influence of the sensitivity of the alignments and the length of the run line on the results of measuring the speed, it is necessary to consider the dependence of the ship's speed on the path and time
v=s/t (1.9)
where v is the arithmetic mean of several speed measurements, m/s; s - arithmetic mean of the path, m; t is the arithmetic mean of the run time, s.
As is known, the error in the result of indirect measurements (the speed is calculated from the measured path and time) is made up of the errors in the results of each direct measurement included in the indirect one. In indirect measurements, the relative error (rms, probable or limit) of each direct measurement is found and the total relative error of the indirect measurement is calculated. Yes, in this case
where εν - relative error of velocity measurement, .%; εs - relative path measurement error; εt - relative error of measurement of travel time.
Expressing relative errors in terms of probable ones, we obtain
or, after substitution, t = s/v .
Where ρs is the probable path measurement error, m; ρt - probable travel time measurement error, s (according to ρt = 0.5 s). Probable path measurement error
if the sensitivity of both alignments is assumed to be the same and equal to half the sum of their sensitivities, and the number of runs in the mode is equal to three.
Substituting these values into formula (1.12) and transforming it, we obtain
Thus, the magnitude of the error will depend on three components: the sensitivity of the secant lines, the length of the run along the measuring line and the speed of the vessel.
As an example, in Table. 3 shows data on the accuracy of measuring the speed of the vessel on one of the measuring lines. Based on these data, it can be concluded that the measured speeds, regardless of the ship's speed, are determined with a high degree of accuracy. So, in the section of the measuring line between the second and third alignments, the errors in measuring the speed are 0.35-0.40%. With an increase in the length of the measuring line (the section between the first and second alignments is one mile, between the second and third alignments - two miles, and between the first and third - three miles), the speed measurement error decreases sharply.
| Vessel speed, knots | Average sensitivity of gates, m | ||
| 12.8 (section between the first and second alignments) | 14.9 (section between the second and third alignments) | 13.0 (section between the first and third alignments) | |
| 8 12 16 20 24 28 32 36 30 |
0,58 0,59 0,61 0,63 0,66 0,69 0,72 0,75 0,79 |
0,33 0,34 0,35 0,36 0,37 0,38 0,40 0,42 0,43 |
0,20 0,20 0,21 0,22 0,22 0,23 0,24 0,25 0,26 |
However, this does not mean that it is more expedient to make runs on long measured lines, since this increases the errors caused by the possible uneven operation of the main mechanisms over a long distance and the influence of disturbing external influences, leading to a course deviation from a straight line.
When assigning the length of the measuring section of the measuring line, it should also be taken into account that during high-speed tests (in the absence of automatic equipment for recording instrument readings), it is sometimes necessary to measure the torque on the propeller shaft at least eight to ten times or to remove indicator diagrams once or twice, as well as several times to measure the frequency of rotation of the propeller shafts and determine some parameters of the operation of the power plant. All this takes at least four minutes. Thus, the minimum run length s on the measured line, which is a function of the time required to perform the indicated measurements and determine the ship's speed, can be calculated by the formula
s = 0.067νs (1.15)
where νs is the ship's speed, knots, s is the ship's mileage, miles.
A dimensional factor of 0.067 corresponds to approximately 4 minutes, i.e. the time required to perform measurements.
The constant knowledge by the navigator of the reliable speed of his vessel is one of the most important conditions for accident-free navigation.
The movement of the ship relative to the bottom at a speed called absolitary, is considered in navigation as the result of the addition of the vessel's velocity vector relative to the water and the current vector acting in the navigation area.
In turn, the speed vector of the ship relative to the water (referbodyspeed) is the result of the work of ship propulsion and the action of wind and waves on the ship.
In the absence of wind and waves, it is most simply determined by the frequency of rotation of the propellers.
Knowing the speed makes it possible to determine the distance traveled by the vessel S about in miles:
S about = V about t, (38)
where V about - the speed of the vessel, determined by the frequency of rotation of the propellers, knots; t- sailing time of the vessel, h.
However, this method is inaccurate, since it does not take into account the change in the state of the vessel (fouling of the hull, change in draft), the influence of wind and waves. The following factors influence the speed of the vessel through the water.
1. The degree of loading, roll and trim of the vessel. The ship's speed changes with draft. Usually, in good weather conditions, a ship in ballast has a slightly higher speed than a fully loaded one. However, with the increase in wind and waves, the loss in speed of a vessel in ballast becomes much greater than that of a vessel in full load.
The trim has a significant effect on the change in speed. As a rule, the trim on the nose reduces the speed. A significant trim to the stern leads to the same results. The optimal trim option is selected based on experimental data.
The presence of the roll of the vessel causes its systematic departure from the given course towards the raised side, which is a consequence of the violation of the symmetry of the contours of the part of the hull submerged in water. For this reason, it is necessary to resort more often to shifting the rudder to keep the vessel on course, and this in turn leads to a decrease in the speed of the vessel.
2. Wind and waves usually act on the ship at the same time and, as a rule, cause speed losses. Headwinds and waves create significant resistance to the movement of the vessel and impair its controllability. Losses in speed in this case can be significant.
Winds and waves in the same direction reduce the speed of the vessel mainly due to a sharp deterioration in its controllability. Only with a weak tailwind and slight waves, some types of ships show a slight increase in speed.
3. Fouling of the hull is observed when ships navigate in any conditions, both in fresh and salt water. Fouling occurs most intensively in warm seas. The consequence of fouling is an increase in water resistance to the movement of the vessel, i.e. speed reduction. In middle latitudes, after six months, the decrease in speed can reach 5 - 10%. The fight against fouling is carried out by systematic cleaning of the ship's hull and painting it with special
growing colors.
4. Shallow water. The effect of shallow water on vessel speed reduction
begins to affect at depths in the navigation area
H≤4Tcp + 3V 2 /g,
Where H - depth, m
Tcp, - average draft of the vessel, m;
V- vessel speed, m/s;
g- acceleration of gravity, m/s 2 .
Thus, the dependence of the ship's speed on the rotational speed of the propellers, determined for specific sailing conditions, will be violated under the influence of these factors. In this case, the calculations of the distance traveled by the ship, performed by formula (38), will contain significant errors.
In the practice of navigation, the speed of a vessel is sometimes calculated using the known relationship
V=S/ t,
Where V- vessel speed relative to the ground, knots;
S - distance traveled at a constant speed, miles; t - time, h.
Accounting for the speed and distance traveled by the vessel is carried out most accurately using a special device - a log.
To determine the speed of the vessel, measuring lines are equipped, the areas of location of which are subject to the following requirements:
lack of influence of shallow water, which is ensured at a minimum depth determined from the ratio
N/T≥ 6,
Where H- depth of the measuring line area, m; T- vessel draft, m;
protection from prevailing winds and waves;
the absence of currents or the presence of weak constant currents coinciding with the directions of the runs;
the possibility of free maneuver of ships.
Rice. 23. Measuring line
The equipment of the measuring line (Fig. 23), as a rule, consists of several parallel secant sections and one leading one perpendicular to them. Distances between secant lines are calculated with high accuracy. In most cases, the line of run of ships is indicated not by the leading alignment, but by buoys or milestones set along it.
Typically, measurements are taken at full load and in ballast for the main engine operating modes. During the period of measurements on the measuring line, the wind should not exceed 3 points, and the excitement - 2 points. The vessel should not have a heel, and the trim should be within the optimal limits.
To determine the speed, the vessel must lie on the compass on a course perpendicular to the lines of the secant alignments, and develop a given frequency of rotation of the propellers. The measurement of the duration of the run is usually made according to the readings of three stopwatches. At the moment of crossing the first secant target, stopwatches are started and every minute they notice the readings of the tachometers. Stopwatches stop at the intersection of the second secant target.
Having calculated the average run time according to the indications of stopwatches, the speed is determined by the formula
V = 3600S/t, (39)
where S is the length of the run between the secant lines, miles;
t- the average duration of the run between the secant lines, s; V- ship speed relative to the ground, knots.
The rotational speed of the propellers is determined as the arithmetic average of the tachometer readings during the run.
If there is no current in the area of the measuring line, then the velocities relative to the ground and water are equal. In this case, it is enough to make only one run. If there is a current constant in direction and speed in the maneuvering area, it is necessary to make two runs in opposite directions. The relative speed of the vessel V 0 and the frequency of rotation of the propellers P in this case will be determined by the formulas:
Vo \u003d (V 1 + V 2) / 2, (40)
n=(n 1 + n 2)/2, (41)
Rice. 24. Graph of the dependence of speed on the frequency of rotation of the propellers
where V 1 , V 2 - the speed of the ship relative to the bottom on the first and second runs; n 1 and n 2
- frequency of rotation of propellers on the first and second runs.
When a uniformly changing current acts in the region of the measuring line, it is recommended to make a third run in the same direction as the first one, and the speed free from the influence of the current is calculated nO approximate formula
V 0 \u003d (V 1 + 2V 2 + V 3) / 4. (42)
If the nature of the change in the current is unknown or they want to get a more accurate result, then four runs are made and the speed is calculated by the formula
V 0 \u003d (V 1 + 3V 2 + 3V 3 + V 4) / 8. (43)
The average rotational speed of the propellers in these cases is calculated for three and four runs, respectively:
n \u003d (n 1 + 2n 2 + n 3) / 4; (44)
n = (n 1 + 3n 2 + 3n 3 + n 4)/8. (45)
Thus, the speed and rotational speed of the propellers are determined for several modes of operation of the main engines in cargo and in ballast. Based on the data obtained, graphs of the dependence of the speed on the rotational speed of the propellers are built for various loading of the vessel (Fig. 24).
Based on these graphs, a table of correspondence between the speed of the propellers and the frequency of rotation of the propellers or a table of correspondence of the rotational speed of the propellers to the speed of the ship is compiled.
If, based on the results of passing the measuring line, any speed and the corresponding rotational speed of the propellers are known, then it is possible to calculate the speed value for any intermediate value of the rotational speed of the propellers using the Afanasiev formula
V And \u003d V 0 (n 1 / n 0) 0, 9, (46)
where V0 - known speed at the frequency of rotation of the propeller n 0 ; V And, - the desired speed for the rotational speed of the propulsor n 1 .
Thus, having determined the speed of your ship according to the graph of its dependence on the rotational speed of the propellers, you can calculate the distance traveled in nautical miles using the formula
where V 0 - vessel speed, knots; t- sailing time, min.
If the distance traveled is known, then the calculation of the sailing time is performed: v
According to these formulas, the tables "Distance by time and speed" and "Time by distance and speed" in MT - 75 appendices 2 and 3, respectively, were compiled.
Calculations of the distance traveled using the speed determined by the frequency of rotation of the propellers V o6 are performed only in the absence of a lag or to control its operation.
Determining the speed of the vessel by the propeller speed mode.
Logs are used to measure the speed of large ships. On small ships, a simple log gives large errors in determining the speed and it is not always possible to apply it. Therefore, for small boats, it is easier to determine the speed from tables or graphs that express the dependence of speed on the number of propeller revolutions. To have such tables or graphs, it is necessary to determine the ship's speed on the measured line for different propeller revolutions (Fig. 59). The determination of speed is carried out in favorable weather. The yaw of the vessel on the course shall not exceed ±2°.
Rice. 59. Scheme of measuring line equipment
The measuring line is equipped with a leading line, along which the vessel keeps its course, and four or more secant lines, the distances between which are accurately measured. Vessel speed on the measured line is measured at constant engine operation. To eliminate errors in determining the speed from the influence of wind and current, two runs are made in the same mode of engine operation - in one direction and the other.
By the stopwatch, they notice the moment the vessel passes the secant alignments. Knowing the time t 1, t 2, t 3 and the distance between the secant sections S 1, S 2, S 3, the speed V S is calculated by the formula:
V S = S
where: V S - vessel speed in knots;
S - distance between secant lines in miles;
t is the time of passage from the target to the target, sec.
During each run, it is important to accurately maintain the specified engine speed. Calculating the individual speeds V 1 , V 2 , V 3 , find the average.
After determining the speed on the measured line, a table or graph of the dependence of the speed of the vessel on the number of engine revolutions is built (Fig. 60).
It is useful to determine the speed of the vessel at different drafts. Then there will be several graphs and tables. They can be depicted on one sheet of paper for ease of use. Having such tables or graphs on the ship, it is possible to find the appropriate speed of the ship from a given engine speed and a known draft.
Sometimes there is no equipped measuring line nearby. However, to determine the speed of the vessel, it is always possible to choose two coastal landmarks, the distance between which is known quite accurately. These distances can be determined, for example, from a plan that contains both landmarks.
The leading alignments can be replaced by a compass on the ship, if there is no fear that the ship will be blown off course by wind or current, for this it is necessary to check and eliminate the influence of a running engine on the compass.
To measure speed, the vessel must be on a straight course on a safe navigation path.
Puc. 60. Vessel speed versus engine speed graph
The direction of the straight line joining the objects can be determined by means of a compass, but it is necessary that runs be made in a direction parallel to the straight line joining the objects.
In advance of approaching the first reference point, the ship develops a certain speed and enters the measured course at a given engine speed, which remain constant during the run to the second reference point. When the first landmark is abeam, a stopwatch is started or the time is noted by the clock. The countdown is made at the moment the vessel passes the traverse of the second landmark. The same observations are made during the reverse run.
§ 27. Simplified method for determining the speed of the vessel.
If it is impossible, especially during navigation, to determine the speed of the vessel using one of the methods described above, another, albeit less accurate, method is used. It is necessary to throw a temporary landmark into the water from the bow of the vessel - a small piece of wood - and at the same time turn on the stopwatch. When a piece of wood reaches the cut of the stern, the stopwatch is stopped. Based on the measured time and the known length of the vessel, the speed is found by the formula:
V S = ,
where V S - vessel speed in knots;
L is the length of the vessel, m;
t- time of passage of an object thrown into the water, sec.
It should be borne in mind that the shorter the vessel, the greater the error will be.
When determining the distance traveled, it must be remembered that the movement of the vessel occurs only relative to the water, and not to the ground. Wind and current are not taken into account, although they constantly affect the speed of the vessel. Therefore, when guiding the laying in the distance calculated by speed, it is necessary to introduce an amendment due to drift by the current and wind. The easiest way to do this is when the course of the vessel coincides with the direction of the current and wind or is opposite to them. With side drifts, the increase or decrease in speed will be approximately proportional to the cosine of the angle between the ship's heading and the lines of action of the current or wind.
The main reasons for the decrease in the speed of the vessel:
1) shallow water, in which, as speed increases, water resistance sharply increases. Therefore, in shallow water, the speed can decrease by 10 - 15%;
2) wind and pitching. With headwinds and waves, as well as with strong tailwinds accompanied by waves, the speed decreases.
With weak tailwinds, the speed increases slightly. Decrease in speed is observed when the vessel is overloaded, rolled and trimmed to the bow. On a wave, at the moments when the propeller leaves the water, the ship abruptly loses speed;
3) fouling of the underwater part of the ship's hull leads to a decrease in speed by 10 - 15% compared to the speed of a ship with a clean hull.
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