5 Piloting and Navigation

Transcription

1 227 5 Piloting and Navigation Introduction To Piloting and Navigation Navigation Generally speaking, marine navigation is the science of determining the position of a vessel, the position of its destination, and how to proceed from the current position to the destination safely and efficiently. Although the science of navigation can be learned in the classroom, the practical application of navigating on the water is as much an art, learned through practice and experience, as it is a science. This section introduces only the concepts and tables needed to get one started and to provide some refresher information for those who have already studied the subject. Navigation is a complex mathematical subject that cannot be adequately covered in a little handbook such as this. Plenty of books have been written that are entirely devoted to the subject, the definitive volume being The American Practical Navigator commonly referred to as Bowditch, so named after the original writer, Nathaniel Bowditch ( ) of Salem, Massachusetts. Bowditch Bowditch was both a ship s master and owner, but more important, he was an outstanding mathematician who was able to substantially advance the science (and art) of navigation on a practical basis. He is considered one of the best (if not the best) mathematicians in the United States of his time. The copyright to American Practical Navigator is now held by the U.S. government and the book is made available through purchase, and also as a free download as an Adobe Acrobat (.pdf) file from Types of Navigation Traditional types of navigation include: Dead reckoning (DR) is where position is determined using estimated distance and direction from a known starting point. Piloting is navigation with visual reference to land, objects, or structures with known positions. Celestial navigation uses stars and other heavenly bodies to determine position. The sextant is the instrument used

2 230 Boater s Pocket Reference: Chapter 5 Symbols used for each are: Examples Degrees: ' 20" Minutes: ' or Seconds: " ' Most large-scale charts use decimal minutes rather than seconds, but be careful since some still use seconds. Parallels of latitude circle the globe in an east west direction parallel to the equator and are measured north and south from the equator ranging from 0 degrees at the equator to 90 degrees at the poles. Parallels are usually written in the form: L N where the L specifies latitude and the N specifies north from the equator. S is used for direction south. Parallels of latitude are evenly spaced and 1 nautical mile is defined (for all intents and purposes) as the distance covered by 1 minute of latitude. Since there are 60 minutes in a degree, it follows that 1 degree of latitude = 60 nautical miles. Parallels of latitude Meridians of longitude FIGURE 5-1: Latitude and Longitude It is worth noting that a minute of latitude expressed to one decimal place equates to nautical miles expressed to one decimal place, which is a precision of 608 feet, or 185 meters. Two decimals gets you within 60 feet and three decimals within 6 feet. Meridians of longitude start from zero at Greenwich (London, England) and run east and west to 180 degrees (the International Date Line). At the equator only, a minute of longitude is equal to 1 nautical mile. Meridians are of the form Lo W where Lo specifies longitude and W

3 236 Boater s Pocket Reference: Chapter 5 The NOS classifies nautical charts by scale as follows: Sailing charts are the smallest scale (largest area) and are used for navigating on the open sea. Scale is generally smaller than 1:600,000. General charts are for coastal navigation offshore. Scales range from 1:150,000 to 1:600,000. Coastal charts are for coastal navigation inshore. These will be used in large bays and harbors and large inland bodies of water. Scale is from approximately 1:50,000 to 1:150,000. Harbor charts are used in harbors and small waterways. Scale is usually larger than 1:50,000 (smallest area). Distance Measurement Distance at sea is usually measured in nautical miles (nm) regardless of whether English or metric units are used for other measurements such as depth, displacement, etc. Speed is expressed in nautical miles per hour and is termed knots. Note: In this book, just the word miles or the abbreviation nm will sometimes be used instead of nautical miles. References to statute miles will always be preceded by the word statute, unless it is clear from the context to which kind of miles we are referring. Since 1 minute of latitude equals 1 nautical mile; to measure distance on a chart spread dividers between the two points to be measured and then place the dividers on the latitude scale at the side of the chart to read the distance. Do not use the longitude scale to measure distance! Measure distance from the latitude scale. This example shows 1.1 nautical miles. FIGURE 5-6: Measurement of Distance on a Chart

4 Piloting and Navigation 243 Lubber Line Flat Card FIGURE 5-10: Fixed Mount Ship s Compass (Photos Courtesy Ritchie Navigation) The compass has a lubber line that must be precisely aligned with the vessel s keel and is used to read the heading. Hand Bearing Compass A boat should also have onboard a hand bearing compass that is used to take bearings on navigation aids and other objects to aid in position determination. The hand bearing compass has other uses as well, which are described elsewhere in this chapter. Bearings are taken with the hand bearing compass by sighting the object through the sights or across the lubber line and reading the compass dial at the sight or lubber line. Fluxgate Compass Often a boat will also have a fluxgate Direct Read FIGURE 5-11: Hand Bearing Compass compass installed, which is basically an electronic compass with components that sense the earth s magnetic field electronically (no moving parts). This type of compass is most often used to feed direction to an autopilot system or to other onboard instruments such as radar. The makers of fluxgate compasses claim an accuracy on the order of +/ 0.5 degrees, which is somewhat better than mechanical compasses. The actual sensor may be mounted remotely away from electrical and other magnetic influences, which further increases accuracy Also a fluxgate compass is self-compensating, which eliminates the need to construct a deviation table (compensating is discussed later in this section). Compensating the fluxgate

5 252 Boater s Pocket Reference: Chapter 5 Some boaters prefer to use a graph rather than a table to read deviation. Figure 5-19 is a graph of Compass Deviation Table 5-4 on the previous page. Note how at 180 degrees apart the deviations are approximately the same absolute value, but with opposite signs. Deviation Compass Heading Degrees FIGURE 5-19: Compass Deviation Table The Deviation Table Using a Hand Bearing Compass An alternate but less accurate method to using the pelorus and range involves using a hand bearing compass to take readings on the boat s heading to compare to the ship s compass headings. The first step is to find a location on the boat where the hand bearing compass can be used without showing deviation in any direction. Start in a location as far as possible from any known magnetic influences and have the helmsman take the boat in a circle while you keep sight on a distant landmark with the hand compass. If the bearing indicated by the hand compass remains the same throughout 360 degrees, then this location can be considered deviation free, and headings taken with the hand compass can be considered magnetic headings. As with the pelorus method, we now proceed in 15- (or 30- or 45-) degree ship s compass heading increments. The hand compass lubber line is aligned with the vessel s heading and the ship s compass heading is subtracted from the hand compass (magnetic) heading to get the deviation. The Deviation Table Using a Fluxgate Compass If you have a fluxgate compass that has been properly compensated (see previous fluxgate compass discussion), and you feel it is working correctly, then the same procedure as for the hand bearing compass could be used. This should

6 256 Boater s Pocket Reference: Chapter 5 accuracy is measured in meters instead of miles as is typical with a sextant. Radar lets the boater see in the dark and fog, which considerably decreases chances of collision under those conditions. Unfortunately, reliability, simplicity, and accuracy have a dark side. Many new boaters never learn what to do in the event of GPS failure, precisely because it s so reliable and easy. For this reason, traditional piloting methods that can be used as a fallback in the event of electronics systems failures, or as adjuncts to electronic systems, need to be learned by the recreational boater. Along with electronics based navigation, these traditional methods are described in this chapter. GPS How GPS Works NAVSTAR is the name of the Global Positioning System (GPS) developed and implemented by the U.S. government for military purposes starting in the 1970s, and later made available for civilian use. Most people refer to the NAVSTAR system as the Global Positioning System or just GPS, and since that s what most people call it, this book will follow that convention. Just keep in mind that there are other GPS systems in this world. Russia has a GPS satellite navigation system called the Global Navigation Satellite System (GLONASS), which is also available for civilian use although it s not particularly useful for recreational boating activities at this time. In addition, Europe is developing Galileo, which will be a third satellite navigation system that is supposed to be operational in The space segment of NAVSTAR has 24 satellites, including three spares, orbiting in six separate orbital planes inclined at 55 degrees to the equator. The satellites orbit at an altitude of about 12,500 miles (20,000 km) and are spaced so that at least six satellites are always in view from any location on earth. The GPS receiver constitutes the user segment. By measuring the time for a radio signal to reach the GPS receiver from each satellite the receiver is able to calculate its distance from each satellite. To accomplish this, accurate clocks are used in

7 268 Boater s Pocket Reference: Chapter 5 FIGURE 5-34: Basic Components of a Radar System. The photos are of a Raymarine open array antenna connected to an E120 Multifunction Display. (Photos Courtesy Raymarine, Inc.) Radar Power, Resolution and Range Radar transmit power is specified in kilowatts (kw) along with a range given in nautical miles. For example a set might be advertised as having a power output of 4 kw and a 36 nm range. Radars for recreational boats normally come with transmission power ratings from 2 kw to 4 kw and have an advertised range from about 16 nm to 48 nm. In general, power relates to range: 2 kw radars have nominal ranges from about 15 to 20 miles. 4 kw radars have nominal ranges from about 35 to 50 miles. The power required to operate the radar is very much less than the transmission power since the transmit pulses are so short (about a millionth of a second) relative to the interval between the pulses (about a thousandth of a second). In other words the radar is only transmitting about a thousandth of the time. Most small boat radars actually only consume around 20 to 75 watts of power continuously. Radar transmits in a straight line, and therefore the distance the radar can function depends on both the height of the radar and the height of the target above sea level. FIGURE 5-35: Radar and the Horizon The following equation from Bowditch can be used to calculate the geographic radar range knowing both the height of the antenna and the height of target.

8 278 Boater s Pocket Reference: Chapter 5 FIGURE 5-43: Compare Digital Raster and Vector Charts. These two screen captures from Nobeltec Visual Navigation Suite show a raster (top) and vector chart (bottom) of the same area. (Courtesy: Don Buddington) To produce a vector chart from the original master, the chart is digitized by someone carefully tracing all the lines and objects from the original master on a digitizing board. Also

9 280 Boater s Pocket Reference: Chapter 5 Types of Personal Computer PCs used for planning and navigating are usually either desktop or laptop/notebook systems; however, marinized versions are available for more permanent installations aboard. To my knowledge, all of the navigation software for these systems only run on the Windows operating system. Laptop computers tend to be the most commonly used since one can do their trip planning at home, then simply pick up the laptop and bring it onboard. Set it by the helm, connect it to a GPS, and you have a real-time navigation system (assuming you have the appropriate navigation software installed). When you re anchored at night, you can take it below to answer your or do some more trip planning. A desktop system is used primarily at home for trip planning, but some larger vessels may have room for a desktop system onboard. Desktop systems are rarely found around the helm, but rather are found in some kind of office setup elsewhere on the boat. Ruggedized notebooks are available that are sealed against moisture and are also able to withstand rough handling. Many of these also have daylight readable displays so they eliminate most of the problems FIGURE 5-44: Computer System Running Maptech Digital Charting Software. conventional associated with laptops. Expect to pay at least double what you would pay for a conventional notebook of similar size and capacity and even more for one built to milspec (military specifications). Fortunately, prices are coming down. Marinized PCs and LCD displays are now on the market but are still quite expensive, particularly the displays. I have found 15-inch XGA daylight readable waterproof displays

10 284 Boater s Pocket Reference: Chapter 5 If you have a laptop, you can pick it up and take it home with you with all the historical data from a trip. Conversely, it s easy to bring onboard and set up; just hook it to a GPS, and you re ready to go. There is no need to upload or download waypoints and trip plans to a chartplotter. Chartplotters Introduction to Chartplotters A dedicated chartplotter, whether linked to or having an integrated GPS unit, is becoming more commonplace on recreational boats. It offers unparalleled ease of use and accuracy, giving a constantly updated plot of the vessel s position, and it works day in and day out, and equally well in the dark. This is a good thing, since it doesn t make as many mistakes as humans, and it allows the captain to concentrate on other tasks like driving the boat and keeping a lookout for hazards. There is a negative aspect in that inexperienced vessel operators are lulled into a false sense of security, failing to see the need for maintaining position plots on paper charts. A chartplotter does pretty much what its name implies; it plots position on marine charts that are displayed on a LCD screen. Screen sizes typically range from a couple of inches on a handheld to around 12 inches (30 cm) on larger fixedmount units. Some display black-and-white images, while FIGURE 5-47: Chartplotter. The chartplotter in this illustration is an MFD concurrently displaying in color, a chart window, a fishfinder window, and a digital data window. The model shown here is a 3010C. (Courtesy Garmin International, Inc.)

11 290 Boater s Pocket Reference: Chapter 5 bottom. Some fishfinders will optionally display the arches as little fish symbols if preferred. Some can even display depth by the fish. Many fishfinders are available that are an integrated with a chartplotter. On these one has the choice of side by side display of the fishfinder and chart data, or just displaying one or the other. Scanning Sonar The best way to think of scanning sonar (also called forward looking sonar) is like radar for underwater use. While scanning sonar has been used for years on military and commercial ships, it has only recently become affordable for the recreational boater. Scanning sonar uses a phased-array transponder that emits a narrow beam scanning either horizontally or vertically ahead. Horizontal mode units scan from 45 to 90 degrees each side of dead ahead depending on the model. Vertical mode sonar scans from horizontally ahead down to directly below the boat. Some of the newer units are capable of both vertical and horizontal scanning and can be switched between either mode by pressing a button. FIGURE 5-52: Scanning Sonar. The model shown is a Color Twinscope. (Courtesy Interphase Technologies, Inc.) The display screen for a horizontal sweep looks somewhat similar to a radar display except the coverage is only 45 or 90 degrees to either side instead of a full 360-degree circle.

12 Piloting and Navigation 293 Integrated systems are cheaper than networks but lack their flexibility. Networked Systems A networked system is a network, much like an Ethernet computer network, where individual devices can be plugged into and out of the network as needed. Network systems use multifunction displays (MFDs) that include chartplotter capability as the central part of the network. More than one MFD can be connected to the network so that devices such as radar can be controlled from various locations such as the pilothouse or flybridge. Some systems combined with NMEA compatible sensors can monitor information such as speed, engine temperature, RPM, and voltage directly on the MFD, although one could argue that these functions should be monitored independently of the network. Currently, networked systems are available from Furuno, Garmin, and Raymarine, and each uses a proprietary network protocol so that devices compatible with one brand are not compatible with another. This generally means that once you decide on one manufacturer s hardware, any devices that will be hooked into the network must also come from the same vendor. Radar GPS Weather fax Heading input Video in Video out Autopilot MFD 1 Hub MFD 2 Depth sounder FIGURE 5-55: A Networked Navigation System Personal computer Most MFDs now accept composite video (television) input so video from remote cameras, a VCR, or even broadcast

13 310 Boater s Pocket Reference: Chapter 5 The course (C) is plotted as a straight line with the course shown above the line and speed in knots, and/or distance in nautical miles, below the line. The dead reckoning (DR) position is plotted on the course line as a semicircle with the time next to the circle. The Power Squadrons plot the time at an angle as shown. A track (TR) is plotted similarly to the course with the intended track direction plotted above the line and the intended speed below. Line of Position (LOP) is plotted as a solid straight line with the time on one side of the line and the bearing or celestial body name on the other side of the line. Here we show the time always on top. LOP advanced -shows the original time of the LOP and the time the LOP was advanced. Plot a fix from the intersection of two LOPs as a circle of about 3/16 inch (50 mm) diameter with the time of the fix horizontally to the side of the circle. Suffix "R" for a running fix as 1756 R. Some use the suffix "FIX" after the time. Other types of fix are labelled with a suffix after the time to identify the type of fix. ie. RADAR, LORAN, etc. Estimated position (EP) is marked as a square on the LOP. Time may also be included if it not readily apparent from a DR or LOP. The Power Squadrons use a triangle to designate a known position. TABLE 5-11: Symbols Used in Plotting C 057 D 26.8 S 5.3 TR 097 S Kochab GPS 1542

14 312 Boater s Pocket Reference: Chapter 5 Dead Reckoning Dead reckoning (DR) involves plotting a course line in the planned or actual sailing direction with DR positions marked at measured distances along the line. Distance along the course line is calculated using the speed and time formulas above or if the boat has a log, then the distance can be read directly from the log at any point. (The log is a device that performs a similar function to the odometer in a car.) Dead reckoning (deduced reckoning) involves calculating position without reference to external objects, using just a starting point and course direction, speed, and travel time. When the GPS and the radar fail and you are out of sight of any landmarks, dead reckoning will be your fallback if you have been keeping your current position plot up-to-date Plot DR at every change in course or speed Plot DR every hour on the hour Distance not usually plotted C 095 S 6.7 D C 045 S 6.5 FIGURE 5-65: Dead Reckoning Plot Example DR Calculations 1. In the DR plot in figure 5-65, the vessel is to travel 8.9 nautical miles at 6.7 knots on a course of 95 to the location of the planned course change to 45. Using the equation for deriving time in minutes we get: T = (D x 60) / S = (8.9 nm x 60) / 6.7 knots = 79.7 min 80 minutes to cover the distance plus 80 minutes = 0937 is the estimated time for the course change. 2. Alternatively if we are underway and it is now 0937, then we calculate distance traveled with the distance equation: = 80 minutes is the time underway. D = (S x T) / 60 = (6.7 knots x 80 minutes) / 60 = 8.9 nm is the distance traveled by 0937.

15 Piloting and Navigation 315 FIGURE 5-68: Trip Plan Chart Using Maptech Chart Navigator Computer Program. Note the course and distance are not plotted for each leg as on the paper chart, but are available as a separate tabulation. To alter a path, waypoints can be moved by dragging with the mouse. Waypoints can be inserted into a leg to subdivide the leg into smaller legs, which can then be moved by dragging them as well. This ease of inserting and moving waypoints leads to a simplified trip planning process, whereby a single leg is constructed from the starting departure point to the final destination point and then waypoints are inserted into the line and dragged into position to construct a multileg plan.

16 322 Boater s Pocket Reference: Chapter 5 Range Fix C 081 S C 083 S FIGURE 5-73: Fix by Two Straight LOPs Intersection of Three LOPs Since it is so easy to make mistakes in navigation observations and calculations, at least three LOPs are recommended if they can possibly be taken and plotted. Figure 5-74 shows three LOPs. In almost all cases there will be a small triangle formed at the intersection of the three LOPs, the size of which indicates the accuracy of the fix. The smaller the triangle, the more accurate the fix. Positioning the fix in the triangle is a matter of some judgment. If one of the LOPs is considered less accurate than the others then the fix might be placed nearer the intersection of the better ones. Or it might just be placed in the center of the triangle. LOP #1 LOP #2 LOP #3 Enlarged View FIX Small Triangle FIGURE 5-74: Intersection of Three LOPs Intersection of Circular LOPs (COPs) The intersection of two COPs constitutes a fix. They are plotted as shown in figure They can be COPs derived either

17 Piloting and Navigation 329 Danger Bearing With GPS The bearing to a known or charted feature is most easily determined with a hand bearing compass, however, a GPS can be used for this purpose as well. This technique can be used in situations where only a handheld GPS with no chartplotter is available and also as a check on the bearings taken with the hand bearing compass. To monitor the bearing to the object, obtain the coordinates of the known feature from the chart and enter these into the GPS as a waypoint. Now when a GOTO for this waypoint is set in the GPS, the bearing to that waypoint will be continuously updated and displayed as the boat moves. As with the compass in the example of figure 5-84, the bearing must be NMT 45 degrees at any time, as the boat approaches the destination. Note that with GPS there is no need to actually see the object or feature, since the GPS is just pointing to the waypoint coordinates and not a real physical object. In fact, the waypoint doesn t really need to be set on a real feature or object. Draw a line that avoids the hazard on the chart and set an arbitrary waypoint at the end of the line. Set the waypoint as a GOTO in the GPS and proceed as before. Using GPS Crosstrack Error to Avoid Danger The perpendicular distance from a leg to a hazard can be measured on a chart, and when underway treated as the maximum allowable crosstrack error. Some GPS units can be made to set off an alarm if a preset maximum crosstrack error is exceeded Maximum allowed crosstrack error FIGURE 5-85: Using Crosstrack Error to Avoid Danger

18 332 Boater s Pocket Reference: Chapter 5 Set EBL on target Target B moves ahead of the EBL. Relative bearing to port is decreasing and B will pass ahead. A B C A B C Target A falling behind Target C staying on the EBL. Relative bearing EBL is on a collision to port is increasing course. and A will pass behind. FIGURE 5-88: Collision Avoidance With Radar Set and Drift Introduction to Set and Drift Current is tidal, ocean or other horizontal movements of water, and it can have a significant effect on the movement of the boat over the ground. Set is the direction of the current and drift is the speed of the current in knots. A vessel is also affected by leeway, which is motion of the boat away from the wind As the boat proceeds along a specific course, she also deviates from the course steered, through helmsman error, compass error, and other factors. With the possible exception of current, these errors are difficult if not impossible to estimate individually, so estimates of combined set and drift from all these factors are made based on actual experience. The combined set and drift is usually referred to as just set and drift, as for current. Determining Set and Drift An approximation of leeway can be deduced by comparing the direction the boat is steered with the direction of the wake. The angle between the two can be used to correct the course steered to try to achieve the desired track (TR). In some cases where leeway is not a significant factor and tide, and current tables are able to provide the speed and

19 Piloting and Navigation 335 Tides and Tidal Currents Introduction Tides are changes in sea level due to forces of the moon and sun acting upon the earth as it rotates. Of the two, the moon exerts about twice the influence on tides as the sun. In addition to the rise and fall, tidal currents also exist and it is important to note that though these phenomena are related, they must be considered separately. Tide refers to the vertical rise and fall of the sea and tidal current refers to the horizontal movement of the water. In most locations, the tide occurs twice daily; that is, there will be two high and two low tides per day. This is a semidiurnal tide. In other areas there will only be one high and low tide per day and these are termed diurnal tides. The tide rises and falls in a manner similar to a sine wave. Figure 5-92 shows an example from Bowditch of a semidiurnal tide Hours -1 FIGURE 5-92: Tide Rise and Fall New York Note how the rise and fall rate is slower at the highs and lows, and is faster in between. Tidal Datums Tide levels, depths, and overhead clearances are measured from averaged tide levels called tidal datums. The mean low water (MLW) is the low tide averaged over many years. If one of the day s low tides is lower than the other, then only the lower level is averaged and we term this

20 340 Boater s Pocket Reference: Chapter 5 Index to Navigation Checklists, Forms, and Tables Navigation Checklist...on this page Sample Daily Navigation Log...on page 342 Convert Meters, Feet, Fathoms...on page 343 Convert Nautical Miles, Miles, Kilometers...on page 344 Natural and Numerical Chart Scales...on page 346 Distance by Vertical Angle from Waterline to Top of Object...on page 348 Distance by Vertical Angle from Horizon to Top of Object Beyond Horizon...on page 352 Distance to Nautical Horizon...on page 355 Distance Object Can be Seen Over Horizon...on page 356 Sines, Cosines, Tangents...on page 358 Distances Between Ports...on page 360 Navigation Checklist Use this as is or as a starting point for you own checklist. This list is downloadable in Excel format at www. anchorcovepublishing.com. NAVIGATION CHECKLIST Permanently Installed Navigation Equipment Ship s magnetic compass installed and adjusted properly Ship s compass deviation curve or table current Fluxgate compass calibrated Autopilot adjusted/checked Pelorus installed/aligned GPS checked out and functioning correctly Chartplotter check Computer with charting software Depth sounder check Tachometer calibrated with speed curves (include even if knotmeter installed) Knotmeter calibrated (GPS or measured mile) Log (odometer equivalent) calibrated Same check all nav equipment on second bridge Loran electronics check Radio direction finding equipment check Portable Navigation Equipment Calculator, two each, preferably scientific

21 Height of Eye Feet Meters Nautical Miles Distance to Horizon Statute Miles Kilometers Piloting and Navigation 355 Height of Eye Feet Meters Nautical Miles Distance to Horizon Statute Miles Kilometers TABLE 5-19: Distance to Natural Horizon

22 Piloting and Navigation 365 Miami Florida Nassau Bahamas Havana Cuba San Juan Puerto Rico St Thomas USVI Tortola BVI Basselterre St Kitts St. John s Antiqua Basse-Terre Guadeloupe Fort-de-France Martinique Bridgetown Barbadoes Kingstown St. Vincent St George s Grenada Port of Spain Trinidad Caracas Venezuela Maracaibo Venezuela Krajendjik Bonaire Willemstad Curacao Oranjestad Aruba Kingston Jamaica Georgetown Cayman Islands Cancun Cancun Panama City Cancun Cayman Islands Jamaica Aruba Curacao Bonaire Maracaibo Caracas Trinidad Grenada St. Vincent Barbadoes Martinique Guadeloupe Antiqua St Kitts Tortola St Thomas Puerto Rico Havana Nassau Distances are scaled from charts. Distances are not shortest distance and tend to follow coastlines and the Windward Isles. TABLE 5-26: Distances Between Ports. Caribbean (nautical miles).