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Catamaran: The Shape of Things to Come
By George E. Meese,
Designer of the Ridgely Warfield
The high-speed catamaran research vessel Ridgely Warfieldan all-welded aluminum vessel, could be the forerunner of a new fleet of low-powered, high-speed naval vessels and merchant ships.
For example, the Warfield’s large deck areas, stability, high speed, and rough- water agility could presage a catamaran aircraft carrier whose wider flight deck will be designed to launch and land aircraft on parallel runways, and whose hanger decks will be spacious compared to present-day carriers.
Built by Bethlehem Steel Corporation’s Shipbuilding Division of Baltimore, Maryland, the Ridgely Warfield is operated by Johns Hopkins University’s Chesapeake Bay Institute of Annapolis, Maryland. In service since 1968, the Warfield’s speed capabilities have permitted research work heretofore impossible for slower vessels.
Catamaran damage stability characteristics are excellent and highly desirable for all types of naval vessels. The Ridgely Warfield, owing to its commer
cial nature, was designed for one- compartment floodability; however, approximately 74% of the length of one hull above the double bottom could be flooded without loss of the vessel.
While many catamarans have difficulty turning, the Warfield’s hull form increases her turning ability to better than that of conventional vessels of the same size. The catamaran offers a tremendous naval advantage for turning aircraft carriers or other types of combatant ships that may come under enemy fire since there is very little heel outboard during a very sharp high-speed turn.
The Warfield’s owner required a vessel which would cruise at 18 knots and work at a minimum speed of one knot. Along with high speed, she had to provide large working deck areas, laboratory space, and a high degree of stability for research tasks while on station.
Feasibility studies revealed that a catamaran approximately 70% of the length of a conventional vessel could meet the owner’s operational require
ments. The cost of building this special catamaran was estimated at two-thirds that of a single-hulled vessel to fulfill the same functions.
A model testing program was outlined in the final design stage to compensate for much past catamaran design which had been done more-or-less by rule of thumb methods.
An increase in wave action between catamaran hulls generally is detrimental to the use of deck wells on research vessels. This difficulty was solved by the use of asymmetric hulls with the straight sides inboard. The resulting wave action between the hulls is almost negligible.
When motions in waves in State 5 seas in the model tanks were studied and the results analyzed, the catamaran was found to have smaller pitch angles than conventional hulls.
Rolling angles were found to be considerably less for the catamaran.
The research catamaran would be able to proceed at higher speed and work for longer periods than a similar conventional vessel.
106 U. S. Naval Institute Proceedings, August 1974
The catamaran Ridgely Warfield is shown below with her extension boom crane rigged over the starboard side in preparation for on-station research work. At right, the vessel is in drydock for final bottom cleaning and painting prior to her trials. The tertiary bow between the twin hulls is designed to reduce pitching. At right, equipment is being prepared for lowering through the center well.
Study of model resistance tests showed the power requirement would be only about two-thirds that needed for an equivalent conventional vessel.
Aluminum was chosen as the material for construction because, in addition to easy fabrication and low maintenance, its light weight permits maximum ship speed while requiring minimum power and fuel. Had high tensile steel been used, the power requirements would have been increased by about 20%, requiring engines above the commercially available standard size and power range for high-speed diesels. The advantages of aluminum—over one-third of structure and fittings weight was saved by the use of welded aluminum—give the Warfield a greater capacity to carry necessary scientific equipment.
As the catamaran developed in the configuration stage, weights had to be eliminated wherever possible. The usual catamaran forecastle arrangement in each hull was considered superfluous and cut away. However, the centerbody structure above deck presented a problem in entering heavy seas. In order to prevent undue slamming on the center- body forward bottom, a third bow, or tertiary bow, was conceived. This reduced pitch angles as well as slamming stresses and weight. It allows the fine catamaran bows to enter a considerable distance into heavy seas before rising and causing excessive pitching. As the bows rise, the heavy seas are thrown aside by the tertiary bow over the two forward decks, causing some additional weight on the decks, which in turn
tends to prevent the rise of the bows in the waves. This reduces somewhat the tendency to pitch.
The vessel—260 gross tons, 106 feet long, with a beam of 33 feet and a draft of 6l/2 feet—was built under cover in order to obtain fine high-class welds for structural strength. The all-welded hulls and superstructure of the vessel contain about 135,000 pounds of aluminum sheet, plate, and extrusions, fabricated from corrosion-resistant marine alloys.
Because the Chesapeake Bay and its tributaries are subject to freezing in the winter months, creating a very thin layer of skim ice which can cut through wood and steel hulls at the waterline, and because the extra weight of the usual protective systems would reduce the
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speed of the Warfield, an ice belt approximately two feet wide, of specially- treated, hardened aluminum was fitted at the waterline of each hull.
Main propulsion engines are two Cummins model VT12-825M tandem marine diesel engines installed as one unit per hull. Engines are rated at 1,150 shaft horsepower, at 1,950 r.p.m. and are fitted for trolling when only the after engine is in operation. Engines are fitted with Capital combining and reduction gears of 3.06:1 ratio, with complete pilot house control. An auxiliary control station is located on the port side at the after end of the boat deck overlooking all scientific operation on the main deck.
At the outset of the design, it was required that all scientific equipment should be operable simultaneously as far as practical in order to speed activity on
stations in keeping with the high cruising speed of the vessel. This was accomplished by locating the coring, dredging, and trawling crane winch so that it operates over the starboard side and aft from the main working deck. The hydrographic winch is arranged to be employed over the port side of the main working deck or through the center well. A bathythermographic winch is workable over the starboard side or through the main deck well. A second BT winch operates over the starboard side from the boat deck.
The heavy lift crane has a capacity for a 2,000-pound lift at 16-foot radius.
One of the research projects in which the high speed of the Ridgely Warfield was engaged was to follow the tides up Chesapeake Bay, and to be on various research stations at the same tide-time.
This effort produced a considerable increase in the knowledge of the tides in this large confined body of water.
The movements of the catamaran are different from those of conventional vessels of similar size. Because of this, a member of the crew usually does not like the catamaran after one short cruise. However, after several five-day cruises he becomes accustomed to the motions and can anticipate them as readily as he could those of the usual vessel.
As these words are being written, more advanced types of catamarans are being considered for naval and commercial service, and conceptual designs and tank tests have been completed. From the experience gained in the Ridgely Warfield, there seems little doubt that better and more efficient catamarans will be riding the waves of the future.
Can We Really Afford Surface ASW Ships?
By Captain Robert J. Alexander,
U. S. Navy (Retired), Former Head of OpNav Oceanography Branch and OpNav ASW Surveillance Branch
Based on the United Kingdom’s experience with an all-volunteer force, we can expect to recruit a Navy of about 250,000 volunteers within the next few years.
Assuming that our present deterrent force of 41 nuclear submarines with their supporting components is about tight, we can deduct 50,000 officers and men from the volunteer force of 250,000. The Blue and Gold crews, tenders, submarine school, nuclear training, Naval Academy, ROTC units, recruiting, supply and repair facilities will require on the order of 50,000 men for our deterrent force. The remainder of the personnel will have to be distributed | among the other forces for Navy- assigned missions.
One of the important missions for the post-1975 Navy will continue to be antisubmarine warfare (ASW) to counter I the estimated 350 enemy FBM and attack submarines that might oppose us.
How has the ASW problem evolved and where do we stand today? At the
beginning of the Cold War, around 1948, the submarine threat consisted essentially of 10,000-yard-range torpedoes launched from diesel-powered submarines. Before the end of the 1950s the threat had changed to longer range torpedoes launched by nuclear powered submarines. The threat we face today has changed to supersonic cruise missiles with ranges of some 20-30 miles when launched from underwater and 200 or more miles when surface-launched.
A 1948 submarine with a submerged speed of, say, 5-10 knots, had a rigorous requirement for exposing a snorkel every day or so to charge batteries. At this point the snorkeling submarine was quite vulnerable to detection by radar and destruction by depth charges from our own surface or air forces. The effectiveness of our ASW units against diesel submarines was fully demonstrated by their success against snorkeling German U-boats in World War II.
The arrival of the nuclear submarine, however, completely changed the rules
of the game. Not only does she not have to expose a snorkel, but she has a submerged speed comparable to that of surface escort vessels and can go to depths where the ocean pressure is so great that depth charges or other thrown weapons will be too bulky or will not function.
The advent of the underwater- launched cruise missile in the late 1960s introduced another factor to complicate the ASW problem. In 1948 we were dealing with an enemy who was forced by the limitations of his weapons to come within a few miles of his target and then had to expose a periscope for his final fire control solution. Today we are dealing with an enemy who can fire many miles from his target, can go to great depths, and can evade at speeds comparable to surface destroyers.
In the early 1960s the initial advantages of the nuclear submarine were partially corrected by the development of airborne surveillance using sonobuoys and torpedoes that could be dropped
from the air. But we have just about hit the limit of improvements which can be made against environmental constraints. The laws of physics in the oceans do not offer any immediate hope or possibility for developing sufficiently improved hull-mounted sensors to deal with the modern submarine threat. In other words, the surface escort role in ASW is fast becoming obsolete. Surface ships can handle the diesel submarine and are marginally effective against submarines that have to get close enough to a target to launch torpedoes, but they cannot be used effectively against submarines capable of launching cruise missiles from ranges in excess of, say, 20 miles.
Such a statement is bound to raise the ire of surface ASW officers, but before crying "Foul,” let us take a quick look at what happens in the oceans when we try to detect, identify, locate, and destroy a modern submarine:
► Electromagnetic radiation is seriously attenuated in saltwater, thereby preventing us from obtaining useful ranges by radar, other electromagnetic sensors, or lasers. Our only usable sensors depend on underwater acoustics.
► Noise from one’s own ship, plus that from waves, fish, and other marine orga
nisms, makes it difficult to use hull- mounted passive sonars. The two-way trip for an active sonar requires excessive power demands for usable ranges and has other technical problems.
► The bending of sound waves at different temperature layers offers shadow zones beneath which submarines hide virtually undetected. Hull mounted sonars cannot get below these layers.
► The increased pressure at great depths makes it difficult to build torpedoes or sensors that will operate effectively at depths below 1,000 feet. Again they have to be heavy, bulky, and cumbersome to withstand great pressures.
Thus, there is only a very limited portion of the oceans where surface ship sensors or weapons can be effective. Our scientists and engineers have come up with some rather ingenious solutions for getting through ocean layers which normally would be impenetrable but they arc not too effective from surface hulls.
Variable depth sonars and air-dropped sonobuoys not only permit sensors to be placed below temperature layers right in the shadow zones where submarines may hide, but also help reduce the noise problem. Lower frequency sonars reduce the attenuation that occurs at higher acoustic frequencies, but this results in bigger sonars and appropriately larger cases to carry the lower frequency sonar. This is one reason why a modern destroyer approaches a cruiser in size.
The use of convergence zones caused by bending of sound rays in very deep water permits detection of submarines out to ranges that are multiples of about 17-18 miles. The main disadvantage, however, is that the detection range is very narrow, so if the submarine stays within 15 miles or beyond 20 miles he
will probably not be detected by convergence zone techniques. Long range detections are possible by putting sonobuoys in the deep sound channel (SOFAR channel), but determining the position of the sound channel requires considerable oceanographic information before dropping the buoy.
The increased use of maritime aircraft and hunter/killer submarines to detect and destroy submarines at great distances from our convoys offers the best hope for solving the submarine problem. It is there that we will have to put our money during the coming decade.
This is far from being a new concept- We have long recognized that a hunter/ killer submarine quietly patrolling a strait or small barrier area has an excellent chance of detecting an intruder and creeping up to destroy her. Similarly, the extremely high speeds of patrol or carrier aircraft with respect to a submarine (250 knots versus 25 knots) offer an additional advantage to air ASW forces which would be formidable if carried out in concert with hunter/killer submarines.
Even with ASW aircraft and hunter/ killer submarines, tremendous technical problems remain. The most important is positively identifying the submarine as being unfriendly.
Another problem is remaining undetected right up to the point when the weapons hit the target. If the enemy sub hears anything, he will evade at full speed, change depth, and go quiet, and that is the end of the attack. To detect, locate, and identify a nuclear submarine is difficult enough—but still possible. To kill a fully evading nuke is just not within the present state of technology because of the wild gyrations that
nuclear submarine can make at high speeds.
Yet another problem is to develop communications equipment and tactics which will permit aircraft and submarines to talk among themselves without alerting the enemy.
It seems clear, then, that surface escorts with hull-mounted sensors cannot be sufficiently effective against a nuclear submarine with long-range cruise missiles, and that we should put our ASW effort into air/hunter killer groups.
1
5
1
t
t
Any analysis such as this, which relies entirely on unclassified information, is subject to gross error. Thus, perhaps the first order of business should be to empanel a group of experts to determine the effectiveness of surface escorts against the nuke/cruise-missile threat. If the study were to support the foregoing analysis, at least five steps would seem to be called for:
► Ship construction and aircraft procurement programs should be reoriented to provide more nuclear hunter/killer submarines, ASW carriers, and patrol aircraft, both land and carrier based.
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i
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► Research and development programs should develop equipment for identifying enemy submarines, for communicating among aircraft and submarines, for silencing ASW weapons so that they are not detectable by the target, and for
Professional Notes 109
ECM devices to decoy or destroy cruise missiles. Such equipment must be ready before 1980, or it will not be useful for the ships or aircraft procured over the next few years.
► Personnel programs should provide a fair transfer of surface ASW personnel into other areas similar to the way diesel submarines have been handled. No one should be hurt professionally who has spent his career in an area which has been overcome by changes in policy or technology. It is no one’s fault that this has happened, so it is the Navy’s duty to see that no one gets hurt.
► Training programs should provide courses in each school from recruit training through Senior War College for detailed instruction in ASW oceanography. All submariners I have known have an excellent knowledge of oceanography and underwater acoustics. I have never had the slightest difficulty in briefing a submarine admiral on how the environment affects his sensors and weapons. That same degree of expertise must be available throughout all ranks in the Navy if we are to control an enemy who really knows the environment in which he is operating.
► All programs involving hull-mounted sensors, surface-launched torpedoes or related items for a purely surface ASW capability should be cancelled and the
effort diverted to more effective areas. This does not mean the end of destroyers or surface escorts. Quite the contrary. Surface ships will always be necessary for carrier defense, and there are many other uses if we will but turn our minds in new directions.
One function comes immediately to mind: convoys could be ringed at 100 miles or so with destroyers that would act as the picket ships did in World War II. These ships could be equipped to destroy or decoy cruise missiles with sophisticated ECM gear. The point, though, is not to create "make-work” roles for surface ASW ships. We should either give them jobs that they can effectively do or decommission them.
In sum, then, the laws of nature obviate hull-mounted sonars; the nuclear submarine can outmaneuver any surface ship; submarine weapons now exceed the range at which a surface ship can engage the submarine. What do we propose to do about it? We have suggested some approaches; others will perhaps be able to come up with better, more acceptable ideas. The purpose of this Note is to stimulate discussion. Hopefully such discussion will provide an answer to the biggest of all our ASW questions: As U. S. resources become ever tighter, can we really afford surface ASW ships?
Reorganizing the Naval Reserve—Again
By Captain Morton Leavitt, U. S. Naval Reserve (Retired), National President, Naval Reserve Association
The Naval Reserve has been in the process of reorganization for many years, particularly over the past year, and the process has all appearances of continuing for some time into the future. The Naval Reserve Command, newly vested in a vice admiral, has established its headquarters at New Orleans and consolidated with the naval air and-surface Reserve commanders. The commander’s additional assignment as Director of the Naval Reserve in his Washington office gives him the position as a major pro-
gram manager necessary for effectiveness. Consolidation of the Naval Reserve support activities is also taking place in the Bureau of Naval Personnel. In November 1973 the Chief of Naval Operations directed actions to restructure the selected reserve.
Essentially, the aim of this latest restructuring process is to establish units which can be more readily mobilized in situations short of all-out war. The organization and training of these new units is more in line with their mobili-
zation functions than the previous setup. Thus they would be able to report as units to specified ships or shore stations if called upon.
These changes in the Naval Reserve organization are understandably hurting force morale by causing present uncertainties and personnel turbulence and instabilities. They must not be long protracted.
In the restructuring of the Reserve, needed emphasis is being given to hardware orientation, and hands-on type
training. These are fine objectives and should all help to attain a better Reserve. The effort to place the Reservists in unit organizations is applauded, but the implication is getting out that Reservists "as individuals” are not a good idea, and should be eliminated. Such thinking is unfortunate. There will always be essential positions in the Reserve for considerable numbers of individuals —as "individual units,” if you prefer.
It has been said that because the Navy has eliminated the former allowance and complement of manning lists and introduced the Ships Manning Documents, that the ships’ augmentation problems have now been solved. The fact is that the problem still remains. There is a Fleet shortage of considerable numbers, particularly in the critical artificer, engineer, and other technical ratings. It can only be solved on "an immediate basis” by the programmed availability of trained and ready Reservists as individual augmentees. This group of essential personnel has sometimes been called a "mobilization pool.” But this phrase has been so extensively disparaged in the unit emphasis that the managers erroneously believe the problem has disappeared. Trained and ready Reservists as individuals are still needed for active unit augmentation, and they should be kept in the program.
Let us examine the needs of the Naval Reserve program:
► Better training facilities. Naval Reserve
training is supported by facilities that vary widely in resources for proper program support. Many are very poor indeed, and some few are quite good and well-equipped. The Readiness Command concept should make it possible to greatly improve this situation. Under the direction of 22 Readiness Commands across the nation, the best- equipped facilities will be used to a greater extent, and the least capable will be phased out. The need to assure that training includes familiarization with the modern equipment and procedures in the Fleet environment depends on efficient transportation. The Navy’s program to enhance Reserve readiness must include ambitious objectives for better facilities, joint use of improved facilities, concentration of modern equipment in a necessary minimum of centers and assured efficient transportation.
► Jet transportation requirements. For the Naval Reserve to be a truly reliable and well-trained force, a jet transport capability is vitally needed in replacement for the C-118 propeller transports now in the reserve logistic squadrons. Support of the Navy during mobilization is the primary requirement for an improved logistic support capability, but training maintenance is also a necessity. The present Reserve C-l 18s have a very short Service life remaining and have become unreliable and very costly to operate. The approximately 30 Reserve C-l 18s should be replaced as soon as possible
with jet transports to provide the transportation needed to maintain Reserve force readiness in training, and the essential emergency logistic support during contingencies leading to mobilization. Naturally, the concern is to find dollars to pay for them and a host of other programs.
► Recruiting initiatives. Since the Navy Recruiting Command took over responsibility for both Regular and Reserve recruiting, results have indicated the ability to attain regular goals of high quality personnel in most programs in the allvolunteer environment. This has required extraordinary leadership and command attention, but there is still the need for improvement. In the Regular Navy there are still some special problems of aviation officers, nuclear submariners, doctors, and some other specialists. In the Reserve program, although success has been achieved in the 3x6 and 2x6 programs, the present 4 x 10 requirements have not been met. If this program is to succeed a very different sales approach and special incentives probably will be needed.
► Organized civilian skills. In the efforts now underway to structure the Naval Reserve organization as units with missions assigned, hardware in hand, trained and ready for prompt response, a disdainful eye is being cast on the individual Reservist. It is often assumed that you don’t need to have Reservists in the program if the same skills are available in the civilian community. Many individual skills spring to mind: doctors, lawyers, constructors, public relations persons, systems analysts, supply experts, administrators, intelligence specialists, communicators, cryptolo- gists, research engineers, port controllers, chaplains, and more. But there is an error in the logic that would try to persuade us that such persons are therefore not needed in the Reserve program. People such as these are in high demand in any emergency, and all too often in short supply. To be available, they must be earmarked for the program, they must accept and recognize their individual commitments. They must become familiar with what to expect and thereby be capable of instant effectiveness. This would overcome the inefficiency and delay of a non-systematic approach. Time-lags are eliminated. Reliable, de-
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pendable performance can be programmed, and vital military operations may have a chance to succeed. Moreover, the President has no authority to call private individuals to duty from the civilian communities without action by Congress. Even the most talented collection of civilians—without any real Service-oriented commitment and having multiple private involvements and future plans—would have no chance of quickly melding into an active operating military unit.
► Deserved service benefits. In order to attract and retain high caliber people, it is important to provide the Reservists benefits and inducements comparable to those enjoyed by regular force personnel. Such a list is not endless, as some negatively-inclined persons have been known to state. It should include such benefits as are reflected in the annual resolutions of the NRA. I particularly want to emphasize the importance of the Survivors Benefit Plan for Reservists, Serviceman’s Group Life Insurance for Reservists, Reserve retirement on an actuarial reduced basis at age 55, and Reservists’ use of exchanges. These several legislative proposals affecting the Reserve components, I believe, fully warrant full support by DoD and early and favorable
action by the Congress.
► Strong management urgent. The general state of the Naval Reserve today is one in which there are many things of which to be proud. But there is, admittedly, a long way to go before the Reserve Force will be completely organized as a responsible, dependable component. The leadership and the organizational concepts are now being applied. Strengthened management is encouraged. The Chief of Naval Reserve should put forward strong and progressive programs to move the Reserve force to its rightful, responsible position alongside the regulars. His budget will have to expand a lot and his control over it will have to be completely fenced and protected if he is to be held responsible to establish and attain a vitally- needed Reserve improvement program. This will take some time to accomplish, but we now have the management opportunity to do the job.
► Goals. Naval Reserve goals must surely include the careful setting of realistic, meaningful manpower levels for mobilization and a well-trained, ready force fully hardware-equipped and supported by interesting, hands-on training programs. The programs seem to be moving in that direction but we need more
hardware-oriented units, and ever-better training, including more opportunities for Reservists to train in association with Fleet units on Fleet operations and deployments.
At present some Naval Reserve units are manned by a high proportion of Regular Navy personnel. This situation is costing the Regular Navy a premium in personnel and dollars, of which they have precious few in today’s much- reduced and fiscally restricted Navy. This is a factor tending to discourage the programming of badly needed additional ships and aircraft. Vigorous and innovative programs should be established with the objective of reducing the regular and increasing the Reserve proportions of manning all Reserve units. This would be a concurrent step in justifying and establishing additional units.
The Reserve program improvements should go forward with continuing energy and imagination and with stronger, more effective management to provide capable modern ships, aircraft and facilities, as well as equality of treatment for the reserve force. In the challenges of today’s uncertain world situation, our Reserve forces must be made more competent, more capable partners in the total force.
Latitude by Observations at Lower Transit
by Warren Norvillc Marine Consultant
To determine latitude, the navigator normally waits until the body he wishes to observe has a local hour angle of 000°. At this time the body is on the Meridian of his zenith. Thus latitude observations arc called meridian altitude observations. The classic example is the traditional observation of the sun at local apparent noon. Meridian altitudes Can be observed and reduced on any cclestial body for which almanacs tabu- kte the necessary data. This data is read- ‘ly available in the Nautical Almanac or d/V Almanac for the sun, moon, planets, ar>d navigational stars.
Some authorities contend meridian altitude sights are not necessary, or even worth learning for navigational purposes any more because tabular methods of sight reduction—which supply lines of position—provide an instant fix any time two or more lines of position are obtainable. Theoretically, this attitude may have merit, but I cannot accept it. Anyone who has contended with trying to get a running fix from two or more sun sights, under some of the adverse conditions experienced at sea, knows how reassuring it can be to be able to get a good meridian altitude of the sun
at local apparent noon.
A typical such adverse condition occurs in the tropics when the declination of the sun and the observer’s DR latitude are nearly equal. In this situation the sun will remain on or near the prime vertical until near noon. Hence the azimuth of the sun will be 090°, or nearly so, the entire morning, and the sun’s azimuth will not change until very near the time of meridian passage. Now please do not call my hand by saying that in such cases the altitude of the sun will be too high for meridian altitude observations anyway. This is not
112 U. S. Naval Institute Proceedings, August 1974
so. After all, the sun can be several degrees away from the prime vertical, but still near enough to prevent the azimuth from changing enough between sights to produce consecutive lines of position that will cross at sufficient angle to give a good running fix. Since the resulting sun lines tend to parallel the meridians, any fix obtained is especially prone to be off in latitude.
Actually, the sun can be this near the prime vertical of the observer, yet still sufficiently off the observer’s DR latitude to provide a usable altitude. This very thing has happened to me. When it does occur, the modern navigator needs his latitude at local apparent noon as much as any oldtimer working a time sight. For this reason, latitude sights will remain a tradition at sea and an essential part of the navigator’s trade. In addition, the methods you use to reduce a meridian altitude of the sun work for other celestial bodies as well.
With modern almanacs and star finders, it is a simple matter to tell by inspection if any observable bodies will be near the meridian at star time. If at twilight a body has a Greenwich hour angle equal to your DR longitude, it is on the meridian. I habitually check for these conditions when precomputing my star time and the altitudes and azimuths of the stars I intend to observe.
I can tell by inspection of the almanac if a body such as the moon or the planets will have a Greenwich hour angle equal to my DR longitude. I can tell from my computations, or from the star finder if a star will have a Zn of 000° or 180° at star time. If the Zn of the body is 000° or 180°, and the Greenwich hour angle is equal to the DR longitude of the observer, the body is on the upper branch of the meridian. If I find a body on the upper branch of the meridian at star time I will observe the body, and reduce the sight by the L = Z+ D formula applied according to Case I, Case II or Case III as appropriate.[1]
Unless specifically stated, anytime a modern navigator speaks of a meridian altitude observation he is speaking of a
meridian observation of the body on the upper branch of his meridian. The body will be in upper transit. Another type of meridian passage occurs approximately twelve hours after the passage of the body at upper transit. This second meridian passage is the passage of the body at lower transit. A body passes in lower transit when it crosses the lower branch of the observer’s meridian. In most cases we think a body in lower transit will be below the visible horizon. This is not always so, however. At lower transit the GHA of a body will equal the observer’s longitude plus 180°. A body visible at lower transit will always have an azimuth of 000° in the northern hemisphere and 180° in the southern hemisphere. When using a star finder, you can tell if a star is at upper or lower transit when its Zn is toward the elevated pole by comparing the computed altitude of the body with your latitude. If the computed altitude is less than your latitude, and the Zn is 000° in the northern hemisphere or 180° in the southern hemisphere, the body is in lower transit.
Since every navigator should be familiar with the three cases in which a body may be observed in upper transit, let’s call lower transit observations Case IV. This is what they were called in the older texts, and lower transit observations were invariably covered in the older navigation works. In Case IV observations, latitude and declination will always be of the same name, and the latitude determined must always be of the same name as the declination. Even where the upper transit observations are explained, most modern texts do not cover lower transit observations. However, Bowditch has covered this in each edition including the 1962 edition. I have never known a modern navigator, including myself, who regularly used lower transit observations for latitude. This is probably another case of a good idea falling into disuse for no apparent reason. There is no reason why Case IV observations should not be used, and if the sky is partially overcast it may be a God-given chance if a body is available for observation at the time of its lower transit. Many navigators never use lower transit observations because they have never heard of them. They limit themselves to upper transit observations.
Those modern navigators who have heard of lower transit observations think i of them as some kind of advanced navi- * gation technique or a technique limited to very high latitudes. Many stars are available for lower transit observations ^ *| in temperate latitudes. Jsit
There are additional factors in under- AB/ standing lower transit observations. A ^ [ body whose declination is the same name as the observer’s latitude, and of \
V
a declination greater than the observer’s colatitude never sets. When on either branch of the observer’s meridian, this body will have an azimuth angle of 000°. The body will scribe a circle around the elevated pole approximately every 24 hours. The body will cross the upper branch of the observer’s meridian at upper transit and the lower branch at lower transit, and it will be above the horizon at all times.
In cases I, II, and III, the navigator is concerned with zenith distance. In reducing lower transit observations, the zenith distance is replaced by the codeclination. By definition, codeclination is 90° minus the declination. In the diagram the circle represents the observer’s meridian on the celestial sphere. The angle D'cPn is the codeclination. It also shows that part of the celestial sphere not visible to an observer because it is below the horizon. The dashed lines represent that part of the celestial sphere that is invisible to the observer. Q is the intersection of the celestial equator with the observer’s celestial meridian below the horizon. D is the position of the body at upper transit, and D' is the position of the body at lower transit.
HH' is the horizon in the diagram.
The angle HCPn is the altitude of the elevated pole. The altitude of the elevated pole equals the latitude of the observer. Angle HCPn equals angle HCD plus angle D'CPn Angle D'cPn is the codeclination of the body observed.
Angle HCD' is the altitude of the body observed. (In the example to follow, this body is Kochab.) Thus the latitude equals the observed altitude plus the codeclination. Expressed as a formula, this is L = H0 -f (90 — D). The reduction of lower transit sights is the easiest of the latitude sight reduction methods.
The steps required to find the zone time of lower transit of a body, and
Professional Notes 113
reduce the sight are as follows:
► You can tell when a body is on the meridian at lower transit by finding in the almanac the GMT at which the GHA of the body will equal your DR longitude plus 180°. Thus, if your DR longitude is 90° 29.6'W, and you want to observe the star Kochab at lower transit on 3 February 1969, you must find the time at which the GHA of the star Kochab will equal 90° 29.6' 4- 180° or 270° 29.6'. Since the GHA of Kochab equals the SHA of Kochab plus the GHA of Aries, the problem is to find what GHA of Aries must be added to the SHA of Kochab to equal 270° 29.6'. To do this simply subtract the SHA of Kochab from 270° 29.6'. Open the almanac to the daily pages for 3 February 1969, and extract the SHA of Kochab of 137° 17.2'. And while you are at it, also extract the declination of Kochab which is N74° 19.2'. Then set up your problem:
DR longitude 90° 29.6'
180° 00.0'
GHA required 270° 29.6/
SHA Kochab 137° 17.2'
GHA Aries 133° 12.4'
The required GHA of Aries is 133° 12(4. When the GHA of Aries reaches this point, Kochab will be on the lower branch of the meridian at longitude 90° 29.6'W. In the Aries column of the almanac find the GHA of Aries nearest to this value, but less than 133° 12.4', and note the GMT. You find that at 00h on 4 February 1969 the GHA or Aries is 133° 10.3'. Subtract 133° 10.3' from 133° 12.4' and get 2.4'.
Latitude =
► Open your almanac to the page with the conversion tables to convert arc into time. You will see 2.4' of arc is equivalent to 08s of time. As we work only to the nearest minute in any meridian altitude observation you can ignore the 08? Thus the GMT of lower transit of Kochab is 00h 00ra the 4 February 1969 or 24h OO"1 the 3 February. Call it 24h 00™ because 90° 29.6'W is in time zone +6. By using 24n 00m 3 February, you avoid change in date. Subtract six hours from 24, and Kochab will be on the lower branch of your meridian at 18h 00m your zone time. This is the time of lower transit of Kochab at your DR.
► Observe Kochab when the star is on the meridian, and correct your sextant altitude of Kochab as you would for any other star. You make this observation exactly as you would any other meridian altitude observation with the very important exception that you observe the star until it is at its lowest altitude. In upper transit observations you observe a body until it reaches its highest altitude. This is one of the beauties of meridian altitude observations; you do not need precise time to the second. You simply observe a body until it quits increasing or dropping in altitude; and read this highest or lowest altitude as the Hs of the body. Assume your corrected sextant altitude of Kochab is 14° 12.3;
► The declination of Kochab is N74° 19.2'. Subtract this from 90° to get the codeclination of Kochab.
89° 60.0' Declination N74° 19.2' Codeclination 15° 40.8'
► Add the codeclination and the H0 of Kochab.
H014° 12.3' 29° 53.1'N
If the lower transit of a star does not occur at star time you cannot get an observation of the star at lower transit. Steps 1 and 2 show the arithmetic involved in finding if a body will be on the meridian at lower transit at star time. As you must find an SHA that will equal your longitude plus 180° when added to the GHA of Aries at twilight, all you need do is determine this SHA as the time of twilight is predetermined.
Then as you glance over the list of navigational stars, if you see a star whose SHA is very near the required SHA of Aries you must add to the GHA to get a GHA approximately equal to 180° plus your DR longitude, you know this star may be at lower transit sometime during star time. When you develop the habit of making this inspection you will find it takes very little effort. Then if you do see a star at lower transit, you can reduce your sight of this star by use of the L = H° + (90° — D) instead of the more involved sight reduction formulae of H. O. 214, 249, or 229. It saves time. If you use one of the popular star finders the problem is even simpler. If the azimuth of a star is toward the elevated pole, its declination exceeds your DR latitude, and the star’s altitude is less than your latitude the star is on the lower branch of your meridian, shoot away.
You can even observe the sun or other bodies in or close to the ecliptic at lower transit if you are in high enough latitude. Now this does require you to be pretty close to the pole. For an observation of the sun at lower transit you would have to be above the Arctic Circle during the summer in the northern hemisphere or below the Antarctic Circle during the summer in the southern hemisphere. If you are in the northern hemisphere the sun will become available for observation at lower transit right at the North Pole as soon as it crosses the equator at the vernal equinox. As the days pass, the sun would be visible at lower transit at lower and lower latitudes until the time of the summer solstice when the sun would be just visible on the horizon at 66° 33.0'N. At this time the sun’s declination is 23° 27.0', and the sun’s codeclination is 66° 33.0'. (Codeclination equals 90° minus the declination.) The latitude 66° 33.0' is the latitude of the Arctic Circle in north latitude and the latitude of the Antarctic Circle in south latitude. To use lower transit observations of the sun and other bodies in the zodiac, you must be in the land of the midnight sun. But lower transit observations of stars can often be used with ease in temperate zone latitudes.
Of course, while you are at it, do not overlook the chance of catching a star or planet at upper transit too.
[1] See Warren Norville, Celestial Navigation Step by Step (Camden, Me.: International Marine Publishing Company, 1973).