The Titanic's Steel test

In 1996, several samples of steel from the Titanic—a hull plate from the bow area and a plate from a major transverse bulkhead—were recovered from the wreck site and subjected to metallurgical testing by Prof. H.P. Leighly at the University of Missouri-Rolla, as well as at the laboratories of Bethlehem Steel and the National Institute of Standards and Technology. Chemical testing revealed a low residual nitrogen and manganese content, and higher levels of sulfur, phosphorus, and oxygen than would be permitted today in mild steel plates or stiffeners. This indicates that the steel was produced by the open-hearth rather than the Bessemer process, most likely in an acid-lined furnace; the steel is of a type known as semi-killed, that is, partially deoxidized before casting into ingots. (Other fragments of the Titanic's hull have yielded slightly different results, suggesting a degree of variability in the chemical and, hence, the mechanical properties of the steel used in the ship.)
Excess oxygen can form precipitates that can embrittle the steel, and will also raise transition temperatures. In the absence of sufficient manganese, sulfur reacts with the iron to form iron sulfide at the grain boundaries; it can also react with manganese, in either case creating paths of weakness for fractures. Sulfide particles under stress can nucleate microcracks, which further loading will cause to coalesce into larger cracks; in fact, this was found to have been the mode of failure in the shell plating of the Titanic. Phosphorus, even in small amounts, has been found to foster the initiation of fractures. Of course, much of this metallurgical information has only been learned in the years since the Titanic went down.
To determine the steel's mechanical properties, it was subjected to tensile testing, as well as the Charpy V-Notch Test, used to simulate rapid loading phenomena; the test used samples oriented both parallel and perpendicular to the original direction of the hull plate. The ductile-brittle transition temperature (using 20 lbs.-ft. for the test) was found to be 20°C in one direction and 30°C in the other, compared with —15°C for a reference sample of modern A 36 steel—and a water temperature of —2°C on the night the ship collided with the iceberg. The Titanic steel was also shown to have approximately one-third the impact strength of modern steel.
When the Titanic samples were also examined with a scanning electron microscope, the grain structure of the steel was found to be very large; this coarse structure made it easier for cracks to propagate. Rivet holes were cold-punched, a method no longer allowed (they must now be drilled), nor were they reamed to remove microcracks.
The steel grain size; the oxygen, sulfur, and phosphorus content of the steel; and the cold-punched, unreamed rivet holes were found to have contributed to the breakup of theTitanic, along with the steel's relatively low ductility at the freezing point of water. The shell plates showed signs of brittle fracture, though some plates demonstrated significant plasticity.
Of course, the science of metallurgy has advanced considerably since the Titanic's day, and William Garzke of Gibbs and Cox and his collaborators emphasized in their report that "the steel used in the Titanic was the best available in 1909-1914" when the ship was built. In fact, they add that when 39,000 tons of water entered the bow, "no modern ship, not even a welded one, could have withstood the forces that the Titanic experienced during her breakup.
By Henry Baumgartner

HOW DID the TITANIC sink?? Part 2

This article is based on a paper, "Titanic, The Anatomy of a Disaster, A Report from the Marine Forensic Panel,"presented at the 1997 annual meeting of the Society of Naval Architects and Marine Engineers, that documents the work of William H. Garzke, Jr. and David Wood, Gibbs & Cox, Inc.; David K. Brown, RCNC; Paul K. Matthias, Polaris Imaging; Dr. Roy Cullimore, University of Regina; David Livingstone, Harland & Wolff; Prof. H.P. Leighly, Jr., University of Missouri-Rolla; Dr. Timothy Foecke, National Institute of Standards and Technology; and Arthur Sandiford, Consultant. Eyewitness accounts are from various sources, including the official transcripts of the 1912 U.S. Senate investigation.
The Death of a Ship
At 2:17 a.m., according to the various investigations after the disaster, the Titanic began to go under, her lights blazing in the cold of the sub-Arctic night and with more than 1,500 people still on board. With a rumbling, crashing noise, the bow of the ship sank deeper into the water and the stern rose into the air.
Believing that the Titanic was invincible, many passengers were willing to board lifeboats only after the bow began to sink below the water's surface.
The stern section remained motionless and high out of the water for 30 seconds or more. The hull fracture was described as the sound of breaking chinaware, but as it continued, it was like a loud roar. A minute later, her lights flickered and then went out.
Then, at 2:20 a.m., the stern settled back into the water. Following a series of explosions, the submerged forward section began to pull away from the stern. As the forward section began its long descent, it drew the stern almost vertical again. Once this began, Titanic picked up speed as she sank below the surface of the pond-still waters of the North Atlantic. Some of the survivors on the stern stated that it was almost perpendicular as it slid silently and with hardly a ripple beneath the surface. William Garzke, staff naval architect at Gibbs & Cox, points out that, had the liner been elevated at 90 degrees, the huge boilers would have been ripped from their moorings, which was not the case. He suggests that the stern section likely rose from the surface to at least 20 degrees but not more than 35 degrees, as it filled with water or was dragged down by the bow section.
Chief Baker Charlie Joughin, who was at the ensign staff at the stern end, later testified that it was like riding an elevator down to the water. With the absence of suction forces, he was able to swim away without even wetting his hair, so swift was the stern's demise.
The failure of the main hull girder of the Titanic was the final phase of her sinking process. This began between 2:00 and 2:15 a.m., starting somewhere between stacks Nos. 2 and 4. The FEA results indicate that the plate failures might have started around the second expansion joint, or just behind it.
Stresses in the hull were increasing as the bow flooding continued and the stern rose from the water. Detailed examination of survivor testimony and underwater surveys has confirmed that the forward expansion joint was opened up while the ship was still on the surface, suggesting the significant stresses induced by the flooding of the forward part of the hull. An FEA review of the stresses in this area confirms that the nominal hull stresses were well above the material yield stress.
Most probably, significant stress developed in the way of the second expansion joint, between its root and the deck structure below it. As the flooding progressed aftward, the hull girder was strained beyond its design limitations, and the local stresses around this expansion joint soon reached the ultimate strength of the material. It is thought that, in the end, a critical structural failure in the hull or deck plates occurred in the area around the second expansion joint.

A stress analysis suggests that the Titanic's hull girder stresses exceeded the yield point of the steel.Once localized fracture began in the way of this joint, additional plate failures and associated fracturing likely radiated out from this joint, toward both port and starboard. The decks, however, with their finer grain structure, were most likely able to deform well into the plastic range of the material before failing in ductile tears. It is speculated, however, that the side shell plates suffered brittle fracture due to their coarser grain structure and manganese sulfide inclusions. This type of failure is evident on the wreck today.
Free field stresses, already at the yield point of the material, may have been increased by a factor of two to four in areas of structural discontinuities, such as large openings or those with small radii, or doubler plate edges. Fractures typically spread in random chaotic paths, following weaknesses in the plate and microcracks already present around rivet holes.
Assuming that the hull girder failed at the surface, then as Boiler Room No. 4 filled with water, the stern rose farther out of the water, resulting in some 76 meters of unsupported hull, which sharply increased the hull girder stresses, in turn accelerating the fracturing of the steel plates. The angle of trim grew to a maximum of 15 to 20 degrees, further increasing the stresses in the hull and deck plating near the aft expansion joint. The stresses continued to build in this area of the ship, where there were large openings for a main access, the machinery casing for the Reciprocating Engine Room, the uptakes and intakes for the boilers, the ash pit door on the port side of Boiler Room No. 1, and the turbine engine casing. As the hull girder continued to fail, the bow was first to begin its plunge toward the seabed.
As the bow and stern sections continued to separate, there were some local buckling failures in the inner bottom and bottom structure. This is what caused the stern section to settle back toward the water's surface as the decks began to fail and the side shell fractured into many small plate sections. The MSC/NASTRAN finite element analysis indicates that the stresses in the region of Boiler Room No. 1 and the Reciprocating Engine Room were elevated.
An additional stress analysis, based on classical beam theory, indicates that the hull girder stresses exceeded the yield point of the steel. When the bow and stern began to separate, the two main transverse bulkheads bounding Boiler Room No. 1 collapsed as they were compressed by the downward movement of the deck structures. The decks, in turn, failed because of the lack of bulkhead support.
When this happened, the unsupported length of the inner bottom suddenly grew to 165 feet, encompassing Boiler Rooms Nos. 1 and 2, as well as the Reciprocating Engine Room. This condition allowed deformation of the inner bottom structure to extend up further into the ship's machinery spaces, while the deck structure failures continued. It is believed that this compression of the hull girder brought about the failure of the side shell plates, and also freed equipment inside the ship, such as the boilers in Boiler Room No. 1, from its foundations.
It cannot be established with any certainty what happened to the ship during its descent to the seabed. However, what is now known is that once the Titanic disappeared below the ocean's surface, it broke into three pieces. The depth where these events occurred cannot be estimated with any precision. The buoyancy of the stern piece also appears to have resisted the downward pull of the bow. The extent of damage evident in the stern wreck implies that the bow section may have pulled the stern section quickly below the water's surface, resulting in structural implosions that caused significant damage. Structural failures ultimately led to the separation of the bow portion, followed by the third or double bottom piece. It is interesting to note that the bow section did not suffer damage similar to that in the stern section. This was likely due to the gradual flooding of the bow section, and its stability during the descent to the bottom. It rests upright on the bottom with little apparent damage directly attributable to impact with the seabed.
The analysis supports some witnesses' testimony that the ship likely began to fracture at the surface, and that the fracture was completed at some unknown depth below the water's surface. The resulting stress levels in the strength deck below the root of the second expansion joint (aft), and in the inner bottom structure directly below, were very high because of the unusual flooding occurring in the forward half of the ship. These patterns of stress support the argument that initial hull failure likely occurred at the surface. Additional work is being performed to investigate this further.
These findings mirror the testimony of Seaman Edward John Buley at the U.S. Senate hearings. Stating that as the bow continued to slip below the surface, "She went down as far as the after funnel, and then there was a little roar, as though the engines had rushed forward, and she snapped in two, and the bow part went down and the afterpart came up and stayed up five minutes before it went down ... It was horizontal at first, and then went down."
In response to what he meant by "snapped in two,"and how he knew this, Buley testified, "She parted in two ... Because we could see the afterpart afloat, and there was no forepart to it. I think she must have parted where the bunkers were. She parted at the last, because the afterpart of her settled out of the water horizontally after the other part went down. First of all, you could see her propellers and everything. Her rudder was clear out of the water. You could hear the rush of the machinery, and she parted in two, and the afterpart settled down again, and we thought the afterpart would float altogether. She uprighted herself for about five minutes, and then tipped over and disappeared ... You could see she went in two, because we were quite near to her and could see her quite plainly."
RMS Titanic, the largest ship of its day, was built in Belfast, Ireland, and was said to be "unsinkable,"a belief so strong that it was to have tragic consequences. Having confidence in the ship's "unsinkability,"many passengers chose to remain on board. The first lifeboats to leave were only half, or one-third full.
The fallacy of the claim itself became tragically apparent during the ship's maiden voyage. Just three hours after it collided with an iceberg, the majestic Titanic vanished beneath the cold waters of the North Atlantic. This ill-founded confidence led to the ignoring of at least 14 warnings of hazardous ice fields, six of which were received on the day of the disaster.
Equipped with only 20 lifeboats, the Titanic went down with the loss of 1,523 passengers and crew. This incredible disaster led to a number of investigations in Great Britain and the United States that resulted in sweeping changes in maritime safety law and ship construction.
The demise of the mighty Titanic was swift, sure, and terrible. Whatever could have gone wrong, did. The engineering marvel that heralded the beginning of the age of technology also displayed, all too clearly, its vulnerability and limits—as well as the need for prudence and safety.
"The analyses, and future analyses we hope to make employing both MSC/NASTRAN and MSC/DYTRAN, help us make critical design decisions about future marine structural features, such as deck openings and expansions joints."Wood said.
"Today, we're changing the way we design ships. In the past, nominal load conditions were averaged. Today, we design for the ultimate stress levels and strength,"says Robert Sielski, senior staff engineer at Gibbs & Cox. "MSC/NASTRAN helps us evaluate and design for increased survivability."

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HOW DID THE TITANIC SINK? part 1

By Dan Deitz, Executive Editor of MEMAGAZINE
This article is based on a paper, "Titanic, The Anatomy of a Disaster, A Report from the Marine Forensic Panel,"presented at the 1997 annual meeting of the Society of Naval Architects and Marine Engineers, that documents the work of William H. Garzke, Jr. and David Wood, Gibbs & Cox, Inc.; David K. Brown, RCNC; Paul K. Matthias, Polaris Imaging; Dr. Roy Cullimore, University of Regina; David Livingstone, Harland & Wolff; Prof. H.P. Leighly, Jr., University of Missouri-Rolla; Dr. Timothy Foecke, National Institute of Standards and Technology; and Arthur Sandiford, Consultant. Eyewitness accounts are from various sources, including the official transcripts of the 1912 U.S. Senate investigation.

When our boat had rowed about half a mile from the vessel, the Titanic—which was illuminated from stem to stern—was perfectly stationary, like some fantastic piece of stage scenery,"recalled Pierre Marechal, a French aviator and a surviving first-class passenger of the ill-fated liner. "Presently, the gigantic ship began to sink by the bows ... suddenly the lights went out, and an immense clamor filled the air. Little by little, the Titanic settled down ... and sank without noise ... In the final spasm the stern of the leviathan stood in the air and then the vessel finally disappeared."
British and U.S. investigations of the Titanic tragedy have resulted in greater lifeboat capacity, improved subdivision of ships, and the creation of an ice patrol.
Elmer Z. Taylor, who watched from Lifeboat No. 5, close enough to the Titanic to observe its final demise, would later write, "The cracking sound, quite audible a quarter of a mile away, was due, in my opinion, to tearing of the ship's plates apart, or that part of the hull below the expansion joints, thus breaking the back at a point almost midway the length of the ship."
"At that time the band was playing a ragtime tune, "remembered Harold Sydney Bride, the surviving wireless operator of the Titanic. "I saw a collapsible boat on deck ... I went to help when a big wave swept it off, carrying me with it. The boat was overturned and I was beneath it, but I managed to get clear. I swam with all my might and I suppose I was 150 feet away when the Titanic, with her aft quarter sticking straight up, began to settle."
"The orchestra belonging to the first cabin assembled on deck as the liner was going down and played 'Nearer My God to Thee.' By that time,"as Miss C. Bounnell, first-class survivor, relived the night, "most of the lifeboats were some distance away and only a faint sound of the strains of the hymn could be heard. As we pulled away from the ship, we noticed that she was hog-backed, showing she was already breaking in two."
Four survivors with firsthand knowledge, remembering probably the most important—certainly the most traumatic—event in their lives, disagreed on one major point, and it has remained a mystery for more than 80 years: Did the Titanicbreak apart at the surface or sink intact?
Although all the officers testified that the ship sank intact, some survivors and crew testified to a hull failure at the surface. Even during the American and British inquiries into the disaster, few questions focused on the structural aspects of the ship. Despite survivors' testimonies, it was concluded that the ship sank intact.


Evidence from the DepthsThe mystery arose again when the wreck of the Titanic was discovered in 1985 and the hull was found in two pieces. Many theories were developed as to how the ship broke apart during the sinking process, and research was begun to determine how this could have happened. The speculation intensified further when the wreck site was revisited in 1986 and a third 17.4-meter section from the midship region of the ship was found.
To help solve this mystery, the Discovery Channel, in developing its award-winning "Titanic: Anatomy of a Disaster"television documentary, approached Gibbs & Cox, Inc., one of the oldest naval architecture and marine engineering firms in the world. Gibbs & Cox agreed to perform a stress analysis to help determine the possibility of hull fracture at the surface.
With funding provided jointly by the Discovery Channel and the Society of Naval Architects and Marine Engineers, Gibbs & Cox conducted a basic study of the breakup of RMSTitanic using linear finite-element-analysis (FEA) software. This study was done in conjunction with materials testing of the Titanic steel by the University of Missouri-Rolla, with advice from Prof. H.P. Leighly Jr., Dr. Timothy Foecke, and Dr. Harold Reemsnyder of the Bethlehem Steel Corp.'s Homer Research Laboratory in Bethlehem, Pa.
Important to the analysis effort was accurate weight and buoyancy data for the ship at the time it struck the iceberg, and then later while it was sinking. These data were provided via a recent study of the ship's breakup undertaken for another technical paper, "The Titanic and Lusitania, A Final Forensic Analysis,"published in a 1996 issue of Marine Technology. The study provided the loading information needed to take "snapshots"of the ship's state of stress during the sinking process. Tests conducted on the steel recovered from the wreck site were performed at the University of Missouri and the National Institute of Standards and Technology in Gaithersburg, Md. The results from these metallurgical tests of Titanic steel and rivets were also input as data for the finite element analysis.
Gibbs & Cox engineers selected MSC/NASTRAN, from the MacNeal-Schwendler Corp. in Los Angeles, to perform the analysis. FEMAP engineering-analysis modeling and visualization software from Enterprise Software Products in Exton, Pa., was used to perform the pre- and postprocessing of the analyses. Gibbs & Cox had been using MSC/NASTRAN for approximately five years. According to David Wood, the firm's structures department manager, MSC worked closely with his team during the development of MSC/NASTRAN Version 70 to provide the special program solutions needed for use in their industry.
Engineers analyzed the stresses in the Titanic as the flooding progressed within the bow region, using modern FEA techniques that simply were not available until the 1960s, and certainly were not known to the structural designers of the ship in the first decades of the century. In the 1960s, engineers started to analyze the stresses in ship hulls using finite-element modeling (FEM). As a pioneer of FEA technology, MSC has been in the forefront of dramatically improving this technique to take advantage of advances in computer technology.
A full-ship model was graphically constructed, employing a modern approach similar to that used for U.S. Navy destroyers and cruisers today. Loadings for the model were developed based on one flooding scenario from the paper, "The Sinking of the Titanic,"by Chris Hackett and John C. Bedford.
The corresponding weight and buoyancy curves, developed by Arthur Sandiford and William H. Garzke, Jr., were used to model the critical flooding conditions believed to represent the hull loading just prior to hull fracture. Since the flooding process took place over several hours, a quasi-static analysis was considered appropriate. The initial modeling effort focused on the determination of the location and magnitude of high-stress regions that developed in the hull while she remained on the surface.
Engineers determined that stress levels in the midsection of the ship were at least up to the yield strength of the steel just prior to sinking. When considered alone, stresses at these levels do not indisputably imply catastrophic failure. Additional analyses, focusing on probable locations of initial hull fracture, are required to indicate that the ship sustained possible catastrophic failure at the surface and began to break apart.
Significant stresses were developed in the vicinity of the two expansion joints, and in the inner bottom of the ship between the forward end of Boiler Room No. 1 and the aft end of the Reciprocating Engine Room. Structural discontinuities, such as expansion joints, result in stress-concentration development. Typically, stress concentration levels are three to four times that of free-field stresses. While these structural discontinuities have not yet been thoroughly investigated, it is believed that stresses developed at these locations were significantly higher than the material yield stress.

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SHIP STABILITY – WHAT IS IT AND HOW DOES IT WORK?

By Patrick J. Bray, Naval Architect 

Stability is the ability of a vessel to return to a previous position.  Positive stability would
then be to return to upright and negative stability would be to overturn.  Stability in it’s most ba-
sic form is the relationship between the center of all floatation in your hull (center of buoyancy,
or CB) and the center of all weight (vertical center of gravity, or VCG).  In other words, the
downward pull of Gravity and the uplifting force of Buoyancy.  These are the primary charac-
ters in this scene and all others play minor roles.  Once you understand how their relationship
works, understanding stability becomes a simple matter



Static stability 

This is stability at rest without active external forces at work.  The VCG for all intended purposes is a fixed point in space and is a reflection of the placement of major weights within the boat affected in part by the amount of superstructure and number of decks. The shape of the hull determines the CB, which is not fixed.  It moves around to balance the loads and keep the forces in equilibrium.  The shape of the hull has a major effect on the path that the CB will take as the vessel heels or trims.  At rest the CB and the VCG align vertically.  If they are not vertically aligned then the vessel will change trim or list until they do come into alignment.  Then all the forces are equalized and the vessel becomes stable.  


Form stability (effects of section shape) 

As the boat heels, sides of the boat previously above water are immersed and bottom sections previously underwater are now exposed, and the CB is on the move.  The battle has begun.  As forces are applied to the vessel changing it’s attitude in relation to the surface of the water.  This causes the CB to move, creating the forces to bring the vessel into equilibrium.  If the CB can not create enough counterforce to right the vessel then it will be over turned.

The amount of counterforce can be determined mathematically from the designer’s
drawings and verified on the finished boat with an inclining experiment.  How are these calcula-
tions performed, you ask?  Someone needs to do a weight calculation, which accounts for every item on the entire boat right down to the weight of the paint and screws, and also accounts for their position in the vessel both vertically and fore and aft.  Usually transverse measurements are considered less critical as most boats are very symmetrical.  Knowing where the VCG is (remember, this is a fixed position) we can now go on to look at where the CB moves for each angle of heel.  This is also calculated, but in a far more complex manner and is best done by and experienced Naval Architect (and their computer).  Both hull form, and ballast if
any, have a major role to play out in this scene.  Both will have a major effect on stability, but more on that later.


Dynamic stability 

Once a vessel is under way the effects of wave train, bottom pressure and change of trim can add or detract from the amount of stability.  Many planing hulls become more stabile at speed due to the change in pressure distribution across the hull bottom.  Because of the V-bottom and the spray rails outboard, unequal pressure builds up on the lower side in a turn.  This pressure restores the vessel to upright when the heeling forces of the turn are over. Some double-ended vessels become less stable at hull speed because of the wave hollow amidship.  With only the fine forward and after sections floating the boat and the Midship section mostly unsupported there is little width to provide stability.  This can also be a problem to any vessel in short steep seas. Active and passive stabilizer systems give the impression of increasing stability; however they actually act to reduce roll.   In fact, active stabilizers are only effective when underway.  
 
The Coast Guard sets minimum standards of stability for various types of commercial craft but no such
standard is set for yachts.  Fishing vessel standards can be used as a guide as they have similar characteris-
tics and are a similar size to many offshore cruising powerboats.  However, these are minimums and do not address comfort.  And even comfort is subject to personal interpretation.  A serious offshore vessel will be tender initially and will tend to have a long easy rolling motion which will not knock crew nor gear off the deck.  For many pleasure boaters this amount of roll and length of time (period of roll) is alarmingly long.
If a boat is very stable at the dock (great as a liveaboard), or in flat water, it can have a very quick motion at sea.  A case in point is a very successful 110 ft motoryacht that does harbor charters seating 95 for dinner cruises.  When taken to sea it has such a quick motion in a seaway that the crew have to go around on their hands and knees to keep from getting knocked off their feet; and this is at 8 knots. Stepping aboard a 60 footer that noticeably settles under your weight (low GM) does not instill confidence.  Having it heel under the press of a strong breeze will only erode that confidence further; yet it can have an excellent sea motion and be extremely comfortable once you get your sea legs.  
            For the general boating public there has to be a middle ground.  Few boaters spend all their time at sea and just as few plan to spend all their aboard time at the dock (although sometimes it works out that way!).  A compromise must be struck: one that provides stability for dockside comfort moving around the vessel and a suitable motion for safe operation in some kind of seaway.  The proportion of compromise depends on the purpose of the vessel and the market segment that it is appealing to.  What is the intended use of the vessel and what level of experience will the purchaser have?    
            Let’s look at the effects various elements of design have on the stability characteristics
of a vessel.  


Ballast 

Generally when people think of stability and ways to increase it they think of adding bal-
last.  Generally this will help - but not in all cases.  If ballast reduces the freeboard to the de-
gree that the deck edge enters the water at a much lower angle of heel, then the overall stabil-
ity is drastically reduced.  Again, it is the relationship between the hull form and the overall cen-
ter of weight that tells the story.  Removing top weight from as high up as possible will have a
greater effect than adding ballast low down.
Also, ballast will not do much to increase initial stability (stability at very low angles of
heel).  It’s real forté is at higher angles of heel.  Once the heel angle starts to reach or exceed
45 degrees ballast comes into it’s own.  Although this may provide real peace of mind, it does
little to improve a day to day  comfort situation.

Beam 

Wide beam will produce high initial stability (and low ultimate stability).  It is great for liveaboard space and comfort at the dock but taken to extremes it will produce an uncomfortable motion in a seaway.  The combination of extremely wide beam and low freeboard in a monohull can be dangerous, as the high initial stability reaches it’s peak at a very low angle of heel.  Once overturned the vessel becomes very stable inverted and has no desire to return to upright.







A catamaran is a classic example of widebeam and no ballast taken to extremes.  The interior volume and low heel angles make thesevessels very comfortable to liveaboard.  On theother hand, if one hull ever leaves the water thechances of seeing it re-enter the water upright isslim to none.  To compensate for this, specific
hull forms have been developed which allow highimmersion of one hull before the other hull leaves Inertia

One of the ways to improve the motion of an existing vessel is to alter the amount of inertia.  If you have a boat with a quick motion and want to slow it down try moving major weights horizontally outboard from the centerline.  This will increase the inertia and dampen out the motion.  By moving the weight horizontally rather than vertically you do not affect the overall stability.  If your vessel has a slow roll that you would like quickened, try moving heavy weights inboard or down as this will reduce the inertia and/or increase the stability.  For a quick demo on inertia try spinning around on a swivel chair and bring your arms or legs in and out.

Superstructure 

           Generally stability increases in strength until 45-60 degrees of heel (occasionally as high as 90 degrees) and then slowly diminishes to nothing at 90-120 degrees, (the critical angle or ultimate angle).  Rarely, the ultimate angle goes all the way to 180 degrees, making the boat self-righting.  Once the deck edge immerses the shape and size of the superstructure becomes a major player in shaping the course the CB will take.  For this reason it is important that the house be well constructed and that there are few openings for water to enter.  If there are wide side decks then the house will enter the water later reducing it’s effect on the stability curve.  If the vessel is flush decked with little in the way of a house then all stability must be gained from the hull form.  

Windage 

The very area of a large superstructure can act as a sail, heeling the boat to some degree.  If the stability is reasonable then this should not be a problem.  In fact, in some boats this press of wind actually acts to dampen rolling motions further producing an even more comfortable ride.  Unfortunately, it is not a sail area that can be reduced as the wind pipes up.

Waves 

All vessels will suffer from loss of stability on a wave face.  If the waves are short and steep, as in a shelving area, the situation becomes worse.  With the majority of the vessel supported by the buoyant ends there is little waterline width.  As the vessel heels the midsection falls into the trough.  This can induce deep rolling motions and even cause capsizing. Even a beam sea can create serious problems if the wave train coincides with the vessel’s natural rolling motion.  Luckily a small change in direction or speed is all that is required to
avert a progressively worsening situation.

Active stabilizers 

These stabilizers are usually gyro controlled and hydraulically activated fins which are placed amidship. Their dynamic forces return the vessel to upright when a heeling force is detected.  They can even compensate for a list.  Generally, they dampen rolling motions more than 80%, however they do not increase the vessel’s actual stability.  Unfortunately, they are not considered workable at speeds below 8 knots through the water.  Their forces and effectiveness diminish as speeds drop until at anchor or at the dock they provide no real effective-
ness.


Passive stabilizers 

These passive systems vary from Paravanes to Fixed Bilge Keels to antirolling ballast tanks.  Their primary feature is that they have few to no moving parts and are mechanically simple and therefore very reliable.  They are not as effective in dampening roll underway as an active system but do tend to be better roll dampeners at low speed and at anchor.  They will do nothing to increase stability but will dampen the forces acting on your
vessel.

Downflooding points 

All of the stability in the world will be of no use if there are openings in your hull or superstructure that will let water in to flood your boat.  Items such as engine room intakes placed low in the hull sides and leading directly into the engine room are a classic problem.  Even if the boat has a good range of stability, what good will it do when water is pouring in a large open vent?  It is like having a gaping hole in your boat waiting to gulp up water. Openings for interior ventilation also need to be placed carefully.  Non-watertight hatches to
hull compartments need to be checked so water cannot enter at low angles of heel. The effects of uncontained water inside a boat are unbelievable!  Water always runs to the low side, making matters worse and it is surprising

how much water weighs.  As the vessel rolls, the water shifts from side to side, producing a pendulum effect which increases the rolling even further.   Even uncontained water on deck has the effect of raising the VCG and reducing stability.  Large freeing ports (drainage openings) in the bulwark are necessary in order to get the water off the boat fast, before the next wave adds further to the burden on deck. Of course, once a hull has been damaged and water is pouring in, there is little that can be done if there are no watertight ubdivisions in you boat.  Commercial vessels not only are required to have watertight bulkheads but must be designed to remain afloat and stable with one and sometimes even two compartments completely flooded.

            Numbers for number crunchers 

            Hard and fast numbers are usually misleading, because there are so many variables that effect stability.  In addition few of the thousands of small craft are designed by a Naval Architect so in many cases these stability numbers are just not known. Having said that, for a mono-hull yacht that is expected to do extended coastal cruising, open ocean cruising, or any vessel that is expected to be caught out in really bad weather you
should look for the following;
• a minimum range of stability to 90° of heel and over 120° preferred
• a maximum righting moment (GZ) that occurs after 30° and preferably after 50°
• and a GM of at least 2 feet and preferably between 3 feet and 5 feet

GM is the most valuable number to consider, as it tells you the initial stability; the stability that you will notice day to day.  GM is actually the measurement between the VCG and the Metacenter (M—the point at which a vertical line taken from the heeled CB crosses the vessel’s centreline) as shown in this diagram. Low GM means the boat may be a bit tender to walk around on and heel a few degrees under wind pressure.  How-
ever, there may be a good overall range of stability, which you can check by looking at the Critical Angle or Ultimate Stability.  
  If it has a higher GM then more than likely there will be sufficient initial stability, andpossibly too much, however Ultimate Stability may be low.  The boat will have a quick jerky motion in a seaway but be very stable at the dock.


Good stability doesn’t ‘just happen’

Good stability can be, and should be, achieved during the design stage.  Careful placement of heavy items down low, and knowledgeable design of a hull form and superstructure that takes stability characteristics into account will produce a sound vessel with a good solid feel to it.  Attention to the numbers that determine the vessel’s final characteristics will produce a successful yacht long before the boat is launched.





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SYMBOL OF HULL CLASSIFICATION - USA NAVY

The United States Navy uses hull classification symbols to identify the types of its ships. See also pennant number, a somewhat analogous system used by the Royal Navy and some European navies.
The combination of symbol and hull number identify a modern Navy ship uniquely. A heavily modified or repurposed ship may receive a new symbol, and either retain the hull number or receive a new one. Also, the system of symbols has changed a number of times since it was introduced in 1907, so ships' symbolssometimes change without anything being done to the physical ship.
Many of these symbols are not presently in use.
The 1975 ship reclassification of cruisers, frigates, and ocean escorts brought US Navy classifications into line with other nations' classifications, and eliminated the perceived "cruiser gap" with the Soviet Navy.

1 Warships

Warships are designed to participate in combat operations.

1.1 Aircraft Carrier Type

All ships designed primarily for the purpose of conducting combat operations by aircraft which engage in attacks against airborne, surface, sub-surface and shore targets. "CV" is from the original description, "Cruiser, Aviation" -- CA was already in use for "Cruiser, Armored".

• CV Multi-purpose Aircraft Carrier
• CVA Attack Aircraft Carrier (retired)
• CVB Large Aircraft Carrier (category merged into CVA, 1952)
• CVE Escort aircraft carrier (retired)
• CVHE Escort Helicopter Aircraft Carrier (retired)
• CVL Light Aircraft Carrier (retired)
• CVN Multi-purpose Aircraft Carrier (Nuclear-Propulsion)
• CVS Antisubmarine Aircraft Carrier (retired)

1.2 Surface Combatant Type

Large, heavily armed, surface ships which are designed primarily to engage enemy forces on the high seas.

• ACR Armored cruiser (retired)

• BB Battleship (originally just "B")

• C Cruiser
• CA (first series) Armored Cruiser (retired)
• CA (second series) Heavy Cruiser, category later renamed Gun Cruiser (retired)
• CAG Guided Missile Heavy Cruiser (retired)
• CB Large Cruiser (retired)
• CBC Large Command Cruiser (retired)
• CC Battle Cruiser (retired)
• CC (second usage) Command Cruiser (retired)
• CG Guided Missile Cruiser
• CGN Guided Missile Cruiser (Nuclear-Propulsion)
• CL Light Cruiser (retired)
• CLAA Antiaircraft Cruiser (retired)
• CLG Guided Missile Light Cruiser (retired)
• CLGN Guided Missile Light Cruiser (Nuclear-Propulsion) (retired)
• CLK Hunter-Killer Cruiser (abolished 1951)
• CS Scout Cruiser (retired)
• CSGN Strike Cruiser

• DD Destroyer
• DDE Escort Destroyer (not to be confused with Destroyer Escort, DE - an Escort Destroyer, DDE, was a Destroyer, DD, converted for antisubmarine warfare) (category abolished 1962)
• DDG Guided Missile Destroyer
• DDK Hunter-Killer Destroyer (category merged into DDE, 4 March 1950)
• DDR Radar Picket Destroyer (retired)

• DE Destroyer Escort (abolished 30 June 1975)
• DE Ocean Escort (abolished 30 June 1975)
• DEG Guided Missile Ocean Escort (abolished 30 June 1975)
• DER Radar Picket Destroyer Escort (abolished 30 June 1975)

A word about the DE type symbol is in order here. There were two distinct breeds of DE, the World War II Destroyer Escorts (some of which were converted to DERs) and the postwar DE/DEG classes, which were known as Ocean Escorts despite carrying the same type symbol as the WWII Destroyer Escorts. All DEs, DEGs, and DERs were reclassified as FFs, FFGs, or FFRs, 30 June 1975.

• DL Destroyer Leader (later Frigate) (retired)
• DLG Guided Missile Frigate (abolished 30 June 1975)
• DLGN Guided Missile Frigate (Nuclear-Propulsion) (abolished 30 June 1975)

The DL category was established in 1951 with the abolition of the CLK category. CLK 1 became DL 1 and DD 927-930 became DL 2-5. By the mid-1950s the term Destroyer Leader had been dropped in favor of Frigate. Most DLGs and DLGNs were reclassified as CGs and CGNs, 30 June 1975. However, DLG 6-15 became DDG 37-46. The old DLs were already gone by that time.

• DM Destroyer Minelayer (retired)

• FF Frigate (retired)
• FFG Guided Missile Frigate
• FFR Radar Picket Frigate (retired)
• FFT Frigate (Reserve Training) (retired)

The FF, FFG, and FFR designations were established 30 June 1975 as new type symbols for ex-DEs, DEGs, and DERs.

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SHIP HULL TERMINOLOGY

Amidships - toward the middle of the vessel.

Bow - strictly, one of the two curved structures where the hull broadens out from the stem (the pointed end). The bows is a term for, the head of the vessel or front of the ship. Compare prow, a more poetical term for the ship's head.

Stern - the after end of the ship.

Aft - towards the stern when the relationship is within the ship.

Astern beyond the stern where the relationship is outside the vessel.

Starboard - the side of the ship which lies to the right when an observer within the ship faces forward.

Port - the side of the ship which lies to the left when an observer within the ship faces forward. (A mnemonic to distinguish port and starboard notes thatleft and port both have four letters. Another incorporates the navigation light: Is there any red port left?)

(Navigation) Bridge - A structure above the weather deck, extending the full width of the vessel, which houses a command centre, itself called by association, the bridge. A bridge usually extends a little beyond the ship's side to enable observation of boats alongside, or the proximity of a dock or lockgate; these projections are called bridge wings. In big vessels, a docking bridge used to be found aft. (See Lord, Walter. A Night to Remember (1976) p.96). It enabled an officer to observe docking manoeuvres before giving orders. RMS Titanic had one but they have been superseded by closed circuit television cameras.

Bulkheads - internal "walls" in a ship. Bulkheads are the vertical equivalent of decks. They have a structural function as well as dividing spaces. They serve to prevent collapse of the hull under stress, to maintain stability, in the event of flooding, and to contain fire. Many bulkheads feature watertight doors which, in the case of certain types of ships, the crew may close remotely. An internal "wall" that is not load-bearing is usually referred to as a "partition". It is to a bulkhead as a flat is to a deck.

Cabin - an enclosed room on a deck or flat.

Capstan - a winch with a vertical axis.

Coaming - Raised edges of hatches in decks for keeping water and articles free on the deck from falling into the hold.

Decks - the structures forming the approximately horizontal surfaces in the ship's general structure. Unlike flats, they are a structural part of the ship.

Deck Head - The under-side of the deck above. Sometimes panelled over to hide the pipe work. This panelling, like that lining the bottom and sides of the holds, is the ceiling.

Draft - The vertical distance from the current waterline to the lowest point of the ship or in the part of the ship under consideration.

Figurehead - symbolic image at the head of a traditional sailing ship or early steamer.

Forecastle - a partial deck, above the upper deck and at the head of the vessel; traditionally the sailors' living quarters.

Freeboard - The vertical distance from the current waterline to the highest continuous watertight deck. This usually varies from one part to another.

Galley - the kitchen of the ship

Gunwale - Formerly a fabricated band placed for strengthening around the ship at the main or upper deck level to accommodate the stresses imposed by the use of artillery. In later use it is the angle between the ship?s side and upper deck. It remained as a structural member, in wooden boats where it was mounted inboard of the sheer strake regardless of the need for gunnery.

Bulwark - the extension of the ship's side above the level of the weather deck.

Hold - In earlier use, below the orlop deck, the lower part of the interior of a ship's hull, especially when considered as storage space, as for cargo. In later merchant vessels it extended up through the decks to the underside of the weather deck.

Hull - the shell and framework of the basic flotation-oriented part of a ship

Keel - the central structural basis of the hull

Kelson - the timber immediately above the keel of a wooden ship.

Mast - a spar (in a ship, a very heavy one stepped in the keelson) formerly designed for the support of one or more sails. In modern ships, it is a steel or aluminium fabrication which carries navigation lights, radar antennae etc.

Prow - a poetical alternative term for bows.

Scupper - a drainage waterway at the edge of a deck, is drained by a pipe or, on the weather deck, a small opening in the bulwarks, leading overboard. It is called a scupper which is distinct from larger openings with hinged covers on the bulwarks, designed for relieving the ship of large quantities of water in a seaway. These are called freeing ports or wash ports..

Windlass - A winch mechanism, usually with a horizontal axis. used where mechanical advantage greater than that obtainable by block and tackle was needed.

Weather deck - whichever deck is that exposed to the weather ? usually either the main deck or, in larger vessels, the upper deck.

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HOVERCRAFT, THE THEORY AND TERMINOLOGY

A hovercraft travels over land and water on a cushion or bubble of low-pressure air.  A hovercraft has one or more blowers that blow air underneath the craft, which is contained by a skirt.  The skirt that is around the perimeter of the hovercraft performs an extremely important function in containing the air cushion.  By using a skirt, the amount of engine power required to lift the craft is considerably reduced and as an added benefit, extra hull surface clearance is obtained.  The skirt is a long strip of material that is mounted onto the underside of the craft.  When the skirt is inflated, it lifts the hovercraft.  The escaping air coming from where the skirt touches the ground is what creates a friction-less cushion of air.  Because the hovercraft has practically no friction, it takes little force to move the craft. 

The diagram above shows the basic principle of a hovercraft.  Air is directed underneath the craft by a blower, and the air that escapes out of the flexible skirt creates a near frictionless environment which enables the craft to hover.  The craft can move forward and turn by using propellers to propel the craft.

The hovercrafts shown on xinventions either use a leaf blower of vacuum cleaner engine for lift.  The larger hovercraft uses an engine for propulsion while the smaller hovercraft has none however one can easily be attached.

Weight and Balance

Center of gravity (CG): is the point at which a hovercraft would balance if suspended.
Center of lift (CL): is the center of area.
Reference datum: is an imaginary vertical plane from which all distances are measured for balance purposes.
Station: is a location along the hovercraft hull given in terms of the distance from the reference datum.
Empty weight: is standard weight plus weight of optional equipment
Maximum weight: is the maximum weight approved for general operation.
Maximum flight weight: is the maximum weight the craft will fly in ground effect.

Operational

Plow in: is when the hard structure in the bow of the craft comes into contact with the water. The contact has the ability to generate a high level of drag causing a controlled deceleration of the craft.
Side plow: is when the hard structure on the side of the craft comes into contact with the water. The contact has the ability to generate a high level of drag causing a controlled deceleration of the craft.
Landing skid: surface on the bottom of the craft in which the craft rests when not on cushion. Landing skids are also used to reduce wear on the bottom of the craft.
Hover height: distance between the lower hull and the surface when the craft is on cushion.
Cushion delay time (cdt): is the time it takes for the hovercraft to go from full hover (or 10 inches of hover height) to the landing skids touching the ground. This action is done by rapidly reducing the engine rpm with the throttle or by turning the key to the off position.
Thrust duct: is the tapered ring around the diameter of the propeller. Thrust ducts add protection; increase overall efficiency and stability of the craft.
Trim wing / elevator: Horizontal wing located behind the thrust duct. This wing helps to maintain proper trim of the craft while operating. When the craft is in Hoverwing™ mode the horizontal wing them becomes an elevator controlling the altitude and trim of the craft.
Plow plane: Angled bottom surface of the craft that first comes into contact with the water. This surface helps to break water from bottom helping to reduce drag.
Skirt drain hole: Hole located in the rear center section of the skirt. This hole remains open allowing water to continuously drain from the skirt.
Scooper hole: Hole or damage to the skirt resulting in water continuously being scooped into the skirt. These holes are typically found in the skirt contact area.
Skirt contact area: Portion of the skirt that comes into contact with the terrain. This portion of the skirt incurs the most wear.
Tail heavy: When the rear of the craft is excessively loaded. The rear of the craft squats in the water causing excess drag and hindered maneuverability.
Hovercraft: transport vehicle that moves on a cushion of air.
Radar: apparatus that detects objects through the use of microwaves.
Pylon: supporting post.
Dynamic propeller: two-bladed apparatus that provides motion.
Fin: steering device.
Rudder: apparatus that prevents drift.
Lift-fan air intake: opening to allow air to enter.
Main level drive gear box: compartment that contains and protects the gear mechanism.
Skirt finger: part of the flexible skirt.
Passenger entrance: opening on the side wall that provides access to the passenger cabin.
Flexible skirt: lower flexible part.
Bow door ramp: opening at the front.
Control deck: cubicle from which a hovercraft is operated.


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SMALL WATERPLAN AREA TWIN HULL (SWATH)

The Small Waterplane Area Twin Hull (SWATH) is a hull form used for vessels that require a ship of a certain size to handle in rough seas as well as a much larger vessel. An added benefit is a high proportion of deck area for their displacement — in other words, large without being heavy. The SWATH form was invented by Canadian Frederick G. Creed, who presented his idea in 1938 and was later awarded a British patent for it in 1946. It was first used in the 1960s and 1970s as an evolution of catamaran design for use as oceanographic research vessels or submarine rescue ships.

Catamarans provide large, broad decks, but have much higher water resistance than monohulls of comparable size. To reduce some of that resistance (the part that generates waves), as much displacement volume as possible is moved to the lower hull and the waterline cross-section is narrowed sharply, creating the distinctive pair of bulbous hulls below the waterline and the narrow struts supporting the upper hull.

Swath designs are more expensive to produce than conventional twin hulled vessels.  The advantage to such a design is that a significant proportion of the hull remains below the surface of the sea.  For this reason wave contact is reduced where it may only act on the thin leg areas.  Wave drag is a major component of the total drag on a vessels hull.  For this reason the hull is more stable, being less prone to pitching and rolling and requires less power for propulsion than conventional designs.    

A Swath design wastes proportionally less energy climbing wave peaks and accelerating down troughs.  The passage is smoother.  The below surface hull (in normal operating conditions) is subject to laminar (or pipe) friction, which is more predictable.  The SSC Radisson Diamond is an example of a commercial ocean liner claiming to offer unrivaled cruise comfort

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