Displacer and glider
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In shipbuilding, a displacer is understood to be a boat that is in the water with the entire underwater hull at all times and displaces it . The opposite of displacers are gliders that rise out of the water with increasing speed and begin to slide on it. A suitable hull shape, moderate weight and powerful propulsion are required for planing. Semi- gliders are between displacers and gliders in terms of construction and drive power.
A hull with a displacer shape cannot become a glider even by increasing the drive power.
Basics
Almost every watercraft, whether raft , flying boat , seaplane , aircraft with support floats , amphibious vehicle , boat or even ship , is a displacer when at rest , since every body floating on the water displaces water of the same weight. This is the content of the Archimedean principle .
In this sense, air-cushion vehicles are not water vehicles, because they do not swim, but rather float - with high energy consumption - due to overpressure under the vehicle. The differences between displacer , semi-glider and full glider only come to light when a watercraft starts moving .
Displacer
A slow watercraft usually drives in displacement mode. As much water is displaced through the hull as it corresponds to its mass. With increasing speed, the resistance increases due to your own bow wave. As a result, this type of locomotion is only possible up to the so-called trunk speed . Classic displacers can be found in the entire range from the dugout canoe to Columbus' Santa Maria to Queen Mary 2 .
Torso speed
No displacer can overtake the wave with the stern that is formed by the bow of the same displacer while driving. In practice, the problem is already noticeable when the stern wave front side, which is a little ahead of the stern, reaches the bow wave rear side, which is lagging behind the bow, that is to say the bow and stern wave systems begin to interact with one another. Due to the extremely slim design (see also the catamaran article ) from a length / width ratio of around 8: 1, the wave systems of the bow and stern no longer interact with each other, but the stern cannot overtake the bow wave. In practice, this means that with short, wide hulls you can only achieve the same speeds with more propulsion power than with a narrow hull of the same length.
Nevertheless, the final speed that can actually be achieved by a displacer depends exclusively on the length of the ship's hull in the waterline, but not on its shape or width. This maximum final speed that exists specifically for each trunk is called the trunk speed .
The trunk speed of a displacer can be calculated approximately. If the hull speed is nevertheless exceeded due to adverse circumstances (e.g. in a storm on the way from a wave crest into a wave trough) or by being towed by a significantly larger ship, dangerous surfing or damage to the ship's hull occurs.
This also applies to displacement hulls that are designed as wave trusses (see below ), even if their hull speeds differ from the classic ones, as they cannot be calculated from the hull length and water depth, but from the energy of the water particles and always asymptotically at around 50 knots end up. An example of a boat is the German speedboat series 140/141 Jaguar class and one for ships is the French series of destroyers of the Le Terrible class .
Glider
Due to their hull construction, gliders are suitable for lifting themselves out of the water by means of a strong drive against the water resistance and sliding on the water. The majority of the vehicle's mass is therefore above the waterline. As the hull rises out of the water, the bow wave drag is reduced. Finally, as the speed continues to increase, the tail begins to slide on the wave. Due to the much lower water resistance, significantly higher speeds are achieved compared to the displacement travel. The transition from displacement travel to plane travel is the equivalent on water to breaking the sound barrier in the air, where an aircraft overtakes its own sound wave.
The moment when the plane begins to plane is called planing . Whether and when a watercraft starts planing depends on the shape of the hull, the weight distribution, the speed and the sea. Surfboards, dinghies and flat motor boats slide relatively easily. A long, flat hull with a wide stern is advantageous for planing. If the speed is too high, each glider can be lifted by dynamic buoyancy and then lift off completely from the water.
Glider types
While planing was previously limited to small boats, there are now even light 30-meter yachts that are built and operated as gliders. However, if the weight is increased in such a yacht through subsequent installations, it can happen that it can no longer be brought into plane, since not only weight and drive power, but also the weight and hull shape of each glider must be coordinated with one another.
Military uses of gliders have also reached astonishing sizes. There is now even an American one-man submarine that can go on plane when surfaced.
Design aspects
The structural and operational catch with every glider is the ability to start gliding in the first place. This ability is not only dependent on the shape of the hull, but above all on the power-to-weight ratio of the ship. However, once the glider slides, you could theoretically increase its weight, for example by loading a helicopter , beyond the maximum weight with which the gliding could still be achieved.
One problem is that, to put it in a very simplified way, the speed at which a ship's hull starts gliding on its journey through the water increases with increasing weight. In order to achieve this minimum sliding speed, an increasing drive power is therefore necessary with increasing weight.
Since every glider is hydrodynamically a displacer up to the moment of gliding and there is the phenomenon of cavitation , the construction of the glider is set a weight limit upwards, which even with hydrodynamic tricks such as those used in a hydrofoil or by the aerodynamic effect is limited upwards in a flying boat. Therefore, there are no really large watercraft that are constructively underway as a glider.
The phenomenon of sticking
If a glider succeeds in gliding, i.e. catching up with its bow wave (by the front of the stern wave system reaching the rear of the bow wave system), but not overtaking the bow wave with the trailing edge at the stern, this is called sticking . When gluing - albeit for a slightly different reason - the same speed limit applies as for a displacer in displacement travel. Asymptotically approximated, the limit is 50 knots. At sea one can “glue”, i. This means that a boat is racing across the water on its own bow wave, especially when watching smaller boats. The reason for gluing a glider hull is always an incorrect hull shape, i.e. a construction error. For small boats (which would have significantly lower hull speeds as a displacer), gluing still means a fast glide, which is, however, much rougher than a real glide, in which the bow wave is not only caught but also overtaken.
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In flying boats and ground-effect vehicles starting from the water, sticking can also occur if the hull is incorrectly constructed, but does not mention the fact that you get stuck while planing, but rather that you are unable to take off from the water while gliding even at the highest speeds. However, sticking is often observed and documented in aviation, especially in the case of particularly large flying boats and ground-effect vehicles, when they attempt to take off from mirror-smooth water surfaces. When attempting to take off on a water surface roughened by wind and waves, the sticking of large flying boats and ground-effect vehicles practically does not occur.
In rough seas, it is also possible for boats that tend to stick as gliders to overcome the sticking and to get into real gliding, as long as the conditions allow small boats to travel through the water at all.
A popular trick to overcome sticking with an adhesive glider hull is to cut the stern sea of another boat or ship, as this allows the bow wave to be torn off from the stern. A flying boat or ground effect vehicle can similarly get into flight from gliding. So z. For example, the flying boat Dornier Do X on its round-the-world flight after 27 unsuccessful attempts in June 1931 only got off to a start in Rio de Janeiro because the stern sea of a freighter was cut.
If the gluing effect is already noticed on a prototype of a boat / flying boat / ground-effect vehicle, it is often sufficient to modify the fuselage in series production with another tear-off edge, referred to as a step in aircraft construction . In small boats, however, there is often no space for this, as additional spoiler edges in relation to the hull width also require certain minimum hull lengths. Other solutions to positively influence the boundary layer (which is responsible for the gluing effect) in terms of gliding are mostly uneconomical for small boats. As a result, despite the knowledge about the hydrodynamic processes, boats are produced and sold that tend to stick as gliders.
Hovercraft
Air-cushion vehicles are not gliders, as they float on a cushion of compressed air while driving, but are not in contact with the surface of the water. A glider experiences so-called dynamic buoyancy from the impact of the water while driving, a hovercraft experiences static buoyancy from the overpressure on its underside.
Semi-glider
Since a glider needs significantly less energy to maintain its speed than a displacer of the same size when planing at the same speed, the idea behind the semi-glider construction is to save energy, i.e. to use less fuel or faster with a sailing boat in less wind to be on the go.
The semi-gliders can be divided into several subgroups:
Classic semi-glider
The following two subgroups are designated as classic semi-gliders:
Displacer type
A displacer that has been made to glide with part of the hull by construction, while the majority of the same hull is still underway as a displacer. Large trawler yachts are underway like this. In the 70s, many manufacturers of small displacement boats (up to about 9 m) started to widen and flatten the stern area of the boats so that you can go into the planing phase with this part of the hull. This is also achieved by the fact that the bow is shaped in such a way that it throws the water to the side when driving fast, so that the sliding area of the hull lying under the water surface can slide at all.
Glider type
A glider with a keel, which displaces the water, i.e. does not completely protrude from it. Large, sea-going racing sailboats are underway like this. In theory, hydrofoils also belong to this group.
Modern semi-planers
There are different hull shapes of modern semi-planers on the market, which work according to different methods.
Parametric Fast Hull
Almost all planing hulls work according to the tilted plate principle. The bottom of a ship is opposed to the current at an angle. This creates a dynamic lift , but also an induced drag and a suction that counteracts the lift. Curving the floor can greatly reduce the negative effects. The Parametric Fast Hull follows this principle. Its soil shape is described by a few parameters that depend on the desired speed, displacement and the length-width ratio. The handling of boats with this hull shape is significantly different from that of conventional shapes. The ship hardly trims and is lifted at the front by the bow wave and at the rear by the concave bottom shape. This is associated with very little wave formation and, due to the small trim, very favorable sea behavior.
The suitability of the hull shape depends on the length and speed of a ship. It is suitable for Froude numbers from 0.6 to 1.3, which is e.g. B. ships with a length of 10 m and speeds of 13 to 25 knots or 30 m in length and 20 to 43 knots. Possible uses are therefore yachts , pilot boats , patrol boats, etc. However, the hull is not suitable for very high speeds (e.g. for racing boats ) because the curvature in the stern increasingly trims the boat forward. This would increase the surface of the hull wetted by the water and reduce the advantages of the hull.
Experience with built boats has shown that the Parametric Fast Hull also has a positive effect on maneuverability and rolling behavior and only causes minimal wave formation. The power saving compared to conventional hull shapes is 20 to 50 percent depending on the quality of the comparison shape. For example, with a 33 knot patrol boat with 170 t displacement and 3 × 1000 kW propulsion power, one of the three propulsion systems can be dispensed with using the pfh hull.
Edersche DG-Hull
The Austrian physicist Theodor Eder was asked by the mayor of Venice at the end of the 1990s to develop a hull that would not throw a wave. Its construction was called DG-Hull ( displacement glider ).
From a physical point of view, in this boat the wave that is generated by the underwater hull while the boat is in motion, “cancels” the waves that are generated by the bow and stern by the trough of one wave coinciding with the crests of the other two waves. The boat works great and has been on the water since 2003.
If you look at it closely, it is a glider with a clearly pronounced displacement keel, which makes the construction a displacer without the displacement-typical hull speed: because where there is no wave, none has to be overtaken, which makes the Eder boat much faster with significantly less energy than the hull speed of a classic displacer of the same size would allow with any engine size. The Eder concept of the DG-Hull combines the advantages of the glider with the advantages of the displacer. In theory, this concept is applicable to any boat and ship size and use, and appears to work best when the boat / hull in the construction waterline (KWL) has a length / width ratio of 3: 1 to 4: 1 .
Alsphere is the owner of all rights to the (hard chine) hull, which does not cause waves, has all rights to this DG concept worldwide and issues licenses. The patent expires in 2023.
Wave tie
The wave tie was invented in 1910 by the boat builder and engineer Claus Engelbrecht and was then improved in the first decades of the twentieth century by the German shipbuilding engineer Arthur Tiller (1884–1957) employed by Engelbrecht . It has the same functional principle as the Edersche DG-Hull. However, there are two main differences between DG-Hull and wave truss:
- The Edersche DG-Hull is still at least 2023 proprietary chine , which in the design waterline in a full-length / width ratio of 3: 1 and is therefore deemed to be genuine Halbgleiter 1 to. 4
- The Tillersche Wellenbinder is a round-frame whose patent rights expired in the course of the Second World War , but which is a little more complicated to calculate and more expensive to manufacture than the DG-Hull, and the one in the KWL in the slim to very slim length / width ratios of 5: 1 to 10: 1 was built. In addition, the design and shape of the hull make the Tillersche wave tie more of a real displacement than a semi-glider.
During Arthur Tiller's lifetime, the hull was not yet calculated - one was not even able to do this, but developed through empirical procedures, i.e. the construction of watercraft as a model and in real size.
In addition, Engelbrecht, Tiller and others of their contemporary European and American boat builders and engineers took a different approach than Theodor Eder. As a physicist, you did not have to protect coastal facilities from the waves of moving boats, but wanted to achieve the highest possible speeds with the pleasure boats, which were only weakly motorized at the beginning of the twentieth century, to the delight of owners and users.
The real shaft truss constructions from that time at around 10 meters in length and a good 20 hp drive drove almost 10 knots (18 km / h), although with the methods of hydrodynamics at that time only about 8 knots (15 km / h) would have been possible, regardless of the motorization of these boats.
A boat built by Arthur Tiller is said to have reached just under 26 knots (48 km / h) as a wave tie with a length of 18 meters with 520 hp, although - as is known today - the hull speed of the construction is only a good 10 knots (just under 18 knots) even with the most powerful engine km / h) Driving through the water is permitted.
The distinction between “real” and “fake” wave trusses was made by Arthur Tiller around 1934, to identify those boats whose underwater hulls only looked like the underside of a wave tie, but only came along as a real displacer or (with sufficient motorization) as a real glider, to be distinguished from those with which it was actually possible to reach high speeds without going over to plane. At that time, the difference could not be determined during the design and construction of a wave tie. Only the test drive could reveal this.
Planing instead of displacement travel was only not wanted by the designers at that time for the two reasons that gliders had to be both very well motorized and very light, which, given the possibilities of choice of materials and motorization at the time, in contrast to the increased demands the comfort of a pleasure boat.
At that time, however, there was a general tendency to build wave trusses based on the principle of wave truss 1 (see picture), since these boats could begin to slide with sufficient motorization and incorrect proportions, which was considered to be the lesser evil when one was the "real one." “It was not possible to build wave trusses, while boats based on the principle of wave truss 2 and with incorrect proportions remained ordinary displacers even with the most powerful engines.
In sales back then (and also on today's used / classic boat market), the distinction between “real” and “false” shaft ties was and is only rarely made due to a certain commercial skill, but often also out of ignorance, and can only be made after complex calculations or a simple test drive. And if wave trusses, built between 1910 and 1940, if they survived the turmoil of the time to this day, are already classic rarities, the “real” wave trusses are to be regarded as very, very rare among them.
In the new builds of boats in the retro look of the 1920s to 1950s, which have been modern since the turn of the millennium , the wave truss is practically non-existent as a construction of the underwater hull, although the 21st century in particular would offer the possibility of designing a wave truss on the virtual drawing board in such a way that that finished boat later also works in Tiller's sense.
Sliding on the crest of waves
One way of overcoming trunk speed is to go down the incline of a crest of waves. Surfers , canoeists , but also sailors strive for this condition because it allows higher speeds to be achieved. Surfboards, most whitewater canoes and modern, light sailing yachts are easy to control in such situations.
However, large vehicles or sailing yachts with unfavorable hulls that are not designed for surfing can be in serious danger if they shoot into a trough. The greatest danger comes from the violent impact in the trough of the waves, which can seriously damage larger ships in particular. In extreme cases, the ship can even undercut, that is, dig its bow into the next wave crest and even "go deep" from this movement. Another danger for large ships is ricocheting. On their flat underside of the hull, they can slide sideways into the trough of the waves, where they are then downright knocked over by the side impact of the next wave.
The wrong fifties
Since around 1950 there have been small recreational boats , often driven by outboard motors , for which the expression "wave tie" was rediscovered. These boats emerged from Canadians with transom with outboard motors, steering wheel and windshield, and quickly took on a shape and size of their own, very much like the motorboat and limousines used in motorboat construction in the Roaring Twenties . On closer inspection, they are also small car boats, only that the classic car boats had inboard engines . These "wave ties" from the 1950s and 60s have nothing in common with the Tiller's and Engelbrecht's wave ties, they do not even have the concave-convex frame shape of a real wave tie, but are real, sharp-cut gliders in the underwater hull with a chine construction. What unites them with the real wave tie is the good straight-line stability with very good maneuverability, even in displacement travel. The fake fifties (named after the 1950s) became popular during the economic boom , as any halfway practically gifted person could build them in their hall, allotment garden or garage.
Today this type of boat is becoming modern again, as they are already real, elegant, small multi-person motorboats and they can still be trailered in modern lightweight construction with the small car driver's license B (if the outboard motor is in the trunk of the car and not while driving on the road located on the boat). There are now numerous associations that deal with the construction, maintenance and operation of the sport boat in the category of the false fifties wave tie.
Web links
- yachtsportarchiv.de - Information on "pan-German" boat building and yachting from around 1910 to around 1950
Individual evidence
- ↑ eB: "Parametric Fast Hull" - Shipping and Technology 05/2007, p. 76. (PDF; 1.9 MB) (No longer available online.) May 1, 2007, formerly in the original ; Retrieved January 3, 2013 . ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
- ↑ Claus Reissig: "The few liters more" - Frankfurter Allgemeine Sonntagszeitung September 5, 2010, No. 35. (PDF; 423 kB) (No longer available online.) September 5, 2010, formerly in the original ; Retrieved January 3, 2013 . ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
- ↑ The early yacht designers , yacht building. Design, construction and calculation of sailing yachts . (1937)
- ↑ Die Yacht , year 1935, issue 30, page 11
