Torso speed

The maximum speed in displacement hulls does not depend on the engine power, but on the length of the waterline.

The hull speed is a seaworthy technical term that has nothing to do with the actual speed of the hull.

It is a theoretical value for the maximum speed that a ship can achieve in displacement travel.

As hull speed , the speed of the vessel is referred to, in which the wavelength of the bow wave reaches the wave-forming length of the ship, and as a result greatly increases the flow resistance. Boats and ships with displacement hulls can hardly exceed this speed even with increased power.

Explanation

With increasing speed of a ship in displacement travel, the wavelength of the bow wave increases. If the structure of the bow and stern waves overlap when the hull speed is reached, the stern of the ship enters the trough formed from it and sinks. The flow resistance to be overcome thus increases sharply with a further increase in speed.

Practical calculation and meaning

To calculate the height of the hull speed , the square root of the length of the ship's waterline in meters is multiplied by a factor that depends on the unit of speed. For kilometers per hour the factor is 4.50, for a result in meters per second the factor 1.25 applies instead, for knots (nautical miles per hour) the factor 2.43 applies : ${\displaystyle v_{\mathrm {max} }}$ ${\displaystyle l_{\mathrm {wl} }}$

${\displaystyle v_{\mathrm {max} }\approx 4{,}50\cdot {\sqrt {l_{\mathrm {wl} }}}\,\mathrm {\frac {km}{h}} \approx 1{,}25\cdot {\sqrt {l_{\mathrm {wl} }}}\,\mathrm {\frac {m}{s}} \approx 2{,}43\cdot {\sqrt {l_{\mathrm {wl} }}}\,\mathrm {kn} }$

The hull speed therefore increases with the length of the waterline : ${\displaystyle v_{\mathrm {max} }}$ ${\displaystyle l_{\mathrm {wl} }}$

• ${\displaystyle l_{\mathrm {wl} }=10\,\mathrm {m} }$ results ${\displaystyle v_{\mathrm {max} }\approx 7{,}7\,\mathrm {kn} \approx 14\,\mathrm {km/h} }$
• ${\displaystyle l_{\mathrm {wl} }=100\,\mathrm {m} }$ results ${\displaystyle v_{\mathrm {max} }\approx 24\,\mathrm {kn} \approx 44\,\mathrm {km/h} }$

The relative speed indicates for each boat at a given speed how close it is to its hull speed , the relative speed is at most . ${\displaystyle v_{\mathrm {rel} }={\frac {v}{v_{\mathrm {max} }}}}$ ${\displaystyle v}$ ${\displaystyle v_{\mathrm {max} }}$ ${\displaystyle v_{\mathrm {rel} }=1}$

The cubic relationship between propulsion power and speed leads to the rule of thumb that with displacement hulls, speeds above or relative speeds are considered uneconomical due to the high fuel consumption, which corresponds to speeds above about 80% of the hull speed. ${\displaystyle v=2\cdot {\sqrt {l_{\mathrm {wl} }}}\,\mathrm {kn} }$ ${\displaystyle v_{\mathrm {rel} }>0{,}82}$

The fact that the hull speed only depends on the length of the waterline is the reason why longer ships - with correspondingly strong propulsion - can reach higher speeds in displacement travel than shorter ships. This is reflected in the phrase “length runs”. In modern shipbuilding , however, the term hull speed no longer plays a role; it has been replaced by more suitable parameters such as length / width ratio and Froude number . In recreational boating, however, the hull speed is still used to indicate the speed potential of a boat.

background

In ships, the wave resistance increases sharply from a Froude number of 0.35. This is due to the fact that, on the one hand, the amplitudes of the transverse waves increase sharply, and on the other hand, that they unfavorably overlap the waves of the ship's own primary wave system. With a Froude number around 0.4, the stern and bow waves always overlap in such a way that the second crest of the bow wave meets the stern wave. With a Froude number around 0.56, the valley of the bow wave meets the stern wave, so that these roughly neutralize each other. At the time when the term hull speed was coined, this was the speed that could not be exceeded with the performance and shape of the ship at the time.

The formula given above for the hull speed results from the approximation formula for the speed of a surface wave in deep water . Here, the speed of the in this case by ship wave generated, the acceleration of gravity at the Earth's surface (e.g., 9.81 / m B. s² for medium widths) and the wavelength that reaches the waterline length of the ship hull at speed. ${\displaystyle v\approx {\sqrt {g\lambda /2\pi }}}$ ${\displaystyle v}$ ${\displaystyle g}$ ${\displaystyle \lambda }$

Exceeding the trunk speed

If an attempt is made to accelerate a boat designed for displacement travel beyond its hull speed, e.g. B. by having it towed by a fast ship, the tow rope can break or the fittings tear off. However, if the material withstands the force, an ever-increasing bow wave builds up instead. This leads to an increasingly steep position in the water until the stern finally dips below the surface of the water.

Racing boat on plane

Using suitable hull shapes, it is possible to exceed the hull speed by far. Such watercraft are referred to as gliders and, with increasing speed, achieve ever higher hydrodynamic lift . This lifts them out of the water and the specific water resistance drops sharply, so that the above formula for the hull speed is no longer applicable. Examples of gliders are practically all racing boats , but also most speedboats. The latter have an embossed sliding surface. They are also available in large sizes and for military use. If the engine is sufficiently powerful, they can also switch from displacement travel to gliding mode and then travel faster than the speed of the trunk.

Very slim hull shapes are implemented in rowing boats

In displacement boats, wave formation and water resistance depend largely on the width (under water), hardly on the length. Fast displacers must therefore be as narrow as possible. Examples are canoes and catamarans or rowing boats, especially skiffs . Along the shape of their hull shapes, these are not very crooked. This means that the wave amplitudes are smaller. In addition, when the tail is pointed, the tail wave is smaller than that of the wide tail. The energy required for acceleration is consequently lower.

This simpler construction principle was followed in the 1920s and 1930s in the field of military shipbuilding also special hull designs for destroyers and torpedo boats , which were only 10 meters wide with hull lengths of 100 to 130 meters and could reach speeds of around 33 to 38 knots. The particularly slim shape with a length: width ratio of 10: 1 was supplemented on the one hand by an elongated bow so that the largest frame cross-section was achieved well behind the center, on the other hand by sharp spoiler edges at the stern. This type of hull is now known as a semi-glider because at least part of it (the front one) can slide.

The Lürssen effect achieves further improvements .

A drastic reduction in water resistance can also be achieved by using hydrofoils (also called hydrofoils) . The fuselage is lifted by wings so that it is above water. A sailing boat thus reached a speed of over 120 km / h in November 2011.

literature

• Joachim Schult: Sailors Lexicon . 13th updated edition. Delius Klasing, Bielefeld 2008, ISBN 978-3-7688-1041-8 .
• Seamanship. Yachting manual. 30th edition. Delius Klasing Verlag, Bielefeld 2013, ISBN 978-3-7688-3248-9 .

Individual evidence

1. ^ Herbert Schneekluth: Hydromechanics for ship design . Koehler, Herford 1988, ISBN 3-7822-0416-6
2. ↑ ^{a b} hull trip . In: Joachim Schult: Segler-Lexikon . 13th updated edition. Delius Klasing, Bielefeld 2008, ISBN 978-3-7688-1041-8 .
3. ^ Seamanship. Yachting manual. 30th edition. Delius Klasing Verlag, Bielefeld 2013, ISBN 978-3-7688-3248-9 , p. 148 f.
4. ↑ Fable world records for Sailrocket 2. In: yacht.de. November 19, 2012, accessed May 10, 2014 .