Hull speed

As the hull speed, the speed of a ship in the displacement mode is referred to, in which the wavelength of the bow wave reaches the length of the waterline of the vessel and its flow resistance increases sharply as a result.

Explanation

With increasing speed of a ship in displacement mode the wavelength of the bow wave grows. If constructively superimposed upon reaching the hull speed bow and stern wave, the stern of the ship comes into the trough formed therefrom and decreases. The ship must therefore start against the building in front of him steep bow wave. In order for the to be overcome flow resistance increases disproportionately.

The hull speed is thus no sharp defined upper limit of the possible speed in displacement mode, the power required to drive increases before and after reaching the hull speed continuously ( approximately the third power ) (see chart). The hull speed referred to within this resistance curve a useful approximation for the maximum achievable with acceptable driving power in stable travel speed.

Practical calculation and significance

For a value of hull speed in kilometers per hour in general the square root of the waterline length of the vessel in meters is multiplied by the factor of 4.5 used. For a result in meters per second respectively, the factor 1.25 is valid for node, the factor 2.43:

Therefore, the hull speed is for a hull with a length to the design waterline

• Of 10 meters about 7.7 knots 14.2 km / h,
• 100 meters about 24 knots 44.4 km / h,
• 300 meters about 42 knots 77.8 km / h

The fact that the hull speed depends only on the waterline length, is the reason why larger ships - under strong driving wind or engine power - can reach higher speeds in planing conditions than smaller ships. This is reflected in the phrase " long runs " contrary.

The above-mentioned cubic relationship between driving performance and speed, however, has that larger vessels will be used for economic reasons in general only with speeds up to approx.

In modern ship the concept of hull speed does not matter, he was replaced by more suitable parameters such as length to width ratio and Froude number.

Background

For ships of the wave resistance increases from a Froude number by 0.35 very strongly. This is because, firstly, that the amplitudes of the transverse waves rise sharply, on the other hand, that they are superimposed on the waves of the ship's primary shaft system unfavorable. At a Froude number of 0.4, the stern and the bow wave superimposed always so that the second wave crest of the bow wave impinges on the rear shaft. At a Froude number of 0.56 the Valley of the bow wave strikes the rear shaft so that neutralize in about. At times where the concept of hull speed was coined, it was the speed that could not be exceeded with former achievements and ship forms.

The above formula for hull speed results from the approximate formula for the velocity of a surface wave in deep water. In this case (eg 9.81 m / s ² for mid-latitudes ) the gravitational acceleration on the Earth's surface is the velocity of the wave generated by the ship, in this case, and the wavelength that reaches the waterline length of the ship at hull speed.

Exceed the hull speed

If a designed for trawler boat with extreme force accelerates through its hull speed (eg by can be a sport boat from a fast commercial vessels tow ), this can in addition to breaking the towline and the fittings by the extreme force and uncontrollable handling, capsizing and above particularly water absorption and thus lead to sink. The tail can sink further and further into this situation by the suction until water runs over the stern of the hull.

This situation may also sailing ships on departure occur by a wave (so-called surfing ). This can cause considerable damage caused to the vehicle when immersed in a wave or when passing wave crests by vibrations and force peaks result of the addition of wave velocity and speed of the hull. The "Surfing" in stormy trip can be avoided by dragging of ropes or a floating anchor.

By means of suitable ship types, it is possible to exceed the hull speed is far stable and without extreme performance overhead.

These are for a so-called gliders, whose hull is shaped so that they reach an ever increasing rate higher hydrodynamic lift, thereby out of the water, so that the wave resistance decreases greatly. Examples include virtually all racing boats, but also military speedboats have a pronounced sliding surface with a constant width to the tail.

A further possibility is to perform the extremely long and slender hull ( length to width ratio 8 ). Thus the wave formation is greatly minimized and the water resistance increases with the speed at significantly lower. Examples are canoes, skiffs and catamarans here especially. These hull forms have their great length to width ratio, only a small curvature along the shape. Thus, the wave amplitudes are smaller. Through the pointed tail, they also have no pronounced tail shaft. The necessary energy must be expended to keep these shafts is therefore low.

This design principle was followed in the 1920s and 1930s in the field of military shipbuilding special hull designs for destroyers and torpedo boats that can reach speeds of about 33 to 38 knots in a hull length of 100 to 130 meters, at the tests even more than 40 knots.

Again came an extremely slim shape with a ratio of length to width to 10:1 for use. For this purpose, a long-drawn bow was used so that the largest Spantquerschnitt is reached only in the posterior half, and sharp separation edges at the rear. The engine power had to be dimensioned accordingly: 22,000 kW to over 51,000 kW were used. This type of hulls complies with today's perspective, half Glides: ships that can come into sliding, at least in part.