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SUPERYACHT #494
June 2003

Article selected from our quarterly magazine dedicated to the largest and most luxurious boats with information, interviews, technical articles, images and yachting news


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Article by
Angelo Sinisi


THE GLIDING UNDERBODY

In previous articles we looked at the limits of the displacing underbody to surpass certain speeds and we saw that to reach a relatively high speed a gliding underbody is necessary. While it would be wrong to claim that continual improvements to engine characteristics, combined with greater experience, studies and rapid progress have solved all the problems relating to gliding underbodies, it is true that they are a better solution for boats not only with a small, but also a large displacement, which until a short time ago was a mere hope. In fact nowadays large partially gliding hulls are used for fast sea transport. A gliding hull is one which, when in motion, is mostly sustained by the dynamic reaction of the water. In essence, it slides over and skims the surface of the sea and this distinguishes it from a normal hull which instead only floats and opens the course in the water. (Figure no. 5). The action of a gliding surface is similar to that of a wedge forced under a weight in order to lift it. A gliding surface leans, forcing the water down and this force creates a field of pressure which raises the level of water up the sides of the surface itself. Thus it is the dynamic push that at high speed supports almost the entire weight of gliding boats.

Figure 5
Wake and wet bottom
of a gliding underbody
speed of 34 knots in rough seas

We could therefore say briefly that the factors in play are: weight, speed, dynamic and static thrust, angle of longitudinal trim, wet surface and resistance to motion (Figure 3). All these factors are interrelated and the trim angle depends on the relative position, in longitudinal terms, of the centre of gravity and the centre of dynamic pressure. From this it can be inferred that the design of a gliding vessel demands more precise, involved study than that necessary for designing a displacement vessel. The change of trim at high speed becomes one of the most important parameters in the design and the need to plan the weight of the boat and the position of its centre of gravity with precision constitutes one of the main difficulties (Figure 2).

The following are some of the substantial differences between the design elements of a gliding hull and those of a displacing boat. The static thrust of the underbody, in a displacement boat, sustains the whole weight, while in a gliding hull this thrust sustains only 1/3 to 1/13 of the weight of the boat. The remainder is sustained by the dynamic thrust. The form of underbody of a displacement boat affects the resistance to motion of the boat itself as well as its manoeuvring qualities and seaworthiness. In the case of the gliding hulls this influence is more marked. The form of underbody has an affect on its gliding possibilities, on the behaviour of the boat during gliding, on pitching, 'porpoising', seaworthiness, on course stability and on the angle of listing when turning. Wind resistance for a displacing vessel represents a small part of the total resistance. For a fast gliding vessel, this resistance constitutes a very important factor and sometimes it is accompanied by considerable vertical force. The design of the bottom of a gliding hull constitutes, therefore, not only a problem which is substantially different from that of a displacement ship but also a more intricate and complex problem because of the interrelation and interaction of the most important factors that govern it. In no other field of naval design does there exist such a strong, mutual dependence of various factors. A 10% increase in the weight of a displacement vessel means that the latter sinks a little more, sails at a lower speed but keeps more or less the same forms of underbody and its behaviour remains essentially the same. The same weight increase for a gliding hull can prevent it from gliding and/or can modify its characteristics of dynamic stability and therefore its seaworthiness.

For this reason, we could find a paradoxical situation where the problems get bigger while hulls get smaller. There have been many studies and tank tests using underbodies with various angles of lift (the angle made by the bottom of the underbody with the horizontal plane passing the keel) and various ratios between chine length and width. The various tests carried out in tanks have demonstrated that the increase of resistance is not only due to an increase in weight or in speed but is particularly due to the angle of lift of the bottom and to the longitudinal trim angle at high speed, which is determined by the respective positions of the thrust centre and the centre of gravity.

Figure 4

Model 2117
Model 2084


Smooth water speed 15 knots


Rough sea speed 15 knots


Smooth water speed 34 knots


Rough sea speed 34 knots

It has been seen that when the angle of lift is increased, resistance increases as well (Figure 1) but seaworthiness is improved also because impact accelerations are considerably reduced. Thus with a rough sea it is possible to achieve greater speed than with an underbody of the same size but with lower angle of lift. Thus the choice of the angle of lift is vital for speed and comfortable sailing. This last quality requires, in addition, particular attention to the forms of the bow which on impact with waves, by reducing accelerations, protect the structure from heavy stress (the various pictures in Figure 4 demonstrate in detail the different behaviour of an underbody with penetrating bow Model 2084, from a V- bottom with very low angle of lift Model 2117).

Unfortunately the shape of hulls most suitable for gliding is not compatible with the classical forms for displacement sailing. Thus at a low speed, or rather, before reaching the gliding speed, the hull is generally subject to strong wave resistance, whirlpools and to annoying sprays of water. Prior to reaching gliding speed the bow remains suspended in the air, resistance increases considerably and, if there is no power reserve, the boat meets a barrier against speed that it cannot overcome. If, on the other hand, there is still extra power available, the hull could overcome the 'bucklè in the resistance curve, its speed would increase considerably and the crest of the bow wave would move to the stern.

Many are right in saying that at this point the boat rides up the bow wave, the longitudinal trim angle decreases and speed increases. The hull increases speed considerably without requiring a further increase of power; we could say that gliding speed has been reached, although it is certainly not the maximum possible speed.

A boat is gliding when the water touches the bottom only inside the chine. The greater the distance between maximum speed and gliding speed, the better the boat will behave because increased resistance due to dirt on the underbody, to a rough sea, to any extra load, to adverse winds, would not be enough to prevent gliding. Thus when one buys a boat it is important to know the maximum speed and gliding speed with full load.

There is a factor that is often forgotten that regards small ships: the roll period. A small stable boat has a roll period of around 2,3 seconds. This means that, for a wide range of sea conditions, the period of the wave will be greater than that of the boat. The period of a wave of 30 metres is, in fact, 4,5 seconds, the period of a wave of 85 metres is around 7 seconds. With a head or following sea the natural period of the boat is not influenced by the wave period; with an athwart or abeam sea, the meeting period between that of the boat and the wave will also depend on the speed of the wave. When this meeting period, which could be thought of as the time passing between two successive wave impacts, is greater than that of the boat, the latter will tend to jump with the period of the wave. So, if the boat has the same period as the wave, the natural periods of the hull and the wave can become synchronised, causing problems which are much more serious than those resulting from a short period of oscillation not in sync with that of the waves. Small boats, therefore, would find themselves in conditions of forced roll and their oscillations will be governed by the sea rather than by the shape of hull, much more often than the big ones, whose natural roll period is greater in the long run. For many sea conditions, then, the boat with greater r-a (index of static stability) will be more stable than that with less r-a. It is important that stability, in particular transverse stability, is sought through the correct hull proportions, sufficient width and not through the empirical correction of the ballast.

The behaviour of a hull that glides over the sea depends on the following factors: surface and lean of the bottom on the sea, distribution of the pressure on the wet surface, longitudinal position of the centre of pressure and the centre of gravity, longitudinal moment of inertia of the figure of wet underbody and on certain other elements linked to the above factors. When the boat is going fast there must be, obviously, balance between all the horizontal forces, the vertical forces and moments on the longitudinal plane. It happens, however, that a disturbance, even a small one, can alter this balance for a moment. If, for example, the resultant of the dynamic pressure on the bottom passes alternatively from a stern to a bow position of the centre of gravity, the hull is likely to pitch and this pitching is associated with an 'up and down' motion that may develop into a series of cyclic oscillations which are held automatically and which are called 'porpoising' because it imitates the way the porpoises swim.

As readers may have noticed, the position of the centre of gravity is a fundamental element in the design of a gliding hull since this position is linked to the overall behaviour of the boat on the sea both regarding the resistance to motion as well as sailing quality. As we have noted, the centre of buoyancy is the fulcrum through which passes the rotation axis when the boat changes attitude because of embarking, disembarking or a shifting of weight. As a result, the loading of mobile weights (fuel, water and consumer goods) must be done so that the centre of gravity corresponds with the centre of buoyancy of the margin line which is near the boat's centre of gravity.

I hope that this provides some extra help to leisure sailing enthusiasts when choosing a boat.