SUPERYACHT #497
September 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 SEMI-GLIDING
AND/OR SEMI-DISPLACING BOTTOM

We have already discussed the 'displacing' bottom, i.e. one which floats and whose dynamic lift is provided entirely by positive buoyancy, and the 'gliding' bottom, where dynamic lift, relative to speed, is provided for the most part by the dynamic thrust generated on the bottom as a result of its inclination and, for the small amount left over, by positive buoyancy.

When optimised, the ideal form of a displacing bottom is, obviously, round because this form offers less total resistance and reaches the maximum speed allowed by all the hydrodynamic coefficients relating to the form of bottom.

Still optimising, the form of a gliding bottom is a chine bottom. Remember that a bottom is gliding when the sides are not wet, i.e. when the speed of the water under the bottom is sufficient to push the flow away from the sides.

These two forms of bottom, then, cater for two different ways of sailing: one is suited to moderate speeds, the other to high speeds. It often happens that, for operating necessities, the customer asks for a speed which, in relation to boat length and displacement, can be achieved neither by a round nor a chine bottom. In this case, the factors of completing dimensions, displacement and speed will lead to the choice of a hybrid or forced bottom, deriving either from a round or chine bottom.

This will be a semi-displacing or semi-gliding bottom.

Since the semi-displacing bottom, with its round bottom, is consequently faster, it will have an undulating form which makes it considerably stern-heavy. The patterns of pressure and depression created under the bottom, with their negative effect on resistance, must therefore be modified. Since the greatest depression is astern, the forms of the stern must be modified so as to increase vertical thrust until the hull achieves a horizontal trim at the desired speed. The forms of the bow will, of course, also have to be modified according to the use of the boat, in order to tackle the waves comfortably and safely at the highest speed possible. Thus the prow of a semi-displacing bottom will be more pointed, while the stern will be flatter. The bilge keel, often chine, will have a small radius so as to achieve greater vertical thrust and an isocarenic trim while sailing. The semi-gliding hull, on the other hand, derives from a chine bottom. These bottoms move at such speed, in relation to length, width, angle of lift of the bottom and displacement, that the water is not pushed away from the sides.

That is, this bottom sails in a channel of water which hugs the sides to varying degrees so that resistance is increased.

Since, from the point of view of hydrodynamics, this bottom is not perfect, i.e. it resembles a gliding bottom in the first stages of gliding and is therefore stern-heavy, it too, like the semi-displacing bottom, requires a more load-bearing stern form in order to achieve a horizontal trim when sailing. Flaps are necessary for this bottom, which would otherwise be less powerful.

From the above it would seem - at least from a hydrodynamic point of view - that these bottoms are unsatisfactory.

However, as the fruit of research aiming to satisfy precise functional requirements, these bottom forms need to be studied even more in depth to find the hydrodynamic coefficients which are certainly beyond or on the limits of the field of use of systematic bottoms.

The principal components of total resistance for semi-displacing bottoms, as for round bottoms, are frictional or viscosity resistance or wave resistance.

Viscosity resistance depends on the wet surface and the length and speed of the boat, while the residual resistance is either calculated by a systematic series such as NPL, series 63 and 64 (see NA 490) or by the statistical approach in which a regression analysis is conducted on a sample of non-systematic bottoms. The most important characteristic of a systematic series is that a considerable number of 'similar' bottoms are obtained from a mother bottom in which there is an ideal compromise between the geometric parameters and resistance to motion.

With the systematic series, the further we get from the mother bottom the further removed we are from the ideal. However, with the equations of regression it is possible to obtain for each case the ideal bottom from those already made provided that they are kept within the limits of the applicability of the equations themselves.

The very forms of the semi-gliding bottom allow it to exploit greater dynamic lift and to reach higher speeds than the semi-displacing bottom. Furthermore, unlike the semi-displacing hull, the semi-gliding hull, with its chine bottom, (principal dimensions being equal and in certain sea conditions), has smaller angles of pitching and yaw, and with the right prow, fewer accelerations.

These features of the semi-gliding bottom have aroused interest in the search for a bottom for large tonnage craft which have to reach high speeds such as the fast ferries transporting vehicles and passengers.

The navy is interested in this latter bottom but with the rapid development of weapons, speed has fallen into second place.

This form of bottom used for large tonnage craft is called "DEEP-VEE".