GB1561124A - Twin hull marine vessels - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO TWIN HULL MARINE VESSELS (71) We, HYDROCONIC LIMITED, a British company of Worting House, Basingstoke, Hampshire, RG23 8PY, do hereby declare this invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to twin hull marine vessels.

Provided twin hull vessels or catamarans are of sufficiently low displacement/length ratio and are operated at sufficiently high speed, they show propulsive power advantages against single or mono hull vessels at the same speed.

It is fundamental that twin hull vessels of the same length and displacement as single hull vessels have appreciably greater wetted surface of the order of 30or" or 40% more.

Thus at slow speeds there is always in such a comparison a disadvantage against the twin hull vessel. However, when speeds reach a level where significant wavemaking arises, the situation can change. If the speed/length ratio is high a considerable proportion of the total resistance of the vessel is produced by wavemaking and this is significantly affected by the displacement/length ratio of the hull. If the displacement is split into two hulls of the same length as a single hull, then the displacement/length ratio is halved and a very considerable saving in the wavemaking resistance is effected.

Furthermore, if the displacement/length ratio is below certain limits, the wavemaking characteristics of a hull become very different from those of a normal displacement hull and the nondimensional wavemaking resistance coefficient can become more or less independent of speed/length ratio at values of that ratio above, say. 1.2. Generally this limit is where the Block Coefficient of the hull is less than about 1.5xl0-3.

Thus at high speed/length ratios a suitably designed load displacement modern twin hull ship may require much less propulsive power than a mono hull at the same speed/length ratio in spite of the higher total displacement caused by the weight of the bridging structure and in spite of the increased wetted surface of the hulls.

However, the wave trains produced by each hull mutually interfere and this causes subsidiary effects making the resistance characteristics of the catamaran to some extent dependent upon the spacing distance apart of the two hulls. The waves between the hulls can coincide to produce an interference crest or they may alternatively produce a reduction in resistance, so that the resistance of the whole is less than twice the independent resistance of the two halves. As a result, it is important, for a given service of the vessel, to derive the correct spacing of the two hulls so as to get a favourable interference effect and this renders design work less flexible than it otherwise might or should be.

Furthermore, the divergent wavetrain itself, although very much smaller than that of a mono hull at the same speed/length ratio, still absorbs a considerable amount of energy. Moreover, there is always a primary wave interference crest where the leading edges of the two bow waves meet between the hulls and this can become very high and embarrassing at high speed/length ratios, and particularly so if the vessel is pitching.

Objects of the invention are to modify and improve the interference wave characteristics between the hulls, rendering the power requirement less sensitive to the spacing of the hulls at a given speed; to reduce the wavemaking of the hulls anyway, thereby decreasing the general level of propulsive power required; and finally and importantly to reduce the interference wave crests between the hulls and hence remove the embarrassment of a high breaking crest between the hulls at high speeds.

The invention may be applied to a double-ended ship; in other words, to a catamaran in which the hulls are symmetrical about amidships such as a double-ended ferry, or alternatively to a single-ended vessel in which the hulls are shaped asymmetrically lengthwise for travel preferentially in one direction. In the case of a double-ended vessel the invention has a subsidiary purpose and advantage in that by supplying a planing surface at the aft end it reduces the tendency of the ship to trim by the stern.

According to the present invention, there is provided a twin hull marine vessel, wherein the bows of the hulls are each fitted on both sides with fixed fins that are either substantially horizontal or normal to the adjacent surface of the respective hull, and at or adjacent and below the static light waterline, being parallel or substantially parallel in the fore and aft direction to the waterline plane, and wherein each fin is located over the forefoot of the respective bow, or over the top of the propeller aperture if the hull is double-ended and has a propeller at the end serving for the time being as the fore end, and extends right up to the stem of the bow or of said fore end as the case may be and for at least 5% of the length of the vessel.

In a double-ended vessel there will be fins at each end of each hull over the top of the propeller aperture if there is one, the forward fins acting to suppress wavemaking at the fore end and the after fins acting as lifting plates tending to suppress the tendency of the vessel to trim by the aft end.

The appearance of the bow or fore end fins is as if, the hull, from a position approximately 5% to 10% of the length of the vessel aft from the stern of the bow or fore end, is provided on both sides with a plate or ledge substantially at or just below the level of the static low waterline and extending forward up to the stem, where the leading edges of the fins are suitably radiused in plan view, this plate or ledge being horizontal fore and aft and either horizontal or downward sloping in an outboard direction. The fins are very similar in appearance, therefore, to the anticavitation fins commonly fitted over outboard propellers. Alternatively, the fins may be swept aft from the leading edges and radiused on the trailing edges.

If each hull has a bow and stern of identical shape, in other words it is double ended, then exactly similar fins will be fitted at both ends. The stern of course becomes the bow when the vessel goes in the opposite direction.

In action these fins are intended to suppress to some extent the bow wave and it has been found that they do so. In fact, the interference waves between the hulls are, in smooth water, virtually totally suppressed, and there is no interference at all but more or less flat, smooth water between the hulls from bow to stern.

On the outboard sides of the two hulls, measurements taken have shown that the divergent wave train produced by the bow is substantially reduced, much as it would be by a bulbous bow which is by comparison an expensive and difficult object to fit to such fine lined vessels. At the stern, the effect of the lift developed by the fins is such that it has been found that the tendency of the vessel to trim by the stern is almost entirely suppressed with beneficial results in terms of resistance.

Measurements of the resistance of the hulls of catamarans fitted with these fins shows that a substantial reduction in the resistance is available at speed/length rations at which appreciable waves are formed normally.

One arrangement according to the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figures 1, 2 and 3 are, respectively, diagrammatic side elevation, plan and end elevation of the bow of one hull fitted with fins according to the invention.

In the arrangement shown, a simple form of fin 11 is fitted to each side of the bow of the hull 12 substantially normal to the adjacent surface of the hull, having a width of the order of a quarter of the beam of the hull, extending between 5% and 10% of the length of the ship from the stem 13 and disposed on a hull waterline. Generally, this waterline is level with the top of where the propeller aperture would be if there were one and just below the static light water line 14. The fins are parallel or substantially parallel in the fore and aft direction to the waterline plane.

If the hulls are of double-ended form, such fins will be fitted at both bow and stern at the top of the propeller aperture in each case. The result of fitting such fins is to suppress entirely in smooth water the wave pattern between the hulls and in particular the interference crest between the hulls.

The effect is also to reduce the size of the divergent bow wave train outboard of each hull and in the case of double-ended ships to reduce the tendency to trim by the aft end.

A further effect and benefit is that the power required to propel the vessel at high speeds is substantially reduced.

WHAT WE CLAIM IS: 1. A twin hull marine vessel, wherein the bows of the hulls are each fitted on both sides with fixed fins that are either substantially horizontal or normal to the adjacent surface of the respective hull, and at or adjacent and below the static light waterline, being parallel or substantially

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. double-ended ferry, or alternatively to a single-ended vessel in which the hulls are shaped asymmetrically lengthwise for travel preferentially in one direction. In the case of a double-ended vessel the invention has a subsidiary purpose and advantage in that by supplying a planing surface at the aft end it reduces the tendency of the ship to trim by the stern. According to the present invention, there is provided a twin hull marine vessel, wherein the bows of the hulls are each fitted on both sides with fixed fins that are either substantially horizontal or normal to the adjacent surface of the respective hull, and at or adjacent and below the static light waterline, being parallel or substantially parallel in the fore and aft direction to the waterline plane, and wherein each fin is located over the forefoot of the respective bow, or over the top of the propeller aperture if the hull is double-ended and has a propeller at the end serving for the time being as the fore end, and extends right up to the stem of the bow or of said fore end as the case may be and for at least 5% of the length of the vessel. In a double-ended vessel there will be fins at each end of each hull over the top of the propeller aperture if there is one, the forward fins acting to suppress wavemaking at the fore end and the after fins acting as lifting plates tending to suppress the tendency of the vessel to trim by the aft end. The appearance of the bow or fore end fins is as if, the hull, from a position approximately 5% to 10% of the length of the vessel aft from the stern of the bow or fore end, is provided on both sides with a plate or ledge substantially at or just below the level of the static low waterline and extending forward up to the stem, where the leading edges of the fins are suitably radiused in plan view, this plate or ledge being horizontal fore and aft and either horizontal or downward sloping in an outboard direction. The fins are very similar in appearance, therefore, to the anticavitation fins commonly fitted over outboard propellers. Alternatively, the fins may be swept aft from the leading edges and radiused on the trailing edges. If each hull has a bow and stern of identical shape, in other words it is double ended, then exactly similar fins will be fitted at both ends. The stern of course becomes the bow when the vessel goes in the opposite direction. In action these fins are intended to suppress to some extent the bow wave and it has been found that they do so. In fact, the interference waves between the hulls are, in smooth water, virtually totally suppressed, and there is no interference at all but more or less flat, smooth water between the hulls from bow to stern. On the outboard sides of the two hulls, measurements taken have shown that the divergent wave train produced by the bow is substantially reduced, much as it would be by a bulbous bow which is by comparison an expensive and difficult object to fit to such fine lined vessels. At the stern, the effect of the lift developed by the fins is such that it has been found that the tendency of the vessel to trim by the stern is almost entirely suppressed with beneficial results in terms of resistance. Measurements of the resistance of the hulls of catamarans fitted with these fins shows that a substantial reduction in the resistance is available at speed/length rations at which appreciable waves are formed normally. One arrangement according to the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figures 1, 2 and 3 are, respectively, diagrammatic side elevation, plan and end elevation of the bow of one hull fitted with fins according to the invention. In the arrangement shown, a simple form of fin 11 is fitted to each side of the bow of the hull 12 substantially normal to the adjacent surface of the hull, having a width of the order of a quarter of the beam of the hull, extending between 5% and 10% of the length of the ship from the stem 13 and disposed on a hull waterline. Generally, this waterline is level with the top of where the propeller aperture would be if there were one and just below the static light water line 14. The fins are parallel or substantially parallel in the fore and aft direction to the waterline plane. If the hulls are of double-ended form, such fins will be fitted at both bow and stern at the top of the propeller aperture in each case. The result of fitting such fins is to suppress entirely in smooth water the wave pattern between the hulls and in particular the interference crest between the hulls. The effect is also to reduce the size of the divergent bow wave train outboard of each hull and in the case of double-ended ships to reduce the tendency to trim by the aft end. A further effect and benefit is that the power required to propel the vessel at high speeds is substantially reduced. WHAT WE CLAIM IS:

1. A twin hull marine vessel, wherein the bows of the hulls are each fitted on both sides with fixed fins that are either substantially horizontal or normal to the adjacent surface of the respective hull, and at or adjacent and below the static light waterline, being parallel or substantially

parallel in the fore and aft direction to the waterline plane, and wherein each fin is located over the forefoot of the respective bow, or over the top of the propeller aperture if the hull is double-ended and has a propeller at the end serving for the time being as the fore end, and extends right up to the stem of the bow or of said fore end as the case may be and for at least 5% of the length of the vessel.

2. A vessel according to claim 1, wherein each fin extends for between 5 /, and 10?' of the length of the vessel.

3. A vessel according to claim I or claim 2, wherein the hulls are double-ended and the fins are fitted at both ends.

4. A vessel according to claim 1 or claim 2 or claim 3, wherein the width of each fin is substantially a quarter the beam of the hull.

5. A twin hull marine vessel having fins fitted at one or both ends of each hull, substantially as described with reference to the accompanying drawings.