The most common form of propulsion for vessels is propeller propulsion.

Independent of the propeller system, in theory the propeller should be located; a) far from the hull in order to avoid vibrations, b) in uniform parallel flow in order to avoid vibrations, cavitation and propeller noise, c) with a view to achieving little pressure reduction, d) in areas with high viscous parallel flow, and e) in such a manner that the greatest possible degree of rotational efficiency is achieved. The propellers should also be located so that the requisite consideration is given to the engine room arrangement, docking or running aground. There are also a number of other factors that influence the propeller system, such as the size of the ship, the desired propulsion and speed, hull design, etc.

A propeller for a vessel is normally driven via a shaft by an engine located in an engine room. Parts of the shaft transmission will be located outside the ideal hull structure of the vessel. Where there is direct shaft transmission from engine to propeller, the shaft may be covered by a bulge in the hull, known in the industry as a skeg. In order to achieve the least possible resistance and disturbances in the flow pattern round the vessel, the skegs are made as small as possible, also for reasons of economy. At the same time the skegs will be designed so as to obtain the best possible flow pattern for the body of water flowing towards the propellers.

A known method for making the skegs as small as possible is to arrange the propeller shaft at an angle relative to a horizontal plane.

The propulsion of a vessel is also highly dependent on the size of the propellers, and it is often the vertical height of the propellers that is limiting for the propeller size, since for most vessels it is not desirable for the propellers to extend lower than the lower point of the hull in case the vessel runs aground. For vessels with a U- shaped hull structure and two propellers, it is known to arrange the propeller shafts at an angle relative to the centre plane of the vessel's hull, with the result that the propellers are located further from the centre plane of the vessel than the engines that drive the propellers.

Where there is no direct shaft between engine and propeller, the propellers can be given the capability of being rotated in several directions. An example of this is illustrated in US 4 493 660.

The flow pattern around the skegs is also vital. US 4 538 537 describes an asymmetrical skeg round a propeller shaft which has a twisted shape in order to

achieve higher efficiency by adapting the flow of water towards the propeller and where the shape of the skeg is such that the tangential components of the flow are distributed in an advantageous manner around the circumference of the whole propeller.

An object of the present invention is to improve the design of the skegs on vessels with at least two propeller shafts with propellers, in order thereby to achieve financial savings in design and operation of the vessel. A second object of the present invention is to be able to exploit the viscous parallel flow to a greater extent than in conventional vessels and reduce the drag by means of the design of the double skeg, the stern and the engine room arrangement. It is also an object to reduce vibration and the risk of cavitation.

These objects are achieved by the features of the invention indicated in the following claim 1, where further details of the invention are indicated in additional claims.

The present invention attempts to solve the preceding objects by means of a vessel with a propulsion system comprising at least one engine and at least two propeller shafts with propellers. The vessel according to the invention has a baseline located in a substantially horizontal base plane touching the bottom of the vessel's hull and a centre line in a substantially vertical centre plane along the centre of the longitudinal extension of the vessel's hull. According to the invention at least two of the propeller shafts are at an angle a relative to the base plane, with the result that the distance of the propeller shaft from the base plane increases in a forward direction from the propeller to the engine. Furthermore, at least one of the propeller shafts is at an angle P relative to the centre plane, with the result that the distance of the propeller shaft from the centre plane increases in a forward direction from the propeller to the engine.

According to the invention the size of the angle a is from and including 0 to 10 degrees, and the angle P is in the range 1-5 degrees, preferably between 1 and 3 degrees. By giving the propeller shaft an angle a, the engine is moved upwards into the vessel's hull when it is a direct extension of the propeller shaft. The engine installation's space requirements are often a limiting factor for the size of the skeg, i. e. the size of the skegs must allow space for the engine installation. By raising the engine installation and the engine, the skegs can be made smaller both in cross- sectional area and longitudinal extension, and a lower increase in resistance is obtained due to the skegs. By giving the propeller shaft an angle (3, slimmer waterlines are obtained at the inner surface of the skegs as well as less variation in the cross-sectional area between the skegs than with conventional parallel propeller shafts. This is advantageous since it results in a decrease in pressure and velocity

variation for the water flow, giving less risk of breakaway with an associated increase in the resistance, vibration, cavitation and propeller noise.

According to the invention the vessel may have more than two propeller shafts. For example, there may be a propeller shaft with a smaller propeller located between the two propellers, where, for example, the propeller shaft for the centre propeller may be at an angle a relative to the base plane, but be substantially parallel to the centre plane. Where there are two intermediate propellers, the two centre propellers, for example, may be parallel to both the base plane and the centre plane while the two outer propellers are at an angle a and an angle P to the base plane and the centre plane respectively.

It may also be envisaged that one of the propeller shafts is at an angle (3 to the centre plane and other shafts are parallel to this shaft.

The invention will now be explained in greater detail by means of an embodiment and with reference to the drawings in which: fig. 1 is a side view of the rear part of a vessel hull with propeller shaft arrangement according to the invention, fig. 2 is a view from below of what is illustrated in fig. 1, fig. 3 is a view of the rear part of the vessel hull illustrated in fig. 1 viewed from the stern towards the propellers, fig. 4 is a view of a cross section along line IV in figs. 1 and 2, fig. 5 is a view of a cross section along line V in figs. 1 and 2, fig. 6 is a view of a cross section along line VI in figs. 1 and 2, fig. 7 is a view of a cross section along line VII in figs. 1 and 2, fig. 8 is a view of a cross section along line VIII in figs. 1 and 2, fig. 9 is a view of a cross section along line IX in figs. 1 and 2.

Figs. 1 and 2 illustrate a side view and a view from below respectively of a rear part of a vessel's 1 hull 2, in which is depicted a propulsion system consisting of two engines 3, two propeller shafts 4 and two propellers 5. The propeller 5 is located with its axis of rotation coincident with the propeller shaft 4 and the engine 3 is located as a direct extension of the propeller shaft 4, at the opposite end of the propeller 5. As illustrated in fig. 1, the vessel 1 has a base line located in a base plane 6 that touches the lower point of the hull 2, and as illustrated in fig. 2 a centre line located in a centre plane 7 of the hull. Round the part of the propeller shaft that

is located outside the hull, in order to cover this while simultaneously containing the propeller shaft, a skeg 8 is provided, round each propeller shaft.

According to the invention, as indicated in fig. 1 the propeller shafts 4 are at an angle a relative to the base plane 6. This angle may be varied from 0 to 10 degrees.

For larger, heavier vessels, this angle is preferably in the range of 1-3 degrees, but for other types of vessel it may well be larger. The choice of the angle a also depends on the type of engine used for propulsion. The angle a may also be envisaged equal to 0 degrees, i. e. the propeller shafts 4 are parallel to the base plane.

When a larger angle a is chosen, the engine 3, when located as an extension of the propeller shaft 4, will be moved upwards in the hull 2 and in some cases located substantially outside the skeg 8. This permits the skeg 8 to be designed with a smaller cross-sectional area in the region around the engine 3, which is advantageous since the volume of the skeg 8 is decreased and thereby the resistance to propulsion of the vessel 1. The alternative to having the propeller shaft 4 at an angle to the base plane 6 is to extend the propeller shaft 4, which is undesirable both with regard to considerations of strength with regard to the torque transmission in the propeller shaft 4 and considerations of space in the vessel 1. At the same time it is not expedient to have the propeller shaft 4 at too large an angle a to the base plane 6 if the propeller is located as an extension of the propeller shaft 4, since too large an angle a will result in too large components of the generation of water velocity in other directions than the propulsion direction of the vessel 1.

The angle P of the propeller shaft 4 may be varied from 1-5 degrees, preferably in the range of 1-3 degrees. As illustrated in fig. 2, both the propeller shafts 4 in this embodiment are at an angle P. For the inner oppositely directed surfaces 9 of the two skegs 8, the waterlines that describe a water particle's flow path along the skegs 6 will be slimmer for a propeller shaft arrangement according to the invention than for a conventional installation. This means that the skegs'8 inner oppositely directed surfaces 9 have a smaller curvature over their longitudinal extension in the fore-and-aft direction than skeg 8 where the propeller shafts 4 are parallel. The angular change for the inner waterline for a skeg 8 round a propeller shaft 4 according to the invention is less and the skegs'8 inner oppositely directed surfaces 9 will be substantially parallel over a larger part of their longitudinal extension, than in conventional skegs. This is advantageous for the flow pattern round the skegs 8.

If the shape of the skegs 8 for the propeller shaft arrangement according to the invention corresponds to the shape of skeg 8 for parallel propeller shafts 4, but are at an angle ß, the waterline along the inner oppositely directed surfaces 9 of the two skegs 8 will be slimmer, while the waterline for the outer surfaces 10 of the skegs 8

will have a larger curvature over its longitudinal extension than in a conventional installation. It is the inner waterlines that are particularly important, so that a larger curvature for the outer surfaces 10 has less influence in a negative direction for propulsion of the vessel 1 than the advantage obtained with slimmer inner waterlines.

Another component for pressure and velocity differences for the flow of water between the skegs is the change in cross-sectional area for the region between the skegs'inner oppositely directed surfaces 9 over the longitudinal extension of the skegs. Both conventional design with parallel propeller shafts 4 and propeller shaft arrangement according to the invention have an increase in the cross-sectional area over the longitudinal extension of the skegs 8, but for the propeller shaft arrangement according to the invention the change may be less. An increase in the cross-sectional area results in a reduction in the velocity of the flow of water between the skegs 8. Less reduction in velocity combined with slimmer waterlines reduce the risk of three-dimensional breakaway or separation. Breakaway results to some extent in a substantial increase in resistance while simultaneously producing parallel flow fluctuations that can result in vibrations, cavitation and propeller noise.

As can be seen in fig. 3 and figures 4-9, the skegs 8 will have a centre plane in their longitudinal extension, which is substantially vertical and substantially parallel to the propeller shaft. This is apparent in fig. 3 where the propellers 5 with the propeller shaft 4 are located closer together than the parts of the skeg 8 located in front viewed in relation to the propulsion direction when the skegs 8 are viewed from the propellers 5.

As can be seen in figs. 4-8, the skegs 8 may be arranged substantially symmetrically about their centre plane through the propeller shaft 4. Round the rear part of the propeller shaft 4 near the propeller 5 the skegs 8 have a small cross-sectional area, as illustrated in fig. 5. This cross-sectional area is gradually and steadily increased in the longitudinal extension of the skegs 8, as illustrated in fig. 6, to the area of the skegs 8 located round the engine 3 and the engine installation, as illustrated in fig.

7. In this area it is the space requirements of the engine installation below the propeller shaft connection to the engine 3 that is the determining factor for the cross-sectional area of the skegs 8. At the front end of the engine installation and the engine 3, i. e. the front part of the skegs 8 viewed in relation to the vessel 1, the skegs 8 are gradually terminated as illustrated in fig. 8 up to the front attachment point between the skegs 8 and the hull 2, where the vessel according to the embodiment has a hull 2 with a flat bottom coincident with the base plane, as illustrated in fig. 9.

If it is more advantageous from the flow point of view, the skegs 8 may also be arranged asymmetrically about their centre plane. One type of consideration that may be taken into account corresponds to that in US 4 538 537 with regard to design of the skegs'8 geometry.

In model experiments with a hull with a propeller shaft arrangement according to the invention, it was found that a combination of the angle (3 and the angle a on the propeller shaft produces less turbulence on the skegs and less drag. This results in less reduction in speed for the vessel and thereby improved energy consumption and possibly higher speed.

The invention has been explained in the above with reference to an embodiment.

The invention is not limited thereby, and a number of variants of the invention may be envisaged within the scope of the invention as it is defined in the following claims. As mentioned, the vessel may have more than two propellers. In the case of two propellers, a first propeller shaft may be at an angle (3 and the second propeller shaft may be parallel to the first. The skegs may be envisaged as a much more integrated part of the vessel's hull, or they may form larger parts of the hull structure, thereby giving the vessel"twin keels". In such cases it may be natural to make the skegs larger and without a centre plane along the propeller shaft. The engine for propulsion of the vessel may be of different types. An engine may be envisaged with power transmission to two propeller shafts, or two or more engines on each propeller shaft. The attachment of the engine to the propeller shaft may also be envisaged at an angle or passing through a transmission.