The object of the present invention is an icebreaking vessel presented in the introduction to claim 1 .

Known icebreaking vessels can have several propellers attached to conventional propeller shafts or to either fixed or azimuthing thruster apparatuses, and engines driving these. High-powered icebreakers can have several propellers in the stern. There may be propellers at the bow of a vessel as well. Azimuthing thruster apparatuses improve the vessel's manoeuvring characteristics and ability to increase the width of the ice channel, for example. The propellers in a thruster can either be pullers or pushers. A single thruster can also have two propellers, which may rotate in opposing directions.

For an icebreaker to create a sufficiently wide channel in the ice, it is a known solution to design a wider bow section of the icebreaker, i.e. equalling the width of the desired channel. The rest of the vessel can then be narrower, which improves the seaworthiness of the icebreaker in open water conditions. It is furthermore a known solution to equip an icebreaker with reamers, i.e. with widened sections intended to break the ice at the edge of the channel even when the vessel proceeds astern. Icebreaking vessels have been presented in publications US4831951 A, US54601 10A and US3931780A.

Similar characteristics can be achieved also in other types of vessels than actual icebreakers by attaching a wider, ice-breaking bow section to the bow of the vessel. If the vessel is reinforced for ice conditions and has sufficient engine power, it can manoeuvre in icy conditions and replace an actual icebreaker, at least if the ice is relatively thin.

A common problem with known icebreaking vessels and icebreakers is, however, that such vessels have difficulties in making turns in compressive ice due to the length of these vessels. An icebreaker in particular should be as agile as possible to be able to move about rapidly in different directions and to effectively assist vessels that have gotten stuck in ice. If an icebreaker also has bow thrusters, in addition to the propellers in the stern, the manoeuvring characteristics of the vessel improve but even so, the icebreaker is not usually capable of sharp turns in compressive ice. The purpose of the present invention is to create a new type of icebreaking vessel that in several respects is more effective and agile in compressive ice than known icebreaking vessels. Another purpose of the invention is to create an icebreaking vessel by attaching a new type of detachable icebreaking bow to a ship.

An icebreaking vessel according to the invention is characterized in what has been presented in the characteristics section of claim 1 .

An icebreaking vessel according to the invention has, at each side, downwards breaking reamers in the midship area, or between the midship and the bow of the vessel. To propulse the vessel, there are propellers in connection with the reamers, which can be attached to the propeller shafts directed towards the stern of the vessel. The propeller shafts can be horizontal or they can be tilted downwards or upwards and, as viewed from above, the propeller shafts can be parallel or they can be directed outwards, towards the sides of the vessel, or inwards, whereby the propeller shafts form an angle relative to one another. The propellers in connection with the reamers can also be attached to either fixed or azimuthing thruster apparatuses. Thruster apparatuses can have one or two pulling propellers ahead of the thruster apparatus, or one or two pusher propellers astern of the thruster apparatus, or at least one propeller ahead of the thruster apparatus and at least one propeller astern of the thruster apparatus. Preferably, the downwards breaking reamers at each side of the vessel and the propellers in connection with them are located in the midship area, or between the midship and the bow of the vessel. The propellers at each side of the vessel are attached in connection with the reamers such that they are located astern of the reamers in the vessel. Downwards breaking reamers have, on the stern side, a inclined icebreaking surface rising astern and upwards, due to which it is preferable to position propellers at the sides of the vessel in this area of the reamer. Because the inclined surface in the reamer is primarily required at the ice to be broken and the propellers are located entirely underwater, the attachment point for a propeller shaft or a fixed or azimuthing thruster apparatus arranged in the reamer does not necessarily have to be inclined, however. This attachment point can be, as adapted to the required joint, also a horizontal or vertical plane surface or an otherwise-aligned area of the hull.

It is preferable to position propellers at the sides of the vessel such that they are located as close to the outboard edge of the vessel as possible, but inside the outboard edge, however, such that the propellers do not touch the pier side while the vessel is moored. Fixed or detachable fenders can naturally be added to the side of the vessel, however, to protect the propellers, which can then be positioned even very close to the side of the vessel. Due to the shape of the vessel hull or other reasons, propellers may in some cases be positioned also significantly closer to midship, under the hull of the vessel. In such a case, however, the distance between the propellers becomes smaller, which decreases the torque created by the propellers when turning the vessel. The turning characteristics of the vessel improve when there is a sufficient distance between the propellers in connection with the reamers located on each side.

In some cases, the propellers in connection with the reamers cause the icebreaking vessel to sway slightly from side to side, which may assist the advance of the vessel in ice. The swaying breaks ice at the sides of the vessel, whereby the vessel does not become wedged to a halt in the ice channel.

An icebreaking vessel according to the invention can also be equipped with propellers positioned in connection with reamers at the sides of the vessel only. It is possible to design the engines driving the propeller at the sides with such a high output that the vessel can operate powered by them only. The thrust force and manoeuvring characteristics of an icebreaking vessel according to the invention can, in this way, be adequate without any other propellers. However, other propellers can also be added to the vessel, for example such that there is at least one propeller in the stern of the vessel. In the stern of the vessel, there may be one or several propellers attached to propeller shafts or thruster

apparatuses. An icebreaker equipped with propellers at the sides of the vessel and with at least a single propeller in the stern of the vessel is very effective and capable of agile turns, particularly if the propellers at the sides are driven in opposite directions.

In order to achieve effective icebreaking characteristics for a vessel according to the invention also when turning in severe ice conditions, the inclination of the hull sides relative to the vertical plane in the bow section of the vessel is greater than 10° at the point where the hull sides of the ship touch the ice to be broken. The hull sides of the vessel can in some cases be perpendicular as well, but if more effective icebreaking characteristics are required, particularly when the vessel is making a turn, the inclinations of the hull sides can be designed to be considerably high.

An icebreaking vessel according to the invention can also be arranged such that a ship is attached to a detachable icebreaking bow section with reamers at the sides and propellers in connection with them. The ship can be an icebreaker, and an upgrade of its icebreaking capability is

accomplished by attaching to it a detachable icebreaking bow section. The objective can be, for example, an improvement in the icebreaking capability of the icebreaker or some other reason. The ship does not have to be an icebreaker, however, because the detachable icebreaking bow section can be attached to any type of ship, such as a regular merchant ship or a cargo vessel that is not incapable of breaking ice independently. Such a combination results in an icebreaking vessel. A vessel equipped with a detachable bow according to the invention can be significantly more effective when such a detachable icebreaking bow section with reamers at the sides and propellers in connection with them is attached to the ship. The total engine power of the propellers in connection with the reamers in the detachable icebreaking bow section can be even greater than or equal to the engine power of the propeller or propellers of the ship that is attached to the detachable bow section. Such a solution is particularly advantageous when the aim is to use the vessel in an icebreaking role part of the time and part of the time as suitable for open water conditions. Thus the detachable icebreaking bow section is attached to the ship when icebreaking is necessary or when the ship is headed to ice-covered water areas, and the bow section is disconnected from the ship when the ship is used in open water areas. The draught of the detachable bow section equipped with reamers and propellers can be equal to the draught of the ship. Their draughts can also be non-equal, however. The draught of the detachable bow section can be greater or smaller than the draught of the ship to be attached to it. If the draught of the ship is greater than the draught of the detachable bow, the ship's hull will function as an ice plow which diverts ice to the sides such that it does not reach the stern propeller.

Preferably, the detachable bow also increases the stability of the ship to be attached to it. If the ship is a short tug which is also used for breaking ice, for example, the bow of the ship, due to the short length of the ship, tends to rise too high while breaking ice. In such a case the detachable bow will increase the total length of the ship, whereby the vessel becomes more stable while breaking ice.

The draught and use of space in both the detachable bow section and the ship can be adjusted, for example, by distributing parts of the propulsion system of the detachable icebreaking bow section, for example an electric motor, generator, diesel engine and the fuel tank of the diesel engine, such that some of the said parts are located in the detachable icebreaking bow section and some in the vessel. According to the invention, propellers at the sides of the vessel are highly preferable because they have a high thrust force in normal conditions, i.e. in open water and in light ice. This is because the propellers at the sides of the vessel are in open water, whereby their propeller wash will not hit the hull of the vessel and thereby reduce the thrust. The propeller wash from propellers located at the sides will also not hit the propellers in the stern of the vessel, which would reduce the thrust from the stern propellers.

Known icebreakers, such as the Finnish icebreaker Urho, have two propeller shafts directed forward in the ship bow. Thus the propeller wash from these propellers hits the ship bow, which reduces the thrust from the propellers. Their propeller wash also hits the stern propellers, reducing the thrust from the stern propellers as well. Similar

disadvantages also result from a vessel having a single azimuthing thruster apparatus, for example, located in the middle in the bow of the vessel.

Propellers located in the bow of a vessel also limit the optimisation of the bow shape. Due to the propellers in the bow, the bow cannot be shaped optimally for breaking ice only, and thus the bow shape of the icebreaker becomes a compromise. Because in a solution according to the invention, there are no propellers in the bow of the icebreaking vessel, the bow shaped can be freely optimised in the best possible manner in terms of breaking ice.

In a solution according to the invention, there are no propellers in the bow of the vessel. The propellers are at the sides of the vessel. The propellers on the sides of a vessel according to the invention are sufficiently close to the bow, however, to be able to function effectively when encountering ice ridges and heavy channels, and particularly at the sides of bow, which is the critical area. Positioning the propellers at the sides of the vessel, in connection with icebreaking reamers, accomplishes a great benefit in that, in a heavy channel, a ridge and in a compressive ice field, propellers close to the vessel sides eat up ice, break down the ridge and flush the sides of the vessel. Propellers at the sides of a vessel will function effectively even when they are attached to conventional backwards directed propeller shafts and do not point forward.

Propellers at the sides of a vessel decrease the resistance of the vessel, make the channel wider, suck down the ice, and blow away ice blocks. Propellers positioned at the sides also make it easier to widen the ice channel by means of thruster apparatuses in the stern or the rudders, because the propellers at the sides have broken the ice at the edges of the ice channel and the ridge, which are the most taxing areas to clear for an icebreaker. Also heeling the ship is very effective in this way.

Steeply sloped sides are possible in the optimisation of the bow shape because it is the propellers that break up the mass of ice at the sides, not the hull. Propellers at the sides of a vessel jointly with the bow shape and a narrow stern make the turns and heeling of a vessel according to the invention more effective than with a ship equipped with normal reamers. When manoeuvring the vessel, the propellers at the sides can be rotated in opposite directions, whereby the large distance between the propellers creates a large torque resulting in a turning action on the vessel. Simultaneously, the sloped sides of the vessel enable the bow to break ice sideways when carrying out a turning manoeuvre. Thus turning a vessel according to the invention even in place in severe ice conditions can be readily accomplished.

The propeller wash from the propellers at the sides of the vessel also effectively suck down ice blocks from under the hull of the vessel to the sides, where there is clear space. This way, only few ice blocks end up at the stern propellers and their thrust force remains at a good level. Even nozzles and a normal diesel-powered system can be used in the stern propellers of a vessel according to the invention because the ice torques are low in the stern when driving ahead.

Due to what was stated above, any combination of propeller systems can be installed in the stern of a vessel according to the invention. The propulsion equipment in the stern can be, for example, 1 to 3 azimuthing or fixed thruster apparatuses or a single or several combinations of a regular propeller shaft, propeller and rudder.

Driving ahead and manoeuvring a vessel according to the invention can be effectively accomplished even if it has regular propeller shafts and rudders in the stern. Only the vessel's capability to proceed astern, to manoeuvre and to spread the channel are improved if the vessel has azimuthing thruster apparatuses in the stern. To enable heeling, also a normal ducted propeller can be used in the stern, in addition to regular propulsion, because the shroud will collect only a little ice.

According to the invention, the conventional propeller arrangement of the propellers at the sides is preferably suitable for detachable bow solutions because the solution is inexpensive compared to thruster apparatuses. The reamer area of the vessel can also be equipped with azimuthing or fixed thruster apparatuses. The additional benefit to be had from them is rather small, however, because regular propeller shafts work very well in the manner described above.

An icebreaking vessel according to the invention can also be such that the vessel has propellers in connection with the reamers only, preferably at the widest point of the vessel. In such a case the vessel has no other propellers in the stern or bow. It is preferable according to the invention that the bow section of the icebreaking vessel is as short as possible. The draught of the bow section of the vessel can be equal to the draught of the stern section of the vessel, but the draught of the bow section can also be greater or smaller than the draught of the stern section. The frame angles and vertical angles in the bow of the vessel are, particularly in icebreakers, preferably small, i.e. the bow is shaped flat. It is also preferable that the sides of the bow are heavily inclined and that the bottom rises towards the sides. However, the side can in some cases also be vertical and the bottom can be horizontal. The wide bow section is followed by a reamer breaking heavily downwards in the midship section of the vessel, in connection with which there is a propeller at both sides of the vessel. On the stern side of the reamer, the waterline of the vessel is narrower and generally narrows down towards the stern.

A propeller in connection with the reamer has a large propeller clearance, whereby broken ice blocks will not get wedged between the propeller and the hull. Most preferably, the propeller shafts are angled outwards and downwards. Such an arrangement is preferable particularly when the shaft of a propeller in connection the reamer is coupled in the conventional manner with an engine located in the vessel. The propellers in connection with the reamers can also be attached to fixed or azimuthing thruster apparatuses. The propeller shafts of both thruster apparatuses can then be separately pivoted either entirely freely or under limitations.

The shape and rather shallow draught of the bow divert the ice blocks also under the bottom of the bow section, which is preferred, however, in a solution according to the invention. It is not advisable to install a plow or a similar apparatus under the bottom because it would divert more ice blocks to the propellers in connection with reamers. In a uniform ice sheet and in light channels, the side propellers receive little ice only, which is preferable because it is not useful to expend energy breaking up the ice blocks.

The propeller wash from the propellers in connection with the reamers, located at the sides of the vessel, sucks up ice from under the bottom to the sides, whereby the propellers in the stern preferably only come into contact with a small amount of ice blocks while proceeding ahead.

The propeller wash becomes more powerful if the draught of the stern section of the vessel is smaller, whereby the suction of outboard turning propellers at the sides of the vessel is directed at a suitable height relative to the bottom of the stern section of the vessel.

Propellers turn outboard when in a propeller under the vessel, a propeller blade directed towards the bottom of the vessel moves towards the side of the vessel. According to the invention, the ratio between the power rating of the propellers at the sides of the vessel and the total power rating of the vessel can vary. If the vessel has no other propellers than the propellers in connection with the reamers at the sides of the vessel, their power rating equals the total power rating of the vessel. If there is at least a single propeller in the stern of the vessel, the total power rating of the propellers at the sides of the vessel can, for example, amount to roughly half of the total power rating of the vessel. In such a case, the impact of ice on the speed remains relatively small even in a uniform ice sheet and in light channels. The ratio between these power ratings can also differ from what is presented above, however. Correspondingly, if an icebreaking vessel has been arranged by attaching a detachable bow to a ship that is not an icebreaker, the power rating of the propellers at the sides of the detachable bow, in connection with the reamers can, for example, roughly equal the total power rating of the propellers in the ship. Even this ratio between the power ratings can also differ from what is presented above, however. In a heavy channel and in ice ridges, the purpose of the propellers at the sides of the vessel is to break up the mass of ice, such ice off from the ridge and break up the ridge while at the same time flushing the sides and blowing the ice mass astern from the ridge. This, combined with the thruster apparatuses in the stern or with turning the rudders to the sides and back, will widen the opening in the ice ridge and break up the ice ridge.

Heeling can be accomplished effectively because, according to the invention, the bow section of the vessel is wider and has a round shape. This gives more space for turning the stern section of the vessel. Similarly, because the propellers at the sides of the vessel break the edges of the ridge, heeling can be accomplished very effectively, even better than with known multi-purpose icebreakers. When surging into a large ice ridge, the bow penetrates deep enough such that the impact of the propellers at the sides of the vessel and heeling make it possible to directly penetrate the ice ridge at a slow speed. This also means that a penetration of an ice ridge is possible even when the displacement of the vessel is small. The relatively shallow draught and the low mid section area coefficient in the bow generally decrease the resistance, i.e. the volume of ice mass to be displaced is relatively small.

When proceeding astern with an icebreaking vessel according to the invention in a uniform ice sheet and in a channel, the propellers in connection with the reamers come into contact with very little ice. Thus it is not disadvantageous to have the propellers attached to propeller shafts and to include propeller skegs.

In a compressive ice field, a wide bow section of the vessel is preferable because it creates a space in the ice channel before the channel closes up and the ice presses against the sides of the ship. The bow section of the vessel only has a short straight portion and heavily inclined sides. The propellers at the sides break up the pressing mass of ice and lubricate the sides. Thus, the performance in a compressive ice field is relatively high even at a low propulsion power.

The broad bow section creates space for turning the vessel. Similarly, a channel wider than the hull of the vessel stern section, created in an uniform ice field, helps to

accomplish this. Additionally the heavily inclined sides of the bow section, small length and the short parallel straight portions of the sides result in having also the bow section break ice sideways when the propellers at the sides are driven in opposite directions or at unequal speeds. In such a case, the propeller wash from the side driven astern also breaks up ice and lifts water on top of the ice. Thus the vessel is highly capable of making turns at a small radius in a uniform ice field, both bow-first and stern-first.

When towing a ship in the towing notch, the great distance between the propellers at the sides provides an advantageous manoeuvring torque compared with traditional icebreakers, which do not have this type of solution. Even though the front portion of the bow resists a turn to the side, the side propeller apparatuses in connection with the reamers lighten up the edges of a heavy channel and an ice ridge and flush the sides of the vessel.

When using a detachable bow to create an icebreaking vessel, the thruster vessel can also be a conventional propeller-rudder ship with a clearly shallower draught than a regular icebreaker. In such a case, the draught of the ship can be smaller than the draught of the detachable bow section.

The propulsion power of the propellers positioned in the detachable bow can be smaller than, equal to, or greater than the output of the propulsion equipment in the actual vessel. If electric motors driven by diesel generators are used as engine systems in the vessel and/or in the detachable bow, the various parts of the engine systems can be distributed in several various ways in the vessel and in the detachable bow section. The propeller is rotated by an electric motor coupled with it, but the electricity required by the electric motor can be transmitted by cables from a more distant location. This way the electric generators required for electricity generation, diesel engines and fuel tanks for the diesel engines can be distributed as required such that any of these components are either in the vessel or in the detachable bow section. In the following, the invention is described using examples with reference to the appended drawings, in which

LIST OF FIGURES

Fig 1 A shows a side view of an icebreaking vessel according to the invention, Fig 1 B shows a schematic view of the vessel in Fig. 1 A as seen from below, Fig 2A shows a side view of a ship to which a detachable bow section can be

attached.

Fig 2B shows the vessel in Fig. 2A as seen from below.

Fig 3A shows a side view of the detachable icebreaking bow section.

Fig 3B shows a schematic view of the detachable bow section in Fig. 3A as seen from below.

Fig 4A shows a side view of the vessel in Fig. 2A and the detachable bow section in

Fig. 3A as attached to one another.

Fig 4B shows a schematic view of the vessel in Fig. 2B and the detachable bow

section in Fig. 3B as attached to one another and seen from below. Fig. 5A corresponds to Fig. 1 A and shows another embodiment of the icebreaking vessel.

Fig. 5B shows the vessel in Fig. 5A as seen from below.

Fig. 6A shows a side view of a ship to which a detachable bow section can be

attached.

Fig. 6B shows the ship in Fig. 6A as seen from below.

Fig. 7 A shows another embodiment of the detachable icebreaking bow section.

Fig. 7B shows the detachable bow section in Fig. 7A as seen from below.

Fig. 8A shows the ship in Fig. 6A and the detachable bow section in Fig. 7A as

attached to one another.

Fig. 8B shows the ship in Fig. 6B and the detachable bow section in Fig. 7B as

attached to one another.

Fig. 9A shows a side view of the vessel in Fig. 8A where the propellers of the

detachable bow section are in a pivoted position.

Fig. 9B shows the vessel in Fig. 9A as seen from below.

Fig. 10A shows a schematic back view of the vessel in Fig. 8A.

Fig. 10B shows a schematic back view of the vessel in Fig. 9A.

DESCRIPTION OF THE FIGURES

Figures 1 A and 1 B show an icebreaking vessel 10 according to the invention. In this example, the icebreaking vessel 10 was built to primarily function as an icebreaker, as it has a wide bow section 20 to open up a sufficiently wide lane or channel in the ice for the ships to be assisted. The stern section 21 is narrower than the bow section 20, whereby the vessel is suitable, due to its seagoing characteristics, also for use in open water conditions. Between the wide bow section 20 and the narrower stern section 21 of the vessel 10, on each side of the vessel 10 there are downwards breaking reamers 23a and 23b with inclined surfaces rising astern and up, which break up the edges of the ice lane when the icebreaker proceeds stern ahead.

According to the invention, propellers 24a and 24b are attached in connection with the reamers 23a and 23b at each side of the icebreaking ship 10 shown in Figures 1 A and 1 B. In this example, the propellers 24a and 24b are installed in the conventional manner using propeller shafts 25a and 25b directed towards the stern section of the ship. Fig. 1 B is shown schematically such that it also shows the locations of motors 26a and 26b and the directions of the propeller shafts 25a and 25b. Most preferably, there is space available for the propellers 24a and 24b in connection with the reamers and their motors 26a and 26b in the bow area, where the hull of the vessel is narrower, but they can also be positioned in the midship section of the vessel if the hull shape of the vessel allows this. If fixed shaft struts are used in connection with the propellers 24a and 24b, they are most preferably tilted 3°-10° in the transverse direction of the vessel.

Fig. 1 B shows that the propeller shafts 25a and 25b are not parallel. They are directed slightly outwards, towards the side of the vessel, such that the propeller shafts 25a and 25b together form a small angle. However, the propeller shafts 25a and 25b can also be parallel or tilted inwards, but the arrangement shown in Fig. 1 B allows better fitting of the motors 26a and 26b driving the propeller shafts 25a and 25b into the hull of the vessel 10. Such an angle between the propeller shafts 25a and 25b is also preferable in some icebreaking conditions. The propellers 24a and 24b in connection with the reamers of the vessel can also be attached to other types of propulsion equipment, such as azimuthing thruster apparatuses, as described below.

The vessel 10 shown in Figures 1 A and 1 B has, in its stern, an azimuthing thruster apparatus 27 and a propeller 24c, whereby all three propellers 24a, 24b and 24c can be used for propelling and manoeuvring the vessel 10. A very effective solution for any type of manoeuvring is to enable the thruster apparatus 27 to pivot a full 360° circle and to enable both the propellers 24a and 24b at the sides to be used separately to propulse the ship ahead or astern. Thanks to the bow shape of the vessel 10, inclined side shells in the bow and the propellers 24a and 24b at the sides, rotating in opposing directions, the vessel 10 is capable of making a turn in place even in severe ice conditions.

Figures 2A and 2B show a ship 30 that can be modified into an icebreaking vessel by means of a detachable bow section 31 shown in Figures 3A and 3B. In other respects, the ship 30 does not have to be in any way specialised, and it can indeed be a regular merchant or cargo ship. However, the ship must have joint members for attaching the detachable bow section to it. The joint members are not shown in the drawings. The ship can also be ice-reinforced for the most severe conditions.

When a detachable bow section is used, both the ship and the detachable bow section can be optimised for entirely different conditions. The ship can be shaped and optimised for use in open water, whereby its hull does not necessarily have to be reinforced for ice. The detachable bow section can be shaped and reinforced to operate as an icebreaker. Thus building and operating the vessel and the detachable bow section becomes more economical. Using a reinforced icebreaking vessel in open water would not be economical because an icebreaker has to break up a wide channel. An icebreaker would make a poor cargo vessel.

Attaching a detachable bow section to a vessel designed for open water does not pose a problem because the mechanical joint members can be made reliable using relatively simple structures. The joint members can also be made such that they allow variation in the draught of the vessel even if there is no change in the draught of the detachable bow section. Such a solution can be preferable when a vessel equipped with a detachable bow section visits a port to unload cargo and returns with the detachable bow section without any cargo.

When any merchant ships or cargo ships have joint members for attaching a detachable bow section to the ship, any merchant ship or cargo ship can chosen from a group of two or several ships, and a detachable icebreaking bow section can be attached to it. In such a case it can be arranged that at least two merchant ships or cargo ships alternate to form an icebreaking vessel, each ship having joint members for attaching a detachable icebreaking bow section equipped with reamers and propellers in connection with them in turns to these ships.

According to the invention, a ship proceeding through the Northeast Passage or a cargo vessel entering a frozen Bothnian Bay, for example, does not require support from an icebreaker because the detachable icebreaking bow section can be attached to the cargo vessel before the ship departs for the ice-covered area. When the ship arrives in open water the detachable bow section can be detached and attached to another ship departing for an ice-covered area. It is much more preferable to maintain icebreaking bow sections that are available as required than to always provide an icebreaker to assist the ship. The detachable bow section 31 attachable to the ship 30 shown in Figures 3A and 3B corresponds with the bow section of an icebreaker shown in Figures 1 A and 1 B, which is wide enough to open up a channel in ice. The detachable bow section 31 is also substantially wider than the ship 30 to which it is attached. Figures 3A and 3B show that the detachable bow section 31 has reamers 23a and 23b and propellers 24a and 24b in connection with the reamers 23a and 23b, similarly to the vessel 10 shown in Figures 1 A and 1 B. Figures 4A and 4B show the ship 30 shown in Figures 2A and 2B, and the detachable bow section 31 shown in Figures 3A and 3B as attached to one another. Figures 4A and 4B illustrate that the ship 30 and the detachable bow section 31 form, as attached to one another, an icebreaking vessel 10 corresponding to what is shown in Figures 1A and 1 B. If the strengths and engine outputs of the ship 30 and the detachable bow section 31 are sufficient, this icebreaking combination vessel 10 is capable of tasks comparable to those of the icebreaking vessel 10 shown in Figures 1 A and 1 B. It can also be equally effective and agile while manoeuvring in ice. Figures 5A and 5B show an icebreaking vessel 10 corresponding to the vessel in Figures 1 A and 1 B. In the vessel 10 shown in Figures 5A and 5B, however, the propellers 24a and 24b in connection with the reamers 23a and 23b are positioned in azimuthing thruster apparatuses 28a and 28b. Even though traditional fixed propeller shafts work very well in a vessel according to the invention, azimuthing thruster apparatuses do have some advantages over fixed shafts.

An advantage of thruster apparatuses is that they leave a larger space for pieces of ice compared with fixed shafts. In some cases pieces of ice may become wedged between the hull and the strut of a fixed shaft. In a thruster apparatus, the propeller shaft can also be directed upwards, whereby the propeller pushes water and pieces of ice upwards along the side of the hull. If a propeller is positioned ahead of the thruster apparatus, the propeller of such a pulling thruster apparatus operates in a clear water flow with no disturbance due to turbulences from the pod of the thruster apparatus. The propeller of a pulling thruster apparatus also effectively breaks up pieces of ice, whereby they will not become wedged anywhere.

According to the invention, all possible propeller configurations can be used in the thruster apparatuses positioned in connection with the reamers. There can thus be a single or two pulling propellers ahead of the thruster apparatus, there can a single or two pushing propellers astern of the thruster apparatus, and a single or two propellers both ahead and astern of the thruster apparatus.

Figures 6A, 6B, 7A and 7B show a ship 30 to which can be attached a detachable bow section 31 according to the invention. The detachable bow section 31 has in connection with reamers 23a and 23b azimuthing thruster apparatuses 28a and 28b. Figures 8A and 8B show the ship 30 and the detachable bow section 31 as attached to one another, which creates an effective icebreaking vessel 10 according to the invention. Figures 9A and 9B show a schematic view of an icebreaking vessel 10 according to the invention where both thruster apparatuses 28a and 28b in connection with reamers 23a and 23b are pivoted outwards, i.e. to the sides. As the vessel 10 proceeds ahead and breaks ice, the pulling thruster apparatuses 28a and 28b at the sides of the vessel 10 can be pivoted slightly outwards. Thereby the flow of water created by propellers 24a and 24b is directed to the edge of ice at the sides of the vessel 10, breaking it up, as a

consequence of which the ice channel broken by the vessel 10 becomes wider. In such a situation, the pivot angle required for the thruster apparatuses 28a and 28b does not necessarily have to be large. Due to the characteristics of the thruster apparatuses 28a and 28b, however, both propellers 24a and 24b can be directed, either simultaneously or independently of each other, also directly to the side and even ahead as required. By using various angles and power settings of the thruster apparatuses 28a and 28b, it is possible to have the vessel either widen up the ice channel, proceed in any required direction or turn around in place.

Fig. 10A shows a schematic back view of the icebreaking vessel 10 in Fig. 8A wherein a detachable bow 31 according to the invention is attached to a ship 30. In connection with the reamers 24a and 24b at the sides of the detachable bow 31 , there are azimuthing thruster apparatuses 28a and 28b equipped with propellers 24a and 24b. Fig. 10A clearly illustrates that, thanks to the detachable bow 31 , the bow section of the vessel 10 becomes relatively wide, whereby the vessel 10 is capable of breaking up a wide ice channel. Fig. 10A furthermore shows that the distance between the propellers 24a and 24b in connection with the reamers 23a and 23b located at the sides of the detachable bow 31 on the transverse direction of the vessel 10 is large. As a consequence of this, the thruster apparatuses 28a and 28b located far apart from each other form a large torque axis, by means of which it is possible to create a very large torque resulting in a turning action on the vessel 10. When the power settings of the thruster apparatuses 28a and 28b or corresponding fixed propellers are adjusted independently of each other, or the propellers 24a and 24b are rotated in different directions, the vessel 10 will easily turn as required, thanks to the large torque, or proceed in the required direction.

Fig. 10B shows a situation where the thruster apparatuses 28a and 28b of the vessel in Fig. 10A are pivoted towards the sides of the vessel 10. Even a small pivot angle of the thruster apparatuses 28a and 28b will direct the flow of water towards the edges of ice on the sides, whereby the ice channel widens up. A further advantage of the thruster apparatuses 28a and 28b, in addition to easily manoeuvring turns, is that the thruster apparatuses 28a and 28b can be tilted both in the longitudinal and transverse direction of the vessel 10, whereby the flow of water under the hull is directed along the hull shape. A preferred tilt of 3°-10° relative to the vertical plane of the thruster apparatuses 28a and 28b, for example, will pivot the propeller shafts for an equal distance, whereby the propellers 24a and 24b will direct the flow of water upwards along the hull shell. This has an advantageous effect on an icebreaking vessel because, in addition to the widening of the ice channel, the flow of water will push away the pieces of ice from under the hull and the sides. LIST OF REFERENCE NUMBERS

10 Icebreaking vessel

20 Bow section

21 Stern section

23a Reamer

23b Reamer

24a Propeller

24b Propeller

24c Propeller

25a Propeller shaft

25b Propeller shaft

26a Motor

26b Motor

27 Thruster apparatus

28a Thruster apparatus

28b Thruster apparatus

30 Ship

31 Detachable bow section