Ice breaker vessel

An ice breaker vessel and a method for increasing maneuverability and protecting the propellers against broken ice when going backwards. The ice breaker has two stern propellers and a steering rudder aligned with each propeller shaft. Each of the rudders is individually rotatably mounted on a vertical rudder stock adjacent half the cord of the rubber blade profile length, and connected to a rudder gear, so that they e.g. may converge or diverge in the direction backwards, in abutment with end stops positioned on the stern of the ice breaker vessel, when the rudders are in their respective end positions. The method comprises turning the steering rudders in opposite directions until they cooperate and for backwards converging rudder surfaces.

FIELD OF THE INVENTION 
The present invention relates to an ice breaker vessel and a method for 
increasing maneuverability and protecting the propellers against broken 
ice when going backwards, wherein said ice breaker has two propeller 
shafts and a steering rudder aligned with each shaft, each of said rudders 
being rotatable about a vertical axis. 
BACKGROUND OF THE INVENTION 
When assisting a ship which no longer is able to progress through the ice, 
e.g. as a result of an ice ridge, an ice breaker vessel has to maneuver in 
along side the ship, break through the ice ridge and turn in immediately 
in front of said ship. The maneuver resembles a close "overtake". Often 
the ice breaker vessel subsequently has to back up towards the ship that 
needs assistance, in order to transmit a tow line. Ice breaking vessels 
are normally equipped with two propellers and a steering rudder in the 
longitudinal direction of each shaft. During backing maneuvers, the 
rudders and propellers are especially vulnerable to the impact of broken 
ice. In order to reduce the stress on the rudders, ice breaker vessels 
normally are equipped with ice wedges, which are mounted at the bottom 
skin, in the longitudinal direction of each propeller shaft, behind the 
rudder. Damages on the rudders may be avoided by positioning the rudders 
in parallel with the centerline of the ship. The propellers will 
nevertheless be worn by contact with broken ice. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a method and means for 
reducing the amount of broken ice that may hit the propellers and rudders 
when going astern with an ice breaker vessel which is equipped with two 
propellers. The invention also aim for enhanced maneuverability for these 
vessels. 
For this object, the method according to the invention comprise turning the 
steering rudders in opposite directions until they cooperate and form 
backwards converging rudder surfaces, at end positions being defined by 
end stop means. 
Preferably the end stop means permit the rudders to be rotated until they 
converge backwards at an angle of about 40.degree.. 
Preferably the end stop means permit the rudders to be rotated until they 
diverge backwards at an angle of about 70.degree.. 
The ice breaker vessel according to invention comprise two stern propellers 
and a steering rudder aligned with each propeller shaft, said rudders 
being individually rotatably mounted on a vertical rudder stock adjacent 
half the cord of the rudder blade profile length, and connected to a 
rudder gear, so that they e.g. may converge or diverge in the direction 
backwards, in abutment with end stop means positioned on the stern of the 
ice breaker vessel, when the rudders are in their respective end 
positions. 
Preferably, when in the backwards converging rudder position, each steering 
rudder abuts the respective end stop means with a stern surface. 
Preferably, each steering rudder abuts the respective end stop means with a 
frontal surface, when in the backwards diverging rudder position. 
According to one preferable embodiment of the invention, each end stop 
means is mounted diagonally behind and transversely inside of the 
respective rudder stock. 
Preferably the upper part of the rudders, adjacent the bottom skin, is 
widened and has an aperture for the end stop means, so that they, in the 
backwards converging rudder position, cooperate and form diagonally 
backwards-inward directed ice wedges. 
Preferably the steering system of the ice breaking vessel is provided with 
means for automatically rotating of the steering rudders to the rearward 
converging rudder position, when the thrusts of the propellers are 
reversed for going astern.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The stem, the midships section and the stern are designated with the 
reference numbers 10, 11 and 12 respectively. The vessel is equipped with 
two shrouded propellers 13 and vertically rotatably journalled steering 
rudders 15, each of which is mounted in the extension of the respective 
propeller shaft axis. The propeller shafts are carried through skegs 16 
which protrude from the stern 12. An ice wedge 18 is mounted adjacent the 
bottom skin 17 of the vessel, diagonally behind each rudder, said wedges 
are more clearly seen in FIG. 2 and 3 of the drawings. 
FIG. 2 shows how the rudder stocks 19 are mounted in the extension of the 
propeller shaft axis. The ice wedges 18, however, are mounted so that they 
are displaced towards midships. Immediately in front of the ice wedges, 
are end stops 20 provided for the rudders 15. In the rudder position shown 
in FIG. 2 the vessel performs a turn to port with maximal rudder angle. In 
this case the port rudder end stop 20a permits the port rudder 15a to take 
a "toe-in" angle of 70.degree., while the end stop 20b, allows the 
starboard rudder to take a "toe-out" angle of 40.degree.. Corresponding 
angles are of course possible for a turn to starboard. The upper part 15c 
of each rudder is widened in the area of the respective ice wedge 18 and 
end stop 20. 
FIG. 3 shows the rudders in position for going astern. In this case both 
rudders 15a and 15b take the position of maximum "toe-out", i.e. a 
40.degree. angle to the respective end stop. The result of this is that 
the entire backwardly projected rudder area forms as deflectors in front 
of the shrouded propellers 13 and protect them from broken ice 21. The 
opposite rudder position, i.e. with the rudders in 70.degree. angle to the 
centerline of the ice breaker vessel, may be used for clearing a broken 
channel of broken ice, as the propeller thrusts is diverged to the sides 
by the rudder blades. 
When going astern with the vessel, i.e. with the rudder blades in the 
position of maximum "toe-out", steering is affected by varying the 
rotation speed or pitch of the propellers. This results in a powerful 
deflection of the propeller thrust, if one of the propeller thrusts is 
directed backwards for a short instant, greatly enhancing maneuverability 
compared to conventionally designed sterns. 
In FIG. 1-3 one embodiment of the invention is shown in which the ice 
wedges 18 and the end stops 20 are separate units. FIG. 4-8 shows another 
embodiment of the invention in which the ice wedges and the end stops are 
built together as one unit 22. This results in a stronger and more sturdy 
design. FIG. 4 shows the rudder 15 seen from the direction of the front 
with the journal 23 for the rudder stock 19 in the bottom skin 17, wherein 
the width relations of the upper wider part 15c and the rest of the rudder 
blade is evident. FIG. 5 shows the combined ice wedge and end stop 22 in a 
side view. 
FIG. 6-8 are sections along the line A--A in FIG. 5 seen from the above, 
and show one of the rudders in a neutral position, in a position of 
maximum "toe-out", and in a position of maximum "toe-in", respectively. 
From this it is clear how the upper wide part 15c of the rudder 15 is 
formed with a recess 24 for the end stop 22, who in the cooperating end 
position (see FIG. 7) form an obliquely backwards-inward pointing ice 
wedge. The end stop is provided with machined contact surfaces 25, 
cooperating with the rudder. 
The steering gear controls of the ice breaker are preferably arranged so 
that the steering rudders are automatically rotated to the rearward 
converging rudder position, when the thrusts of the propellers are 
reversed for going astern. Since the rudder stocks 19 are positioned more 
close to the centre of the profile than normally in the field of the art, 
the drag forces on the rear halves of the rudders will be reduced when 
going astern. Besides, since the rudders cooperate with the end tops when 
going astern, forces on the rudder stock journals will be reduced. 
The steering controls 27 are shown schematically in FIG. 1 located in the 
bridge 28 and connected to a steering machine 29. The interconnection of 
the steering controls 27 with the propeller controls is shown 
schematically at 30. 
The invention is not limited to the above described embodiments, but 
several modifications are possible within the scope of the accompanying 
claims. For example, the end stops 20, 22 may be positioned for other 
maximal rudder angles.