Date: 11/7/00 10:19:38 AM Pacific Standard time
From: [email protected] (Gordon Bain (on Aurora)
At 09:52 AM 2/11/2000 -0500, Betty wrote:
I have another question for you, this one from my Father. I was telling him all about your expedition and you, and the ship, and ALL of that last night on the phone. He is a retired engineer who worked for the U.S. Government for years. He is very interested in the ship, and I told him I was going to mail him copies of your journals. Anyway, he wanted to know HOW the ship's propellers (he had a different name and I can't remember what the term was) do not get damaged when you break through the ice. He knows where they are located, but said the "screws" (is that the term) would be hit by blocks of ice once you break through. Once the ship goes up on to the ice and comes down to break it, wouldn't this damage those propellers or screws or whatever?
Now that is a very good question and I think I am going to have a considerable amount of difficulty trying to describe in words the effect of 2m thick fast ice which few of you would have seen, on a ship even fewer of you will have seen, much less traveled on, in an area which few are privileged to travel. OK, lets see what I am like at building some word pictures.
Picture a pregnant goldfish -very rounded and fat in the front but flattening out and tapering quickly to a narrow tail. At the bottom of the throat, below the chin and mouth is a vertical 'knife edge'. Make the whole 4000 tonnes deadweight, 8000 or so tonnes displacement, of steel and bright orange. The hull, or bottom, of this ship is also very different from many other ships in that it doesn't have a prominent keel, but is rather more rounded so that if sideways pressure is applied by ice, the vessel will tend to be pushed upward like a cork rather than be crushed. A disadvantage of the rounded hull is that the ship will "roll on a wet sponge", that is, its performance in the open ocean can be worse than in more conventional hulled ships.
Imagine the ice - stretching in a continuous unbroken surface to the horizons. Try to imagine that ice being 1.8 metres thick (the height of a tallish person). T he ice is strong enough to land large planes on but not strong enough to handle a relatively minor swell.
Think of very powerful ship's engines which together can drive the ship forward at 18 knots in open water. Think of a single propeller or screw near the stern of the ship, and it's much BIGGER than that on an outboard motor. The propeller assembly broadly comprises a hub to which the blades are attached. The hub has a diameter of nearly 2 metres and weighs 19 tonnes. The hub carries all the connections back into the engine spaces on the ship via shafts, pitch servo units and complex hydraulic systems. For the technically minded (I'm taking this from a manual) -The propeller hub contains a hydraulic cylinder yoke which actuates the blades by means of a pin-slot mechanism. A follow-up valve, built on the pitch servo-unit, feeds the cylinder yoke through pipes in the hollow shafting. This follow-up valve is mechanically linked to a servi cylinder, which is controlled by the remote control system." There are four blades attached to the hub - these are made of stainless steel with a mass of 2.9 tonnes each.
The assembly is a four-bladed variable pitch propeller, the blades swivel according to whether the ship is going ahead or astern. It has an overall diameter of 5 metres and a mass of 35 tonnes. The propeller has a nominal power of 10,000 kW at 125 rpm. The propeller shaft is 20 tonnes, three intermediate shafts a total of 45 tonnes, sleeve coupling 3 tonnes.
A bit further back than the propeller is the rudder, which is what allows the ship to change direction when it is moved on its axis. The rudder is a cast one, is spade shaped, 5.3 metres long, 2.6 metres wide at the bottom, close to a metre thick at its fattest point, and surprisingly is hollow to give it a bit of buoyancy in water so as not to put too much weight on its housing. The rotary-vaned system's weight is held by the steering gear and is an integral part of the steering system. Unfortunately I don't have a weight on it probably somewhere between 5 and 10 tonnes.
Apart from being partially sheltered by the drive shaft housing and the shape of the hull the propeller is unguarded - as ice could 'jam' between the propeller and the guard. Also in ice breaking mode, as the power of the engines is increased, there is a decrease in the pitch of the propeller and this reduces the prospect of damage as blocks of ice collide with the propeller.
Now, that description might not be quite right for those technically minded but will give you some feel as to just how strong and powerful the 'undercarriage' of this ship is. The rudder assembly, the propeller and the bow are the strongest parts of this ship. I will have to refer any technical questions to the ship's Chief Engineer!!!
When the ship meets fast ice it cannot push it aside (it's got nowhere to go) - and the front of the ship cannot 'cut' the ice - it has a very blunt front (the goldfish below the mouth). For softer pack ice or ice floes, the ship can rise a bit and use the 'ice knife' to split the ice in front, but more often the blunt bow tends to split the ice and the parts are brushed to the port or starboard sides of the ship.
So, in fast ice, the ships engines drive the ship forward and the curved front starts to rear up on to the ice while still going forward. As more of the ship goes forward more weight comes onto the ice, the weight of the ship causes the ice to bend and then to break and the ship settles a bit deeper into the water while still going forward, rising up and breaking more ice as more weight comes down on it.
As the ice breaks it is sort of under the ship's foot and comes into contact with the 'knife edge' which does some splitting but basically pushes the broken blocks to the right and left sides of the ship's hull - but still under the hull. Some of the broken ice is pushed outwards and rises to the surface under the fast ice off to the sides. Some of the ice close to the side of the ship also breaks off and tips sideways so it is on edge jammed between the side of the ship and the broken ice off to the side. But a lot of the ice doesn't escape those ways and is effectively trapped under the hull. Remember the ship is continuing to move forward. Also the effect of the turning of the propellers is to draw water from the front toward the stern and the chunks of ice with it. Most of the ice tends to fan away from the centre line of the hull. As the chunks go just over halfway down the ship, they meet the part of the ship where the 'pregnant goldfish' part gives way to a narrower bit. The blocks of ice get into this area, rise a bit, get caught up in the rush of water generated by the propeller and are squirted out the back many missing the propeller and rudder. Those that get near the propeller get chunks chewed off and spat out. If there is a lot of ice it tends to also take up the centre line space and these get hit by the propeller and have chunks taken out or otherwise mangled and spat out the back. The churning at the stern of the ship carries the blocks away pretty quickly so they don't concentrate in the area. The rudder is also massive but the way it is mounted is such that any ice that does not slip by can give it very heavy blows without damage to the mounts.
So, it is a combination of hull design, the shape and structure of the bow, the construction, positioning and sheer strength of the propeller and rudder which allow ice to pass down the length of the hill without causing damage except to the ice itself. But it's not to say the ship and its gear never get damaged - there have been various troubles in the past but I am hazy on detail and such incidents were never life-threatening.
I hope I painted enough of a picture for you to understand how this part of the vessel operates. And it has certainly worked well for us. We are now into ever easing ice conditions - we are starting to be influenced by the ocean swell again so the open ocean cannot be too far away. There is still
Ice about, but getting thinner and more open water.
I think our next episode was going to be about continental ice and icebergs. That will have to be it for now.
Berkley, OzGold, Dreena, Puff, Oily, Millie and Syd and not forgetting our host Gordon