Propeller shafts and propellers.

PROPELLERS & SHAFTS

Balmoral has two engines, two propeller shafts and two propellers. She is described as a 'twin screw' ship as propellers are also called screws.

PADDLES V PROPELLERS

Early ships operating on the Bristol Channel had paddle wheels. These are huge wheels ... with individual paddles .... on each side of the whip and they push water behind as they turn and create thrust to push the ship forward. They are almost as efficient as a propeller, despite what you may read in some books, but they are a lot more complicated and only suitable for very low speed engines. Paddles push a lot of water slowly, so they are better for slower speeds and propellers push smaller amounts of water much faster, so they are easier to steer and generally the ship will have a higher top speed ( but not always ! ). Paddles were excellent for early steam powered ships with very slow turning engines, and they can be used in shallow water as they do not stick down below the hull and risk hitting the bottom, but they do not work very well with a diesel engine as it turns much faster and thus needs lots of gearing ( or an electric drive system ) to slow the final drive to a speed that works with a paddle wheel. A paddle will typically turn 80 times in a minute and a screw propeller might turn ten times faster.

These photographs show the difference between paddles and propellers - paddles push a lot of water slowly along the sides and propellers push less water faster from underneath the back of the ship. They do the same job but in a different way. Balmoral has four bladed propellers. Paddles can get really complicated with 'feathering' floats ( the flat plate that pushes the water ) so that they are always at 90 degrees to the water surface but the ones shown here are basic, fixed float paddles.

Paddles may be almost as efficient, but as you will see in the engine section, steam power is not. One of the reasons Balmoral was designed as a propeller driven ship was to save money and also to enable her to manoeuvre in tight spaces - and saving money, even in 1949 meant using diesel engines. Any paddle steamer that carries passengers must by law have the paddles linked so that they turn together, this was after an accident on the Thames in the early 20th Century, and thus they are difficult to turn at low speed as there is no water passing the rudders. A propeller pushes directly on the rudder behind it and thus the ship can turn more easily. Also, by going forward with one propeller and backwards with the other, the ship can almost turn in its own length which can be really handy in a tight spot. A paddle steamer carrying passengers can't do that !

Thus, if you want to operate in shallow water and can afford more crew and more fuel, paddles are good, but if you want to operate at low cost, visit places that are limited in turning space and want to be able to start and stop easily, you want diesel engines and propellers. Sadly today, paddle steamers are not used other than for historic excursions and pleasure cruising - they just cost too much to operate !

PROPELLER SHAFTS

Propeller shafts transmit the power from the engine to the propellers at the stern of the boat. The propeller has angled blades that literally screw it through the water and this produces thrust that pushes the ship along.

The shafts run half the length of the ship and are strong enough to transmit the power of the engines at full speed. They are supported in large bearings so that they are held exactly in line with the opening at the back of the ship where they pass through the hull. Any misalignment would cause very rapid wear and lead to leakage. This assembly is known as the stern gland and is a very heavy duty waterproof bearing and seal that keeps the water out of the ship and lets the shaft turn smoothly.

Outside the hull of the ship, the propeller shaft has a further length of tube which also needs to be held steady or it would flex and vibrate, so there is a large steel 'A bracket' that acts as a final bearing ( tail shaft bearing ) and holds the end of the shaft firmly in place. Directly behind the a bracket is the propeller. Behind the propeller is the rudder, positioned so that water from the propeller flows past it to increase the effect of the steering.

This is one of Balmoral's propeller shafts and bearings - the shaft is about 200 mm in diameter.

The big ring on the right is a coupling - if the shaft has to be removed this is undone and the whole shaft pulled out of the back of the ship ...but only when it is in drydock !

This is the shaft 'tunnel' looking forward towards the engines. It isn't really a tunnel as the floor of the lounge forms the top and it has no sides either!

The structure of the hull, the ribs and the actual bottom of the ship are clearly visible.

The round thing with big bolts is a coupling to join parts of the shaft together.

This has to be very accurately aligned so that there is no movement and the entire shaft is in a direct line from the engine crankshaft to the propeller . It is supported by bearings throughout its length.

As well as turning the propeller the shaft has another purpose. There is a force of several tons on the shaft at full speed. The propeller transmits the power of the engine to the water, but the water is pushed backwards as the propeller blades try and move forwards. This creates a forward pressure on the propeller and this in turn pushes on the shaft. If the pressure is not absorbed into the structure of the ship, the whole propeller shaft would try and move forward into the engine room, and wear out the main engine bearings by putting a huge force on them - they are not designed to take this ! Additionally the movement would destroy the tail shaft bearing, stern gland and cause a major water leak !

Thus every propeller shaft has a thrust bearing assembly. This is a large bearing that is supplied with oil to lubricate and cool it. The outer part of the bearing is firmly attached to the hull of the ship and this is the point at which all the power of the engine is absorbed and where the force generated by burning fuel actually becomes the force that moves the ship.

Balmoral has twin screws and twin rudders. The water moving away from the screws when going forward passes the rudders and makes the steering more effective.

The white blocks on the rudders are zinc anodes designed to reduce corrosion of the steel hull.

You can see the four bladed propellers and the edge of the 'A bracket' immediately ahead of it.

If the tail shaft bearing or stern gland fails, the ship will be unable to move and may leak water. To make matters worse, it can't be repaired while the ship is afloat and has to be done in a 'dry dock' .

Thus it is essential that the stern glands are kept in very good condition.

If the propeller shaft needs repair, it has to be removed from the ship by taking off the rudder behind the propeller and then pulling the shaft backwards out of the ship .

This is an horrendous job and although the safety authorities insist that the bearings are checked regularly, it costs a lot of money and takes a long time.