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Technical sheet

length 53,77 m
breadth 6,40 m
height 2,60 m
draft 1,35 m
displacement 286 t
maximum load 900 persons
engine power 600 CV
maximum speed 28 Km/h

Characteristics (^)

The twin steamers "Patria" and "Concordia" were put in service during the twenties by the "Societa' Lariana", the company that at that time ran the navigation service on the lake Como, to face the need for fast steamers. Built by the firm Odero from Sestri, they were launched in 1926 at the Dervio dockyard with the names "Savoia" and "28 Ottobre" respectively. The hull is made by riveted steel plates. They measure 53,77 m in length, 6,40 m breadth (12.30 including the paddle wheels), 2,60 m height, 1,35 m draft, and, respectively 209,95 t and 286,14 t empty and full load displacement. Originally they could carry 900 people and 10 crews, including two engineers and two firemen.

They are vessels with only one covered deck, of the type called "half saloon" , because the first-class saloon on the stern lies below the covered deck. The paddle-wheel propellers are powered by a diagonal three-cylinders compound steam engine with the "Caprotti" valve distribution, developing 600 HP at the maximum speed of 28 km/h at 60 rpm at the trials by the consumption of 450 Kg of coal an hour. The boilers with two furnaces each, are of the "Scotch" type with smoke tubes and were later adapted to oil firing.

The engine (^)

Passengers can still admire the operation of the Concordia's big engine. The wheels are joined by a steel-forged crankshaft with three huge cranks at 120 degrees that take motion, through three connecting rods, from three inclined cylinders unequal in size because of the different steam pressure they work with. The great size of these engines, if compared to modern ones of equal power, is due to the low rotation speed of the wheels. In fact, the power provided by an engine can be expressed as the product of the force and the velocity at which the force is exerted, so that low speed requires high forces. When paddle-steamer engines were developed reduction gears for such powers weren't yet a mature technology. As a consequence the engine had to be coupled directly to the paddle shaft and, thus, had to be large. Later, the use of reduction gearing and screw propellers, that run at higher speed, allowed for reduced forces and less bulky engines.

The large size of the engine has always been a problem in designing paddle-wheel steamers because it subtracted room to the payload. On the other side, cylinders could not be fitted directly underneath the crankshaft because of the small room available between the shaft itself and the hull. For this reason, several smart solutions were proposed in the last century, starting from the "side-lever engine" to the "oscillating-cylinder" engine.

The Patria's engine has three inclined-cylinders, the type that became common for paddle wheelers at the end of the past century. The cylinders have different size because they implement compound steam expansion. In direct expansion, three equal amount of steam would be completely expanded in three cylinders of equal size. With compound expansion, a triple amount of steam is expanded in cascade. That is, the steam is partially expanded first in the high pressure cylinder, then it passes to a medium pressure cylinder of larger dimensions to expand further, and finally it terminates its work in the low pressure cylinder, which is the bigger one (see Figure 1 for two-stage expansion cylinders).

Compound expansion is normal practice in marine engines, where savings in fuel are very important. In fact, splitting the steam expansion into three stages avoids the wide change of temperature in cylinders, and the consequent heat loss, that occurs with single expansion. As a further advantage it avoids complete expansion in a single cylinder, which is difficult to obtain.

Notice that compound expansion complicates the engine operation and starting the engine may become difficult. In fact, since steam is admitted into the engine through the high pressure cylinder only, steam cannot pass to the other cylinders until the first expansion in the high pressure cylinder is completed. Thus, the very first movement is attained by the high pressure cylinder only. Further problems arise if the crank is close to the dead point. If this is the case, fresh steam must be directly introduced into the medium and low pressure cylinders. This practice was mandatory in steam locomotives that adopted compound expansion because, unlike boat, train service requires full power on starting.

Three-stage expansion was common practice in marine engines for screw propellers, where the most popular engine was the three-cylinder compound inverted vertical type, i.e., with cylinders above the shaft, like the internal-combustion motors, as shown in Figure 2 . However, paddle wheelers almost always wrere fitted with two-cylinders compound diagonal engines (Figure 3 , Figure 4), so that the Patria's and Concordia's engines can be considered rare.

The paddle wheels (^)

The wheels derive from a model introduced in England by William Morgan in 1830. They have floats that can turn on their axis. Each float has a crank arm that is driven by a rod connected to a ring that turns with the paddle wheel but is not concentric with it. As the wheel rotates, the connecting rod turns the float so that it enters and leaves the water almost in vertical position. In fact, links that keep the float in exactly the vertical position were found unsatisfactory, as, in fact, were the wheels with fixed radial floats.

The steam cycle (^)

The Patria's power generation is based on "open" steam cycle. In fact, the water for the boilers is directly taken from the lake and after being purified is fed into the boilers by a feed pump which is driven by an eccentric on the crankshaft. The water is converted into saturated steam at a pressure of 12 Kg per sq cm., that corresponds to a temperature of 187 degrees centigrade. The saturated steam is collected into the steam drum, a cilindrical vessel placed transversally above the boilers, and from there it is fed into the superheater, where its temperature is raised to about 230 degrees centigrade by the smoke that flows from the smokebox to the funnel. Superheating has several beneficial effects. First of all, the heat absorbed to increase the steam temperatures increases the work given by the steam during the expansion in the cylinders and in this way it helps saving fuel. Also, when saturated steam loses part of heat it condenses into water. This can be dangerous if happening in the cylinders, because too much water can damage the piston during the exhaust phase. On the contrary, superheated steam cannot condense until it has lost all its superheat so that the inconvenient cited above cannot happen.

From the main steam valve steam is admitted into the engine and expanded. The low pressure cylinder exhausts into the condenser, a device that maintains the exhaust pressure below the atmospheric one. Such a low pressure can be achieved because the pressure at which the steam condenses is dictated by temperature. Thus, in a condenser the steam is cooled by cold water. Since room is precious aboard the Patria, the condenser is one of the jet type, in which the steam is mixed to a sprinkle of water. The water in the condenser is then extracted, taken to the atmospheric pressure and thrown off board by a pump, this too directly driven from the engine. Since this water is at higher temperature than the boiler feed water, the efficiency of this cycle is less than in the "closed" cycle, in which the condensed water is stored in the "hot well" and re-used, as is usual in marine practice.

The boilers (^)

The steam production is assured by two "Scotch" boilers that are arranged as shown in Figure 5 and Figure 6. This kind of boiler originates from England about 1860 as an evolution of the preceding cylindrical boilers of the "Cornish" and "Lancashire" type. The boilers are fitted with two cylindrical furnaces each , coal fired in conception. The furnace tubes are of the Fox corrugated construction, which offers great resistance to the collapsing pressure exerted by water outside the furnaces. At the rear of each furnace is a combustion chamber , from which the nests of return fire tubes run parallel with the furnaces, within the water section, out to the front of the boilers where the smokebox is placed (not shown in figure). The boilers are completed with horizontal stays to strengthen the flat ends. The inner part of the furnaces is fitted with bricks. During the fifties coal firing was converted into oil firing.

(^)

Short history of the paddle-steamer engines from the University of Toronto.

More technical details can be found in:

  1. J.T. van Riemsdijk and K. Brown, "The Pictorial History of Steam Power", Octopus Books Limited, London, 1980.
  2. H.P. Spratt, "Marine Engineering", Descriptive Catalogue, Science Museum reprint series, London.
  3. R. J. Law, "The Steam Engine", Science Museum booklet, London.

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Created: November 1996. Last update: April, 1998