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In the early days of trawling, the sailing smacks towed a beam trawl. This trawl had its mouth held open in a lateral direction by a heavy wooden beam. Vertically it was held open by two large iron hoops fixed to the ends of the beam. This weight placed severe restrictions as to the weather that the smacks could fish in, not enough wind to power the sails and the fishing gear would act as an anchor and the boat wouldn't move. Too much wind and there was a danger of being dismasted.
It was the advent of the steam engine that allowed a quantum leap in fishing methods. No longer was the weather as significant as the steam engines developed enough power to tow the heavy gear. Once the steam engine had been coupled with the invention of the Otter Door. the basic design of fishing gear was to remain unchanged down through the years to the present day. We have James Watt to thank for his development of the steam powered, triple expansion engine that was the heart of trawlers until overtaken by the diesel engine. Bu steam has left its indelible mark. Even today a vessel underway is said to be 'steaming'. Let's face it, to say that a vessel is 'dieselling' for the fishing grounds doesn't sound right, does it?
James Watt became instrument maker for the University of Glasgow in 1757. It was while he was repairing the university's model of the Newcomen engine that he realised that it was inefficient and could be made to work much better. He know that much of the energy of the steam was being used up in reheating the cylinder after each inrush of water. In his own words "...the idea came into my mind that, as steam was an elastic body, it would rush into a vacuum, and, if a communication were made between the cylinder and an exhausted vessel, it would rush into it, and might be there condensed without cooling the cylinder".
Watt built a first engine model and decided that the steam would to go in the cylinder above the piston. The cylinder was closed with a cap with a stuffing box for the rod to pass through.
The steam helped atmospheric pressure to drive the piston down.
In 1769 Watt sought and obtained a patent for a "new method for lessening the consumption of steam and fuel in fire engines". When Watt gained support for developing his engine from the great Birmingham manufacturer, Matthew Boulton, he started manufacturing engines.
In 1776, he built an engine with a cylinder of 127 cm diameter to pump water at Bloomfield Colliery. In the same year Wilkinson, iron master, built a new type of lathe.
This lathe made possible to bore a cylinder with a precision which had never before been seen.
The final version of the new Watt engine worked in 1778. In 1782 Watt made his double-acting engine. With this improvement the engine had double the power with the same displacement. To further save coal, the steam was admitted inside the cylinder for a fraction of the stroke which continued by steam expansion. Watt never developed engines that were powerful for their weight, because he refused to use high-pressure steam. He feared that could not make the boiler and engine strong enough to withstand such pressure with the iron and workmanship of the time. He did, however, make minor improvements, such as the steam governor that now is named "Watt's governor".
From these small beginnings the steam engine progressed through several incarnations. Perhaps the most significant of these was the triple expansion theory. This engine, made possible by advances in metalurgy that led to stronger materials, used the same steam three times through a series of cylinders. The steam would be fed at high pressure into the first cylinder and be exhausted, at a reduced but still considerable pressure, into an intermediate cylinder where it did its work again. Upon exhaustion it went into a third (low pressure) cylinder where it used the last of its energy up.
This is the basis of the engines that powered the old trawlers. Coal fed into the boiler heated the water which produced steam. This the entered the high pressure cylinder and on through the intermediate and low pressure cylinders. The only change to this configuration was when the coal was replaced by oil which was burned to produce the steam. This last picture shows the final configuration of the triple expansion engine although there was one variation in which a fourth cylinder was employed making a quadruple expansion engine.
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