Propellers

Screw Propulsion - A Victorian Revolution

Posted March 21, 2012, 2:24 p.m.
SCREW PROPULSION – A Victorian Revolution By Chris Kidd Screw propulsion! Depending on where you put the emphasis on those two words it could be a question, or invective. In this case it is a question. It is a question that I had not even given a moments thought to before I was asked to write this article, even though I have been around boats for the past 30 years. In all that time I had never asked myself why the technical term for the form of propulsion that uses a propeller is screw propulsion. Having asked a few other people who should also know, including the very knowledgeable editor of this publication, it became clear that lots of other people involved in the boating business did not know either. Eventually I found someone who was able to get part way to the answer. Embarrassingly he is a friend who happens to be an optician, and who knows as much about boats as I do eyes but was able to tell me it was Archimedes. Having said it in that annoying, 'I know pretty much everything', way of his, he was left floundering as to how Archimedes screw, a device developed by that very clever man in the 3rd century BC, could have much to do with a fan shaped object that first appeared on boats around the 1830's. The point is, it has everything to do with Archimedes, his screw, and a chance accident. Archimedes developed his screw in the first instance to lift water from lakes, streams and other areas of low lying water into irrigation ditches. The screw, which incidentally does look just like any screw that you would put into wood, and certainly unlike any propeller you have ever seen, was positioned inside a hollow pipe and was then turned by manual labour or windmills. Water is lifted up on the bottom spirals of the screw and as it is turned it eventually reaches the top and is disgorged into the waiting pipes of the relevant irrigation system. Archimedes would no doubt be delighted to know that his invention, which went on to be used by the Romans to remove water from mines, is still widely in use today. Significantly more modern in appearance, but still essentially the same design that he came up with over 2000 years ago you can find in many varied uses from sewage treatment plants, draining the polders in Holland to the transport of granular materials in factories such as plastic granules used in injection moulding or the movement of cereal grains. At this point you could be forgiven if you still could not see the relationship between the screw and today's modern propeller, and this is where our one in a million chance accident comes in. Fast forward 2000 plus years to the late 18th century and the invention of the steam engine; an invention that presented naval architects with the first practical means of delivering power to an Archimedes screw on a ship. Yes, the first screw driven ships still had shortened versions of the same implement that for 2000 years had been lifting water into irrigation ditches. Several inventors where racing to try and produce the first efficient screw driven ship of any note but it fell to one Frances Petit Smith, an Englishman, and a farmer by trade who just happened to have a fascination with screw propulsion, to have the lucky accident which would make him famous. Smith secured a patent for his screw propeller on the 31st of May 1835. After building a small model to test his theory and having secured some finance Smith built a 30 foot, six horsepower canal boat which used a wooden screw propeller of two turns. He used this to run trials and demonstrate his invention to interested parties on the Paddington Canal between November 1836 and September 1837. It was on one of these trips that his screw propeller broke, effectively in half, and became a single full turn rather than two; to Smiths amazement the little vessel doubled its speed from around four miles an hour to eight; not surprisingly Smith rushed to amend his patent. When one sees how long it sometimes takes us to progress new inventions in the 21st century, the speed with which they moved in the 19th was truly amazing. The Navy had already come to the conclusion, for some unknown reason, that screw propellers would be unsuitable for seagoing service. They were already developing paddle wheel propelled ships but Smith was determined to prove them wrong. In September 1837, having re-propped his little vessel with an iron propeller of one turn he undertook a demonstration trip from Blackwall in London to Hythe in Kent with stops at Ramsgate, Dover and Folkstone. The Navy was not that interested however until the return trip, when good fortune again visited Smith in the shape of stormy weather. His little craft continued to make good progress in difficult sea conditions; such good progress that the Admiralty, though still not so fully confident as to put up Government money, did nonetheless encourage Smith to build a much larger vessel. Smith had little trouble getting backers and only one year later the SS Archimedes was launched at Millwall; a vessel of 125 feet and 237 tons. After fitting out was completed Archimedes made her maiden voyage on the 2nd of May 1839 and was able to attain 10 knots. The navy trialled her against one of their fastest ships HMS Vulcan, a paddle wheel driven Frigate, and Archimedes won the day, further enhancing the navies rapidly growing appreciation of the potential of screw propulsion. Archimedes had one major problem however; her original full helix, single turn, single threaded propeller created huge amounts of vibration at the stern of the ship. Changes to design where much more on a trial and error basis than they are today and again Smith was fortunate in his choice. He replaced the original propeller with a double-threaded, half turn propeller with two distinct blades; a change that dramatically reduced the vibration to acceptable levels. For those of us brought up on thinking that folding and feathering propellers are a product of the 20th century, while that might well be true it did not mean that the naval architects and engineers nearly 200 years ago where not fully aware of the significant effect of the drag of the fixed propeller right from day one when used on a sailing vessel. The solution on Archimedes; a fully retractable propeller; an operation that took fifteen minutes to accomplish. The Navy, now much more interested in the abilities of screw propellers, put Archimedes up against the fastest Dover – Calais mail packets, all paddle steamers. These were much more 'official' trials than the previous one and where overseen, on behalf of the Admiralty, by Captain Edward Chappel. The fastest of the mail packets, the 'Widgeon', was just able to beat Archimedes on smooth water but Chappels report concluded that 'screw propulsion' was the way forward, offering several important advantages for fighting ships, not least the 'propellers' much reduced vulnerability to enemy fire compared with paddle wheels, and the ability to carry more guns for broadsides than was obviously the case with the intruding paddle wheels. From this point forward the future of screw propulsion was assured and Archimedes continued to be used by the Navy and other designers and engineers as the demonstrator for the new propulsion system. Most notably Smith agreed to lend Archimedes to the Great Western Steamship Company who were then constructing the world's largest steam ship. Brunel, the Great Westerns principal engineer took the opportunity to test Archimedes with various propeller designs and again it was Smith who produced the wining version, in a few short years having gone from two blades to four. Brunel however, right at the last moment, installed a six bladed 'windmill' propeller which he had designed. It was a costly failure! The Navy, in order to make a final decision decided on trials between two identical ships. Identical that is, in all but propulsion method. The ships were specially constructed for the purpose, HMS Rattler having the screw propulsion and HMS Alecto the paddle wheels. As a result of the trials, which included a famous 'Tug of War' between the two vessels when 'Rattler' succeeded in pulling 'Alecto' backwards though the water, the Navy adopted screw propulsion as its preferred propulsion method. Only ten years after the trials between 'Rattler' and 'Alecto' were completed the Navy had 174 screw propulsion driven ships in service, including 52 'Ships of the line'. ...and what of Smith. Like so many inventors he lost money rather than became rich and was forced to return to farming, but he was not forgotten, and in 1855 he was one of five inventors, all of whom were working on the marine propeller, who were awarded £4000 each by the House of Commons. In 1858 a group of patrons presented him with some silverware and a financial award which collectively amounted to £2,678. In 1860 he was appointed Curator of the Patent Office Museum in South Kensington and in 1871 was given a Knighthood. Contact: chris@marketingconnections.biz Chris is the marketing co-ordinator for Bruntons Propellers who have supplied the modern propeller illustrations in this article.
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