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www.sname.org/sname/mt April 2013 FFOLLOWING AN 1872 PRESENTATION by Sir Frederick Bramwell, reporting sea trial results of John I. ornycrofts steam yacht Miranda , which had achieved twice hull speed, noted naval archi- tect J. Scott Russel commented, I see here a set of phenomena exhibited in direct contradiction to all of the received laws of dynamics and hydrody- namics.? Naval architects in that day believed that waterline length of a vessel was the dominant fac- tor limiting maximum speed; this was possibly a mental block evolving from a history of sailing ships and underpowered steam-driven vessels. Russels comment was related to the accepted theory of the day that maximum possible hull speed? occurred when V/(L) 1/2 = 1.34, which is the dimensionless Froude speed, F nL = 0.40.In 1863, ornycroft, an English designer and constructor, had developed a steam launch, Ariel , which achieved a speed of FnL = 0.57?the earli-est record Ive found of a powered boat or craft substantially exceeding hull speed.? One can imagine the disbelief of this speed by the naval architectural community in 1863. us, when ornycroft launched Miranda in 1871, dem-onstrating a trial speed of F nL = 0.82, he wanted to have unquestionable performance docu- mentation. He therefore retained independent consulting engineer Sir Frederick Bramwell to conduct and present his ndings to the naval architecture community. What were the evolutionary factors that Ariel and Miranda had that might have contributed to speed substantially in excess of hull speed? One might make inferences about Ariel from an early photograph of Nautilus , hull number 1 of ornycroft, launched in 1859, and sketches of Miranda . (Note: Nautilus achieved a reported speed of FnL = 0.46 in 1859, but a second source for confirmation could not be found.) Both Nautilus and Miranda had the following features: ? round bilge and double ended, sailboat-like underwater hull ? high length-to-beam ratio ? single, xed-pitch screw propeller ? propeller and engine were directly connected ? rudder on centerline, forward of propeller ? power ? reciprocating steam engine ( Miranda had an engine having total indicated horse- power (IHP) of 64, while Ariel s power is unknown). Their underwater hull geometry did not encourage hydrodynamic lift, in fact, just the opposite, and was not a factor for exceeding hull speed. My view is that the combination of recip- rocating steam engines and some knowledge of xed-pitch propeller characteristics was the seed that gave birth to performance vessels. Propeller shafts were directly connected to the engines crankshaft, so there were no gears. Backing down required engine rotation to stop and then restart with reverse rotation. Also, steam engines had good torque at low rpm to drive relatively high- pitch ratio propellers. Another fantasy worthy of consideration is that ornycrofts design and construction (with the assistance of family members) of Nautilus in