Symon -> RE: SOT: Naval Architects of Lore May Have Been Right After All (6/1/2015 2:51:11 PM)
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I’m a bit skeptical. And a bit annoyed, but more at the idiot who wrote the article. There are no 100 year old myths to debunk, here. Software is a nice tool, but overated. I certainly agree that there’s a great deal we don’t fully understand about their functional hydrodynamic qualities. But many permutations of bulbous fore-bodies have been extensively evaluated in empirical model/tank testing for decades, all over the world: Taylor Basin, Stevens Institute, NJ; Maritime Research Institute (MARIN), Netherlands; CEHIPAR, Spain; Bassin d'essai des Carènes, France; HSVA, Germany; SSPA, Sweden, etc.. The empirical test results are unambiguously clear and have been verified in actual ships. There was a test performed on identical C2 cargo ships, some with various bulb configurations, some with a stock bow, by Todd Shipyards (now Vigor) in the mid ‘60s under Ralph Anselmi. Once again, the results were unambiguously positive. The different bulb configurations helped promote an understanding of just what the heck was going on. Many, many things are happening, and many, many things are still in dispute (the math/understanding just isn’t there). “Roughly”, and I do mean roughly, some things are “known”. One thing, certain, is that the “known” effects relate to the reduction of wave-making resistance, and are most pronounced at the first Froude limit. 1) a bulb adds an arbitrary volume which allows for a consequent reduction in volume in the remaining bow sections, in turn, allowing a finer entry with reduced wave-making resistance while maintaining coefficients (@jmalter, there’s where your prismatics come to play pal). 2) depending on shape, length, etc.. a properly designed bulb sets up an interference wave pattern that destructively interferes with W-1 at Fr-1 and reduces wave-making resistance by “spoofing” a larger [Sqrt]L. Obviously, if one is below the Froude limit, wetted surface area is the prime determinant of hull resistance. A bulbous bow at low speeds don’t hep ya none, might even hurt ya some (but even here, the volumetric compensations limit the effects to negligent quantities). But for straight line tracking, at-speed, it demonstrably works. It works best at that break point in the power curve at Fr-1. For stuff like this, the math is simple, just plug in a slightly higher [Root]L and you get either lower Power for the given V, or a higher V for the given Power. Sorry for the pedantic nonsense. It's just that this is very interesting to me and is probably more important to me than my profession. Ciao. JWE
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