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Hull speed theory?
Barry Palmer wrote: Mathematical modelling, even with things as simple as earth surface gravity, always involves a hypothesis regarding the physical principles involved. If the apparent results line up with the hypothesis, the "theory" could be said to be correct. In the case of Hobie Cat versus 45 footer, one only has to look at the theory in a different way, one that takes slenderness into account. Obviously, as a given hull becomes finer, less wavemaking potential exists, and load carrying ability of the hull diminishes. As the extreme of a knife edge in the water is approached, load carrying capability approaches zero, and friction with the water becomes the dominant drag force, making wave drag less relevant, as friction becomes greater and greate compared to wave drag. A hobie is a different study. The side of the hull may be considered as part of a system most easily understood as one hull lifts clear of the water, balanced somewhere between the sideways, upwards and downwards forces acting on the sail and the keel effect of the more deeply submerged lee hull. The balancing act works because it uses the keel area of the side of the hull as a planing surface. The lift from planing is not vertical, but presents a vertical component in a vector analysis of all the forces at work. If you have ever ridden one, you can see the flat water behind the lee hull. A hobie on one hull is not in pure displacement mode any more. The narrownwess of the hull means it does not make a large hole in the water, requiring energy to move a lot of water aside. A hobie does not climb over it's bow wave, it slices like an edge on a ski on ice. It works more like a skipping stone. I do not know why someone has not built a catamaran with flat bottom rails angled so as to enable a more stable balancing act, with perhaps 10 % of the weight still on the windward hull, supported by the flat portion on plane. There would still be a possibility that a lump in the water would launch the windward hull towards orbit. It would still require twice the level of concentration as driving a car on only two wheels. The opposite extreme is found in hovercraft, where there is little friction (in smooth water), and the body making the wave is broad, and wave making theory comes into play, with friction of minor concern. (High drag in rough water is a problem with hovercraft, which may be overcomeable, but not at all explored by the industry.) To analyse the wave making problem on my surface skimmers, I took a different approach from classical marine analysis, and got roughly the same results. I assumed the craft influences a semi-circle section of water under it and gives that body of water a downwards force, the vertical momentum of which is the dynamic lift on the hull, as lift from displacement diminishes as forward speed builds. Whadiya know, it works for broad barge-like hull bottoms with no friction. After all, the theory of earth surface gravity breaks down when you start to get away from the earth's surface, yet we use the theory in many engineering endeavors. (When you leave the planet, Neutonian physics "theory" breaks down.) Subject: Hull speed theory? From: (JAXAshby) Date: 7/20/03 5:52 AM Pacific Daylight Time Message-id: it ain't any theory whatsoever. It was merely a "scientific" explanation given to 19th century British naval brass to "help" them understand why putting 2x the power in a boat didn't make the boat go 2x the speed. The "theory" sounded scientific and it had numbers in it and it was called a theory so the brass accepted it. most sailboats built in the last several decades will easily exceed "theoretical" hull speed. in fact, a deep vee hullled Hobie cat (displacement hull by any standard) at 18 feet will under many points of of sail easily pass a 45 foot sailboat (also a displacement hull for most every 45 sailboat out there). Barry Palmer, for A HREF="http://members.aol.com/sevtec/sev/skmr.html"Sevtec/A -- Terry K - My email address is MY PROPERTY, and is protected by copyright legislation. Permission to reproduce it is specifically denied for mass mailing and unrequested solicitations. Reproduction or conveyance for any unauthorised purpose is THEFT and PLAGIARISM. Abuse is Invasion of privacy and harassment. Abusers will be prosecuted. -This notice footer released to public domain. Spamspoof salad by spamchock - SofDevCo |
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